Submarine Landslides: Selected 

. Studies in the U.S. Exclusive 

Economic Zone 

U.S. GEOLOG~CAL SURVEY BULLETIN 2002 
Submarine Landslides: Selected 

Studies in the U.S. Exclusive 

Economic Zone 

Edited by W.C. SCHWAB, H.j. LEE, and D.C. TWICHELL 

U.S. GEOLOGICAL SURVEY BULLETIN 2002 
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Any use of trade, product, or firm names 

in this publication is for descriptive purposes only 

and does not imply endorsement by the U.S. G_overnment 

UNITED STATES GOVERNMENT PRINTING OFFICE: 1993 

For sale by 

U.S. Geological Survey, Map Distribution 

Box 25286, Bldg. 810, Federal Center 

Denver, CO 80225 

Library of Congress Cataloging in Publication Data 

Subma-rine landslides : selected studies in the U.S. exclusive economic zone I 

U.S. Department of the Interior, Geological Survey ; edited by W.C. 

Schwab, H.j. Lee, and D.C. Twichell. 

p. em. - (U.S. _Geological Survey bulletin ; 2002). 

Supt. of Docs. no.: 119.3:2002 

1. Mass wasting-United States. 2. Marine sediments-United States. 

I. Schwab, William C. II. Lee, Homa j. Ill. Twichell, David C. 

IV. Series. 

[QE598.5.U6] 

557.3 s-dc20 

[551.3'07] . 91-33678 

CIP 
FOREWORD 

With each passing year, the United States becomes increasingly dependent upon 

resources from the oceans. Historically, the sea has provided food resources, recreational 

resources, and a defensive barrier in times of war. During the last few decades·, we have 

turned to the sea to provide other resources such as oil, gas, and construction aggregate 

which had been almost exclusively extrac.ted from onshore areas. With an ever-growing 

population demanding more resources to sustain a high quality of life, the United States 

will have to turn more frequently to the oceans to meet its increasing requirements for 

fresh water, energy minerals, industrial minerals, agricultural minerals, waste disposal, 

recreational diversions, military exercises, and other activities. All this must be 

accompanied and balanced with environmental con~erns. 

Recognizing the growing importance of the oce~ns to the future of the United 

States, President Reagan in 1983 expanded the United States to include those parts of the 

oceans extending 200 miles off our shores. This enormous new territory, the Exclusive 

Economic Zone (EEZ), is over 3 million square nautical miles in size which is about 30 

percent large than the land area of the United States. 

The United States Geological Survey (USGS) through its Offshore Geologic 

Framework program was directed by the President and the Congress to explore the sea 

floor of this new frontier. In partnership with c~lleague_s from other Federal agencies, the 

States, academia, industry, and scientists from the international community, the USGS 

began a program in 1983 to map the EEZ. To date, these recoimaissance-scale studies 

have mapped approximately 2 million square nautical miles of the EEZ and unveiled a 

fascinating, but extremely complex, sea floor. These discoveries hold the promise of 

potential resources that ultimately will help meet the needs of all Americans. This promise 

will be realized, however, only through continued re~earch, technology adv.ancement, and 

informed decision making. · 

Although our studies discovered significant potential resources, they also identified 

geologic processes that not only threaten the availability of these potential resources but 

also threaten human life directly. This volume is dedicated to increasing the understanding 

of one such process-submarine landslides. The authors focus on documenting our new 

understanding of the geologic processes involved in submarine landslides and the 

consequences of these processes. 

Submarine landslides fall within a general category marin~ geologists call geohazards. To evaluate fully the significance of any geohazard, it is necessary to develop an 

understanding of the basic geological processes involved as well as the geographic 

distribution, frequency of occurrence, scale, magn1tude, and consequences of the 

processes. All these topics and more are covered as the authors share their insights into 

submarine landslides occurring in diverse geologic settings throughout the EEZ from New 

England to Hawaii, from the arctic to the tropics, and from shallow water to deep ocean 

basins. 

Without question,. submarine landslides involve ~ignificant geologic processes 

which must be understood and documented for policy makers to make informed decisions 

as to when and how the marine resources within the United States EEZ are to be exploited. 

This volume is a substantial and unique contribution to expanding the knowledge base of 

submarine landslides. 

Gary Hill 

Foreword Ill 

CONTENTS 

Foreword III 

Submarine Landslides: An Introduction 1 

By H.J. Lee, W.C. Schwab, and J.S. Booth 

U.S. Atlantic Continental Slope Landslides: Their Distribution, General Attributes, and 

Implications 14 

By J.S. Booth, D.W. O'Leary, Peter Popenoe, and W.W. Danforth 

Submarine Mass Movement, a Formative Process of Passive Continental Margins: The 

Munson-Nygren Landslide Complex and the Southeast New England Landslide 

Complex 23 

By D.W. O'Leary 

The Cape Fear Landslide: Slope Failure Associated with Salt Diapirism and Gas 

Hydrate Decomposition 40 

By Peter Popenoe, E.A. Schmuck, and W.P. Dillon 

Ancient Crustal Fractures Control the Location and Size of Collapsed Blocks at the 

Blake Escarpment, East of Florida 54 

By W.P. Dillon, J.S. Riseh, K.M. Scanlon, P.C. Valentine, and Q.J. Huggett 

Tectonic and Stratigraphic Control on a Giant Submarine Slope Failure: Puerto Rico 

Insular Slope 60 

By W.C. Schwab, W.W. Danforth, and K.M. Scanlon 

Slope Failure of Carbonate Sediment on the West Florida Slope 69 

By D.C. Twichell, P.C. Valentine, and L.M. Parson 

Slope Failures in\an Area of High Sedimentation Rate: Offshore Mississippi River 

Delta 79 

By J.M. Coleman, D.B. Prior, L.E. Garrison, and H.J. Lee 

Salt Tectonics and Slope Failure in an Area of Salt Domes in the Northwestern Gulf 

of Mexico 92 

By B.A. McGregor, R.G. Rothwell, N.H. Kenyon, and D.C. Twichell 

Slope Stability in Regions· of Sea-Floor Gas Hydrate: Beaufort Sea Continental 

Slope 97 

By R.E. Kayen and H.J. Lee 

Mass Movement Related to Large Submarine Canyons Along the Beringian Margin, 

Alaska 104 

By P.R. Carlson, H.A. Karl, B.D. Edwards, J.V. Gardner, and R. Hall 

Comparison of Tectonic and Stratigraphic Control of Submarine Landslides on the 

Kodiak Upper Continental Slope, Alaska 117 

By M.A. Hampton 

Submarine Landslides That Had a Significant Impact on Man and His Activities: 

Seward and Valdez, Alaska · 123 

By M.A. Hampton, R.W. Lemke, and H.W. Coulter 

Processes Controlling the Style of Mass Movement in Glaciomarine Sediment: 

Northeastern Gulf of Alaska 135 

By W.C. Schwab and H.J. Lee 

Contents V 
VI Contents 

Liquefaction of Continental Shelf Sediment: The Northern California Earthquake 

of 1980 143 

By M.E. Field 

A Submarine Landslide Associated with Shallow Sea-Floor Gas and Gas Hydrates 

off Northern California 151 

By M.E. Field and J.H. Barber, Jr. 

Sur Submarine Landslide, a Deep-Water Sediment Slope Failure 158 

By C.E. Gutmacher and W.R. Normark 

Seismically Induced Mudflow in Santa Barbara Basin, California 167 

By B.D. Edwards, H.J. Lee, and M.E. Field 

Submarine Landslides in a Basin and Ridge Setting, Southern California 176 

By M.E. Field and B.D. Edwards 

Giant Volcano-Related Landslides and the Development of the Hawaiian Islands 184 

By W.R. Normark, J.G. Moore, and M.E. Torresan 

Submarine Slope Failures Initiated by Hurricane lwa, Kahe Point, Oahu, Hawaii 197 

By W.R. No~ark, Pat Wilde, J.F. Campbell, T.E.· Chase, and Bruce Tsutsui 
SUBMARINE LANDSLIDES: SELECTED.STUDIES IN THE U.S. EXCLUSIVE ECONOMIC ~ONE 

Edited by W.C. Schwab, H.j. Lee, and D.C. Twichell 

Submarine Landslides: An Introduction 

By H.j. Lee, W.C. Schwab, and j.S. Booth 

INTRODUCTION 

Consider these events: In the late afternoon of March 

27, 1964, one of the most violent earthquakes of all time 

rocked southern Alaska. In the community of Valdez, 

shortly after the shaking began, a cargo ship unloading at 

the town dock began to toss violently, with vertical motion 

in excess of 10m (meters) and rolls of 50°. Next, the docks 

vanished into turbulent water. The entire waterfront followed, along with warehouses, a cannery, heavy equipment, and 30 people. Ten-meter waves propagated back and 

forth across the bay on which Valdez is situated, surging 

over 500 m inland and flooding much of the town. For 

months following these catastrophic events, the shore area 

near Valdez continued to subside. 

In August 1969, Hurricane Camille, one of the 

century's most intense hurricanes, struck the coast of 

Louisiana. At an oceanographic station near the South Pass 

portion of the Mississippi Delta, wave heights of 21 to 23m 

were recorded before the instruments used to measure wave 

height failed. Three offshore drilling platforms were badly 

damaged or destroyed. Later, one of the platforms was 

found, half-buried in mud and displaced 30 m downslope. 

On November 19, 1929, a major earthquake occurred 

on the continental slope south of the Grand Banks, off the 

eastern coast of the United States and Canada. During the 

earthquake, many submarine communications cables failed 

near the earthquake epicenter. For 13 hours following t~e 

earthquake, cables continued to fail in a systematic 

sequence: with the passage of time, cable breaks occurred 

farther and farther to the south. 

On the Hawaiian island of Lanai, limestone-bearing 

gravel blankets the coastal slopes and extends to an altitud.e 

of 326 m. The characteristics of the gravel show that It 

originally formed at sea level. The island is sinking. so 

rapidly that the deposit could not have been deposited 

during previous worldwide highstands of sea level. Most 

probably, the deposit was placed by a giant wave. 

These four examples share a common element: they 

all show the devastation that can result from submarine 

Landslides, a general term used to refer to downslope 

movement of sea-floor material en masse (also termed 

Manuscript approved for publication July 12, 1991. 

"mass movement"). At Valdez, a large portion of a submarine delta-front failed during the Alaska earthquake and 

slipped into the bay, carrying with it the town's waterfront 

and generating massive waves. During Hurricane Camille, 

the large wav.es generated by the storm loaded the weak 

sea-floor muds, causing them to fail and move downslope. 

On the Grand Banks, the magnitude 7.2 earthquake caused 

slope failures on the continental slope and rise. Some of 

these slope failures transformed into high-velocity turbidity 

currents that traveled over 720 km (kilometers) from the 

source area, breaking communication cables along the way. 

Off the coasts of the Hawaiian Islands, the remains of giant 

slope failures have been found using a modern side-looking 

sonar system. Some are more than 200 km long with 

volumes up to 5,000 km3

. The great displacement of the 

landslide blocks from their original source suggests that 

some of the failures were cataclysmic events, capable not 

only of removing landmass from the islands' coastlines .but 

also of generating tremendous sea waves. One of these gi~nt 

landslides may have been the cause of the wave that carried 

gravel from sea level up to an elevation of 326 m on the 

island of Lanai. 

Most submarine landslides are not so spectacular. In 

fact, most undersea slope failures probably occur without 

any person being aware of them, but as man reaches farther 

into the oceans to extract oil and gas and mineral resources, 

searches for environmentally safe areas to dispose of waste, 

and occupies more coastal areas, the potential impact of 

undersea landslides on humanity increases. Major loss of 

property and life has already occurred; more losses may 

occur in the future. Even when particular landslides have no 

direct impact on man's development, they are still of basic 

interest to geologists in understanding how sediment and 

rock are transported to deeper water. Mass instability acts as 

a fundamental process in the formation and modification of 

continental and insular margins. The occurrence of such 

features tells us about the environments in which they form 

and allows us to use fundamental geologic knowledge to 

exploit resources and increase our knowledge of the importance of environmental hazards such as earthquakes and 

hurricanes. 

The purpose of this report is to introduce the subject 

of submarine landsliding through selected studies that have 

been partly or fully supported by the U.S. Geological 

Submarine Landslides: An Introduction 
RECORD . ~ - I 

111 I 1 I 

Sea Floor Return (Echo) Acoustic Noise I I I I I 

J I - ~- -~-- ---=:! 

~,::~-: -·~:~~{f=S: ¥4-• .. ~· ~- ::_-~ • .1:.:. Subbottom Returns (Echoesr~--.. r 

n 

t 

Figure 1. High-resolution profiling technique. The ship is crossing a series of slump blocks (arrows showing the 

direction of relative movement). The record in the upper right is a real profile showing what this idealized mass 

movement looks like on a seismic-reflection profile. 

Survey. The report is not meant to_ be comprehensive but 

rather is an attempt to provide a cross' section of the type of 

research going on within the academic, government, and 

ocean-engineering communities. The selected studies 

clearly demonstrate that marine investigators are just beginning to appreciate that submarine landslides are a common, 

widespread feature of the U.S. Exclusive Economic Zone 

(EEZ) and impose many constraints on the future of 

sea-floor utilization. The title of each section reflects the 

major point exemplified by the landslide being described. 

The sections are mainly visual in format and are intended 

for individuals with a general interest in the offshore areas 

of the United States, either from a basic scientific or from a 

seabed utilization point of view. 

METHODS FOR FINDING SUBMARINE 

LANDSLIDES 

Submarine landslides are occasionally recognized 

when they directly involve the shoreline. The catastrophic 

failure at Valdez is one of the best examples of this type. 

Other landslides disrupt or destroy offshore facilities, for 

example, the offshore platform lost during Hurricane Camille or the submarine cables broken off the Grand Banks. 

Some landslides are identified when major changes in water 

depth or sea-floor relief are observed, perhaps by fishermen 

or divers. The detection and recognition of most submarine 

landslides, however, relies on remote observation of seafloor and subbottom morphology. That is, does the sea floor 

have the appearance of slope failure? Are features observed 

that look like landslides that have been seen on shore? 

Development in offshore remote-sensing technology 

over the last 3 to 4 decades (such as precision depth 

recorders, high-resolution seismic-reflection systems, and 

sidescan-sonar systems) has allowed marine investigators to 

determine the appearance of the sea floor accurately and 

map areas of landslides. The basic principle of seismicreflection techniques involves detection of sediment stratigraphy and structure as indicated by variations in acoustic2 Submarine landslides: Selected Studies in the U.S. Exclusive Economic Zone 
reflection characteristics (fig. 1). In general, the higher the 

frequency of the signal sent out by the shipboard seismic 

system, the higher the resolution and the smaller the depth 

of penetration below the sea floor. For example, a seismic 

system that uses an outgoing signal with a frequency of 12 

kHz (kilohertz) is typically used as a precision depth 

recorder (high resolution with little, if any, sub bottom 

penetration) while a 3.5-kHz system, although yielding 

a slightly lower resolution, is capable of penetrating over 

100 m of soft sediment and defining subbottom landslide 

geometry. High-power, low-frequency systems (for example, air guns and sparkers) can penetrate up to 15 km 

beneath the sea floor and resolve reflecting surfaces 100 to 

200m apart .. 

Probably the most significant recent development in 

mapping sea-floor morphology is the evolution of sidescansonar systems (fig. 2). The principle of sidescan sonar is 

relatively simple. An instrument towed by a research vessel 

sends out two sound beams which are directed to either side 

of the towed vehicle. The sound beams are tuned to create 

a beam that is very narrow in the fore and aft direction but 

very broad in the across-track direction. The transmitted 

sound beams image, or "insonify," a narrow slice of sea 

floor perpendicular to the ship's track. The acoustic energy 

returned from the sea floor (acoustic backscatter) is then 

sensed, or "heard," by the instrument and transmitted to a 

shipboard recorder which displays the data in twodimensional form as an image. As the instrument is towed 

along, it insonifies adjacent slices of the sea floor and, in 

this way, builds an acoustic image. The level of acoustic 

backscatter is a function of, among other things, the 

sea-floor topography, roughness, and composition. Using 

computer-processing techniques, it is now possible to produce true-scale mosaics of large areas of sea floor similar in 

appearance to aerial photographs. 

Conventional sidescan-sonar systems can insonify 

-swaths of sea floor typically less than 1 km wide. These 

high-resolution systems are inappropriate for use as a 

reconnaissance tool because of the huge amount of time 

required to survey large areas. Two other systems, SeaMARC I (Sea Floor Mapping And Remote Characterization) and GLORIA (Geological LOng-Range Inclined 

Asdic) are able to obtain much broader swaths of the sea 

floor (up to 5 km for SeaMARC and 45 km for GLORIA). 

The GLORIA system is towed only 50 m below the sea 

surface at relatively fast speeds (8 knots) and can cover an 

area of as much as 27,700 km2 per day. Thus, GLORIA can 

obtain true large-scale reconnaissance views of the sea floor 

which are needed in asses_§ing the extent and style of mass 

movement processes in the deep-water sections of the EEZ. 

Many examples of the use of both seismic-reflection and 

sidescan-sonar techniques in the investigation of submarine 

slope instability are presented in the sections of this report. 

HISTORY 

Our awareness of the importance and extent of 

submarine slope failure and mass movement has evolved 

over a long period. It has become apparent to marine 

geologists and ocean engineers that the largest landslides on 

Earth have occurred on the bottom of the sea. As recently as 

the 1960's, however, submarine landslides were thought to 

be rare in ocean basins except on steep slopes, on the soft 

sediment of river deltas, or in seismically active areas. The 

first evidence for large-scale submarine slope failure and 

mass movement came primarily from studies of anomalous 

sea-floor topography (for example, see Shepard, 1955; 

Moore, 1961; Inderbitzen, 1965) or displaced material (for 

example, see Heezen and Ewing, 1952, 1955) and from 

studies of ancient strata from many locations (Jones, 1937; 

Beets, 1946; Kuenen, 1949; van Straaten, 1949). Development of seismic-reflection, sidescan-sonar, and sea-floor 

sampling techniques since the 1950's coupled with increasing interest in the continental margins as a resource area has 

resulted in the accumulation of a large amount of data 

pertinent to the evaluation of submarine slope failure and 

mass movement. Large volumes of sediment and rock are 

involved in these slope failures, and they occur on seismically active and inactive submarine slopes of varied steepness (Dill, 1964; Heezen and Drake, 1964; Menard, 1964; 

Kelling and Stanley, 1970; Moore and others, 1970; Lewis, 

1971; Booth and Dunlap, 1977; Carlson and Molnia, 1977; 

Embley and Jacobi, 1977; Hampton and Bouma, 1977; 

Embley, 1980; Booth and others, 1984). 

CAUSES OF SLOPE FAILURE AND ANALYSIS 

TECHNIQUES 

As recently as the mid 1970's, the few actual observations of submarine slope failure in the literature were 

mainly qualitative, because slope stability analysis requires 

knowledge of the slope topographic profile, the shape of the 

major slip surface (failure plane), and a variety of engineering properties of the material that composes the sea 

floor-information that is seldom available in the case of 

submarine landslides. Early investigators recognized from 

qualitative studies that several mechanisms can induce or 

"trigger" submarine slope failure. Heezen (1956) suggested 

that "over-steepening" of the slope due to rapid deposition 

of sediment was responsible for submarine slope failure 

near the mouth of the Magdalena River in Colombia. Dill 

(1964) suggested that the generation of gas associated with 

the decomposition of organic matter could lead to slope 

failure. Submarine slope failure triggered by earthquakes 

first was recognized from qualitative observations (for 

example, see Gutenberg, 1939; Hee.zen and Ewing, 1952, 

1955; Ryan and Heezen, 1965). Henkel (1970) showed that 

large storm waves could load the sea floor and cause slope 

Submarine Landslides: An Introduction 3 
Figure 2. Principles of sidescan-sonar for mapping sea-floor landslide deposits. The top panel illustrates how sidescan 

sonar operates, and the bottom panel sketches the resulting sidescan-sonar image. 

4 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
MASS MOVEMENT 

------. MASS FLOWS 

.-.-..----~~- ~ -~~---__,__. 

TRANSLATIONAL ROTATIONAL(SLUMP} DEBRIS AVALANCHE MUD FLOW 

~ DEBRIS FLOW LIQUEFACTION 

~ TURBIDITY CURRENT 

Figure 3. General landslide classification (modified from Varnes, 1958). 

failure. This failure mechanism was observed clearly during 

the passage of Hurricane Camille (Bea and others, 1983). 

Quantitative analysis of the mechanics of submarine 

slope failure requires making assessments of the strength of 

sediment and the magnitude of stresses acting downslope. 

An important quality of sediment is that the strength 

decreases as the excess pore-water pressure (that is, in 

excess of hydrostatic pressure) increases. Excess pore-water 

pressures commonly exist in deposits that are accumulating 

rapidly and can be evaluated using simplified sedimentation 

models (Terzaghi, 1956; Gibson, 1958). Excess pore-water 

pressures can also be caused by bubble-phase gas and by 

repeated loading from earthquakes or storm waves. 

Downslope stresses result from gravity, earthquakes, 

or storm waves. Because, typically, so little is known about 

the properties and geometry of submarine slopes, investigators use analysis methods that are very simplified. The 

slopes are generally approximated by a tilted plane that 

extends for considerable distance in all directions (the 

so-called "infinite slope"). Earthquakes are represented by a 

constant lateral force that is proportional to the peak 

horizontal acceleration resulting from the shaking (Morgenstern, 1967). Wave-induced stresses are related to wave 

height, water depth, and other factors (Henkel, 1970; Seed 

and Rahman, 1978). 

By evaluating the magnitude of downslope stresses 

relative to sediment strength, investigators have shown how 

steep slopes, high deposition rates, earthquakes, and storm 

waves can lead to slope failure (for example, Morgenstern, 

1967; Almagor and Wiseman, 1977; Hampton and others, 

1978; Schwab and Lee, 1983; Booth and others, 1984; and 

Lee and Edwards, 1986). 

More sophisticated models for evaluating sediment 

deformation on slopes have also been developed. The 

quantitative accuracy of any deformation prediction is, 

however, limited by the quality of the sediment or rock 

stress-strain relation. Choosing the appropriate stress-strain 

relation for a seabed material poses major problems. For 

example, a significant change in the stress-strain behavior 

occurs when a sediment is subjected to repeated loading 

(Esrig, Ladd, and Bea, 1975; Anderson, 1976; Wright, 

1976). Thus, the dominant influence of repeated loading 

(stresses associated with earthquakes or storm waves), 

along with other factors such as poor sample quality (Lee, 

1985), ·Severely limit the accuracy of these more advanced 

methods for offshore slope stability analysis, as a common 

practice. For site-specific studies, however, in which adequate resources are available for field borings, laboratory 

testing, and advanced computer modeling, accurate analyses of sea-floor deformations can be made using the 

finite-element approach (Arnold, 1973; Wright, 1976). 

TERMS USED TO DESCRIBE SUBMARINE 

LANDSLIDES 

Studies of sedimentary structures associated with 

slope failure and mass movement have shown that during 

and following slope failure, sediment can achieve a broad 

range of styles of mobility, from rigid block motion to 

turbulent flow. These styles of mobilization involving 

various types of material can be referred to by different 

terms. In this report, we follow the terminology recommended by. Varnes (1958) with some modification (fig. 3). 

Slope failure occurs when the downslope driving 

forces acting on the material composing the sea floor are 

greater than the forces acting to resist major deformations. 

Following slope failure, the failed mass moves downslope 

under the influence of gravity and possibly other forces. 

Thus, mass movement (also known as mass wasting) is 

defined as the movement of the failed material driven 

directly by gravity or other body forces, rather than tractive 

stresses associated with fluid motion. If the moving sediment takes. a form that resembles a viscous fluid, the 

process is termed mass flow. Such a failure has considerable 

internal deformation with innumerable invisible or shortlived internal slip surfaces. Slides are translational or 

rotational movements of essentially rigid, internally undeformed masses along discrete slip planes. In the literature, 

all forms of mass movement are occasionally referred to as 

slumps. Correctly, slumps are a special kind of slide in 

which blocks of failed material rotate along curved slip 

surfaces (rotational movement). In each of these types, 

movement can be fast or slow. Extremely slow movement is 

called creep. 

Submarine slides can become mass flows as the 

sliding mass progressively disintegrates and continued 

downslope movement occurs (Morgenstern, 1967; Hampton, 1972). End-member products of disintegrating slides 

have been given special names. Debris flows are flows of 

Submarine Landslides: An Introduction 5 
sediment in which the sediment is heterogeneous and may 

include larger clasts supported by a matrix of fine sediment. 

Mudflows involve predominantly muddy sediment., Turbidity currents involve the downslope transport of a relatively 

dilute suspension of sediment grains that are supported by 

an upward component of fluid turbulence. Turbidity currents are often generated by the disintegration and dilution 

of slides or debris flows, although they may be generated 

independently from other mass-movement events. Liquefaction occurs when a loosely packed sediment collapses 

under an environmental load. Here, the grains temporarily 

lose most contact with one another, and the particle weight 

is temporarily transferred to the pore fluid. Excess porewater pressures are induced by this behavior. They eventually dissipate, although sometimes very slowly, and the 

material may flow downslope under the influence of gravity 

or spread laterally under the influence of stresses induced by 

earthquakes or perhaps storm waves. 

Recent surveys using modem remote-sensing equipment have revealed truly giant submarine landslides that 

involve the failure of thousands of cubic-kilometers of rock 

and sediment (for example, see Moore and others, 1990). 

When these landslides have disintegrated into relatively 

smaller pieces (which may still be quite large) and have 

clearly moved rapidly, they are referred to as debris 

avalanches. Debris avalanches may occur on a smaller 

scale, too. In particular, when some sandy sedimentary 

deposits fail, most commonly as a result of slope oversteepening, they avalanche downslope as a thin grainflow. 

ENVIRONMENTS 

Submarine landslides are not distributed uniformly 

over the U.S. EEZ. Instead, they tend to occur most often 

where there are thick bodies of soft sediment, where the 

slopes are steep, and where the environmental loads are 

high. These conditions are ·met in fjords, deltas, and 

submarine canyons and on the continental slope. 

Fjords 

Of all the environments of U.S. EEZ, Alaskan fjords 

(and areas within Puget Sound, Washington) with high 

sedimentation rates are likely the most susceptible to slope 

failure, both in terms of the proportional areal extent of 

deposits that can become involved in mass movement and 

also in terms of the recurrence interval of slope failures at 

the same location. Fjords are glacially eroded valleys that 

have been inundated by the sea. They are typically fed by 

sediment-laden rivers and streams that drain glaciers. These 

factors lead to environmental conditions that are highly 

conducive to slope failure. The submerged sides of glacial 

valleys are commonly very steep and may extend to great 

depths, perhaps 1,000 m or more. There is typically a delta 

at the head of the fjord formed by streams that may drain the 

glacier that initially eroded the valley. These fjord-head 

deltas have beds that dip at 5° to 30° between 10 and 50 m 

water depth. In greater water depths, the submerged part of 

the delta dips at angles of 0.1° to 5° to the flat basin floor, 

typically at depths between 100 and 1,000 m (Syvitski and 

others, 1987). The glacial streams feeding these deltas carry 

rock flour and coarse sediment whose deposits easily lose 

strength when shaken by an earthquake. The sediment may 

also be deposited so rapidly that pore-water pressures 

cannot dissipate completely as sedimentation continues. 

The resulting underconsolidated state of the sediment 

causes abnormally low strength. Organic matter brought 

down with the glacial debris may be abundant, and it can 

decay and produce bubble-phase gas that also may lead to 

elevated pore-water pressure and low strength. Some fjorddelta deposits are so near instability that they fail during 

particularly low tides, during which the supporting forces of 

seawater are temporarily removed from the sediment. Some 

of these steep slopes with weak sediment fail seasonally or 

semicontinuously yielding numerous small-scale slope failures. Slope failures of a seasonal or semicontinuous nature 

have been reported in many fjords in Canada; for example, 

Bute Inlet (Syvitski and Farrow, 1983; Prior and others, 

1986), Knight Inlet (Syvitski and Farrow, 1983), and North 

Bentinck Arm (Kostaschuk and McCann, 1983) and are 

probably common in Alaska, as well. Fjord-head delta 

slope failures occasionally produce catastrophic effects, 

such as occurred in Valdez (Coulter and Migliaccio, 1966; 

Hampton and others, this report), Seward (Lemke, 1967; 

Hampton and others, this report) and Whittier (Kachadoorian, 1965) .in the 1964 Alaska earthquake or in Kitimat 

Arm, Canada (Prior, Bomhold, and others, 1982; Prior, 

Coleman, and others, 1982). 

The sidewalls of fjords can also be unstable. Deposition of suspended sediment on the steep ( 10° to greater 

than 90° overhangs) submerged valley sides can frequently 

lead to small slope failures (Farrow, Syvitski, and Tunnicliff, 1983). Even more important are slope failures on 

side-entry deltas that build out rapidly onto the sidewall 

slopes. Failures in Howe Sound, Canada (Terzaghi, 1956; 

Prior and others, 1981), are of this type. 

Finally, .the deep fjord basins, which tend to have 

slopes of less than 0.1, commonly receive failed sediment 

masses and flows from the side walls and fjord-head deltas. 

If these landslides incorporate enough water during their 

movement, they can progress through mass flows into 

turbidity currents. These mass flows and turbidity currents 

can be fed into and across the basins by channels (Syvitski 

and others, 1987). 

Slope failures in fjords can generate large waves 

which cause major damage to coastal communities. During 

the 1964 Alas~a earthquake, much of the damage and many 

of the fatalities in Valdez (Coulter and Migliaccio, 1966; 

Hampton and others, this report), Seward (Lemke, 1967; 

Hampton and others, this report) and perhaps Chenaga 

(Plafker and others, 1969) resulted from giant waves 

6 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
generated by submarine landslides in fjords. Waves 8.2 m 

in height were generated during the major sea-floor failure 

at Kitimat Arm, Canada, in 1975 (Murty, 1979). 

Because fjords are found in rugged, mountainous 

terrain, the fjord-head deltas and side-entry deltas are 

frequently the only flat land available for development. 

These marginally stable to unstable locations become the 

sites of coastal developments. Not only do these developments become vulnerable to natural slope failure, which 

might occur frequently even if the developments were not 

present, but man's activities can also lead to additional 

slope failures. For example, a river channel stabilization 

program at Howe Sound (Terzaghi, 1956) caused rapid 

delta growth to be localized and probably contributed 

ultimately to slope failure. 

Active River Deltas on the Continental Shelf 

Active river deltas are the next most likely sites for 

slope instability in the U.S. EEZ. Many rivers contribute 

large quantities of sediment to relatively localized areas on 

the continental margins. Depending on a variety of environmental factors, including wave and current activity and 

the configuration of the continental shelf and coastline, 

thick deltaic deposits can accumulate fairly rapidly. These 

sediment wedges and blankets can become the locations of 

sediment instability and landsliding partly because of their 

thickness. To create large, deep-seated landslides, a thick 

deposit of comparatively low-strength sediment is needed. 

Because most of the continental shelves were subaerially 

exposed during the last glacial cycle, most sediment on the 

shelves from that time or before has been eroded or 

desiccated. Accordingly, the strengths of these older deposits are commonly high enough to resist downslope gravitational stresses on the gentle shelves, and all but the very 

greatest storm~ and earthquake-induced stresses as well. 

Only in areas of continued modem deposition where significant thicknesses (tens of meters) of younger sediment 

have been deposited above the strong, old sediment do 

landslides occur. These younger deposits tend to have 

relatively low strength because of rapid deposition rates. In 

addition, decaying organic matter can produce bubblephase gas that can further reduce strength. These locations 

may fail under gravitational loading (due to the slope 

steepness alone) or during storms or earthquakes. 

The locations of the major sedimentary depocenters 

provide some information on where undersea landslides 

might be expected on the continental shelf. For example, 

the Yukon River is a major source of sediment (60 million 

tons per year) in the northern Bering Sea. Although the 

prodelta is nearly flat, there are indications of localized 

liquefaction of sandy silt (Clukey and others, 1980) during 

large storms. 

The occurrence of slope failures on active deltas of 

the continental shelf off southern Alaska is not surprising. 

Glacially fed rivers debouching into the Gulf of Alaska or 

adjacent sounds and inlets contribute 450 million tons of 

sediment per year (Milliman ·and Meade, 1983). Slope 

failures have been identified in modem sediment all along 

the Alaskan margin (Reimnitz, 1972; Molnia and others, 

1977; Carlson, 1978; Schwab and Lee, 1983, 1988, this 

report; Schwab and others, 1987; Lee and Edwards, 1986). 

The landslides are likely induced by either storm waves or 

earthquakes (Schwab and Lee, 1988, this report); the high 

incidence of landsliding arises because of the intensity of 

earthquake and storm-wave loading, the thickness of modem sediment, and the tendency of the glaciomarine sediment to lose strength when cyclically loaded. 

The west coast of the United States has relatively 

short, steep rivers with small drainage basins. The sediment 

yield of these rivers can be high but is extremely variable 

(Milliman and Meade, 1983). There are indications of slope 

instability on some of the deltas, for example, that of the 

Klamath River (Field and others, 1982; Field, this report). 

The Mississippi River contributes the most sediment 

to the EEZ of any single river within the United States (2 to 

7 x 108 tons of sediment per year; Coleman and others, 

1980; Milliman and Meade, 1983). Most of this sediment is 

deposited on the shelf fronting the modem bird-foot delta, a 

delta-lobe that has been in existence for only 600 to 800 

years (Fisk and others, 1954). The distributary mouths of 

the modem delta build seaward at rates varying from 50 to 

100 m per year. Offshore of these distributaries, the 

sediment accumulation rates are very high, ranging to 1 m 

per year or more (Coleman and others, 1980). The sediment 

deposited consists mostly of clay-sized particles and is also 

rich in organic matter that is rapidly degraded to gas (mainly 

methane and carbon dioxide). Rapid sedimentation and gas 

charging lead to high excess pore-water pressures and a 

state of extreme underconsolidation. Although the slopes of 

the delta-front are very low, less than 1.5°, evidence of 

slope failure is widespread. Submarine gullies were first 

described by Shepard ( 1955). More recently, sidescan sonar 

mosaics have shown extensive fields of sediment instabilities all along the delta-front (Coleman and others, 1980; 

Coleman and others, this report). 

Elsewhere along the coast of the Gulf of Mexico and 

Atlantic Ocean, the rate of sediment accumulation on the 

continental shelf is low and landslide features are rare. Most 

modem deposition occurs in bays that formed when valleys 

were drowned by rising sea level after the most recent 

glacial cycle (ice age). 

Deltas of some of the major rivers that deposit large 

amounts of sediment along the margins of the United States 

do not display geomorphic evidence of submarine landsliding. Such lack of evidence does not automatically preclu~e 

previous slope failure in these deposits because waves and 

burial can erase the failure effects. The lack of slope failure 

features likely indicates, however, that the occurrence of 

slope failure is less frequent or less extensive in these 

Submarine Landslides: An Introduction 7 
deltas. Most notable is the delta of the Columbia River 

(Nittrouer and Sternberg, 1981) whose surface appears to be 

devoid of failures. Evidently, a combination of circumstances is needed for slope failure to occur on the continental shelf. High sedimentation rates are needed so that a 

sufficiently thick bed of modem sediment can develop. 

Relatively low permeability (fine-grained sediment) can 

allow high pore-water pressures to be retained and produce 

low sediment strength. Also ~eeded, however, are environmental factors, such as storms or earthquakes, that can 

generate stresses in the sediment that exceed the strength. 

These can be augmented by strength degradation during 

cyclic loading, such as seems to be particularly common 

with glacially derived sediment. Finally, the configuration 

of the continental shelf, including its slope, can influence 

the stability of deltas. The interaction of all of these factors, 

rather than one factor alone, ultimately determines whether 

mass movement will occur. 

Submarine Canyon and Deep-Sea Fan Systems 

Submarine canyon systems serve as conduits for 

passing large amounts of sediment from near the continental 

shelf to deep-sea fans. Although more active at times of 

lowered sea level during periods of extensive continental 

glaciation, the presence of extensive, thick sediment deposits on the continental rise surrounding the United States 

testifies to the importance of mass movement mechanisms 

associated with these systems. Mass movement mechanisms are capable of bringing sand-sized and even coarser 

particles to locations hundreds of kilometers from shore. 

Landsliding appears to be one system element that allows 

massive deposition on submarine fans to occur. According 

to one model (Hampton, 1972), sediment accumulations in 

· canyon heads begin to move as coherent landslide blocks 

following some triggering event, such as an earthquake or 

storm. As the blocks move downslope, the resulting jostling 

and agitation cause disintegration and subsequent incorporation of ambient water. The debris flow produced displays 

increasingly fluidlike behavior. As the debris flow continues on its path, further dilution by surrounding water 

occurs, particularly as sediment is eroded from the front of 

the flow. Ultimately, a dilute turbulent cloud of sedim~nt, 

o.r turbidity current, is created. The turbidity current has a 

density slightly greater than the surrounding seawater and 

can flow for long distances (up to hundreds of kilometers) at 

moderate to high velocities (1 to 8 m/s, Shepard, 1963; 

Bowen and others, 1984; Reynolds, 1987). When the 

turbidity current enters a depositional phase, it leaves 

deposits that have distinctive textural characteristics; 

namely sandy sediments at the base that decrease in grain 

size to silt and clays at the top of the deposit (Bouma, 1962; 

Middleton and Hampton, 1976). 

Landsliding, particularly within submarine canyons 

(Carlson and others, this report), appears to be an important, if not essential, part of the process of building deep-sea 

fans, which are among the most extensive sedimentary 

features of the Earth's surface. The circumstances surrounding these slope failures and their subsequent conversion into 

turbidity currents are poorly understood, however. Storms 

cause sediment movement in canyons, perhaps by inducing 

slumping near the canyon heads (Marshall, 1978) or perhaps by introducing or resuspending enough sediment to 

form a density current directly (Shepard and Marshall, 

1973; Reynolds, 1987). Earthquakes also cause landslides 

in canyon heads and subsequent turbidity current flow 

(Malouta and others, 1981; Adams, 1984), but details of 

this process are lacking. Major earthquakes and other 

shocks do not always cause canyon-head landslides (Shepard, 1951; Dill, 1969). Landsliding in canyon heads and 

turbidity current mobilization were likely more common 

during glacial cycles (Nelson, 1976; Barnard, 1978) 

because of lowered sea level, increased sediment supply 

and possibly, increased storm wave loading. Thus, many of 

these submarine canyons and deep-sea fans, unlike fjords 

and river deltas, are not active areas of slope failure today. 

The Open Continental Slope 

A final common environment for undersea landsliding in the U.S. EEZ is the area between submarine canyons 

on the continental slope. Landslides have been reported all 

around the U.S. margin along slopes removed from submarine canyon systems. Included are slopes off southern 

California (Buffington and Moore, 1962; Haner and 

Gorsline, 1978; Field and Clarke, 1979; Nardin, Edwards, 

and others, 1979; Nardin, Hein, and others, 1979; Ploessel 

and others, 1979; Field and Edwards, 1980; Field and 

Richmond, 1980; Hein and Gorsline, 1981; Thornton, 

1986; Edwards, Lee, and Field, this report; Field and 

Edwards, this report), central and northern California (Field 

and others, 1980; Richmond and Burdick, 1981; Field and 

Barber, this report; Gutmacher and Normark, this report), 

off Alaska (Marlow and others, 1970; Hampton and 

Bouma, 1977; Carlson and others, 1980; Hampton, this 

report; Kayen and Lee, this report), in the Gulf of Mexico 

(Lehner, 1969; Woodbury, 1977; Booth, 1979; Booth and 

Garrison, 1978; Twichell, Valentine, and Parson, this 

report; and McGregor and others, this report), and along the 

Atlantic coast (Embley and Jacobi, 1977; McGregor, 1977; 

Knebel and Carson, 1979; McGregor and others, 1979; 

Malahoff and others, 1980; Booth and others, 1984, 1988; 

Cashman and Popenoe, 1985; O'Leary, 1986; Booth and 

others, this report; O'Leary, this report; Popenoe, 

Schmuck, and Dillon, this report; Dillon and others, this 

report). The failures are found near river mouths and far 

removed from them, as well as in both arid and humid 

8 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
0 

Q 

80°W 

Figure 4. Locations of landslide studies described in this report. Numbers refer to sections listed in table 1. The shaded 

area is the U.S. EEZ. 

climates. Ages of the slope failures are seldom known, so 

we cannot determine whether they occurred under glacial or 

interglacial conditions, nor can the recurrence interval be 

estimated with any degree of accuracy. Many were probably seismically induced because typical continental slopes 

have gradients of 5° or less which would be expected to be 

statically stable. Storm-wave loading is seldom a major 

factor much below the shelf break (Lee and Edwards, 

1986). Occurrence of slope failures seems to correlate with 

sedimentation rate, slope declivity, seismicity, and presence of bubble-phase gas and gas hydrate, but the relationship is complex (Field, 1981). 

SECTIONS IN THIS REPORT 

This report contains 20 sections that discuss landslides within the U.S. EEZ (fig. 4, table 1). The locations 

of these landslides are distributed fairly uniformly around 

the United States. This compilation is not exhaustive, 

however. Rather, the goal is to show the variety of 

landslides that have been found, the associations of the 

landslides with other features, methods that have been used 

to understand the causes and mechanisms of the landslides, 

and the potential that some of these features have as a 

hazard to coastal and offshore development. One of the 

Submarine Landslides: An Introduction 9 
Table 1. Sections included in this report 

No. Authors Study area Major theme of section 

2 Booth and others · Continental slope, Atlantic--margin 

3 O'Leary Continental slope, southeast New England 

Variety and distribution of Landslides on Atlantic margin 

Large-scale landsliding is a principal formative process of 

the New England continental margin 

4 Popenoe, Schmuck, Cape Fear landslide, continental slope, 

and Dillon North Carolina 

5 Dillon and others Lion's Paw landslide, Blake Escarpment, 

Florida 

6 Schwab, Danforth, Insular slope, Puerto Rico 

and Scanlon 

7 Twichell, Valentine, Continental shelf and and West Florida 

and Parson Escarpment, Florida 

8 Coleman and others Mississippi River Delta, continental 

shelf, Louisiana 

9 . McGregor and others Continental slope, western Gulf of 

Mexico, Texas 

10 Kayen and Lee Continental slope, Beaufort Sea, Alaska 

11 Carlson and others Beringian margin, submarine canyon 

Bering Sea Alaska 

12 Hampton Kodiak continental margin, Alaska 

13 Hampton, Lemke, Fjords, Valdez and Seward, Alaska 

and Coulter 

14 Schwab and Lee Continental shelf, Gulf of Alaska 

15 Field Continental shelf, northern California 

16 Field and Barber Continental slope, northern California 

.17 Gutmacher and Sur submarine landslide, continental 

Normark slope, central California 

18 Edwards, Lee, Gaviota mud flow, basin slope, 

and Field southern California 

19 Field and Edwards Southern California borderland 

20 Normark, Moore, Hawaiian Ridge, Hawaii 

and Torresan 

21 Normark and others Insular slope, Oahu, Hawaii 

sections in this report- (Hampton, Lemke, and Coulter) 

considers sediment failures in fjords, three sections consider 

failures in rapidly deposited sediment on the continental 

shelf (Coleman and others; Field; Schwab and Lee), one 

section (Carlson and others) considers mass instability in 

submarine canyons, and the remaining fifteen sections are 

concerned with failures of varying size on continental, 

insular, or basin slopes. The slope environment is emphasized because the largest and most spectacular landslides 

occur here and more nearly complete coverage with 

sidescan-sonar mapping has been obtained during the U.S. 

Geological Survey EEZ-SCAN Project (Gardner, 1986). 

More landslide deposits are preserved in the slope environment through glacial stages when the continental shelves are 

subaerially exposed and coastal fjords are filled with glaSlope failure induced by salt tectonics and decomposition of 

gas hydrate during low sea level 

Erosional processes cause carbonate escarpments to fail 

Debris avalanche controlled by structure and regional tectonic setting 

Erosional processes cause carbonate slopes to fail 

Slope failures are common on active deltas and can affect 

offshore development 

Salt diapirism related to the style of slope failures 

Slope failures induced by decomposition of gas hydrate 

during low sea level 

Mass movement is the principal process dominating the 

submarine canyon environment 

Slope failures controlled by structure and regional tectonic 

setting 

Effects of earthquake-induced slope failures on communities 

along fjords 

Different slope failure morphologies result from different 

failure mechanisms 

Earthquake-induced liquefaction 

Slope failures associated with gas-charged sediment 

Large slope failure on a margin with a low sedimentation 

rate 

Processes controlling earthquake-induced mud flow 

Regional distribution of landslides in a well-studied area 

Giant slope failures are a primary process in the formation of 

many volcanic islands 

Storm-wave-induced slope failure of carbonate sediment 

ciers. That is·, more landslide deposits are preserved on 

continental and insular slopes even though the frequency of 

landslide recurrence may be much greater in deltaic deposits 

on the continental shelf and in fjords. 

The sections in this report provide insights into the 

presence of mass movement features and processes that 

control submarine mass movement. Although marine 

researchers and engineers are attempting to resolve the 

problems associated with slope instability through the use of 

more quantitative methodologies, the results of recent 

sea-floor mapping surveys provide a constant reminder that 

there is much yet to be known. The marine researcher will 

be tasked with the responsibility to solve these unknowns in 

the 1990's and beyond as the economic significance of the 

sea floor increases. As many of the sections in this report 

10 Submarine Landslides: Selected Studies in the U.S. ·Exclusive Economic Zone 
suggest, however, marine research will not only provide 

solutions but will continue to reveal new mysteries. 

ACKNOWLEDGMENTS 

The authors wish to thank Monty Hampton and David 

Drake for their helpful reviews of the manuscript. All 

illustrations and photography were provided by Jeff 

Zwinakis and Dann Blackwood. 

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12 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
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·~ .. '. 

Submarine Landslides: A!l. Introduction 13 
U.S. Atlantic Continental Slope Landslides: Their 

Distribution, General Attributes, and Implications ' . 

By j.S. Booth, D.W. O'Leary, Peter Popenoe, and W.W. Danforth 

INTRODUCTION 

Because of its relatively gentle slopes and tranquil 

environment, the sea floor off the Atlantic coast was not 

considered a likely place for landsliding. In fact, many 

researchers associated with the offshore geologic studies 

that preceded the oil- and gas-lease sales of the 1980's 

believed that the continental slope would be shown to be 

basically inert-a quiescent environment which was gradually, over long periods or perhaps at rare intervals, 

undergoing some erosion but, in general, becoming increasingly more stable. Recent systematic surveys, culminating 

with a Geological LOng-Range Inclined Asdic (GLORIA) 

sidescan-sonar survey (EEZ-SCAN 87, 1991), and studies 

of geologic processes have revealed, to the contrary, that 

not only have there probably been hundreds of landslides on 

the U.S. Atlantic continental slope but that most of the 

mappable landslides have occurred in relatively recent 

geologic time. 

Here, we focus on 179 individually mapped landslides on the U.S. Atlantic outer continental margin. Our 

purpose is to provide a qualitative overview and a statistical 

summary of the distribution and characteristics of these 

landslides and a brief analysis of observed tendencies, 

associations, trends, and correlations that may give insight 

into the causes and processes of marine landsliding. A 

complete listing of the data used in this chapter may be 

found in Booth and others (1988). The largest landslides 

and areas of extensive mass movement are shown on the 

GLORIA imagery (pl. 1, envelope inside the back cover). 

GEOLOGIC SETTING 

The continental slope off the Eastern United States is 

a relatively steep and narrow rampart between the Atlantic 

Ocean basin and the North American continent. In terms of 

water depth, it extends from the edge of the continental 

shelf (75 to 200 meters (m) water depth) to approximately 

2,000 m, where it is bounded by the continental rise. From 

Georges Bank south to Cape Hatteras, the physiography of 

the continental slope is dominated by the presence of 

numerous canyons and canyon systems. South of Cape 

Hatteras, the slope's typical ramp like appearance is interrupted by the broad Blake Plateau, which separates the 

inner Florida-Hatteras slope from the Blake Escarpment 

(outer slope), thus giving the margin a stairstep appearance 

in this region. 

This complex morphology is a reflection of a continental margin that has been extensively modified through 

time (see, for example, Schlee and others, 1979). Regional 

slope angles may be as low as 1° or 2°, but they commonly 

approach 10° .(~eller and othe_rs, 1979) and may be greate~ 

than ·15°, as on the Blake Escarpment (Emery and Uchupi, 

1972). Locally, declivities frequently exceed 20° and some 

canyon walls are practically vertical (see, for example, 

O'Leary and Twichell, 1981). In the Tertiary and Quaternary Periods (that is, the Cenozoic Era, or approximately 

the last 66 million years of Earth history), the slope 

experienced a series of transgressive-regressive cycles 

(gradual submergence then _re-emergence of land areas in 

response to the, relative rise and fall of sea level), during 

which it was subjected to progradation (building s~award) 

as well as large-scale net erosion. 

Near the top of the continental slope, the Quaternary 

sedimentary section (which represents about the last 2 

million years of Earth's history) typically is 200 to 300 m 

thick (Poag and Sevon, 1989). It is composed chiefly of 

muds (silts and clays) which were derived primarily from 

erosion of the adjacent land mass. Calcareous sediment 

(commonly composed or derived from shells and shell 

fragments) is locally dominant off parts of the Southern 

United States, whereas sands and gravels derived from 

melting glaciers or related processes are locally dominant 

off New England. The Quaternary sediment section, in 

general, thins and becomes more uniformly layered to 

seaward; in places on the lower slope, particularly in 

canyons, it is ~bsent. Thus, viewed in cross section, the 

Quaternary sediment, which is frequently involved in slope 

failure and mass movement, is a tilted, wedge-shaped mass 

lying atop the Tertiary sediment beneath it. 

The Tertiary sediment section is more varied in 

composition aq4 stratification. Schlee and others (1979), 

Robb and oth~r~ (1981), and Poag and Sevon (1989) have 

shown that th~-:lower part of this approximately 3-km-thick 

section comprises mainly indurated chalk or limey clay. 

The sediment at this stratigraphic level also has a high silica 

content; in places it resembles porcelain. Higher in the 

Tertiary section, the 'sediment is more typically muddy, but 

sands containing a distinctive green mineral (glauconite) are 

present as well. Tertiary sediments are also involved in 

landslides. 

MAPPING METHODS AND LIMITATIONS 

_ A variety of data sources was used to identify and 

enumerate the landslides and related features. Primarily, 

14 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
high-resolution, seismic-reflection profiles and sidescansonar images served as the basis for our compilation (for a 

description of these techniques see Lee, Schwab, and 

Booth, this report). The many investigations (dating from 

1952) used to compile the landslides data base encompass a 

considerable range in technology, navigational accuracy, 

acoustic resolution, and data-processing sophistication. 

Consequently, there are various degrees of data quality and 

confidence associated with the identification and interpretation of the previous studies. Furthermore, because much 

of the systematic survey work was conqucted in areas of 

pending oil- and gas-lease sales, the coverage is regionally 

biased; that is, coverage of 'th~ continental slope is not 

·uniform. The plotted distribution of landslides is, thus, not 

necessarily representative of the true distribution and areal 

extent of the largest landslides. 

The spacing of the seismic-reflection profiles is 

another limitation. As examples, lanqslides that have 

din:tensions considerably less than the grid spacing used in 

the surveys have a relatively low probability of detection, 

and the actual size (length and width) of the slope failure 

features are bracketed by the grid spacing rather than known 

precisely. 

Although most data sources were used in the general 

compilation, the data base used for statistical description 

and analyses was limited to those values or interpretations 

that were judged to be of relatively high quality. Still, the 

accuracy or utility of the preponderance of the data used for 

the statistical work cannot be verified. Nonetheless, we 

assume that data used in our analyses is of acceptable 

quality and is the best data set available in any case (the 

reader is referred to Booth and others ( 1988) for more 

details on the methods, limitations, and data base). 

OVERVIEW OF LANDSLIDES 

Distribution 

Evidence of landsliding exists across the entire 

breadth of the U.S. Atlantic continental slope (fig. 1). The 

data suggest that each of these landslides occurred within 

the last 2 million years and that almost all occurred within 

the last 200,000 years (Booth and others, 1988). Despite the 

inference created by the presence of landslide complexes, 

which to some extent reflect biased survey coverage, slope 

failures are apparently not limited to specific areas nor are 

·they uniformly distributed. Also evident in figure 1 is the 

great extent of the largest landslides. Most of these major 

landslides comprise debris flow or other types of flow 

deposits, which extend 100 km or more out onto the 

continental rise. The larger landslides and landslide-related 

features have been outlined on the Atlantic U.S. Exclusive 

Economic Zone (EEZ) GLORIA sidescan-sonar imagery 

(pl. 1, in envelope). A more complete map of landslides and 

related features is presented in Booth and others (1988). 

Other maps showing the distribution of landslides have been 

published by Embley and Jacobi (1986) and Tucholke 

(1987). 

Landslides have originated in all but the greatest 

water depths. Figure 2 shows their distribution with respect 

to bathymetry; it also shows that the mid-slope region (800 

to 1 ,000 m water depth) is favored as an origination depth. 

Landslides have even originated at the base of the slope and 

at the top of the rise (2,000 to 2,200 m water depth); in fact, 

the Depth-of-Origin histogram (fig. 2) implies that there is 

a mode (increased tendency for landslides to originate) in 

these water depths. This is somewhat of a paradox. Intuitively, we would expect that landslides would terminate, 

rather than originate, at those depths because there is a 

pronounced reduction in slope angle at the top of the 

continental rise (from an average of 5° to 6° on the 

continental slope to less than 0.1 o farther out on the rise); 

that is, we would expect that landslides would tend to slow 

down and stop on the flatter surface of the rise. In fact, 

despite the slight tendency for landslides to originate at the 

slope-rise boundary, the data (fig. 2) show that a large mode 

in termination depth (depth of the toe of the landslide 

deposit) is, in fact, present at 2,000 to 2,200 m. As is the 

case for origin depth, however, landslide termini have been 

mapped over the entire range of depths investigated, and as 

shown in figure 2, another, broader mode is centered at 

about 1,100 m. 

Dimensions and Declivities 

There is a considerable range in the dimensions of the 

areas affected by landslides. Measured lengths vary from 

0.3 to 380 km (most common length: 2 to 4 km); widths 

vary from 0.2 to 50 km (most common width: 1 to 2 km). 

The thickness of the sedimentary section involved in the 

landslides (at or near its origin) varies from 10 to 650 m. 

Given the stated limitations concerning survey line spacings 

and resolution of the acoustic data, there are probably many 

landslides areas even smaller or thinner than those measured. As shown in figure 3, landslide areas range from less 

than 0.1 km2 up to approximately 19,000 km2 , which is 

roughly the size of New Jersey'. Nearly two-thirds of all 

landslides, however, have an area of 10 km2 or less. 

Numerically, most landslides have occurred at declivities of 4° or less, there is a dramatic drop in landslide 

frequency at angles greater than 4°, and landslides on slopes 

greater than 10° to 12° are rare (fig. 4). Moreover, although 

landslides took place at slope angles of up to 30°, it is 

apparent that steep slopes are not necessarily associated 

with the slope failures despite the presence of the numerous 

submarine canyons, which tend to have steep sidewalls (the 

slight rise in landslide frequency at slope angles of 13° and 

14° is, however, attributed to submarine canyons). 

U.S. Atlantic Continental Slope Landsliiles: Their Distribution, General Attributes, and Implications 15 
so· 

CAPE COO 

40' 

36• 

72' 

64' 

68' 

40' 

EXPLANATION 

"e; A reo of slide rubble or displaced sed1menl, bcunded by 

,, scarps and (locally) by slide toes (0'Leory,1986o,l986b) 

~ Areas mferred Ia be disturbed by creep (O'Leary, 198Gb) 

~ Areas of non- slrollfled surf ic1al cover, portly scarp 

bcunded (Molohof f and olhers,1980), portly overlapped 

by moss- flow deposits (Embley, 1980) 

0 "Moss - flow" deposits (Embley,l980) 

~ Scarps (Embley, 1980) 

~ Slump and (or) sl1de scarp (Popenoe and others,l982) 

36' 

'• Slump foul Is or downward rololed beds (f'openoe and olher.;,l982) 

-;? Area of moss wasting ossocioled w1lh submor1ne 

~ canyons (Popenoe and olhers,l982) 

'<.-~ Slump depos11 (Ayers and P11key,l981) 

.. Sl1de depos1t (McGregor and Bennell,l981) 

D Th1ck blocks 

liD Thm layers 

= Bathymetric contours, depth '" meters 

Locollon of moss movements 

200 KILOMETERS 

68' 

Figure 1. Distribution and style of landslides and related features (modified from Booth and others, 1988). 

Style 

The frequency distribution of the landslide types is 

shown in the histograms (fig. 5). The prevalence of the 

types of mass movements that involve a breakup or fragmentation of a sediment section indicates that once a slope 

failure has taken place, the failed ection of slope tends to 

disintegrate into a structureless mass. To achieve this 

condition, the sediment must be in a "steady state." The 

concept of steady state and its relevance to disintegrative 

failures are presented in Schwab and Lee , this report. 

Despite the prevalence of disintegrative mass movements, 

slides and other varieties of landslides also are common 

(fig. 5), which suggests that slope failures are not restricted 

to or a ociated with a unique set of initial sediment 

condition . In fact, the variety of styles shown probably 

suggests that landsliding has occurred in a variety of 

sediment types as well as under a variety of conditions. The 

modest dominance of translational (straight slip plane) over 

rotational (curved slip plane) failures (fig. 5) is not considered meaningful because of the uncertainties in data quality. 

Morphologic Setting 

Slope failures have occurred on gentle, smooth 

slopes; on steep walls of submarine canyons; on ridge 

crests; and on a variety of other topographic features, 

including mounds, alcoves, small gullies, slopes with rough 

terrains, and deeply embayed regions (fig. 6). Because each 

of the three major morphologic settings shown in figure 6 

16 Submarine landslides: Selected Studies in the U.S. Exclusive Economic Zone 
Landslide Termination 

Figure 2. Origin and termination of landslides. 

Slope failures have occurred at all but the greatest 

depths shown but most commonly at about 900 

meters. Note also the mode in origin below 2,000 

meters. Termination depths of landslides (depth at 

ha experienced numerous landslides, we suggest that 

landsliding is not just ubiquitous but has happened with 

significant frequency in each setting. Canyons, because of 

their relatively steep sides, would intuitively seem to be the 

most likely site for landslides, but, as also indicated in 

figure 6, at least on a numeric basis there have been more 

landslides on smooth, gentle slopes (hereafter called open 

slopes). 

Summary 

The occurrence of landslides on the U.S. Atlantic 

continental slope is widespread: landslides have taken place 

in all geographic areas, all water depths, and at all slope 

angles. They may be found associated with all types of 

topographic features, and they occur in a variety of styles or 

landslide types and range in size from virtually imperceptible to nearly 20,000 km2 . Although the attributes of these 

landslides vary widely, some generalizations can be made. 

Table 1 is a tabulation of characteristics of a "typical" 

landslide. 

ASSOCIATION AND TRENDS 

Analysis of the data base shows that several variables 

either change systematically in association with other vari15 

12 

z 

9? 

0 

--+. 

6 () 

Q) 

C/) 

CD 

C/) 

3 

0 

toe of landslides) have two primary modes: at 

mid-slope water depths (approximately 1,100 

meters) and at the base of the continental slope 

(approximate ly 2,100 meters, where the slope 

angle is reduced). 

ables or are somewhat constrained by the presence or 

absence of certain conditions. These relationships can 

reveal fundamental mechanisms that control landsliding 

(Lee, Schwab, and Booth, this report) and may provide 

insight into how and why landslides occur. Although a 

complete treatment of all the various tendencies is beyond 

the scope of this paper, we believe that two in particular are 

worthy of discussion: the relationship between slope angle, 

morphologic setting, and the size of the landslide; and the 

tendency for landslides to be disintegrative (that is, for the 

displaced mass of sediment to continue to lose its internal 

structure or change its shape). 

Table 1. Characteristics of a "typical" landslide 

Length 

Width 

Thickness 

Origin water depth 

Termination water depth 

Slope angle (at origin) 

Morphologic setting 

Style (type of landslide) 

2 to 4 km 

1 to 2 km 

50 m 

900 m 

1 , 100 and 2, 100 m 40 

Open slope 

Mass flow generated from a 

translational failure plane 

U.S. Atlantic Continental Slope Landslides: Their Di~tribution, General Attributes, and Implications 17 
35 -

23% 

~ 

30 - 21% 20% c:::::;::; ~ 

25 r--~ -- -

C/) 

Q.) '--- -- C/) 

C1j 20 

u 

-

12% 

-0 15 0 

z 

10 

5 

0 

~ -- -r-- -

8% 7% c...._._ 1---r-- i---- 1---1-- - ~ Largest landslide 

3% = 19,000 km2 

-- t--r-- ,_____ -r-- - 2% 2% n. 2% 

~- -· .... -· -· • I I I I I I I 

IIIII I ~ I ln. I 

<0.1 0.1-0.5 0.5-1 1-5 5-10 10-50 50-100 100-500 500-1000 >1000 

Landslide Area (km2 ) 

Figure 3. Landslide area frequency distribution. Because of survey methodology, highest frequency of occurrence may 

not be in the 10 to 50 square kilometers interval (as shown) but in a smaller size class. The largest (19,000 square 

kilometers) is equivalent in area to New Jersey. Values rounded to nearest whole percent. 

35 - 19% 

~ 

18% 

30 - t:=. 

1-

25 -

11% - 20 C/) 

- cz Q) 

C/) 9% ctl 

u 8% ,___ - £!1). QO/_ 

-0 15 

0 -~ 

z 

-6% 

10 1- 1- - -

- - 5% 

5 

4m 4% 4% 

.. 3% t:1i3 - I- I- - 1- 1- - - ~ 2% 

0 

1% 1% 

II 

I 

II 

I • I 

II 

I • I • I • I • I 

go 

• I .. I 

~ 

-I 

fl. I /\d;~~ Slope Angle 

Figure 4. Slope angle frequency distribution at sites of landslide origin. Most occur at very low angles. Values rounded 

to nearest whole percent. 

18 Submarine landslides: Selected Studies in the U.S. Exclusive Economic Zone 
40 -

......__ 

35 - f 

/ 

30 1---

- 57% 

43% 

1--- .. (/) 

25 - I 

-I 

Translational Rotational 

Q) 

(/) 

co 20 u - 17% '+-

0 / 

0 15 -

z 11% / 

10 - 8% / 

I---

5 -

0 ... I I 

Block Slide Slab Slide Carpet Slide 

Figure 5. Frequency of occurrence of different styles or 

types of landslides (based on dominant characteristic). 

The percentages shown for each category reflect combined totals from both translational and rotational origin. 

Brief definitions: Block slide-displaced mass is structurally intact and cubical; Slab slide-displaced mass is 

structurally intact and tabular; Carpet slide-displaced 

mass is basically intact, typically tabular, and parts are 

folded or "wrinkled"; Debris slide-displaced mass consists of large rock fragments or rubble (similar to the 

Associations of Angle, Setting, and Size 

Comparison of landslides found on open slopes with 

those found in canyons shows that the average-sized openslope landslide (= 100 km2

) tends to occur at lower slope 

angles and is much larger (by an order of magnitude) than 

the average canyon landslide = 6 km2

). Furthermore, 

regardless of morphologic setting or other characteristic , 

large-scale landslides (area greater than 100 km2

) tend to be 

associated with gentle slopes (approximately 3° to 4°). 

Similarly, small-scale landslides (average area less than 5 

km2

) tend to be associated with steep slopes (greater than or 

equal to 1 0°). The basic associations between area and slope 

angle for both open-slope landslides and canyon landslides 

are shown in figure 7 . Note that in contrast to the trend 

showing a decrease in area of open-slope landslides with 

increasing declivity, the area of canyon landslides remains 

fairly constant until the slope angle exceeds about 15°. 

That the sizes of canyon landslides would be less than 

their open- slope counterparts is somewhat intuitive: canyons are relatively confined areas themselves. Moreover, 

the total area of the continental slope occupied by steep 

35% 

/ 

20% ~ 

8% f 

1---

...., I 1 I I 

Debris Slide Flow Bowl Debris Flow 

debris avalanche defined in Lee, Schwab, and Booth, this 

report); Flow bowl-a depression associated with a sediment mass that has collapsed or liquefied at depth, or 

possibly throughout its entire thickness; Debris flow-a 

completely deformed, displaced mass that has moved as a 

viscous fluid. The insert shows the percentages of landslides that originated on straight (translational) and curved 

(rotational) failure planes. Values rounded to nearest 

whole percent. 

slopes is likewise relatively small and mostly associated 

with canyons. On the other hand, if we focus on the 

open-slope data, it is evident from figure 7 that in addition 

to the apparent general correlation between slope angle and 

landslide area, there is also a pronounced difference in area 

between the landslides that occurred on slopes greater than 

or equal to 9°, which have areas = 100 to 1,000 km2 , to 

those that took place on slopes of greater declivity ( = 1 

km2

) . In summary, the landslides that took place on open 

slopes not only were much more frequent in regions of low 

declivity but also tended to be greater than or equal to two 

orders of magnitude larger than in regions of high declivity. 

The Tendency Toward Disintegrative landslides 

A second aspect of the landslides is that most landslide products are disintegrative in nature. After initial slope 

failure, whether it be translational or rotational, most 

landslide bodies tend to develop large strains, lose their 

internal structure, and consequently, flow, collapse, or 

generally break up into debris or rubble . As indicated on 

U.S. Atlantic Continental Slope Landslides: Their Distribution, General Attributes, and Implications 19 
100 

80 

~ 60 C/) 

co 

u 

'+-

0 

0 40 

z 

20 

0 

-

47% 

- .r 

-

-

-

Open Slope 

37% 

14% 

L 

2% 

F .. I I I I 

Canyons Ridges Other 

Morphologic Setting 

Figure 6. Common morphologic settings and the assoCiatea rrequenLy ui::,muuuon ot landslides in each. Frequency 

determined on the basis of occurrence only; frequency on the basis of the area of each morphological feature not 

considered. Values rounded to nearest whole percent. 

10000 

10000 

1000 

1000 r 

Ql 

::J 

Q. 

~ 

100 Q. 

100 Cl> 

)> 

(i) 

Ql 

10 til 

10 .0 

"' 3 

0.1 

0.1 

Slope Angle 

Figure 7. Association between landslide size and slope angle (ar site of landslide origin) for open slopes and canyon 

landslides. Areas shown within each 3° interval are average values. 

20 Submarine landslides: Selected Studies in the U.S. Exclusive Economic Zone 
(/) 

Q.) 

(/) 

cu 

80 

60 

u 40 

-0 

0 

z 

20 

36o/o 

Non-disintegrative 

Figure 8. Individual and cumulative percentages of disintegrative landslides (that is, the displaced mass of sediment continues to lose its internal structure or change its 

shape at stresses less than those that caused the initial 

figure 8, nearly two-thirds of the landslides have suffered 

this fate. This implies that the sediments tend to weaken 

considerably once they experience stresses greater than their 

natural strength can withstand. Such weakening can happen , in some cases, even if that excess stress is only 

momentary (that is, from an earthquake). 

SOME POSSIBLE CAUSES OF lANDSliDES 

Both the relationship between angle, etting, and 

landslide size and the tendency for landslides to be disintegrative bear on landslide genesis. We consider the associations of low slope angles, large landslides, and open slopes 

to be significant. The occurrence of large-scale landslides 

on gentle slopes implies that regional , rather than local, 

factors have been dominant. Furthermore, slope angle , 

which is commonly an important factor controlling the 

initiation of a landslide , does not appear to be important for 

the eland !ides. In fact, the angles involved are well below 

those normally required to initiate landslides. Specifically, 

tatic forces alone at angles less than 1 0° would not be 

enough to cause slope failures except in the most anomalously weak sediments; angles exceeding 15° to 20° would 

64o/o 

Debris Flow 

Flow Bowl 

Disintegrative 

slope failure) and non-disintegrative landslides (that is, 

the displaced mass of sediment retains its original shape 

and internal structure after the initial failure). 

be more typical. The fact that the landslides tend to be 

disintegrative is compatible with the assertion that regional 

mechanism may be involved and may also suggest that a 

relati vely rapid stress increase or strength reduction took 

place within the sediment column. 

We have seen that (I) landslides are pervasive and 

that there are no apparent exclusionary factors based on 

location , slope angle, depth, or morphologic setting, (2) 

slope angles associated with the landslides are generally 

much less than those normally required to influence slope 

stability under static conditions, (3) the lower-angle landslides tend to be larger in area, and ( 4) the landslides tend 

to be disintegrative. Although it is evident that there has 

been a variety of causes for the landslides, transient forces 

(especially those that could affect large areas) likely played 

a significant role in producing many of the slope failures 

documented here. Earthquake loading is the most common 

transient force affecting these slopes. 

The east coast continental margin, although not prone 

to as frequent or intense earthquakes as the Californian or 

Alaskan margins, is not aseismic. Large earthquakes that 

have occurred at Cape Ann, Mass. (1638, 1755), Charleston , S.C. (1886), and more recently, in the Grand Banks 

U.S. Atlantic Continental Slope Landslides: Their Distribution, General Attributes, and Implications 21 
region off the Canadian Maritimes (1929) testify to this. 

Apparentfy, they would have approached or exceeded a 

magnitude of 7 on the Richter scale. The Cape Ann and 

Charleston earthquakes caused extensive damage locally. 

The Grand Banks earthquake caused a slope failure which 

was directly responsible for a series of breaks in transAtlantic cables on the sea floor. Moreover, since their 

relatively recent emplacement (around 1970), seismic stations have recorded many small tremors off the coast of the 

northeastern United States (for example, Yang and 

Aggrawal, 1981). These tremors may contribute, through 

ground shaking and a reduction in the strength of the 

sediment, to the development of an unstable condition on 

some sections of the continental slope or rise. Earthquakes 

can easily produce the levels of loading necessary to fail 

these lower-angle slopes (Morgenstern, 1967). 

Accordingly, we hypothesize that earthquakes are a 

major cause of landslides on the continental slope. Earthquakes fit the implied criteria derived from the landslides 

data base. They can induce slope instabilities over broad, 

morphologically varied areas of low slope inclination; they 

promote disintegrative landslides; and they are sufficiently 

frequent on this "passive" continental margin to be a factor 

in numerous observed slope failures. , . 

Gas may be abundant in frozen form (gas hydrates) 

beneath the southern part of the U.S. Atlantic outer continental margin (Popenoe, Schmuck, and Dillon, this report). 

If frozen gas were converted to free gas by a change in 

pressure, such as by a lowering of sea level during an ice 

age, it could trigger landslides by lowering sediment 

strength (Kayen and Lee, this report). We suggest, therefore, that gas hydrates also may have played a role in some 

of the observed slope failures. 

REFERENCES CITED 

Ayers, M.W., and Pilkey, O.H., 1981, Piston core and surficial 

sediment investigations of the Florida-Hatteras slope and 

inner Blake Plateau, in Popenoe, Peter, ed., Environmental 

geologic studies on the southeastern Atlantic Outer Continental Shelf, 1977-1978: U.S. Geological Survey Open-File 

Report 82-582A, p. 5-1 to 5-89. 

Booth, J.S., O'Leary, D.W., Popenoe, Peter, Robb, J.M., and 

McGregor, B.A., 1988, Map and tabulation of Quaternary 

mass movements along the United States-Canadian Atlantic 

continental slope from 32°00' to 47°00' N. latitude: U.S. 

Geological Survey Miscellaneous Field Studies Map 

MF-2027. 

EEZ-SCAN 87, 1991, Atlas of the U.S. Exclusive Economic 

Zone, Atlantic continental margin: U.S. Geological Survey 

Miscellaneous Investigations Series, v. 1-2054. 

Embley, R.W., 1980, The role of mass transport in the distribution and character of deep-ocean sediments with special 

references to the North Atlantic: Marine Geology, v. 38, 

p. 23-50. 

Embley, R.W., and Jacobi, Robert, 1986, ·Mass wasting in the 

western North Atlantic, in Vogt, P.R., and Tucholke, B.E., 

eds., The Western North Atlantic Region: Boulder Colorado, 

Geological Society of America, The Geology of North 

America, v. M, p. 479-490. 

Emery, K.O., anci Uchupi, Elazar, 1972, Western North Atlantic 

Ocean: topography, rocks, structure, water, life, and sediments: American Association of Petroleum Geologists Memoir 17, 532 p. 

Keller, G.H., Lambert, D.N., and Bennett, R.H., 1979, Geotechnical properties of continental slope sediments-Cape Hatteras to Hydrographer Canyon, in Doyle, L.J., and Pilkey, 

O.H., eds., Geology of the Continental Slopes: Society of 

Economic Paleontologists and Mineralogists Special Publication 27, p. 131-151. 

Malahoff, Alexander, Embley, R.W., Perry, R.B., and Fefe, 

C. F., 1980, Submarine mass-wasting of sediments on the 

continental slope and upper rise south of Baltimore Canyon: 

Earth and Planetary Science Letters, v. 49, p. 1-7. 

McGregor, B.A., and Bennett, R.H., 1981, Sediment failures and 

sedimentary framework of the Wilmington geotechnical corridor, U.S. Atlantic Continental Margin: Sedimentary Geology, v. 30, p. 213-234. 

Morgenstern, N .R., 1967, Submarine slumping and the initiation 

of turbidity currents, in Richards, A.R., ed., Marine Geetechnique: Urbana, Ill., University of Illinois Press, p. 189-

220. 

O'Leary, D.W., 1986a, Seismic structure and stratigraphy of the 

New England continental slope and the evidence for slope 

instability: U.S. Geological Survey Open-File Report 

86-118, 7 maps. 

---1986b, The Munson-Nygren slide, a major lower slope 

slide off Georges Bank: Marine Geology, v. 72, p. 101-114. 

O'Leary, D.W., and Twichell, D.C., 1981, Potential geologic 

hazards in the vicinity of Georges Bank Basin, in Grow, 

J .A., ed., Summary Report of the Sediments, Structural 

Framework, Petroleum Potential, and Environmental Conditions of the United States Middle and Northern Continental 

Margin In Area of Proposed Oil and Gas Lease Sale No. 76: 

U.S. Geological Survey Open-File Report 81-765, p. 48-68. 

Poag, C.W., and Sevon, W.D., 1989, A record of Appalachian 

denudation in postrift Mesozoic and Cenozoic sedimentary 

deposits of the U.S. middle Atlantic continental margin: 

Geomorphology, v. 2, p. 119-157. 

Popenoe, Peter, Coward, E.L., and Cashman, K.V., 1982, A 

regional assessment of potential environmental hazards and 

limitations on petroleum development of the southeastern 

United States Atlantic continental shelf, slope, rise, offshore 

North Carolina: U.S. Geological Survey Open-File Report 

82-136, 67 p. 

Robb, J.M., Hampson, J.C., Jr., and Twichell, D.C., 1981, 

Geology and potential hazards of the continental slope 

between Lindenkohl and South Toms Canyon offshore MidAtlantic United States: U.S. Geological Survey Open-File 

Report 81-600, 21 p. 

Schlee, J.S., Dillon, W.P., and Grow, J.A., 1979, Structure of 

the continental slope off the eastern United States, in 

Dowle,L.J., and Pilkey, O.H., eds., Geology of the Continental Slope: Society of Economic Paleontologists and Mineralogists Special Publication No. 27, p. 95-118. 

Tucholke, B.E., )987, Submarine geology, in Milliman, J.D., 

and Wright, W.R., eds., Marine Environment of the U.S.Atlantic Continental Slope and Rise: Jones and Bartlett Publishers, Boston, p. 56-113. 

Yang, Jih-Ping, and Aggarwal, Y .P., 1981, Seismotectonics of 

northeastern United States and adjacent Canada: Journal of 

Geophysical' Research, v. 86, no. B6, p. 4981-4998. 

22 Submarine Landslides: Seiected Studies in the U.S. Exclusive Economic Zone 
Submarine Mass Movement, a Formative Process of 

Passive Continental Margins: The Munson-Nygren 

Landslide Complex and the Southeast New England 

Landslide· Comple~~ 

By D.W. O'Leary 

INTRODUCTION 

Evidence of extensive submarine mass movement 

along continental slopes rimming the Atlantic Ocean began 

to appear in the 1970's (Embley, 1976; Jacobi, 1976; 

Embley and Jacobi, 1977). High-resolution seismicreflection or echo-sounder profile data from that time shows 

irregular, disrupted stretches of sea floor and chaotic 

deposits upon, or not deeply buried beneath, the sea floor. 

This geophysical evidence has been confirmed by cores and 

photographs which show sediments at the sea floor with 

embedded fragments of rock far removed from their original 

sites of deposition. 

During the time this evidence began to accumulate, 

the notion of a "passive continental margin" was developed 

(Burk and Drake, 1974). Continental slopes around the 

Atlantic (with the exception of the Caribbean region) were 

considered to be passive, which meant simply that geological processes conducive to sea-floor instability, such as 

subduction of oceanic crust, active faulting, an~ slope 

steepening, do not occur. Rather, passive margins are sites 

of more or less gradual subsidence in response to sediment 

deposition and crustal aging. To explain extensive mass 

movement on passive margins, geologists appealed to 

processes such as failure of unstable masses of deltaic 

sediment deposited on the upper part of the continental 

slope during times of low sea level (during ice ages), to 

oversteepening of the lower part of the slope by deep 

current erosion, to infrequent but major earthquakes, to gas 

generation, and even to bioerosion (Saxov and Nieuwenhuis, 1982; Lee, Schwab, and Booth, this report). 

By the middle 1980's, marine survey data included 

sidescan-sonar images which showed that submarine landslides along the Atlantic margin are more common than 

previously thought and that some are of enormous extent 

(Kidd and others, 1985; Bugge and others, 1988; Booth and 

others, this report). The new data implied that mass 

movement might be an integral phenomenon in the evolution of passive continental margins. 

The geological problem that large submarine slides 

represent in the passive margin setting is examined in this 

section with reference to two centers of mass movement 

located on the continental slope off New England: the 

southeast New England landslide complex and the MunsonNygren landslide complex (fig. 1). The descriptions of 

these submarine landslide complexes and their geological 

setting are based mainly on data obtained during a Geological LOng-Range Inclined Asdic (GLORIA) sidescan-sonar 

survey conducted in 1987 (O'Leary, 1988a); however, the 

search for an understanding of causes and processes 

depends on additional and more varied data, only some of 

which can be cited here and much of which still remains to 

be gathered. 

SUBMARINE LANDSLIDE COMPLEXES OFF 

NEW ENGLAND 

The southeast New England landslide complex and 

the Munson-Nygren landslide complex appear, on the basis 

of the GLORIA sidescan-sonar data, to be the only large 

(>6,000 square kilometers (km2

) areas of mass movement 

along the continental slope off New England. Evidence of 

mass movement within the area of the southeast New 

England landslide complex was first recognized by Uchupi 

(1967); subsequent seismic-reflection surveys revealed that 

a wide area of the continental rise in this region is covered 

by mass-movement debris (Macllvaine and Ross, 1979; 

Embley, 1982; Vassallo and others, 1984). Evidence of the 

Munson-Nygren landslide complex was first reported by 

O'Leary (1986). 

It was clear from the earlier work that large masses of 

strata had been subject to slope failure within these two 

areas, but the spatial relationships of individual failures and 

mass-movement deposits that characterize each area were 

unknown until data were acquired by a GLORIA sidescansonar survey (O'Leary, 1988a). Sidescan-sonar images and 

seismic-reflection profiles show that small individual landslides as well as exceptionally large ones contribute to each 

landslide complex and that mass movement has recurred 

both within and adjacent to previous slope failures. Because 

of the apparent chronological and spatial complexity, the 

term landslide complex is applied to the two broad areas of 

mass movement. 

SOUTHEAST NEW ENGLAND LANDSLIDE 

COMPLEX 

The southeast New England landslide complex 

extends along the New England continental slope from 

Submarine Mass Movement, a Formative Process of Passive Continental Margins 23 
~~ ~~ 

~~ 

~0 ~" 

" 41 ° + + 

+ 

/ / / / 

~«' /// be\)~ / / / 

..,....... ....... ,..../ 

, ______ __ _ + + + 

0 

E3 E3 

+ 

/ 

/ 

/ 

-v/ ~ / 

I 

I 

I 

+ 

// U.S. EEZ boundary 

+ + 

60 

KILOMETERS 

Figure 1. Areas of the New England continental slope and rise occupied by mass movement complexes: (A) 

Southeast New England landslide complex; (B) Munson-Nygren landslide complex. 

Hud on Canyon east to 69°30'W. (fig . 2), a distance of 200 

kilometers (km). The complex comprises four major landslides and numerous lesser areas of mass movement, as well 

as masses of sediment that have issued from canyons and 

that coalesce with the larger mass-movement deposits. The 

four major landslides (or centers of mass movement) within 

the southeast New England landslide complex, the Block 

Canyon landslide, the Atlantis Canyon landslide, the Nantucket Canyon landslide, and the Veatch Canyon landslide , 

are named for the largest canyons that seem to have been 

involved in or modified by each slope fa ilure (fi g. 2). Each 

landslide complex is represented along the upper rise by 

broad depressions bounded by scarps as much as 150 meters 

(m) in relief (fig. 3). 

Slope failure generally originated on the continental 

slope in water depths greater than 750 m. Ev idence includes 

irregular scarps and cliffs that range in relief from 10 to 200 

m (fig 4). Most strata of Pleistocene age (10,000 to 2 

million years old) have been removed , along with an 

unknown amount of older strata . Chalky sediment of 

Eocene age (38 to 54 million years old) has been sampled 

from the sea floor below the landslide scarps (Gibson, 

1965), and blocks of chalk have been sampled from debris 

on the rise , indicating that hard, deeply buried strata have 

been exposed by mass movement and , at least locally, 

involved in the transport. 

Despite the gentle slope inclination (4° to 8°), mass 

movement has been remarkably complete. The lower slope 

is nearly bare of rubble except for tongues of debris that 

have formed below some of the larger scarps, probably as a 

result of rockfalls or parti al collapse of sections of scarp. 

There is no clear evidence of any large slabs that have 

survived transport across the slope nor any slabs that have 

been arrested close to the scarps, whereas the rise is 

24 Submarine landslides: Selected Studies in the U.S. Exclusive Economic Zone 
EXPLANATION 

Undifferentiated mass 

movement deposits 

Area shown in numbered 

figure 

Subtle rise of ocean floor 

Axis of named canyon 

Farnella 87 tracklines 

contour interval 250 meters 

30 60 KILOMETERS 

Figure 2. Generalized map of the southeast New England landslide complex. Numbered heavy line segments indicate 

locations of seismic-reflection profiles shown in corresponding figures. 

blanketed by sheets of debris as much as 50 m thick and 50 

km wide (figs. 2, and 5). Some mass-movement deposits 

are separated vertically from others by intervals of welllayered strata, suggesting geologically significant periods of 

quiescence and normal deposition between mass movements (fig. 5). 

Debris from the landslides has spread across the 

upper rise to form broad, flared deposits which have sharply 

defined, irregular borders (figs. 2 and 6). The GLORIA 

images show the debris as light gray areas (areas of high 

acoustic backscatter intensity) and the undisturbed sea floor 

as dark gray (areas of low acoustic backscatter). The 

Submarine Mass Movement, a Formative Process of Passive Continental Margins 25 
r 

to.. 

w 

0 3000- a: 

w 

t-

~ 

3750-

0 

E 

~= ;: g 

.i~ Atlantis Canyon 

Alvin Canyo II ~ 'I ' l n l 'Nantucket Canyon 

I 

J Veat<;_h Canyon 

40km 

Figure 3. Seismic-reflection profile along the upper rise at water depths of about 2,250 meters shows relationships 

between landslide-complex depressions and canyons. 

SE NWI ~E sw 

1950-

" : ~ 0 : 

r: 

.. :· 

(/) 

- = a:: 

w 

~ 

w 2100- ::E 

z 

I 

~ 

CL 

w 

a 

a:: 2250-

w 

~ 

<t: 

3: :J;<, . ' 

. ,, . 

. · •• • ~-· !'," • 

·,· :. ' 

·:: 

·,. 

0 IOkm 

Figure 4. Part of the escarpment along the head of the southeast New England landslide complex. The features shown 

here, in a water depth of about 2,250 meters, have a relief of almost 100 meters. The central gap is choked with debris, 

and the two stacks at the right end of the profile segment are partly overridden with debris. 

26 Submarine landslides: Selected Studies in the U.S. Exclusive Economic Zone 
SE 

2250-

NW 

- :G older debris C/) 2325-

0:::: 

! younger debris ! - - :"' 

w ::a tw 

~ 

z 

I 

t- 2400 a_ 

w 

0 . -

r • I •,• • • ;"'" , • .o 

0:::: 

w 

t-

<! 

r., ' , 

t· ~ 

-:. i . 

3: 

. ~. . 

0 5km 

Figure 5. Echo-sounder profile shows two generations of mass-movement debris separated by remnant of normally 

layered sediment. 

boundaries of the mass-movement deposits mapped from 

the GLORIA images show that transport of debris was 

halted or diverted by subtle rises of the ocean floor more 

than 100 km from the landslide heads (figs. 2 and 6). The 

debris typically pinches out at the contacts with little relief. 

A number of submarine canyons feed into the south-

_east New England landslide complex. The larger canyons 

appear to predate mass movement, because some are 

evidently partly filled with debris, and sediment that drapes 

their upper walls has been partly removed by mass movement (fig. 7). Other canyons, however, clearly postdate the 

major mass-movement events, because they slice through 

some of the failure scarp on the slope (fig. 8). GLORIA 

images of the continental rise show that mass-movement 

debris partially dammed canyons. Veatch Canyon was 

actually overridden by debris of the Veatch Canyon landslide (fig. 9), whereas Hudson Canyon was partly filled by 

debris of the Block Canyon landslide that entered through 

alcoves along the east bank (fig. 10). Carstens Valley, 

which originates on the continental rise itself, was partly 

choked by the Block Canyon landsl ide debris; its head and 

upper region are nearly obscured by debris (fig. 11). 

MUNSON-NYGREN LANDSLIDE COMPLEX 

The Munson-Nygren landslide complex occupies the 

lower slope off Georges Bank between Munson and Nygren 

Canyons and just west of Munson Canyon (figs. 1 and 12). 

A sequence of at least three separate landslides has occurred 

between Munson and Nygren Canyons at a water depth of 

about 1,900 m. Little is known about the oldest landslide; 

evidence of its existence consists of a rubble deposit hown 

in a seismic-reflection profile (fig . 13). This deposit is 

truncated by a 325-m-high scarp that represents a younger 

(and the major) landslide. The scarp that is associated with 

the second phase of mass movement fom1s a narrow alcove 

6 to 10 krn wide that extends 45 km downslope to where it 

opens on the continental rise and loses topographic definition (fig. 12). Displaced slabs as much as 250 m thick, and 

Submarine Mass Movement, a Formative Process of Passive Continental Margins 27 
Figure 6. GLORIA sidescan-sonar imagery shows a broad distribution of debris within the southeast New England 

landslide complex (area A, fig. 1); debris shows as relatively light gray tones in the images. Wispy light and dark tones 

oriented north-south in eastern half of the image are debris flow lines. See figure 2 for further interpretation. 

locally back-tilted blocks represent a third phase of landsliding toward the central Munson-Nygren landslide trough; 

the zone of these displaced slabs extends laterally for 

distances as much as 5 to 10 km back from the scarp (fig. 

14) and apparently led to breaching of the east wall of 

Munson Canyon and to pattial filling of the canyon with 

debris (figs. 15 and 16). The GLORIA images show that 

mass movement along the west flank of Munson Canyon 

constitutes a distinct landslide comparable in size to the 

Munson-Nygren landslide. It is called the Retriever landslide, because it spreads its debris directly downslope 

toward Retriever Seamount (fig. 12). 

28 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
.. downslope I km 

Figure 7. SeaMARC I sidescan-sonar image shows embayed, collapsed west wall of Atlantis Canyon. This image has been 

enhanced to emphasize the arcuate scarps forming the canyon rim as well as steep outcrops and cliffs along the wall. 

Dark tones indicate steep and (or) highly reflective surfaces. 

Debris from the Retriever landslide seems to be older 

than sediment issued from Munson Canyon; whereas sediment from Munson Canyon merges rather completely with 

debris of the Munson-Nygren landslide, indicating coeval 

activity (fig. 17). Debris from the Munson-Nygren landslide complex extends across the rise for a distance of at 

least 200 km and over a width of sea floor as great as 80 km 

(fig. 12). The total combined debris sheet, not much thicker 

than about 35 m, has flowed around Retriever seamount, 

partly burying it. 

COMPARATIVE FEATURES OF THE LANDSLIDE 

COMPLEXES 

The southeast New England landslide complex and 

the Munson-Nygren landslide complex share important 

similarities in structure and form, but there are significant 

differences in their physiographic settings and in the character of the respective debris deposits. The similarities and 

the differences present interesting implications about origin 

and process. 

In both areas, landsliding originated on the middle to 

lower part of the continental slope (the headwall scarps are 

at 750-m to I ,900-m water depths) and involved thick 

sections (less than or equal to 300 m) of consolidated, 

well-bedded strata. Both areas of mass movement also show 

evidence of deformed layers deep in the sedimentary section 

(intrastratal deformation; figs. 3 and 14). Mass movement 

has been complex in both areas, involving multiple and 

sequential displacements that have cut into or occurred 

within older mass movement deposits. 

In both areas, landsliding has been geologically 

recent; the youngest strata resolvable by echo-sounding, 

strata of late Pleistocene age (about 20,000 years old), have 

been cut. Landsliding in both areas also postdates the 

episode of major canyon cutting along the slope, although it 

Submarine Mass Movement, a Formative Process of Passive Continental Margins 29 
w t upslope E 

I km 

Figure 8. SeaMARC I sidescan-sonar image shows small canyon cut through a mid-slope head-wall scarp of the 

southeast New England landslide complex. The scarp faces downslope and is in acoustic shadow (white tone). 

30 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
U) 

a: 

3100-

~ 3250-

w 

~ 

z 

SE 

~ 

~ 

I 

. , 

• 

• 

J 

.J 

0 

NW 

, 

... 

;o 

'·1, ,1;! I .. ::····:l:··:i 

1.' 

- - ;;; ... - i "" 

IOkm 

Figure Y. Echo-sounder profile shows Veatch Canyon nearly filled by debris of the Veatch Canyon landslide. The debris 

crossed the canyon from the north, partly filling it, and extended for some uncertain distance to the south. 

is clear that in the southeast New England landslide complex, at least, some canyons have formed along with or 

following some of the mass movement, and Munson Canyon has been active following some deposition in the 

Munson-Nygren landslide complex (fig. 17). 

In both areas, mass-movement debris is almost totally 

disintegrated; only small blocks of hardened sediment have 

survived transport. The strata subject to mass movement 

must have been unable to withstand the stress of movement 

and must have fragmented in the manner of subaerial 

landslides. The disintegrated debris was remarkably 

mobile. In both areas, it spread out, away from the landslide 

head across a sea floor inclined at less than a degree. 

The two landslide complexes occupy distinctly different physiographic settings. The southeast New England 

continental slope is an area of mainly parallel stratification 

in which the lower slope steepens with depth and forms an 

abrupt contact with a more gently sloping continental rise 

surface (fig. 18). The Munson-Nygren landslide complex 

and the Retriever lands! ide occur along a segment of slope 

which is steepest above the headwall scarp (fig. 13) and 

which otherwise curves gradually into the continental rise. 

It is easy to see how slope steepness in the former setting 

controls the extent of mass movement and the location of 

the slope failure scarps of the southeast New England 

landslide complex. Given the more or less uniform physical 

properties and sediment type, however, it i difficult to 

understand how slope failure could occur in the latter 

setting, because failure has taken place on the more gently 

inclined and presumably more stable lower part of the 

slope. Most of the slope along Georges Bank (fig. 1), a 

distance of 350 krn, features physiographic characteristics 

typical of the Munson-Nygren area; near Munson and 

Nygren Canyons, there do not appear to be any distinctive 

environmental factors which could have localized recurrent 

mass movement. Most of the Georges Bank slope is deeply 

cut by canyon erosion, and although landsliding has 

occurred at many places along the Georges Bank slope, 

nowhere does it reach the proportions of the MunsonNygren landslide complex. 

There are interesting differences as well in the gross 

aspect of the debris deposits. Compared to debris of the 

Submarine Mass Movement, a Formative Process of Passive Continental Margins 31 
/ 

filled part of canyon 

canyon rims 

/i 

ship track 

Figure 10. GLORIA sidescan-sonar image shows debris partly filling Hudson Canyon on the upper rise at a water depth of 

about 260 meters. Toward the south end of image the canyon has about 200 meters relief. 

southeast New England landslide complex, debris of the 

Munson-Nygren landslide complex, as seen in the GLORIA 

images, does not have well-defined margins; debris of the 

Munson-Nygren landslide complex has a more flared distal 

aspect and a generally more homogeneous internal acoustic 

character. The less well-defined margins of the MunsonNygren debris deposit and the apparently deeper cuts into 

the sea floor suggest higher velocity transport with erosive 

powers. The more clearly defined and intricate borders of 

debris deposits of the southeast New England landslide 

complex, the existence of flow lines and patches of undistlll·bed strata beneath thin debris in this area implies a less 

destructive emplacement, perhaps involving lower velocities or less viscosity. 

SUMMARY OF INTERPRETATIONS AND 

CONTROVERSIES ON ORIGIN 

An explanation for the origin of one landslide complex must in some way account for the origin of the other, 

32 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
NW 

2250-

~ 2400-

w 

..__ 

w 

~ 

z 

:r: 2550 ..__ 

CL 

w 

a 

0:: 

w 

~ 2700- ~ 

2850-

0 

SE 

I I' I' ;1'1 ,, 'I ~ 1: Ill!,, •I ·II' I '1' '1' i llll l1 1 ·' Ill · · ,,,.l .l l. llll1\ t !· 1

'111 1 1t ,l 11, ·•' -

IOkm 

Figure 11. Echo-sounder profile shows mass-movement debris filling the head of Carstens Valley at water depths of 

about 2,600 meters. 

since we wish to explain both the common features and the 

differences. Additionally, an acceptable explanation must 

conform to what we know about the geological histories of 

the two complexes. The accumulated geophysical data from 

the New England continental margin place strong constraints on any such explanation. 

The concept of erosional undercutting of the lower 

continental slope by bottom currents is given no support by 

the GLORIA data off New England. The data show no 

evidence for any significant erosional or depositional features aligned along the New England slope, but they do 

show a nearly ubiquitous alignment of erosional and depositional features extending downslope and across the upper 

rise. The geological history of the New England continental 

slope, for the last 120,000 years at least, is dominated by 

sediment transport (mass movement) and erosion trending 

across the slope and rise rather than parallel to it (O'Leary, 

1988b; Danforth and Schwab, 1990). 

The concept of slope instability due to gas accumulation or to rapid sediment deposition on the upper slope is 

negated in the New England region by the fact that mass 

movement has occurred at water depths from 750 m to 

2,000 m, in stacks of strata tens to hundreds of meters thick, 

many of which had become compacted and indurated 

through time. Furthermore, these causal mechanisms are 

predicated on sea level having been lowered to at least the 

shelf edge, and while there is good evidence that such 

lowering did occur in Pleistocene time (O'Leary, 1988c), 

the data show that both canyon cutting and mass movement 

postdate any deposition that attended the latest drop in sea 

level. This means that shelf-edge deposition did not directly 

cause the slide complexes; shelf-edge deposits had attained 

stability before sliding occurred. 

Likewise, "bioerosion" (Hecker, 1982) cannot 

account for failure of apparently stable stacks of strata tens 

of meters thick over areas of tens of hectares, because 

boring or digging organisms cannot penetrate deeply 

enough into the compacted strata. Bioerosion is, at most, a 

surficial process. 

Of the various hypotheses put forward for mass 

movement along the New England slope, only earthquake 

shaking could account directly for the slide complexes. The 

case for earthquake-induced mass movement seems wellestablished for the Atlantic passive margin setting because 

of the widely cited 1927 Grand Banks earthquake, which is 

infamous for having set off mass flows that snapped a 

number of submarine cables sequentially downslope on the 

continental rise (Heezen and Ewing, 1952). Earthquake 

Submarine Mass Movement, a Formative Process of Passive Continental Margins 33 
X 

40oo 

vo 

EXPLANATION 

Undifferentiated massmovement deposits 

:·-----. Area shown in numbered 

~-----~ figure 

~ • - - Slide scarp; position inferred 

except at ship track crossings 

•••-• Axis of named canyon 

- Seismic profile segment 

shown in numbered figure 

-- Farnella 87 tracklines 

contour interval250 meters 

0 10 20 KILOMETERS t:=="1 E3 E"3 I 

Figure 12. Generalized map of the Munson-Nygren landslide complex. Numbered heavy line segments indicate locations 

of seismic-reflection profiles shown in corresponding figures. 

34 Submarine landslides: Selected Studies in the U.S. Exclusive Economic Zone 
0-

750- ~ ,. ' ' i 

NW 

i" 

P.==E ~ !I!! 

. . ! 

~-

(f) 

0::: 

w 

~ 

w 

~ 

border of fig. 12 

1 . 

z 

SE 

2250-rise 

I 

1 a 

... 

. . I . 

·: 

f : 

0 5km 

Figure 13. Seismic-reflection profile across the continental slope shows scarps and rubble associated with the 

Munson-Nygren landslide complex. 

shaking is not necessarily the entire answer, however; there 

are other phenomena involved that must be explained. 

These phenomena include the involvement of slope canyons 

as debri conduits to the southeast New England landslide 

complex, the widespread intrastratal deformation, the recurrent rna s movement at a particular spot and over an 

apparently short-and recent-interval of geologic time , 

and in the case of the Munson-Nygren landslide , the fact 

that mas movement occurred on a relatively gentle slope 

rather than on the steeper upper slope which we might 

expect would be more liable to fail. 

The earthquake hypothesis could possibly be used to 

explain some of these difficulties. It is reasonable to 

suppose that short-lived episodes of seismicity have 

occurred, a part of the ongoing geological evolution of the 

Atlantic margin and that "swarms" of earthquakes have led 

to recurrent slope failure in localized areas (Woodcock, 

1979), including recurrent or sustained collapse of canyon 

Submarine Mass Movement, a Formative Process of Passive Continental Margins 35 
(f) 

a::: NE 

w 1875- . ~ ·: ~Intrastratal 

~ ~ 

sw 

~is placed , slob_. tiL>: *::. _ side scarp .. 

· . .)I Munson -Nygren : · ........ ..... ·..... ....... . · ;~ landslide trough :>. · 

~\1!?~~· ,-. J: · .· ~· '{~ 

Figure 14. Sei smic-reflection profile shows collapsed, displaced strata to either side of the main Munson-Nygren 

landslide. 

I 

1-

a.. 

~ 2400 

0:: 

w 

1-

<( 

~ 'I 

! ·• : I. 

0 

mass-movement debris 

;: 

,j 

5km 

' ,. 

sw 

I 

~.' . 

Figure 15. Sei smic-reflection profile along lower slope shows location of Munson Canyon between Munson-Nygren 

landslide and Retriever landslide. Note the scarp that faces crossing position of profile 125 (fig. 16); the scarp backs the 

upper wall of Munson Canyon. 

36 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
Cf) 

0:: 

NW 

1875- i 

~ 2025--~~:;...,__ ....... w 

~ 

z 

I 

1---

Cl. 

w 

0 

ffi 2175 

~ 3: 

2325-

0 

___ \ 

.I 

I 

,, 

SE 

5km 

Figure 16. Seismic-reflection profile 125 extending along the upper east side of Munson Canyon shows that profile G25 

(fig. 15) crosses a breach in the upper wall of Munson Canyon. 

walls. It is even plausible to assert that some of the 

intrastratal deformation could be a consequence of ground 

shaking. Some unconsolidated, water- aturated layers may 

undergo strength los and local failure during earthquake 

loading (Lee, Schwab, and Booth, this report). Other 

layers, deeper in the section, may weaken over a long 

period due to creep and a gradual loss of strength . Such 

layers may also be susceptible to earthquake shock. Little is 

known about this inferred weak-layer phenomenon, and the 

full story awaits results of ongoing research; however, 

failure of a laterally extensive weak layer could lead to 

nearly instantaneous collapse (liquefaction at depth within 

the strata) and disintegration of large slabs of strata. Such 

disruption is implied by the finely fragmented, thin debris 

strewn across the rise below the landslide scarps. 

The contrast in depositional style between the 

Mun on-Nygren landslide complex debris and the southeast 

New England landslide complex debris can be attributed, at 

least in part, to sediment composition. The southeast New 

England landslide complex is derived from a finer-grained, 

more clay-rich, probably more cohesive sediment 

(O'Leary, 1988c); the debris may have regained some 

cohesive strength below a certain transport velocity, or the 

debris may have behaved more like a debris flow than a 

turbidity current. The Munson-Nygren landslide is derived 

from slightly coarser, less cohesive, more poorly sorted 

sediment that may have been more readily dispersed to form 

a turbid flow than debris of the southeast New England 

landslide complex. 

CONCLUSIONS 

The southeast New England landslide complex and 

the Munson-Nygren landslide complex are similar to large 

landslide complexes found elsewhere on continental slopes 

that rim the Atlantic Ocean basin. The landslide complexes 

show recurrent mass movement in stacks of strata that were 

otherwise stable for some time following deposition. Landsliding has resulted in nearly complete fragmentation of the 

failed strata and emplacement a thin sheets of debris over 

huge areas of the upper rise. 

Mass movement of this nature seems to be a common 

feature of passive continental margins (Booth and others, 

Submarine Mass Movement, a Formative Process of Passive Continental Margins 37 
Figure 17. GLORIA sidescan-sonar image segment shows sea floor in the vicinity of Retriever seamount (water depth 

about 3,700 meters). Sediment debouched from Munson Canyon trends across the Retriever landslide debris field, but 

it seems to merge with, or perhaps be diverted by, the Munson-Nygren landslide debris. 

0 

en 

a:: 750- w 

1-

w 

~ 

~ 

:r:: 1500-

1-

a.. 

w 

a 

a:: 

w 

1- 2250-

~ 

3000-

shelf s 

I . : ~ ~ . 

0 IOkm 

Figure 18. Seismic-reflection profile shows typical continental slope/rise profile in area of the southeast New England 

landslide complex. 
this report); it does not seem to be a result of local 

environmental factors, including rapid deposition at the 

shelf edge. Rather, the origin of the landslide complexes 

would seem to be found in the structural and tectonic 

characteristics of the passive margins themselves. The 

salient characteristics include generally uniform, deposition 

and lithification of fine-grained sediment upon a very 

slowly and gently subsiding surface, the stability of which 

is punctuated by sporadic, large earthquakes. Accordingly, 

a hypothesis of origin for the mass-movement complexes 

described above centers on a gradual weakening, due to 

creep, of key beds buried beneath the lower slope. Strainweakened beds would be susceptible to failure by earthquake shock; with structural support destroyed, the overlying section would thereupon fail and fragmentation of the 

detached mass would ensue during mass movement. 

This simple hypothesis requires much more scrutiny 

in the light of more data. The actual mode of debris 

transport remains problematic, and the genetic relationships 

between slope failure and canyon erosion offer unexplored 

complexities. The period of active canyon erosion that 

preceded and postdated mass movement implies a different 

set of processes and conditions than existed for the large 

landslides, yet these conditions obviously intersected and 

jointly contributed to sedimentation on the New England 

continental rise for some significant period. Whether canyon activity reflects environmental phenomena related to 

the advance and retreat of Pleistocene ice sheets, and 

whether these climatic events contributed directly to submarine sliding on the New England slope remains to be 

seen. Despite present uncertainties, evidence indicates that 

large-scale submarine landsliding is an inherent part of both 

the depositional history and the tectonic processes that 

formed the Atlantic passive margin. 

REFERENCES CITED 

Bugge, Tom, Belderson, R.H., and Kenyon, N.H., 1988, The 

Storegga slide: Philosophical Transactions of the Royal 

Society of London (A), v. 325, p. 357-388. 

Burk, C.A., and Drake, C.L., 1974, Continental margins in 

perspective, in Burk, C.A., and Drake, C.L., eds., The 

geology of continental margins: New York, Springer-Verlag, 

p. 1003-1009. 

Danforth, W.W., and Schwab, W.C., 1990, High-resolution 

seismic stratigraphy of the upper continental rise seaward of 

Georges Bank: U.S. Geological Survey Miscellaneous Field 

Studies Map MF-2111, 9 sheets and 10 p. 

Embley, R. W., 1976, New evidence for occurrence of debris flow 

deposits in the deep sea: Geology, v. 4, p. 371-374. 

---1982, Anatomy of some Atlantic margin sediment slides 

and some comments on ages and mechanisms, in Saxov, 

Svend, and Nieuwenhuis, J .K., eds., Marine slides and other 

mass movements: New York, Plenum Press, p. 189-214. 

Embley, R.W., and Jacobi, R.D., 1977, Distribution and morphology of large submarine sediment slides and slumps on 

Atlantic continental margins: Marine Geotechnology, v. 2, 

Marine Slope Stability, p. 205-228. 

Gibson, T.G., 1965, Eocene and Miocene rocks off the northeastern coast of the United States: Deep-Sea Research, v. 12, p. 

975-981. 

Hecker, Barbara, 1982, Possible benthic fauna and slope instability relationships, in Saxov, Svend, and Nieuwenhuis, J .K., 

eds., Marine slides and other mass movements: New York, 

Plenum Press, p. 335-348. 

Heezen, B.C., and Ewing, Maurice, 1952, Turbidity currents and 

submarine slumps and the 1929 Grand Banks earthquake: 

American Journal of Science, v. 250, p. 847-873. 

Jacobi, R.D., 1976, Sediment slides on the northwestern continental margin of Africa: Marine Geology, v. 22, p. 157-173. 

Kidd, R.B., Simm, R.W., and Searle, R.C., 1985, Sonar acoustic 

facies and sediment distribution on an area of the deep ocean 

floor: Marine and Petroleum Geology, v. 2, p. 210-221. 

Macllvaine, J.C., and Ross, D.A., 1979, Sedimentary processes 

on the continental slope of New England: Journal of Sedimentary Petrology, v. 49, no. 2, p. 563-574. 

O'Leary, D.W., 1986, The Munson-Nygren slide: a major lowerslope slide off Georges Bank: Marine Geology, v. 72, p. 

101-114 

---1988a, GLORIA reveals rise: Geotimes, v. 33, p. 22-24. 

---1988b, GLORIA shows late Cenozoic sediment distribution and erosion patterns of the New England continental rise: 

Abstracts with programs, Geological Society of America 

Annual Meeting, v. 20, no. 7, p. A131. 

---1988c, Shallow stratigraphy of the New England continental margin: U.S. Geological Survey Bulletin 1767, 40 p. 

Saxov, Svend, and Nieuwenhuis, J.K., 1982, Marine slides and 

other mass movements: Series IV: Marine sciences, v. 6, 

New York, Plenum Press, 353 p. 

Uchupi, Elazar, 1967, Slumping on the continental margin southeast of Long Island, New York: Deep-Sea Research, v. 14, p. 

635-639. 

Vassallo, Kevin, Jacobi, R.D., and Shor, A.N., 1984, Echo 

character, microphysiography, and geologic hazards, in 

Ewing, J.l., and Rabinowitz, P.O., eds., Eastern North 

American Continental Margin and Adjacent Ocean Floor, 34° 

to 41° N. and 68° to 78° W., Atlas 4, Ocean Margin Drilling 

Program: Regional Atlas Series, Marine Science International: Woods Hole, Mass., p. 31. 

Woodcock, N.H., 1979, Sizes of submarine slides and their 

significance: Journal of Structural Geology, v. 1, p. 

137-142. 

Submarine Mass Movement, a Formative Process of Passive Continental Margins 39 
The Cape Fear Landslide: Slope Failure Associated 

with Salt Diapirism and Gas Hydrate Decomposition 

By Peter Popenoe, E.A. Schmuck, and W.P. Dillon 

INTRODUCTION 

The Cape Fear landslide is one of the largest and best 

documented of the U.S. Atlantic continental margin massmovement features. The landslide occurred on the continental slope and rise southeast of Cape Fear, N.C., with the 

main headwall scarp centered on the lower slope at 2,600 

meter (m) water depth (fig. 1). The amphitheater-shaped 

headwall scarp, over 50 kilometers (km) long and 120 m 

high, encircles two large salt diapirs that protrude above the 

sea floor within the failure area. (A diapir is a fold in which 

a mobile core has risen and broken through brittle overlying 

rock and sediment.) A complex of slumps extends more 

than 40 km upslope from the major headwall scai-p (fig. 2), 

and mass-movement deposits can be traced for more than 

400 km downslope to a water depth of over 5 .4 km on the 

Hatteras Abyssal Plain. 

The landslide is unique along the Atlantic continental 

margin, both because of its coincidence with the extruded 

salt diapirs, and because of its association with a gas 

hydrate in the shallow subsurface (fig. 1). Oas hydrate is an 

icelike material that forms, when a mixture of methane (or 

other gases) and water satisfy certain pressure and temperature conditions (see Kayen and Lee, this report). Both the 

intrusion of salt (Cashman and Popenoe, 1985) and decomposition of the gas hydrate caused by past global warming 

or sea-level lowering (Carpenter, 1981; Paull and others, 

1989), have been proposed as possible causes of the slope 

failure. In this section, we document some of the features 

associated with the Cape Fear landslide and review evidence relating to the possible mechanisms and timing of the 

slope failure. 

The Cape Fear landslide was discovered in the . .late 

1970's, when mass-movement deposits on the continental 

rise below the area of slope failure were first identified from 

echo-sounder profiles and cores (Embley, 1980). The main 

headwall scarp created by the landslide was mapped by the 

U.S. Geological Survey (USGS) during geologic hazards 

investigations related to offshore petroleum lease activity in 

1979 and 1980 (Popenoe and others, 1981, 1982; Carpenter, 1981; Cashmen and Popenoe, 1985). Most recently the 

lower headwall area of the landslide was surveyed by 

investigators (Paull and others, 1989) from the University 

of North Carolina (UNC). The USGS and UNC network of 

ship tracklines in this study area are shown on figure 2. 

REGIONAL GEOLOGIC SETTING 

The continental slope and upper rise between Cape 

Hatteras and Blake Spur (fig. 1), overlie the eastern edge of 

the Carolina Trough. The Carolina Trough is a narrow 

sediment-filled basin that formed following the initial rifting (separation) of Africa and North America over 140 

million years ago (Jurassic Period). In its early history, the 

Carolina Trough was the site of deposition of a thick layer 

of salt (Dillon and others, 1982; Hutchinson and others, 

1982; Dillon and Popenoe, 1988, and references cited 

therein). Later, in Jurassic and Cretaceous time (140 to 65 

million years ago), the landward part of the basin was 

covered by seaward-prograding sediment of the continental 

shelf and slope. This progradation across the basin loaded 

the salt layer and squeezed it seaward in front of the 

advancing wedge of sediment. When the displaced salt 

encountered a shallower rock basement that forms the 

eastern edge of the Carolina Trough, it flowed upward, 

forming a line of salt diapirs (fig. 1). Although there are 

several salt diapirs that extend above the sea floor on the 

continental rise, the two diapirs exposed in the scar of the 

Cape Fear landslide have the most relief and are the only 

diapirs that are associated with slope failure. 

A well-developed gas-hydrate layer that in many 

areas has free gas trapped beneath it is present throughout 

the area of the crest of Blake Ridge and beneath the 

continental slope and upper rise off North Carolina (fig. 1), 

approximately coincident with the location of the Carolina 

Trough. Gas hydrate looks much like ice; in fact, its crystal 

structure consists of an ice lattice that incorporates gas 

molecules of the appropriate size into its structure. Commonly, this gas is methane or carbon dioxide. Temperature 

and pressure conditions of the deep ocean are conducive to 

the formation of gas hydrate within the upper several 

hundred meters of the sedimentary section (Kayen and Lee, 

this report) and, if adequate gas is available, gas hydrate 

will fill the pore spaces between sediment grains. As the 

sediment gets warmer with increasing depth (geothermal 

heating), eventually a temperature is reached in the subsurface below which the gas hydrate cannot exist. Thus, there 

is a distinct base to the gas-hydrate-cemented zone. This 

base reflects sound energy, so the base of the gas hydrate 

can be seen on seismic-reflection profiles as a reflector that 

tends to parallel the sea floor (bottom-simulating reflector 

or BSR). The .BSR is easily seen to the south of the 

landslide area (fig. 3), where the continental slope is 

undisturbed by structures associated with the landslide. 

Also seen on seismic-reflection profiles are amplitude 

anomalies (bright spots) caused by free gas trapped beneath 

the gas hydrate. In comparison, the gas hydrate and 

40 Submarine landslides: Selected Studies in the U.S. Exclusive Economic Zone 
CAROLI A 

Figure 1. Regional setting of the Cape Fear landslide and 

the location of figures 2 and 4. The delineation of the 

landslide beyond the EEZ boundary is from Embley (1980). 

Also shown are the locations (black dots) of salt diapirs 

underlying trapped gas are less obvious or are discontinuous 

in the area of the Cape Fear landslide. Perhaps structural 

complexities caused by slope failure have obscured the 

BSR, or the gas hydrate is actually discontinuous in this 

area. 

DESCRIPTION OF THE CAPE FEAR LANDSLIDE 

Figure 4 is a Geological LOng-Range Inclined Asdic 

(GLORIA) sidescan-sonar image of the continental slope 

and rise between 32° and 34°N latitude (EEZ-SCAN 87 

Scientific Staff, 1991). On this image, the path of the Cape 

74 c\V 72(W 

(from Dillon and others, 1982) and the known distribution 

(stippled) of the gas hydrate layer (Dillon and others, 

1980; Popenoe, 1984). 

Fear landslide is easily traced across the continental rise by 

its light-toned (stronger) acoustic return that contrasts with 

the darker (weaker) return produced by surrounding undisturbed sedimentary deposits. The surface of the landslide is 

marked by longitudinal lines that represent both debris 

chutes between the salt diapirs at the head of the landslide 

area (A, fig. 4) and flow paths of debris further downslope 

(B, fig. 4). Near the head of the landslide there is an area of 

irregular, hummocky sea floor that has been crumpled and 

buckled by mass movement (C, fig. 4). The distal end of the 

Cape Fear landslide extends across an older mass movement, the Cape Lookout landslide (figs. 1 and 4). 

The Cape Fear landslide: Slope Failure Associated with Salt Diapirism and Gas Hydrate Decomposition 41 
HUMMOCKY 

SEA FLOOR 

(•CARPET" 

LANDSLIDE) 

Figure 2. Headwall area of the Cape Fear landslide showing ship tracklines and major features. Also shown are the 

locations of seismic-reflection profiles (heavy lines) and sidescan-sonar images (shaded areas) referenced in text. 

The Headwall Area 

The main area of slope failure, expressed by a lower, 

main headwall scarp up to 120 m high, is located near the 

base of the continental slope just above two salt diapirs 

(lower, main headwall scarp, fig. 2). Above this lower, 

main scarp, higher on the slope, there are two arcuate scarps 

of lesser relief that encircle shallower landslide scars; one 

on the mid-slope expressed by a 20- to 30-m-high scarp 

(mid-headwall scarp, fig. 2), and one on the upper slope 

expressed by a 15- to 25-m-high scarp (upper headwall 

scarp, fig. 2). There is an additional small embayed area of 

42 Submarine landslides: Selected Studies in the U.S. Exclusive Economic Zone 
w E 

-750 

FREE GAS TRAPPED BENEATH GAS HYDRATE 

-

u 2-§§§§~ w 

(f) 

.__.. 

w 

~ 

t-

_J 

w 

~ 3 

0::: 

t-

>-

~ I 

0 

~ 4-''···, 

5-

0 20 

Figure 3. Airgun seismic-reflection profile across the slope 

south of the Cape Fear landslide showing a strong bottom 

simulating reflector (BSR) indicative of the base of the gas 

hydrate (see fig. 2 for location of this profile). The gasslope failure that occurs just above the lower headwall 

scarp. 

Figure 5 is a seismic-reflection profile which reveals 

the deeper geologic structure of the slope beneath and 

adjacent to the landslide complex. This profile shows that 

upslope of the lower headwall scarp there are a series of slip 

planes (listric faults) that flatten with depth and sole into a 

stratigraphic layer that is characterized by irregular bedding 

and thickening and thinning of strata; interpreted as a zone 

of relatively weak strata. This zone of weakness is coincident with the base of a discontinuous gas hydrate. The 

lithologic unit that the deformed zone of weakness follows 

is inferred to be the top of lower Miocene (about 20 million 

years old) strata (fig. 5; Popenoe, 1985). 

Detail of the shallow subsurface and sea-floor character is shown by high-resolution seismic-reflection profiles 

(figs. 6 and 7) and Sea Mapping And Remote Characterization (SeaMARC) I side can- onar images (figs. 8 to 10). 

The seismic-reflection profiles show a slope strewn with 

debris and rubble and marked by sea-floor depressions 

II BRIGHT SPOTS II 

.. .... ,. 

-1500 (/) a:: 

w 

.._ 

w 

~ 

z 

-2250 I 

.._ 

CL 

SPOTS" w 

0 

a:: 

w 

.._ 

<( 

3000 3: 

3750 

hydrate extends from the sea floor to the BSR and acts 

as a seal beneath which free gas is trapped, visible on 

this profile as high-amplitude reflections (bright spots) 

beneath the BSR. 

caused by downward rotation of blocks of strata along the 

underlying listric faults. These listric faults are actually slip 

planes of a large slump and the rotated blocks are slump 

block of undeformed sediment which have moved along 

discrete shear surfaces. The SeaMARC I images (figs. 8 to 

10) along with the seismic-reflection profiles (figs. 6 and 7) 

show that indurated, well-bedded strata have been truncated 

along the lower headwall scarp. Promontories and recesses 

mark the scarp face (fig. 8) where large blocks of strata 

have broken free to cascade downslope. Below the headwall 

scarp (figs. 8 and 10) downslope-trending 5- to 15-m-deep 

tracks are furrowed into the sea floor. Some of these tracks 

can be followed downslope to large, unbroken displaced 

slide blocks that have come to rest against the upslope side 

of the protruding salt diapirs (fig. 6). The largest of the 

displaced slide blocks has an irregular edge that can be 

virtually "fitted back" into the headwall scarp from which it 

originated (fig. 8). In other places below the lower headwall 

scarp, both landslide tracks and rubble lobes caused by the 

downslope disintegration of lide blocks are evident. 

The Cape Fear Landslide: Slope Failure Associated with Salt Diapirism and Cas Hydrate Decomposition 43 
Figure 4. GLORIA sidescan-sonar image of the Cape Fear 

landslide (see fig. 1 for the location of this image). Scarps 

are outlined by hachures pointing downslope. Massmovement deposits produce the lighter acoustic return 

seen on the image. Undisturbed sedimentary deposits 

produce a dark return. Locations of seismic-reflection 

profiles shown in figures 11 and 12 are indicated. The 

The Mass Flow 

High-resolution seismic-reflection profiles (figs. 11 

and 12) show four transverse crossings of the Cape Fear 

landslide track on the upper continental rise. In much of this 

area, only a thin layer of mass-movement debris overlies 

landslide-scoured bedrock, although near the north end of 

the profiles (figs. 11 and 12), mass-movement debris has 

been covered by younger well-layered sediment. Sidescansonar images (figs. 9 and 10) show that the bottom near the 

head of the mass flow is characterized by a rubble field 

containing large unbroken boulders and clasts that project 

above the bottom. Further downslope (fig. 11 B), a flatbottomed trough over 25 km across and over 100 m deep 

has been cut across the rise (locations shown on figs. 2 and 

4). Within the trough, the bottom is rough due to the 

presence of mass-movement debris and the seismicreflection profile shown on figure 118 crosses a large 

displaced slide block about 20m high. The southern end of 

the profile crosses the area of disturbed bottom that correlates with rough, hummocky bottom on the GLORIA image 

surface of the landslide is marked by longitudinal lines 

that represent both debris chutes between the salt diapirs 

at the head of the landslide area (AL and flow paths of 

debris further down slope (B). Near the head of the 

landslide there is an area of irregular, hummocky sea floor 

that has been crumpled and buckled by mass movement 

(C). 

(fig. 4) and with an irregular crumpled sea floor on figure 

13. This area of sea floor was probably deformed due to 

downslope movement of a shallow layer of the sea floor, 

similar to a rumpled carpet, thus the term carpet slide. Slide 

tracks seen on GLORIA images upslope of the carpet slide 

(figs. 2 and 4) suggest downslope displacement of about 30 

km for the head of this landslide mass. 

Figure 12 shows echo-sounder profiles across the 

Cape Fear landslide 50- and 70-km-downslope from the 

headwall scarp. At 50 km (fig. 12A) the central trough is 

over 30 km wide and 150m deep. The northern boundary of 

the trough is not as abrupt as farther upslope, and the 

southern boundary gradually slopes upward from the trough 

axis. Seventy km downslope from the headwall scarp (fig. 

128) the trough has widened to over 40 km, and its relief 

has decreased to just over 100m, largely because the trough 

is partly filled with mass-movement deposits. The trough is 

not present toward the distal end of the landslide (east of 74° 

longitude) where mass-movement debris forms an elevated, 

lenslike depositional high where it overlies older deposits 

(fig. 4). 

44 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
w 

2 

3 

4 

c_j 

w 

(/) 

w 

~ 

1- 5 __J 

w 

~ 0::: 

1-

~ 

~ Miocene I 

0 

~ 

1-

2 

3 

0 10 km 

4 

5 

Figure 5. Airgun seismic-reflection profile that crosses 

the head of the Cape Fear landslide (see fig. 2 for the 

location of this profile). The middle slope is underlain by 

slumps whose slip planes (listric faults) terminate in a 

zone of weakness at about 400 meters below the sea 

floor. The zone is coincident with the top of lower 

Miocene strata and is nearly coincident with the base of 

E 

750 

1500 

2250 

--115m high scarp 

-. ; ~ed t diapir 

3000 

(/) 

0::: 

w 

1-

w 

3750 ::E 

~ 

I 

1-

0... 

750 w 

0 

0::: 

w 

~ ~ scarp 

1500 

headwall scarp 

Lower headwall scarp 

2250 

3000 

3750 

the gas hydrate which is identifiable only beneath the 

base of the slope on this profile. The zone of weakness 

intersects the sea floor near the base of the slope, below 

the lower headwall scarp of the Cape Fear landslide. The 

profile also crosses a salt diapir enclosed by the headwall 

scarp. Ages of strata are from Popenoe (1985). 

The Cape Fear Landslide: Slope Failure Associated with Salt Diapirism and Gas Hydrate Decomposition 45 
w 

MID HEADWALL SCARP 

2.50-~,~ , 

....... 

........ , .. 

I 

E 

-1875 

DISRUPTED. RUBBLE -STREWN 

SLOPE FAILURE SCAR 

hj 

~ 3.00-

w 

~ 

..._ 

_j 

w 

~ 0:: 

..._ 

~ ~ I 

~ 3.50-

._ 

4.00-

I i/ 

SLUMP BLOCK 

0 5 

KM 

Figure 6. A 3.5-kilohertz frequency seismic-reflection profile that crossed the mid and lower headwall area of the 

Cape Fear landslide (see figs. 2 and 8 for the location of 

this profile). The part of the profile that correlates with the 

mid-range sidescan-sonar image shown in figure 8 is 

indicated. The slope above the lower headwall scarp is 

Sediment cores taken in the lower part of the Cape 

Fear landslide area (Embley, 1980) contain a fabric representative of mass-movement deposits, such as inclined beds 

and small convolutions and clasts within a finer matrix, as 

well as a relatively large proportion of sand. In contrast, 

cores taken from the stratified continental rise adjacent to 

the landslide consisted primarily of homogeneous gray silty 

clay. All cores taken within the landslide area had a 

1-m-thick capping surface layer of younger brown clay. 

Two radiocarbon datings at the base of this clay layer 

gave age dates of 20,830 and 12,125 years, indicating that 

there were possibly two landslide events, both of late 

Pleistocene age. 

FIGURE 8 U) 

-2250 e5 

~ 

w 

~ 

z 

..-115-M-HIGH LOWER HEADWALL_ SCARP I 

~ 

a.. 

w 

0 

I DISPLACED SLIDE BLOCK 

~-~-- f ;¥ .. \" 

BREACHED 

SALT DIAPIR a:: 

~-2625 ~ 

~\\\."' i~ ··.,i -

I 

-3000 

disrupted both by rubble and sea-floor depressions above 

slump blocks. A strong reflector in the subsurface is 

possibly caused by free gas. The lower part of the profile 

crosses a large displaced slide block, about 25 meters high 

and 5 kilometers across, that can be seen on the lower part 

of the SeaMARC I sidescan-sonar image (fig. 8). 

DISCUSSION 

Analysis of submarine slope stability requires making 

assessments of the strength of sediment and the magnitude 

of stresses acting downslope. Downslope stresses result 

from gravity, earthquakes, or storm waves (Lee, Schwab, 

and Booth, this report). An important behavioral aspect of 

sediment is that its strength decreases as the excess porewater pressure (in excess of hydrostatic pressure) increases. 

Excess pore pressures commonly exist in fine-grained 

sedimentary deposits that are accumulating rapidly and can 

also be caused by the occurrence of free gas (Kayen and 

Lee, this report) or by repeated loading from earthquakes or 

46 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
-

u 

w 

Cf) 

-

w 

~ 

...... 

_J 

w 

> 

~ 

w 

2.5-

3.0-

20-M- HIGH UPPER HEADWALL SCARP 

/fREE GAS ZONE ? 

'. THICKER SEDIMENT 

IN FAULT TRACE 

I FIGURE 8 

SCARP 

EMBAYMENT SCARP 

60-M- HIGH LOWER HEADWALL SCARP 

E 

-1875 

-2250 

Cf) 

a:: 

w 

tw 

~ 

z 

I 

ta.. 

w 

I ·'' , . 

~ . . .,i··~ -r~ 

• :. ~ 1: I 1 ' ',' -

1 

' ' 

0:: 

...... 

r 

~ 3.5- I 

0 

~ 

0 

a:: 

w 

-2625 t-

~ 

'I j , ; \!II !:&: , .. , . I ! "'- -~ 

t-

,I I ,( ,~'r~'t;,:~

, I ! il'l i '. I' jj· 

i: · ' i :, \ 

:, ''. 1 1,' 'II' 

I. 

i I 

I •l - ! 

4.0- I I 

0 10 

KM 

Figure 7. A 3.5-kilohertz frequency seismic-reflection profile shows the thicker sediment in sea-floor depressions 

above listric faults (see figs. 2 and 8 for the location of this 

profile). Note the lowermost two listric fau l s that indent 

the bottom about 2 kilometers above the lower headwall 

storm waves (Schwab and Lee, this report). Slope destabilization due to storm wave probably can be ruled out as a 

triggering mechani m for the Cape Fear landslide because 

their effects would be minimal at the water depths encountered (from 900 to over 2,000 m). 

High-amplitude reflections (bright spots) observed on 

seismic-reflection profiles (for example, figs. 3 and 5) point 

to abundant free gas beneath the gas-hydrate layer off Cape 

Fear and near and beneath the landslide area. These areas of 

high-amplitude reflections are zones of high pore-gas pressure and presumably low sediment trength. Bright spots 

al o exist within the layer that we infer is cemented by gas 

hydrate, suggesting that the gas hydrate may be partially 

decomposed above the BSR. 

' \ 

,. I i 

•I 

I 

I 

I' I 

1

!' 

I 

.j I 

I 

' ,·, 

;J, 1: • I I I ' 

i. 

scarp. These active features suggest that a slide block 2 

kilometers across, 80 meters high, and of unknown 

length, is slowly failing. The part of the profile corresponding to figure 8 is indicated. 

During periods of global warming or lowered ea 

level, such as those that occurred during the Pleistocene, 

additional decomposition of gas hydrate could be initiated 

by lowering the base pressure and/or raising the temperature 

out of the hydrate stability field (Carpenter, 1981; Kvenvolden, 1988; Kayen and Lee , this report). This condition 

would release additional large volumes of free gas into the 

sediment column directly under the gas hydrate, leading to 

even lower sediment strength within this zone. The coincidence of the base of the gas hydrate with a major slope 

failure slip plane into which many of the listric faults off 

Cape Fear are rooted strongly suggests that free gas beneath 

the gas hydrate seal is a contributing mechanism to the slope 

instability. The presumed dates of lope failure , 12,125 and 

The Cape Fear landslide: Slope Failure Associated with Salt Diapirism and Gas Hydrate Decomposition 47 
Figure 8. SeaMARC I sidescan-sonar image and interpretive drawing of the headwall area of the Cape Fear landslide (see fig. 2 for the location of this image). The upper 

part of this image shows the rugged lower headwall scarp, 

at places over 120 meters high, that truncates and exposes 

bedded sediments in the scarp face. The bottom below 

the scarp is furrowed by landslide tracks, cut 5 to 15 

20,830 years before present (Embley, 1980), both fall 

within the late Wisconsinan Age when sea level was over 70 

m below the present level (Dillon and Oldale, 1978). Thus, 

decomposition of the gas hydrate caused by lowered sea 

level probably contributed to the slope failure. 

Earthquakes are an effective trigger mechanism for 

initiation of slope failure (Lee, Schwab, and Booth, this 

report). Because the Cape Fear landslide is near the site of 

the 1886 earthquake that did major damage to Charleston, 

S.C. (Bollinger, 1977), it is tempting to cite this historic 

earthquake as the possible triggering mechanism for the 

Cape Fear landslide (Embley, 1980; Carpenter, 1981). The 

evidence, however, indicates that the main slope failure 

predates the 1886 Charleston earthquake by thousands of 

years. The ubiquitous 1-m-thick layer of younger sediment 

that overlies the mass-movement deposits and age dates of 

12,124 and 20,830 years before present taken at the base of 

this sediment layer (Embley, 1980) suggest that the last 

major slope failure occurred in late Pleistocene time. The 

SHIP 

COOR S£ 

I 

I 

I 

I 

I 

I 

I 

__....-,:===-- N--<:::::=-

1 

1/ 

I 

1: 

;: 

FIG. 7 

UfitDltTUIIIlD 

S[ H tOOit 

meters deep, that extend downslope to large displaced 

slide blocks and to debris lobes. Shallower landslides have 

furrowed tracks into the bottom above the main scarp 

leaving remnant areas of high bottom. The location of two 

seismic-reflection profiles that cross this image area are 

indicated by the dashed lines. 

thickened layers of sediment observed within the sea-floor 

depressions above listric fault traces (fig. 7) suggest a long 

history (thousands of years) of retrogressive slumping since 

the main failure event. Moreover, landslide tracks and slide 

blocks below the lower headwall scarp (fig. 8) indicate that 

episodic calving has occurred since the major failure. The 

evidence does not rule out a large, late Pleistocene earthquake as a cause of the main slope failure, or the 1886 event 

as a cause of minor calving. 

The presence of the elevated salt diapirs centered 

within the scar of the major slope failure also suggests a 

cause-and-effect relationship. Although a number of salt 

diapirs underlie the continental rise (fig. 1), the two within 

the scar are the only diapirs that actually underlie the slope. 

Initial fracturing of the slope strata caused by intrusion of 

salt could have created an area of weakened slope above and 

near the salt diapirs. Oversteepening of the slope by upward 

movement of the salt diapirs to some critical angle could 

have initiated a failure of the lower slope. Once support was 

48 Submarine landslides: Selected Studies in the U.S. Exclusive Economic Zone 
5 N 

2 KM 

Figure 9. Detail of part of the SeaMARC I image presented in figure 8 showing outcropping bedding 

on the scarp face (a) and rubble along the edges of landslide tracks (b) below the scarp face. 

The Cape Fear landslide: Slope Failure Associated with Salt Diapirism and Gas Hydrate Decomposition 49 
l..l.J 

A.. 

C) 

-.....J 

\/) 

~ * 

ltt~ ""-..... C) 

~ 

5 KM 

Figure 10. Detail of part of the SeaMARC I image presented in figure 8. This image shows the northern part of 

the headwall scarp where the scarp (a) trends diagonally 

upslope. Slides and mass flows (b) have stripped bedding 

layers above the headwall and have furrowed the slope, 

leaving areas of remnant high bottom (c). Rough bottom 

~ 

~ 

L{) 

within the slide tracks (d) probably represents rubble left 

by the descending landslide. The smooth featureless 

slope on the north side of the image (e) is typical of the 

undisturbed continental slope surrounding the Cape Fear 

landslide. 

50 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
u 

w 

U) 

w 

~ 

f= 

_j 

w 

> <l: 

0::: 

I-

~ I 

0 

~ 

Is 

3.0-

3.5-

4.0-

4.0-s 

I 

' 

4.5 -

5.0-

A 

0 

KM 

.: 

8 

II; 

"CARPET" LANDSLIDE 

0 10 

KM 

Figure 11. A. A 3.5-kilohertz seismic-reflection profile that 

crossed the Cape Fear landslide scar and the two salt 

diapirs just below the main headwall scarp. Within the 

central and northern part of the landslide scar only a thin 

cover of mass-movement debris overlies a bottom 

scoured to bedrock. In the southern part of the scar, 

younger sediment overlies the scoured bottom, suggesting that slope failure to the south predates the most recent 

slumping event in the northern part of the landslide scar. 

B. A 3.5-kilohertz seismic-reflection profile across the 

landslide-cut trough 30 kilometers below the headwall of 

removed from the base of the slope, retrogressive failure 

could have occurred on zones of weakness upslope, as 

suggested by Cashman and Popenoe (1985). In addition, 

arching of sediment above the rising salt diapirs may have 

contributed to the slope failure by creating a trap into which 

free gas could accumulate, causing elevated pore pressures 

immediately over and surrounding the diapirs. The elevation of the salt diapirs above the sea floor within the slump 

scar above the level of failed sediment, suggests that the 

diapirs uplifted the bottom prior to the failure, and then 

were excavated by the slide. 

20 

t•• \ 

:. I 

YOUNGER 

SEDIMENT 

\ 

N 

-2250 

-2625 

t;Q;::-3000 

U) 

0::: 

w 

1-

w 

~ 

~ 

I 

1-

Q_ 

N-3000 ~ 

YOUNGER 

DISTURBED BOTTOM SEDIMENT -3375 -~ ~ ,,. ... ~- c- ,.:. ,··· .. • " .,. ·. ~ 

-3750 

0::: 

w 

I-

~ 

the Cape Fear landslide. At this crossing, the north wall of 

the trough has about 150 meters of relief, whereas the 

south wall has about 90 meters of relief. The trough is 

filled with mass movement deposits including several 

slide blocks. The south end of the profile crosses an area 

of disturbed bottom that corresponds to an area of hummocky acoustic return on the GLORIA image. This irregular return is interpreted as having been caused by crumpled shallow strata (carpet slide) that have moved as a 

relatively intact body a distance of 30 kilometers downslope. See figure 2 for the locations of these profiles. 

CONCLUSIONS 

Although we cannot cite one particular mechanism as 

the cause of the initial slope failure, a number of classic 

conditions exist in the area of the Cape Fear landslide that 

probably contributed, in combination, to the slope failure. 

These include decomposition of a gas hydrate which caused 

high pore-fluid pressures to build, salt diapir intrusion that 

initially fractured and could have oversteepened the lower 

slope, and a location where earthquakes have occurred 

historically , and no doubt also occurred in the distant past. 

The Cape Fear Landslide: Slope Failure Associated with Salt Diapirism and Gas Hydrate Decomposition 51 
4.5-~ ii I I I 

Ht A 

DISTURBED BOTTOM 

EDGE OF TROUGH 

Ed 

~ 

w 

::2: 

5.0-

i= 5.5- __j 

w 

~ 0::: 

t-

>-

~ 

~ t0 

5.5-

6.0-

10 

KM 

8 

EDGE OF TROUGH 

r~•.,:,..,,,... .,ltr. I 

0 

KM 

""'"'"''" >:·r..· ,, ~/ 

... 

MASS- MOVEMENT DEPOSITS 'J BOTTOM OF TROUGH 

'~- ·- / -

10 

Figure 12. A 3.5-kilohertz seismic-reflection profiles 

across the Cape Fear landslide trough approximately 50 

kilometers (A) and 70 kilometers (B) below the lower 

headwall area. On both profiles the trough is broader and 

u 

w 

w 

3.5-

BOTTOM OF TROUGH . 

\ 

: 1 . i T :. 

EDGE OF TROUGH 

~~~~~'" '·· ,;•jF- 'It 

MASS-MOVEMENT DEPOSITS 

~7"""~ '' ._L ·~ ::.. :,;.. ). " • 

-4125 

-4500 

(f) 

0::: 

w 

tw 

::2: 

-4125 ~ 

I 

tCL 

w 

0 

0::: 

w 

~ 

shallower than farther upslope and is partly filled with a 

thick layer of mass movement debris. Locations of these 

profiles are shown on figure 4. 

E 

-2625 

en 

a::: 

!{]. DENUDED SLIDE SURFACE w 

1-

w w 

~ 4.0- I 

I-

_J 

w 

~ a::: 

1-

'I 1''111" 

0 

KM 

10 

-3000 ~ 

z 

11 CARPET 11 SLIDE ::I: 

I 1--

·,~ ~·- ~-.~~~- I ~ .. ~..,_ ' ~ I~ I ' I . -, ~-3375 w 

1--

<l: 

~ 

Figure 13. Echo-sounder profile across the carpet slide area below the Cape Fear landslide headwall scarp (see fig. 2 for 

the location of this profile). 

Whatever the combination of initial mechanisms, once 

initial slope failure had removed support from the lower 

slope, retrogressive slope failure proceeded upslope to the 

edge of the Blake Plateau, causing slumping of the middle 

and upper slope and expansion of the landslide scar area by 

two additional shallower slides. 

52 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
REFERENCES CITED 

Bollinger, G.A., 1977, Reinterpretation of the intensity data for 

the 1986 Charleston, South Carolina Earthquake, in Rankin, 

D.W., ed., Studies related to the Charleston, South Carolina 

earthquake of 1886-A preliminary report: U.S. Geological 

Survey Professional Paper 1028, p. 17-32. 

Carpenter, G.B., 1981, Coincident sediment slump/clathrate complexes on the U.S. Atlantic slope: Geo-Marine Letters, v. 1, 

p. 29-32. 

Cashman, K.V., and Popenoe, Peter, 1985, Slumping and shallow 

faulting related to the presence of salt on the continental slope 

and rise off North Carolina: Marine and Petroleum Geology, 

v. 2, p. 260-270. 

Dillon, W.P., Grow, J.A., and Paull, C.K., 1980, Unconventional gas hydrate seals may trap gas off southeast United 

States: Oil and Gas Journal, v. 78, p. 124, 126, 129-130. 

Dillon, W.P., and Oldale, R.N., 1978, Late Quaternary sea-level 

curve: reinterpretation based on glaciotectonic influence: 

Geology, v. 6, p. 56-60. 

Dillon, W.P., and Popenoe, Peter, 1988, The Blake Plateau Basin 

and Carolina Trough, in Sheridan, R.E., and Grow, J .A., 

eds., The geology of North America, v. 1-2, The Atlantic 

Continental Margin: U.S.: Geological Society of America 

DNAG Series, p. 291-328. 

Dillon, W.P., Popenoe, Peter, Grow, J.A., Klitgord, K.D., 

Swift, B.A., Paull, C.K., and Cashman, K.Y., 1982, 

Growth faulting and salt diapirism: their relationship and 

control in the Carolina Trough, eastern North America, in 

Watkins, J.S., and Drake, C.L., eds., Studies in continental 

margin geology: American Association of Petroleum Geologists Memoir 34, p. 21-46. 

EEZ-SCAN 87 Scientific Staff, 1991, Atlas of the U.S. Exclusive 

Economic Zone, Atlantic continental margin, U. S. Geological Survey, Miscellaneous Investigations Series 1-2054, 174 

p. 

Embley, R.M., 1980, The role of mass transport in the distribution 

and character of deep-ocean sediments with special reference 

to the North Atlantic: Marine Geology, v. 38, p. 25-50. 

Hutchinson, D.R., Grow, J.A., Klitgord, K.D., and Swift, B.A., 

1982, Deep structure and evolution of the Carolina Trough, 

in Watkins, J.S., and Drake, C.L., eds., Studies in continental margin geology: American Association of Petroleum 

Geologists Memoir 34, p. 129-152. 

Kvenvolden, K.A., 1988, Methane hydrates and global climate: 

Global Biogeochemical Cycles, v. 2, p. 221-229. 

Paull, C.K., Schmuck, E.A., Manheim, F.T., and Bralower, 

T.J., 1989, Carolina Trough diapirs: salt or shale?: EOS, 

Transactions, American Geophysical Union, v. 70, p. 370. 

Popenoe, Peter, 1984, Summary geologic report for the South 

Atlantic Outer Continental Shelf (OCS) Planning Area: U.S. 

Geological Survey Open-File Report 84-476, 27 p., 10 

maps, scale 1:1,000,000. 

---1985, Cenozoic depositional and structural history of the 

North Carolina margin from seismic stratigraphic analyses, in 

Poag, C. W., ed., Stratigraphy and depositional history of the 

U.S. Atlantic margin: Stroudsburg, Pa., Van Nostrand Reinhold Publishing, p. 125-187. 

Popenoe, Peter, Cashman, K.V., Chayes, D., and Ryan, W.B.F., 

1981, Mid-range sidescan sonar images covering parts of 

proposed tracts for OCS Lease Sale 56 and contiguous areas, 

Manteo, Cape Fear, and adjacent quadrangles off North 

Carolina: U.S. Geological Survey Open-File Report 81-554, 

6 p. 

Popenoe, Peter, Coward, E.L., and Cashman, K.V., 1982, A 

regional assessment of potential environmental hazards to and 

limitations on petroleum development of the United States 

Atlantic continental shelf, slope, and rise, offshore North 

Carolina: U.S. Geological Survey Open-File Report 82-136, 

67 p. 

The Cape Fear Landslide: Slope Failure Associated with Salt Diapirism and Gas Hydrate Decomposition 53 
Ancient Crustal Fractures Control the Location and I 

Size of Collapsed Blocks at the Bl~ke Escarpment, 

East of Florida 

By W.P. Dillon, J.S. Risch, K.M. Scanlon, P.C. Valentine, and Q.J. Huggett1 

INTRODUCTION 

The Blake Escarpment is a steep, high submarine cliff 

off the Southeastern United States that extends for about 

500 kilometers (km) from northern Florida to the Bahama 

Islands. Water depth at the base of the escarpment is about 

5,000 meters (m) and, at the top, somewhat more than 

1,000 m (figs. 1 and 2). The cliff is far higher than the walls 

of the Grand Canyon (fig. 3). We have long been convinced 

that the present morphology of the Blake Escarpment is the 

result of erosion of the seaward side of a reef-fronted, 

limestone platform (Paull and Dillon, 1980; Dillon and 

others, 1985; Freeman-Lynde and Ryan, 1985), and this 

erosion is confirmed by Geological LOng-Range Inclined 

Asdic (GLORIA) sidescan-sonar images. The images disclose for the first time, however, that whereas most of the 

cliff-face is an erosional surface resulting from scour by 

ocean currents, the erosion of the southern segment of the 

escarpment took the form of slope failure of massive 

limestone blocks (fig. 4; EEZ-SCAN 87, 1991). How were 

these blocks formed, why did they fail, and why do they 

appear along only a short stretch of the escarpment? Indeed, 

off Eastern North America, this is the only place where they 

exist. The causes seem to involve the entire history of 

continental margin formation. 

GEOLOGIC SETTING-FORMATION OF THE 

CONTINENTAL MARGIN EAST OF _FLORIDA 

The continental margin east of Florida is dominated 

by the broad, flat Blake Plateau, which is terminated to the 

east by the steep cliff of the Blake Escarpment (fig. 1). The 

plateau is underlain by one of the four main sedimentary 

basins of the Eastern U.S. continental margin. These basins 

are regions of basement rock subsidence that are now filled 

with sedimentary rocks. Basin locations were controlled by 

inherited weaknesses in the old continental crust that was 

rifted apart when Africa and North America separated. 

These crustal weaknesses also caused offsets in the rifting 

pattern, which are still evident as offsets in the present 

Mid-Atlantic Ridge, the present site of sea-floor spreading. 

The rocks of the Blake Plateau platform pro~ably are mostly 

1 Institute of Oceanographic Sciences, Wormley, Surrey, U.K. 

limestones that accumulated at sea level (at a water depth of 

only a few meters) in conditions similar to the present 

Bahama Banks. Although no samples of the earliest, now 

deeply buried, rocks are available, we infer that sediments 

began to accumulate in the Blake Plateau region not long 

after North America and Africa separated, about 170 

million years ago (Dillon and Popenoe, 1988). Between 

then and now, the basin has subsided more than 14 km 

under the influence of cooling and loading. The initially hot 

continental margin rocks cooled, causing them to become 

more dense and to sink deeper into the soft layer below; the 

accumulating sediment weighted down this continental 

margin region ~s well. Accumulation of sediment kept up 

with this subs~,jence and maintained the surface of the 

plateau near se~ level until about 100 million years ago. At 

that time, the gFowth rate of the carbonate platform slowed, 

the reefs died (perhaps due to stress from climate change), 

and the upbuilding of the platform surface no longer was 

able to keep up with subsidence. Since then, the region has 

sunk to become an intermediate depth plateau, now covered 

by about 1,000 m of water. 

The seaward (eastern) side of the carbonate bank that 

formed the Blake Plateau probably was being eroded during 

most of its history, but two episodes of cutting-away of the 

slope are notable. One occurred during intense erosion of 

the sea floor of the Western North Atlantic Basin in 

Oligocene time (about 30 million years ago; Mountain and 

Tucholke, 1985), and another is going on now. Both 

apparently resulted from intensification of southwarddirected bottom. currents that flowed along the western side 

of the North Atlantic basin. Such currents are formed by the 

chilling and resulting increase in density of water in the 

northernmost part of the basin. This dense water then sinks 

and flows southward, while being squeezed against the 

western side of the basin as a result of the rotation of the 

Earth (Coriolis .effect). The earlier episode left an eroded 

bench that appears in seismic-reflection profiles that cross 

the escarpment (fig. 5). After that earlier period of erosion, 

the bench was covered with sediment and is now buried. A 

more recent pul~e of erosion, which has continued to the 

present day, has cut into that sediment and eroded the 

escarpment back farther. A photograph of the surface of the 

Blake Escarpment (fig. 6) taken from the research submersible Alvin demonstrates the effects of the active erosion in 

the area of the profile shown in figure 4. Here, truncated 

54 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
-- -; 

( 

~ 

~OUTH 

'\ 

1., 

GEORGIA 

NORTH CAROLINA 

CAROLINA 

'\ 

" 

BLAKE 

PLATEAU 

/ 

/ 

Figure 1. Blake Escarpment: location (left panel) and bathymetry (right panel). The locations of the seismic profiles shown 

in figures 5 and 7 and the extension of the jacksonville fracture zone also are shown. 

horizontal strata crop out on the straight cliff and small, 

broken fragments of strata (talus) accumulate on ledges, or 

at the bottom of the cliff, until they are eroded away. 

COLLAPSE OF THE SOUTHERN PART OF THE 

ESCARPMENT 

Along most of the Blake Escarpment's length, the 

scour by southward-flowing deep-sea currents (possibly 

aided by chemical dissolution of the carbonate rock) produces a cliff that is steep, straight, and smooth (figs. 1, 2, 

and 5), except for the small-scale, steplike irregularities that 

are caused by small differences in resistance of strata to 

erosion (fig. 6). The adjacent sea floor also is smoothly 

scoured by the current. Near the southern part of the 

escarpment, however, just north of Little Bahama Bank and 

Great Abaco Island (fig. 1), the deep sea floor is characterized by enormous agglomerations of blocks, commonly 

10 km across, that appear to have fallen from the face of the 

cliff and that are shown both by seismic-reflection profiles 

(fig. 7) and by GLORIA sidescan-sonar imagery (fig. 4). 

The outer part of the Blake Plateau in the southern region 

also differs from the plateau farther north by being cut by 

submarine canyons that trend northwest, parallel to the 

northern side of Little Bahama Bank (figs. 1, 2, and 4). 

Several seismic profiles show faults in the rocks in the 

southern area; faults do not appear in the rest of the plateau. 

WHY IS THE SOUTHERN BLAKE PLATEAU 

FALLING APART? 

Because faults seem to exist in the southern region of 

the outer Blake Plateau and not to the north, we infer that 

the rock that slipped from the eroding escarpment probably 

Ancient Crustal Fractures Control the location and Size of Collapsed Blocks at the Blake Escarpment, East of Florida 55 
- 'oo0 -2ooo 

-:~ooo 

-4uoo -~oo0 

2() 

Figure 2. Perspective image of the Blake Escarpment for 

the area shown in the right panel of figure 1. View is to the 

southwest and vertical scale is in meters below sea level. 

Latitudes are indicated along the lower left side of the 

block and longitudes along the lower right. For most of its 

length, the escarpment is smooth and straight and the 

consists of pre-extstmg blocks, bounded by faults. 

Although the bottom-current erosion (and possible dissolution) that is scouring the outer platform rocks and steepening the cliff shows no evidence of change from north to 

south, the response of the rocks to the steepening created by 

the erosion is different. In the south, being weakened by the 

pre-existing fractures, large blocks slide off when the cliff 

becomes very steep, whereas the massive, unfaulted limestones farther north are strong enough to maintain very 

steep slopes. 

Why should such a set of faults exist in a region such 

as the Florida-Bahama Platform, that is far away from zones 

of active plate interactions and notable for its lack of 

earthquakes? The answer may be given by the orientation of 

faults that cut the rocks, an orientation that probably is 

indicated in figure 1 by the well-defined northwest trend of 

channels and canyons in the area (for example, Great Abaco 

Canyon) and the linear, northwest-oriented flank of Little 

Bahama Bank. This orientation is exactly the trend of the 

Jacksonville Fracture Zone (fig. 1; Klitgord and Schouten, 

1986), a fracture zone that is identified in the oceanic crust 

to seaward. This fracture zone was generated at an offset in 

the initial rifting pattern when North America and Africa 

were ripped apart; the offset formed a boundary between 

different types of crust and, thus, controlled the location of 

PLATEAU 

Blake Plateau behind it is smooth and flat. At its southern 

end, however, the face of the escarpment is more complex. There, it is incised by submarine canyons and valleys 

that trend northwest parallel to the northern flank of Little 

Bahama Bank, and displaced blocks of limestone litter the 

sea floor at the foot of the escarpment (see fig. 4). 

the southern limit of the Blake Plateau Basin. Initially, 

subsidence was much greater and much faster on the basin 

side (north), where the crust was probably thinner and 

hotter. After the cessation of rapid sediment build-up on the 

Blake Plateau, the relative motion direction may have 

reversed. Since then, the sediment has built up faster to the 

south, where the Bahama Banks have succeeded in maintaining themselves at sea level by limestone deposition as 

Figure 4. Perspective image of the southern Blake ~ 

Escarpment, looking toward the northwest. The image 

was produced from the sea-floor reflections acquired by 

GLORIA sidescan-sonar system, which were "draped" 

on a computer image of the shape of the sea floor that 

was determined from echo sounder measurements of 

bathymetry along many ship tracks. The variations in 

tone result from variations in reflectivity of sound from 

the sea floor (see Lee, Schwab, and Booth, this report). 

Light areas, indicating strong acoustic reflections, occur 

where landslides (in landslide scars) or current erosion 

(in canyons) have stripped away loose sediment. Displaced blocks have slipped from the face of the Blake 

Escarpment as the escarpment was cut back and steepened by erosion. The blocks were defined by 

pre-existing fractures (and faults) that cut the limestone 

of the southern Blake Plateau, and thus predestine this 

response. 

56 Submarine landslides: Selected Studies in the U.S. Exclusive Economic Zone 
_J 

w 

> 

w 

_J 

<[ 

w 

(f) 

3: 

0 

_J 

w 

CD 

(f) 

0::: 

w 

1--

w 

~ 

2 

3 

~4 ~ 

(f) 

::::::> 

0 

w 

u 

<[ 

1--

w 

0::: 

u 

a:: 

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0 

APTIAN AND ALBIAN 

BLAKE 

ESCARPMENT 

~ 

? --?--?----.~MISS SSI PPIAN/ =-- . . ~ AND j 

BERRIASIAN(?)~ DEVONIAN _-=-....:... 

GRAND 

CANYON 

_J 

w 

> 

w 

_J 

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2~ 

w 

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0 

CD 

<[ 

~ CAMBRIAN ==-=-- - ---------------------?--?- ~ .·· .;(J(J 53 PRECAMBRIAN ~ ~;;p 

(f) 

0::: 

w 

1--

w 

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0 5 _J JURASSIC (?) 

Figure 3. Comparison of cross-sections of the central 

Blake Escarpment and the south rim of the Grand Canyon. 

Vertical exaggeration is 2 to 1, that is, vertical distances are 

stretched 2 times compared to horizontal distances. The 

NORTHERN FLANK OF 

LITTLE BAHAMA BANK 

;..>~ 

~ ~;..>~ - - ?-- _; ~ 

rocks indicated for the Blake Escarpment may range from 

roughly 60 to 200 million years in age. Those in the Grand 

Canyon are older, perhaps from about 250 million to more 

than 600 million years in age. 

'-

' SCOUR DEPRESSION 
(f) 

BLAKE PLATEAU 0::: 

w 

1-

w PROFILE TD4 ~ 

~ 0 10 ~ 

z 

km I 

1-

a.. 

w 

a 

0::: 

w 

1-

~ 

Figure 6. Surface of the Blake Escarpment at profile TD4 (fig. 5), at a water depth of 3,892 meters. The 

outcrop shown is about 2 meters high. The stepped pattern is created by the erosion of the essentially 

flat-lying limestone beds of the Blake Plateau. 

58 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 

0 

(f) 

a 

z 

0 

Erl 

(f) 

z 

w 

5 ~ 

~ 

.....J 

w 

> <{ 

0::: 

1-

>-

<{ 

~ I 

0 

~ 

1-

10 
~ Figure 5. Multichannel seismic-reflection profile TD4 

across the central Blake Escarpment. Location of this 

profile is shown in the right panel of figure 1. Seismicreflection profiles are cross-sectional images of the 

upper layers of the Earth (see Lee, Schwab, and Booth, 

this report). Because sound waves are used to make the 

images, the vertical scale is indicated in two-way travel 

time of sound, and actually, it is a variable scale in 

depth, as a result of the variable velocities of sound in 

the strata. Although the profiles are distorted by this 

effect, they are useful presentations of the subbottom 

structure. The buried bench at the base of the escarpment shows the extent of erosional retreat of this part 

of the escarpment. The strata of the plateau extend 

nearly horizontally to the escarpment, where they are 

truncated by erosion. The apparent downward curve of 

strata as they come close to the escarpment surface is 

an artifact of the seismic-reflection method, and should 

be disregarded. The multiple also should be disregarded; it does not represent a real subbottom structure. The profile is displayed with a vertical exaggeration of 4 to 1 for the sea floor. 

the region subsided, and by doing so have placed a greater 

new load on the crust south of the fracture zone. These 

tendencies for differential vertical motions at the basin 

boundary may have continually reactivated faults there, and 

although the motions probably have been small in recent 

times, they have been enough to fracture the limestone 

continually. 

Thus, active processes that form the present continental margin are influenced by a boundary that probably was 

inherited from a crustal weakness that predated the breakup 

of North America and Africa. Future block collapse prob-

(/) 

0 

2 

0 

0 

w 

(/) 

w 

~ 

...... 

_J 

w 

> <( 

a::: 

...... 

>-

~ 0 

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4 

6-

8-

BLAKE 

ESCARPMENT 

SCAR 

Figure 7. Seismic-reflection profile TD2 through the 

southern part of the Blake Escarpment, and extending 

through the image shown in figure 4. Location shown in 

figure 1. The displaced block situated on the deep sea 

floor apparently slid from the escarpment, and may, in 

ably will occur at this same site, and, despite the steep 

slopes of the Blake Escarpment farther north, landslides 

probably are much less likely there. 

REFERENCES CITED 

Dillon, W.P., Paull, C.K., and Gilbert, L.E., 1985, History of the 

Atlantic Continental Margin off Florida; The Blake Plateau 

Basin, in Poag, C.W., ed., Geologic evolution of the United 

States Atlantic margin: New York, Van Nostrand Reinhold, 

p. 661-676. 

Dillon, W.P., and Popenoe, Peter, 1988, The Blake Plateau Basin 

and Carolina Trough, in Sheridan, R.E., and Grow, J.A., 

eds., The Atlantic Continental Margin, U.S.: Geological 

Society of America, The Geology of North America, v. 1-2, 

p. 291-328. 

EEZ-SCAN 87 Scientific Staff, 1991, Atlas of the U.S. Exclusive 

Economic Zone, Atlantic continental margin, U.S. Geological Survey, Miscellaneous Investigations Series I-2054, 174 

p. 

Freeman-Lynde, R.P. and Ryan, W.B.F., 1985, Erosional modification of the Bahamas Escarpment, Geological Society of 

America Bulletin, v. 96, p. 481--494. 

Klitgord, K.D., and Schouten, Hans, 1986, Plate kinematics of 

the central Atlantic, in Vogt, P.R., and Tucholke, B.E., eds., 

The Western North Atlantic region: Geological Society of 

America, The Geology of North America, v. M, p. 351-378. 

Mountain, G.S., and Tucholke, B.E., 1985, Mesozoic and Cenozoic geology of the Atlantic continental slope and rise, in 

Poag, C.W., ed., Geologic evolution of the United States 

Atlantic margin: New York, Van Nostrand Reinhold, p. 

293-341. 

Paull, C.K., and Dillon, W.P., 1980, Erosional origin of the 

Blake Escarpment: An alternative hypothesis: Geology, v. 8, 

p. 538-542. 

PROFILE TD2 

5 KM 

(/) 

a::: 

w 

...... 

w 

~ 

0 

_J 

:::r:::2 

2 

part, have come from the landslide scar indicated. Of 

course, the displaced block also may have moved across 

the plane of this section (into or out of the page). Vertical 

exaggeration is 4 to 1. 

Ancient Crustal Fractures Control the Location and Size of Collapsed Blocks at the Blake Escarpment, East of Florida 59 
Tectonic and Stratigraphic Control on a Giant _ 

Su~marine Slope Failure: Puerto Rico Insular Slope 

By W.C. Schwab, W.W. Danforth, and K.M. Scanlon 

DESCRIPTION OF THE LANDSLIDE 

An amphitheater-shaped scarp, approximately 55 

kilometers (km) across on the insular slope north of Puerto 

~ico represents the removal of more than 1 ,500 km3 of rock 

and sediment. A review of the regional stratigraphy and 

tectonics (Schwab and others, 1991) suggests that the scarp 

formed as a result of slope failure of carbonate strata 

(limestone); a giant submarine landslide. The study area 

(fig. 1) lies along the plate boundary zone between the 

North American and Caribbean lithospheric plates (Sykes 

and others, 1982) (fig. 2). Strong earthquakes affect the 

entire region due ~o this tectonic setting (Mann and Burke, 

1984) and were most likely responsible for triggering this 

slope failure. 

During a 30-day cruise aboard the R/V Farnella in 

November and December 1985, the GLORIA (Geological 

LOng-Range Inclined Asdic) sidescan-sonar_ system was 

used to obtain a reconnaissance . view of the sea floor 

surrounding Puerto Rico and th~ U.S. Virgin Islands 

(EEZ-SCAN 85 Scientific Staff, 1987). One spectacular 

feature revealed by the GLORIA imagery was a large 

~mphitheater-shaped scarp north of Puerto Rico (figs. 3, 4, 

5, and 6). The amphitheater-shaped scarp has its crown (or . 

rim) at a water depth of about 3,000 meters (m), and 

extends to a water depth of about 6,700 m (described on 

figs. 2and 3 as "base of slope"). The average slope of the 

amphitheater-shaped scarp is 8.5°. A system of talus chutes 

has developed on the scarp and acts as a series of conduits 

for the seaward transport of sediment. Most of these talus 

chutes appear to have little relation to the submarine 

canyons that stripe the insular slope above the crown of the 

amphitheater-shaped scarp (figs. 3 and 4) and thus are 

~elated to the mass wasting (downslope movement of 

surface materials) of the amphitheater-shaped scarp itself 

(fig. 7). 

GEOLOGIC SETTING 

Puerto Rico has a central core of folded and faulted 

metamorphic, sedimentary, and volcanic rocks of Late 

Jurassic to Eocene age (150 to 45 million years old). 

Flanking these older rocks on the north and south are 

carbonate strata of Eocene to Pliocene age ( 45 to 4 million 

years old). The carbonate rocks on the north flank of the 

island form the Arecibo basin strata (Meyerhoff and others, 

1983), which covers an area about 300 km long and 55 

km wide, and have a maximum thickness of about 4,000 m 

(fig. 1)". These Arecibo basin strata are exposed on the 

amphitheater-shaped scarp. 

The offshort: . stratigraphy in the area · of the 

amphitheater-shaped scarp can be divided -into three units 

based on seismic-reflection profiles (fig. 8; Meyerhoff and 

others, 1983). The lowest unit, unit 1, consists of basement 

rocks of Late _Jurassic to middle Eocene ag~. Unit 1 is offset 

by several large normal faults. The middle unit, unit 2, 

forms the low~r part of the Arecibo basin strata. Unit 2 

rocks are tho~;~ght to be late middle Eocene to middle 

Oligocene ( 45 to 30 million years old) limestones _that were 

originally deposited in shallow water. The upperrripst part 

of the Arecibo basin strata, unit 3, consists of reef and other 

shallow-water ilimestones. Unit 3 rocks range in age from 

middle Oligocene to Pliocene (30 to 4 million years old). 

The unit. 3 strata, both onshore and offshore, have a 

northward dip of 3.5° to 5° (Meyerhoff and others, 1983). 

These strata extend offshore to a water depth of 3,300 m, 

where the section is downfaulted (fig. 8). Because the faults 

cut the entire unit 3 section, they were active after deposition of the unit 3 strata; within the last 4 million~ years. 

Extrapolation of these faults along isobaths (bathymetric 

·contour lines) eastward from the seismic-reflection profile 

(fig. 5) suggests that the location of the scarp is controlled 

by the faults themselves (Schwab and others, 1991). The 

slope failure that created the amphitheater-shapecl scarp 

removed most, if not all, of the Arecibo basin strata. 

The sea floor below the amphitheater-shaped· scarp, 

between the base of slope (water depth 6,700 m) and the 

Puerto Rico Trench (water depth greater than 8,000 m), 

exhibits several hundred meters of local relief while disFigure 1. Study area showing the regional tectonic set- ..,.. 

ting (modified from Schwab and others,.)991). The 

Earth's lithospheric plates interact in three ways; they 

diverge, converge, or they· slide past each other. 

Arrows in the inset indicate relative motion of the 

North American and Caribbean plates. Although the 

relative motiol'il between the North American and Caribbean plates is pred<;>minantly strike-slip ·(the two 

plates move parallel to each other in <?PP~site directions), the pre~ence of the Puerto Rico Trench s~ggests 

that a component of the motion between the two plates 

is convergent (Sykes and others, 198~; Mann and 

Burke, 1984). As a result of this convergence, the North 

American plat~ is being obliquely thrust under. the 

Caribbean plate (also see fig. 2). The ramifications of 

this oblique convergence is the northward tilting of the 

northern insular slope of Puerto Rico and the formation 

of a region of high seismic risk. 

60 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
2000 

4000----

0 100 

KILOMETERS 

Tectonic and Stratigraphic Control on a Giant Submarine Slope Failure: Puerto Rico Insular Slope 61 
NORTH 

AMERICAN 

PLATE 

TRENCH 

AXIS 

Figure 2. A three-dimensional depiction of the bathymetry of the sea floor north of Puerto Rico, a segment of the 

boundary between the North American and Caribbean lithospheric plates. Major physiographic features shown are the 

amphitheater-shaped scarp, the insular slope of Puerto Rico, and the axis of the Puerto Rico Trench. 

playing a regional slope of only 1.7° (fig. 2). GLORIA 

imagery of this 50-km-wide area of sea floor displays areas 

of high and low acoustic backscattering intensity (fig. 4). 

The areas of low backscatter have been interpreted from 

seismic-reflection profiles and sediment cores to be ponded 

turbidite basins (fig. 3). Some areas of high backscatter on 

the GLORIA imagery from the area of sea floor near the 

base of the slope have been identified based on dredged 

samples (Fox and Heezen, 197 5; Perfit and others, 1980) as 

displaced carbonate debris (units 2 and 3) from further 

upslope, thus substantiating an inference that large volumes 

of rock from the insular slope have moved downslope. 

CAUSE OF THE LANDSLIDE 

A number of indicators suggest that slope failure of 

the Arecibo basin strata is principally responsible for the 

present morphology of the amphitheater-shaped scarp. 

These indicators include ( 1) the tectonic setting of the study 

area, a plate boundary and region of high seismicity; (2) the 

occurrence of shallow-water carbonate debris similar to the 

Arecibo basin strata that have been transported downslope 

and are now found in the area near the base of slope; and (3) 

the truncation of the strata by faults. Although thought to be 

the principal factor, it is unlikely that slope failure is the 

sole causative process. For example, the effects of erosion 

by abyssal currents or turbidity currents and/or bioerosion 

(the accumulated action of boring organisms over millions 

of years) are unknown. 

In general, slope failure occurs when the environmental forces (stres ) acting on the strata are greater than the 

resistance to deformation offered by the strata (strength). 

The two dominant driving forces of submarine slope failure 

applicable to the study area are downslope gravitational 

stress, due to the dip of the strata, and earthquake-induced 

shaking (Lee, Schwab, and Booth, this report). 

Downslope gravitational stresses affecting the 

Arecibo basin strata have varied with time. The northern 

part of Puerto Rico slowly subsided due to crustal thinning 

and sediment loading (Birch, 1986) as unit 2 was being 

deposited . At this time, a strike-slip boundary existed 

between the Caribbean and North American lithospheric 

plates (Sykes and others, 1982) presumably in the vicinity 

of the present Puerto Rico Trench. During deposition of unit 

3 in middle Oligocene to early Pliocene time (from 30 to 4 

million years before present), the plate boundary seems to 

have been relatively stable. Seismic-reflection profiles 

62 Submarine landslides: Selected Studies in the U.S. Exclusive Economic Zone 
67°001

W 20°00 N~----------------------~------------------------~------------------, 

0 

r 

:-:-:> -~ . ·.·."."/ 

:>K 

50 

0 

EXPLANATION 

Submarine Canyons; Offshore 

sediment I debris mass transport 

conduits. 

Mass-Movement Deposits; 

Composed of insular shelf sediment 

and debris from Arecibo Basin 

strata. 

a 

@l 

Ponded Turbidite Basins 

Displaced Blocks of Arecibo 

Basin strata (units 2 and 3) and/or 

outcrops of units I or 2 rocks . 

~ Downslope-facing scarps on the 

insular slope. 

Figure 3. Interpretive sketch (from Schwab and others, 1990) of the GLORIA imagery of the northern 

insular margin of Puerto Rico. The location of this interpretive sketch is shown on figure 1. 

Tectonic and Stratigraphic Control on a Giant Submarine Slope Failure: Puerto Rico Insular Slope 63 
DISPLACED BLOCKS OF 

ARECIBO BASIN STRATA? 

SUBMARINE 

CANYONS 

0 30 KM 

Figure 4. Digitally processed and enhanced GLORIA sidescan-sonar imagery of 

the amphitheater-shaped scarp north of Puerto Rico (following the compute r 

processing routines outlined by Chavez, 1986; modified from EEZ-SCAN 85 

Scientific Staff, 1987). The location of this figure is shown on figure 3. For a 

complete description of the GLORIA system, see Somers and others (1978). 

The principle of sidescan-sonar is described in Lee, Schwab, and Booth (this 

report). Areas of high acoustic backscatter on the GLORIA imagery are 

expressed as light tones while areas of low backscatter are dark. Geologic 

interpretation of this imagery is presented on figure 3. 

showing unit 3 have continuous parallel reflections that are 

displaced by only a few faults (Meyerhoff and others, 

1983). Therefore, approximately 4 million years ago, an 

extensive shallow-water carbonate platform existed north of 

the present coast of Puerto Rico. 

Sometime since the early Pliocene (within the past 4 

million years), slight convergence between the Caribbean 

and North American plates caused renewed subduction 

north of Puerto Rico. At least a small component of the 

relative motion between the two plates resulted in the 

thrusting of the North American plate beneath the Caribbean Plate (Sykes and others, 1982). Tectonic erosion 

induced by this oblique subduction is suggested to have 

thinned the island basement from underneath (basement 

rocks of Puerto Rico where abraded by the downgoing 

North American plate and subducted beneath the Caribbean 

plate), causing the northern insular margin of Puerto Rico to 

tilt rapidly to the north (Birch, 1986). As a result, the 

southern part of unit 3 has been uplifted a few hundred 

meters, whereas the northern part has subsided more than 

4,500 m, thus generating a 4.5° regional slope on the 

Arecibo basin strata. The present sea floor of the insular 

64 Submarine landslides: Selected Studies in the U.S. Exclusive Economic Zone 
~SEISMIC REFLECTION PROFILE (FIGURE 8) 

0 

KILOMETERS 

4ooo- Bathymetry in meters ---vz.r-R/V Starella ship trocklines ~ __ -- Normal Fault (dashed where 

U inferred). U, upthrown side. 

D, down thrown side. 

Figure 5. Bathymetry of the amphitheater-shaped scarp in meters (modified from Schwab and others, 

1990). The location of the seismic-reflection profile (fig. 8) and faults that cut the sea-floor surface are 

from Meyerhoff and others (1983). 

slope of Puerto Rico to a water depth of approximately 

3,000 m is essentially the upper surface of unit 3 covered by 

a thin veneer of sediment. Therefore, the sea-floor slope, 

and thus the downslope gravitational stress affecting the 

Arecibo basin strata, has been steadily increasing due to the 

northward tilting of the northern insular margin during the 

past 4 million years. 

Earthquake shaking has also affected the Arecibo 

basin strata. Earthquake-induced horizontal accelerations 

directly increase the downslope-directed stresses which act 

on strata. In addition, the cyclic ground motions increase 

pore-water pressure. An elevation of pore-water pressure 

directly decreases the strength of the strata and, therefore, 

reduces its ability to resist slope failure (Lee, Schwab, and 

Booth, this report). Furthermore, the strength of the 

Arecibo basin strata was probably significantly reduced by 

pre-existing faults that underlie the amphitheater-shaped 

headwall scarp (figs. 5 and 8). 

The increase of the sea-floor slope with time, when 

coupled with earthquake shaking and associated possible 

strength reduction during major earthquakes, could be 

expected to exceed the strength of the Arecibo basin strata, 

particularly along the fault-induced pre-existing zones of 

weakness (Schwab and others, 1991). The result would be 

a slope failure, such that the northern section of the Arecibo 

basin strata would have broken off, disintegrated, and slid 

into the area between the base of slope and the Puerto Rico 

Trench. Mass wasting of the newly exposed failure plane 

(the amphitheater-shaped scarp) resulted in the formation of 

the talus chutes. Subsequent mass-flow activity transported 

debris from the talus chutes and from the insular slope 

above the amphitheater-shaped scarp and deposited this 

material north of the base of slope as turbidites (fig. 3). 

Thus, the mass-movement deposit associated with the slope 

failure that formed the amphitheater-shaped scarp has been 

partially buried by these younger turbidites and is not fully 

discernible on the GLORIA imagery (fig. 4). 

APPLICATION TO OTHER AREAS 

Amphitheater-shaped scarps have been identified and 

verified as the consequence of slope failure in various 

subaerial and submarine settings (for example, Brunsden 

and Prior, 1984; Mullins and others, 1986; Lee, 1989 and 

references therein). Although the amphitheater-shaped 

scarp north of Puerto Rico represents the removal of a huge 

amount of material (about 1,500 km3 of strata), submarine 

slope failures of this large scale may not be particularly 

unusual. Recent surveys using modern remote-sensing 

instruments show that the world's largest slope failures have 

occurred on submarine slopes. These include slope failures 

on the steep submerged volcanic slopes of the Hawaiian 

Tectonic and Stratigraphic Control on a Giant Submarine Slope Failure: Puerto Rico Insular Slope 65 
E 

I 

........ 

CL 

w 

Cl 

~N 

Figure 6. Three-dimensional perspective view of the bathymetry shown on figure 5. 

Islands (Moore and others, 1989; Norrnark and others, this 

report), the continental slope of Norway (Bugge and others, 

1987) and along the middle and lower slopes of convergent 

margins (Lewis and others, 1988, von Huene and others, 

1989). 

It appears that the slope failure of the northern margin 

of the Arecibo basin strata resulted from oversteepening of 

the strata in a region of high seismicity. As this oversteepening continues, the relative stability of the insular slope 

decreases. Further east along the insular slope near longitude 66°W. (fig. 3), a smaller area of high backscattering 

intensity on the GLORIA imagery has been interpreted to be 

another amphitheater-shaped scarp (EEZ-SCAN 85 Scientific Staff, 1987), suggesting that the entire northern edge of 

the Arecibo basin strata is failing under the present tectonic 

regime. 

In addition to the amphitheater-shaped scarp and its 

possible smaller counterpart to the east, several other 

smaller scarps have been identified on the slope north of 

Puerto Rico in water depths less than 3,000 m (fig. 3). 

These smaller scarps may represent incipient slope failures 

and may indicate that the slope failure represented by the 

amphitheater-shaped scarp is retrogressive and will continue to be active. 

SEISMIC SEA WAVES FROM GIANT 

EARTHQUAKES 

The existence of large submarine landslides has 

significance for understanding of the generation of great sea 

waves. These giant waves are called seismic sea waves or 

tsunami (the Japanese word with the same form in both 

singular and plural). Tsunami have devastated oceanic 

islands and continental coastlines throughout history. 

Although tsunami can be caused by large earthquakes (for 

example, the 1964 Alaska earthquake (Plafker and others, 

1969; von Huene and Cox, 1972) and the 1960 Chilean 

earthquake (Kajiura, 1979)), they also can be generated by 

other mechanisms, including submarine slope failure 

(McCulloch, 1985; von Huene and others, 1989; Hampton, 

Lemke, and Coulter, this report). We do not know if the 

slope failure that resulted in the amphitheater-shaped scarp 

off Puerto Rico was catastrophic (the entire mass disintegrating rapidly) or took place over a substantial period of 

time (block by block). If the amphitheater-shaped scarp was 

the result of a catastrophic slope failure, it was of sufficient 

size to have generated a large tsunami that must have had 

major impact on the nearby (35 km) coastal areas of Puerto 

Rico. Because the condition of slope instability that allowed 

an extremely large slope failure continues to develop north 

of Puerto Rico, and because the seismic risk remains the 

same as that when the studied slope failure took place, a 

repetition of the event is quite possible, with potential for 

generation of a catastrophic tsunami. 

ACKNOWLEDGMENTS 

The authors thank the captain and the crews of the 

R/V Parnella and R/V Starella for their cooperation at sea. 

Discussions with Kim Klitgord, William Dillon, Chris 

Polloni, and Douglas Masson were helpful during this 

research. All illustrations and photography were provided 

by Sarah Griscom, Patricia Forrestal, Dann Blackwood, 

and Jeff Zwinakis. Any use of trade names is for descriptive 

purposes only and does not imply endorsement by the U.S. 

Geological Survey. 

66 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
0 

A 

8 30 KILOMETERS 

Figure 7. Three-dimensional perspective views of the amphitheater-shaped scarp using digitally 

merged GLORIA (from fig. 4) and bathymetric data (from fig. 6); (A) viewed from the northwest. (8) 

viewed from the north. (C) viewed from the northeast. See figures 3 and 4 for interpretation. 

Tectonic and Stratigraphic Control on a Giant Submarine Slope Failure: Puerto Rico Insular Slope 67 
EXPLANATION 

~ Early Pliocene to middle Oligocene shallow- water limestone 

<: ::j Middle Oligocene to middle Eocene shallow- water limestone 

[0 Middle Eocene to late Jurassic basement rocks 

'-.._ Normal Fault 

0 10 

KM 

0 

1000 

2000 ~ 

-I 

3000 ta... 

w 

4000 0 

0:: 5000 w 

t6000 ~ 

7000 

Figure 8. An interpretive sketch of a representative seismic-reflection profile collected over the Arecibo basin (modified 

from Meyerhoff and others, 1983). See figures 3 and 5 for the location of this profile. 

REFERENCES CITED 

Birch , F.S., 1986, Isostatic, thermal, and flexural models of the 

subsidence of the north Coast of Puerto Rico: Geology, v. 14, 

p. 427-429. 

Brunsden, Denys, and Prior, D.B., 1984, Slope instability: 

Wiley-Interscience Publication, 620 p. 

Bugge , Tom, Befring, Stein, Belderson, R.H., Eidvin, Tor, 

Jansen, Eystein, Kenyon, N.H., Holtedahl, Hans, and 

Sejrup, H.P. , 1987, A giant three-stage submarine slide off 

Norway: Geo-Marine Letters, v. 7, p. 191-198. 

Chavez, P.S., Jr., 1986, Processing techniques for digital sonar 

images from GLORIA: Photogrammetric Engineering and 

Remote Sensing, v. 52, p. 1133- 1145. 

EEZ-SCAN 85 Scientific Staff, 1987, Atlas of the U.S. Exclusive 

Economic Zone, Eastern Caribbean: U.S. Geological Survey 

Miscellaneous Investigations Series 1- 1864 B, 59 p. 

Fox, P.J., and Heezen , B.C., 1975, Geology of the Caribbean 

crust, in, Nairn, A.E.M., and Stehli, F.G., eds., The ocean 

basins and margins, Volume 3: New York, Plenum Pre s, 

706 p. 

Kajiura, K.T., 1979, Tsunami generation, in, Hwang, L.S., and 

Lee, Y.K., eds., Tsunami: Proceedings of the National 

Science Foundation Workshop, May 1979, p. 15-36. 

Lee, H.J., 1989, Undersea landslides: extent and significance in 

the Pacific Ocean, in Brabb, E.E., ed., Landslides: extent 

and economic significance: Proceedings of the 28th International Geological Congress Symposium on Landslides, 

Ba1kema, Rotterdam, p. 361-374. 

Lewis, S.D., Ladd, J.W., and Bruns, T.R., 1988, Structural 

development of an accretionary prism by thrust and strike-slip 

faulting: Shumagin region, Aleutian Trench: Geological 

Society of America Bulletin, v. 100, p. 767- 782. 

Mann, P.T., and Burke, K. W., 1984, Neotectonics of the Caribbean: Review of Geophysics and Space Physics, v. 22, p. 

309-362. 

McCulloch, D.S., 1985, Evaluating tsunami potential, in, Ziony, 

J .I., ed., Evaluating earthquake hazard in the Los Angeles 

Region: U.S. Geological Survey Professional Paper 1360, p. 

325-382. 

Meyerhoff, A.A., Krieg, E.A. , Cloos, J.D., and Tanner, I.S., 

1983, Petroleum potential of Puerto Rico: Oil and Gas 

Journal, v. 81, December, p. 113-120. 

Moore, J.G., Clague, D.A., Holcomb, R.T. , Lipman, P.W., 

Normark, W.R., and Torresan, M.E., 1989, Prodigious 

submarine slides on the Hawaiian Ridge: Journal of Geophysical Research, v. 94, p. 17 ,465-17,484. 

Mullins, H.T., Gardulski, A.F., and Hine, A.C., 1986, Catastrophic collapse of the west Florida carbonate platform 

margin: Geology, v. 14, p. 167-170. 

Perfit, M.R., Heezen, B.C., Rawson, M.P., and Donnelly, T.W. , 

1980, Chemistry, origin and tectonic significance of metamorphic rocks from the Puerto Rico trench: Marine Geology, 

v. 34, p. 125- 166. 

Plafker, George, Kachadoorian, R.N., Eckel, E.B., and Mayo, 

L.R., 1969, Effects of the earthquake of March 27, 1964 on 

various communities: U.S. Geological Survey Professional 

Paper 542-G, 53 p. 

Schwab, W.C., Danforth, W.W., Scanlon, K.M., and Masson, 

D.G., 1991, A giant submarine slope failure on the northern 

insular slope of Puerto Rico: Marine Geology, v. 96, p. 

237-246. 

Somers, M.L., Carson, R.M., Revie, J .A., Edge, R.H. , Barrow, 

B.J., and Andrews, A.G., 1978, GLORIA II-An improved 

long-range sidescan sonar: Proceedings of the IEEE/IERE 

Subconference on Ocean Instruments and Communication, 

Oceanology International, London, BPS Publications, p. 

16-24. 

Sykes, L.R., McCann, W.R., and Kafka, A.L., 1982, Motion of 

the Caribbean plate during the last 7 million years and 

implications for earlier Cenozoic movements: Journal of 

Geophysical Research, v. 87, p. 10,656-10,676. 

von Huene, Roland, Bourgois, Jacques, Miller, John , and Pautot, 

Guy, 1989, A large tsunamogenic landslide and debris flow 

along the Peru Trench: Journal of Geophysical Research, v. 

94, p. 1703-1714. 

von Huene, Roland , and Cox, C.K., 1972, Locally generated 

tsunami and other local waves in the great Alaska earthquake 

of 1964, oceanography and coastal engineering: National 

Academy of Sciences, Washington, D.C. , p. 211-222. 

68 Submarine landslides: Selected Studies in the U.S. Exclusive Economic Zone 
Slope Failure of Carbonate Sediment on the West 

Florida Slope 

By D.C. Twichell, P.C. Valentine, and L.M. Parson 1 

INTRODUCTION 

Previous studies have shown that slope failure has 

been an important process in the development of the West 

Florida Slope in the eastern Gulf of Mexico throughout the 

last 65 million years of its history. Slope failures have left 

large headwall scarps preserved in the sedimentar~ record 

and, in places, still exposed on the sea floor (Mttchum, 

1978; Holmes, 1985; Doyle and Holmes, 1985; Mullins and 

others, 1986; Mullins and others, 1988). Whether· these 

headwall scarps are the result of a few large catastrop~ic 

mass-wasting events widely separated in age or whether 

they resulted from numerous smaller events were questions 

we were previously unable to answer and that have ·an 

important bearing on the origin of these features and on 

future economic use of this region. In this section, GLORIA 

(Geological LOng-Range Inclined Asdic) sidescan-so.nar 

images and high-resolution seismic-reflection proftles 

(EEZ-SCAN 85 Scientific Staff, 1987) are used to document an area of sea-floor instability ·on the West Florida 

Slope in the eastern Gulf of Mexico (fig. 1). The new 

GLORIA imagery of the study area shows that these large 

headwall scarps, which extend for great dista11ces along the 

slope, are composite features resulting from multiple 

smaller mass-wasting events (fig. 2). Tht?se data also show 

that the extent of mass wasting in this area is more vast than 

was previously thought. These failures lie within. an ~ea of 

potential offshore petroleum exploration, so their presence 

may prove a hazard to the development of this resourc~ .. 

The study area is a 300-kilometer (km)-long section 

of the West Florida Slope in water depths of 500· to 2,000· 

meters (m) (fig. 1). The West Florida Slope is bounded on 

its shoreward side by the nearly flat West Florida Shelf and 

on its seaward side by the precipitous Florida Escaipment 

(fig. 3). The West Florida Slope itself is 30:-50 km wide and 

has average gradients of less than 1° to 5°. Surface -sediment 

on the slope is calcium carbonate mud (Doyle and Sparks, 

1980). Seismic-reflection profiles sho~ that the mud forms 

a westward-thinning, wedge-shaped deposit (fig. 4) that is 

as much as 1,000 m thick (Mitchum, · 1978).· These 

Cenozoic-aged carbonate sediments are less than 65 million 

years old, and overlie limestone rocks exposed on the. face 

of the Florida Escarpment that are older than the N_ltddle 

Cretaceous (about 100 million years ago) (Freeman-Lynde, 

1983; Paull and others, 1990). · 

1 Institute of Oceanographic Sciences, Wonnley, Surrey, U.K. 

METHODOLOGY 

GLORIA is a long-range sidescan-sonar system that 

provides an acoustic map vie~ o( the sea floor (Somers and 

others, 1978). The GLORIA data were collected in swaths 

along ship tracklines which were close enough together to 

provide complete coverage of the study area. These data 

were processed and combined by computer techniques 

(Chavez, 1984, 1986) to create a sidescan-sonar mosaic 

(fig. 2; EEZ-SCAN 85 Scientific Staff, 1987). On the 

GLORIA images, a strong acoustic return from the sea floor 

(referred to in the text as high-backscatter) is light and a 

weak return (low backscatter) is dark (fig. 2). 

High-resolution seismic-reflection profiles were collected along the survey tracklines simultaneously with the 

GLORIA data. The seismic-reflection data show sea-floor 

topography and provide cross-sectional profiles of the 

shallow stratigraphy below the sea floor. The combination 

of the sidescan-sonar and seismic-reflection data provides a 

three-dimensional view of the geology of this area. 

A NEW VIEW OF WEST FLORIDA SLOPE 

MORPHOLOGY 

The GLORIA images and high-resolution seismic 

profiles reveal that the surface of the West Florida Slope is 

scarred by numerous slope failures. The seismic profiles 

show many scarps here (figs. 5, 6); on the GLORIA images 

(fig. 2) these scarps appear as light or dark stripes depending on whether the sonar is looking upslope at them (light) 

or downslope over them (dark). Discontinuous scarps can 

be traced along the eastern edge of the survey area (figs. 1, 

2), ·and they have 50- _to 150-m r~lief where crossed by 

seismic profiles. (Doyle and Holmes, 1985; Holmes, 1985). 

·Downslope. of the scarps, the northern and central 

parts of the West Florida Slope are heavily dissected by 

narrow, downslope-trending valleys (figs. l, 3). These 

valleys are flat-floored and have steep sidewalls with as 

much as 100-m relief (fig. 5). For the most part, the floors 

of these valleys show high-backscatter returns on the 

GLORIA images which are inferred to represent mass-flow 

deposits (fig. 2). The valleys of the central part of the study 

area are subparallel, and they commonly converge on the 

heads of large canyons that incise the Florida Escarpment 

(fig. 2, · 3). The whole set of valleys in the central area is 

bounded along its northwest and southeast sides by scarps 

having as much as 200-m relief (figs. 1, 3, 5B, southern 

end). 

·slope Failure of Carbonate Sediment on the West Florida Slope 69 
0 

CONTOURS IN METERS 

) . 

50km 

EX PLAN AT 10 N 

T'f"1"'t"" S CA R PS 

MASS-MOVEMENT DEPOSITS 

1----4 SEISMIC-REFLECTION 

PROFILE LOCATIONS 

70 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
~ Figure 1. Bathymetric map showing the West Florida 

Slope in the southeastern Gulf of Mexico. Location of 

the study area is given in the inset map. Contours are in 

meters and have been created using the soundings 

collected during the EEZ-SCAN project in combination 

with the GLORIA images which were used as a guide to 

defining the shape of the sea floor between the track 

lines. Locations of scarps and mass-wasting deposits identified from the GLORIA images are shown as well as the 

locations of 3.5-kilohertz and multi-channel seismic profiles shown in figures 4, 5 and 6. 

VALLEY 

SLOPE VALLEY 

0 

I 

50 km 

I 

MASS- MOVEMENT 

DEPOSIT 

Figure 2. GLORIA sidescan-sonar mosaic showing the precipitous Florida Escarpment, failure scarps on the West 

Florida Slope, mass-flow valleys of the central part of the study area, and mass-flow deposits of the southern part of 

the study area. The area shown is the same as that given on figure 1. 

Slope Failure of Carbonate Sediment on the West Florida Slope 71 
N~ 

CENTRAL 

AREA 

ABYSSAL PLAIN 

0 IOOkm 

SOUTHERN 

AREA 

Figure 3. Computer-generated perspective view of the study area showing the high-relief Florida Escarpment, the flat 

abyssal Gulf of Mexico at the base of the escarpment, and the distribution of valleys on the West Florida Slope above the 

escarpment. 

By contrast, the morphology of the slope failures in 

the southern part of the study area is different. A series of 

intersecting scarps, rather than a single one, is seen along 

the upslope edge of the zone of failed sediment (fig. 1). 

These scarps have as much as 100-m relief (fig. 6A). 

Downslope of these scarps, the bathymetry shows few 

valleys (fig. 1), and the sonar images show irregular patches 

of high and low backscatter (fig. 2) rather than the linear 

valley floors seen in the central and northern areas. These 

irregular patches coincide with an area of rough sea floor on 

the seismic profiles (fig. 6C, 6D, and 6£) and are interpreted to be mass- flow deposits derived from the scarps 

upslope. 

Slope failures cover approximately 60 percent of the 

part of the West Florida Slope under study here. If the shape 

of the slope prior to failure resembled the sections that have 

not yet failed, then an average thickness of 150 m of 

material has been removed based on the heights of the 

scarps that bound the failed areas. The volume of sediment 

removed is in excess of 840 km3

. We speculate that upon 

failure, the poorly cemented material disintegrated and that 

most of it was transported over the Florida Escarpment and 

now resides on the abyssal plain along the base of the 

escarpment. The small amount of mass-flow material that 

remains on the West Florida Slope forms a thin, rough 

veneer in the valley floors of the northern and central areas 

and rough, irregular patches in the southern area. 

COMPARISON OF SLOPE FAILURE REGIONS 

Although sediment type (Doyle and Sparks, 1980), 

shallow stratigraphy, and structure (Shaub, 1984; Doyle 

and Holmes, 1985) appear to be relatively uniform in the 

study area, the sidescan sonar results suggest that different 

parts of the West Florida Slope have been affected by the 

mass flows in different ways. The headwall scarps in the 

central area are in 500 to 700 m of water while the headwall 

scarps bounding the southern area are in 1,000 to 1,200 m 

of water (fig. 1). The regional sea-floor gradient in these 

two areas differs also. Gradients in the central area exceed 

4°, whereas in the southern area, gradients are less than 1°. 

Box canyons cut the escarpment downslope of the central 

area, and many of the slope valleys feed into the heads of 

the box canyons (figs. 3, 8). These box canyons are absent 

on the escarpment downslope of the southern area (fig. 1). 

Downslope of the scarps in the northern and central 

areas, the sea floor is dissected by valleys. In the northern 

area, scarps cutting across the valley floors suggest that 

several episodes of slope failure were involved in forming 

the valleys. At the northern end of the central area, 

discontinuous scarps are dissected by some of the slope 

valleys. The cutting of these scarps by the slope valleys 

suggests that at least two stages of slope failure were 

involved in the development of the central area. 

In the southern area, bathymetric, seismic, and 

sidescan-sonar data indicate that the sea floor downslope of 

the intersecting scarps is not cut by valleys (fig. 1), but 

rather it is covered by a layer of mass-flow deposits. The 

surface of the sea floor is rough, and one of the profiles 

crossed an individual block 50 m high and 1 km wide (fig. 

6C). The cross-cutting relationship of the scarps in the 

southern area is another example of the multistaged development of the failures on the West Florida Slope (fig. 7). 

The shallowest scarp is interpreted to be the oldest (because 

it is truncated by scarps that extend into deeper water); 

younger failures occur in progressively deeper water. The 

youngest scarps (stage 4) cut into the mass-flow deposits 

derived from the older scarps farther upslope. Thus, 

although individual scarps have only 30 to 100 m of relief, 

the total relief of the scarps created by the failures of stages 

1 to 3 is as much as 200m (fig. 6A). There may have been 

additional episodes of slope failure, but evidence of these 

events has been masked by more recent failures. 

72 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
0 

1000 

2000 

3000 

.5 4000 

I 

1-

a.... 

w 

0 

a:: 

w 

~ ~ 

1000 

2000 

3000 

4000 

10 km 

PLEISTOCENE AND-_-_-

P Ll 0 C ENE DEPOSITS -~--.---'----.------'-----.--_.__ 

Figure 4. Multichannel seismic-reflection profile across 

the West Florida Slope and Escarpment (Institute of Geophysics of the University of Texas at Austin). Location of 

this profile is shown in figure 1. Note the sequence of 

POTENTIAL GEOLOGICAL HAZARDS 

Causes of the sediment failures on the West Florida 

Slope cannot be derived from these data, but some inferences about the triggering mechanisms can be made. Storm 

waves, a common triggering mechanism of slope failures in 

shallow water (Lee and Edwards, 1986; Lee, Schwab, and 

Booth, this report), probably did not contribute to the 

initiation of these failures because of the great water depths 

in which they occur. Even during periods of glaciation, 

prograding reflectors underlying the West Florida Slope 

(part of which has been removed by mass wasting) and the 

steep Florida Escarpment. Vertical exaggeration is about 

1Q X . 

when sea level was as much as 200 m lower than now, these 

scarps would have been in at lea t 300 m of water, below 

the effect of storm waves. Earthquakes are an important 

triggering mechanism in tectonically active areas (Lee and 

Edwards, 1986), but in areas such as the Gulf of Mexico 

where the occurrence of earthquakes is infrequent, their 

importance has not been documented. If earthquakes were 

the triggering mechanism, the multistaged development of 

these failures suggests that several earthquakes rather than 

just one would have been needed. Another alternative is that 

Slope Failure of Carbonate Sediment on the West Florida Slope 73 
N s 

800- · · - · · SCARP .. . .;_ ,.~.A'~~/ . 

. . ''"""''"' . "'"' . . '\· ··-. ; 

900-.......... 

- 8 VALLEY CONTAINING 

· · : · MASS-MOVEMENT- !:£POSITS .o:"!fR l'lM 1000- I . . '-. ~~ · . . 

..,.r-'''f""~'\ ! . " . . . 1100- .: ~ 

1200-- . ·i 

1100-

E 

:I: 

1-

a... 

l.&J 

0 

Q: 

w 

tc 

1200-_p~ .·· ·y ~,- "\ + I f~~~· . . . I . . . 

~ 

1300- ·\ I 

~~t \ I 1400-

1400-

0 A,

: /~0-r-< . . . "';"'~t-..1/. p~ ~ ._· ~-·, , ·.· :-,.,t·' ' ;' - . / . _· '\: 1 

1500-

1600- ·~ :+ . _.-( . /\.; ..... ' . yo, 

. > __,r.S:;;. );i- . .,.f, .p..or~ N • 

·""~~-·;,. 1700- _ .. , _.;;:;;.:ll'r · / . . 

/ 

I ' 'i 

Figure 5. A series of 3.5-kilohertz seismic-reflection profiles from the northern part of the study area. Profile 

locations are shown in figure 1. In the left part of all four 

profiles is the flat-floored valley shown on the GLORIA 

image. The sidewalls of this valley have 50 to 150 meters of 

relief. The floor is covered by an acoustically transparent 

local oversteepening resulted from physical erosion by the 

Loop Current which sweeps over this area or chemical 

dissolution of the carbonate rocks by acidic bottom waters 

or pore waters. Measurements of the strength of bottom 

currents and the chemistry of the bottom waters and pore 

waters do not exist for this area and will need to be 

measured before these alternatives can be considered. 

The differences in the morphological expression of 

the slope failures in the southern and central areas may be a 

consequence of differences in the sea-floor gradient and 

IOkm 

layer that has a hummocky surface and is interpreted to be 

a mass flow deposit. The southern ends of profiles A, 8, 

and C cross the scarp that marks the northern limit of the 

central mass wasting area. Vertical exaggeration is about 

25 X . 

perhaps differences in sediment age and compositiOn. 

Following slope failure, the sediment disintegrated and 

flowed. Although disintegrative failure occurred in both 

areas, the potential for flow mobilization was probably 

higher for the more steeply sloping northern and central 

areas than for the southern area. The mass flows would have 

more readily carved the sea floor and become more channelized. Alternatively, the scarps and slope valleys of the 

central area may represent two different styles and episodes 

of slope failure. The initial failures may have been like 

74 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
NW 

900-A 

1000- : 

1100- : 

: 1200- : . 

·.• · 

! I 

~ ' 

J I . 

I 

I 

SE 

T\v· 

~ 

-~ "<- \ 

./ 

a · 

1100-. ----""'~~~~"'·"-~'!W~-~------- 1200-· : ....... ,;.,....._ 

1300-· ·"' 

1100- c 

E 

:I: 

1--

Q.. 

~ 1300-

a:: 

w 

1-- 1400-

~ 

1200- 0 , • ~OUGH SEA FLOOR 

1300- : r' . , ...... . ' . ·~ ... 

1400- -~ { j : . '-~

1500 

_ __..../' :r t r t 11 ~ · · ~!!"\,-...;.... ~, /' 

1600-

1700-

1800-

10 km 

Figure 6. A series of 3.5-kilohertz seismic-reflection profiles across part of the southern mass-wasting area. 

Profile A crosses the head-wall scarps of some of these 

slope failures. Profile locations are shown in figure 1. 

Profiles 8 , C, 0, and E show the hummocky surface of 

"--~· . r 

. <. 

\ 

the mass-flow deposit downslope of the scarps. Most of 

the blocks in this deposit are too small to be individually 

resolved on these profiles or on the GLORIA image (fig. 

2), but a large block (50 m high and 1 km wide) is shown 

on profile C. Vertical exaggeration is about 25 x . 

Slope Failure of Carbonate Sediment on the West Florida Slope 75 
0 

~ 

. Zb:( •• •• STAGE 1 

--STAGE 2 

-·-STAGE 3 

\) •. -STAGE 4 

,

) ! @ MASS-WASTING 

: DEPOSIT 

~-- ·. \.. . .,· .. I . • ' ~ ~ l' .. : 

• I : ,. \ ··;·, ... , 

,.,., ', -·-·~ 

(~r 

~ -tt,o 

l. ~ 

"'"· l 

50km 

Figure 7. Summary interpretation of the multistaged evolution of the mass 

flows in the southern area based on the cross·cutting relationships of 

scarps revealed by the GLORIA images. The headwall of the oldest failure 

is farthest upslope, and younger scarps are progressively farther down· 

slope. The youngest, stage 4, appears to cut the mass·flow deposits that 

originated further upslope. 
Figure 8. Perspective view of a part of the GLORIA mosaic 

showing the convergence of the mass-flow valleys of the 

central failure area on the heads of the box canyons that 

incise the Florida Escarpment. The convergence of these 

those in the southern area and created the linear scarps along 

the eastern edge of the study area. Subsequently, these 

scarps could have been incised by slope valleys that formed 

by retrogressive failure. 

The extent of the mass wasting on the West Florida 

Slope indicates that these carbonate sediments can become 

unstable on very gentle gradients in response to environmental conditions that have existed in the eastern Gulf of 

Mexico. From a practical point of view, if these slope 

failures continue to be active, they will be a hazard to 

petroleum exploration and related development of this area. 

Geologically, they are of interest because the morphology 

and failure mechanisms of carbonate sediments are poorly 

understood (Lee, 1985). This advance in our knowledge of 

carbonate sediment failure was made possible by our ability 

to depict large areas of the sea floor with sidescan-sonar 

imagery. 

ACKNOWLEDGMENTS 

Reviews by W.C. Schwab and K. Scanlon considerably improved an earlier version of this manuscript. Drafting and photography for the figures were done by J. 

Zwinakis and D. Blackwood. 

valleys on the heads of the box canyons may suggest that 

the valleys are the result of headward erosion that was 

initiated by erosion of the heads of the box canyons. 

REFERENCES CITED 

Chavez, P.S., 1984, U.S. Geological Survey mini image processing system (MIPS): U.S. Geological Survey Open-File 

Report 84-880, 12 p. 

---1986, Processing techniques for digital sonar images from 

GLORIA: Photogrammetric Engineering and Remote Sensing, v. 52, p. 1133-1145. 

Doyle, L.J., and Holmes, C.W., 1985, Shallow structure, stratigraphy, and carbonate sedimentary processes of West Florida Upper Continental Slope: American Association of Petroleum Geologists Bulletin, v. 69, p. 1133-1144. 

Doyle, L.J., and Sparks, T.N., 1980, Sediments of Mississippi, 

Alabama, and Florida (MAFLA) continental shelf: Journal of 

Sedimentary Petrology, v. 50, p. 905-916. 

EEZ-SCAN 85 Scientific Staff, 1987, Atlas of the U.S. Exclusive 

Economic Zone, Gulf of Mexico: U.S. Geological Survey 

Miscellaneous Investigation Series I-1864-A, 104 p. 

Freeman-Lynde, R.P., 1983, Cretaceous and Tertiary samples 

dredged from the Florida Escarpment, eastern Gulf of Mexico: Transactions-Gulf Coast Association of Geological 

Societies, v. 33, p. 91-99. 

Holmes, C.W., 1985, Accretion of the South Florida Platform, 

Late Quaternary development: American Association of 

Petroleum Geologists Bulletin, v. 69, p. 149-160. 

Lee, H.J., 1985, State of the art: Laboratory determination of the 

strength of marine soils, in Chaney, R.C., and Demars, 

Slope Failure of Carbonate Sediment on the West Florida Slope 77 
K.R., eds., Strength Testing of Marine Sediment: Laboratory 

and In Situ Measurements, ASTM, STP 883: American 

Society for Testing and Materials, Philadelphia, p. 181-250. 

Lee, H.J., and Edwards, B.D., 1986, Regional method to assess 

offshore slope stability: Journal of Geotechnical Engineering, 

American Society of Civil Engineers, v. 112, p. 489-509. 

Mitchum, R.M., 1978, Seismic stratigraphic investigation of West 

Florida Slope, Gulf of Mexico, in Bouma, A.H., Moore, 

G.T., and Coleman, J.M., eds., Framework, Facies, and 

Oil-trapping characteristics of the upper Continental Margin: 

American Association of Petroleum Geologists Studies in 

Geology 7, p. 193-223. . · 

Mullins, H.T., Gardulski, A.F., Hinchey, E.J., and Hine,' A.C., 

1988, The modem carbonate ramp slope of central West 

Florida: Journal of Sedimentary Petrology, v. 58, p. 

273-290. 

Mullins, H.T., Gardulski, A.F., and Hine, A.C., 1986, Catastrophic collapse of the west Florida carbonate platform 

margin: Geology, v. 14, p. 167-170. 

Paull, C.K., Freeman-Lynde, R.P., Bralower, T.J., Gardemal, 

J.M., Neumann, A.C., D'Argenio, B.P., and Marsella, 

E.A., 1990, Geology of the strata exposed on the Florida 

Escarpment: Marine Geology, v. 91 , p. 177-194. 

Shaub, F.J., 1984, The internal framework of the southwestern 

Florida bank: Transactions-Gulf Coast Association of Geological Societies, v. 34, p. 237-245. 

Somers, M.L., Carson, R.M., Revie, J.A., Edge, R.H., Barrow, 

B.J., and Andrews, A.G., 1978, GLORIA 11-An improved 

long-range sidescan sonar, irf.Proceedings, IEEEIIERE subconference on Offshore Instrumentation and Communications, Oceanology International, 1978, Technical Session J: 

London, BPS Publications, p. 16-24. 

78 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
Slope Failures in an Area of High Sedimentation 

Rate: Offshore Mississippi River Delta 

By j.M. Coleman1 , D.B. Prior2 , L.E. Garrison3

, and H.j. Lee 

INTRODUCTION 

Large sedime~t-laden rivers debouching onto the 

continental shelf rapidly build sedimentary bodies that can 

be highly susceptible to slope failure (submarine landsliding). Within the offshore waters of the United States, the 

deltaic sediment placed by the Mississippi River is the best 

example of such an unstable body. It has received extensive 

attention from both industry and the academic community 

because of the presence of major offshore oil resources and 

the massive scale area of submarine slope failure. The 

destruction of one offshore platform and the damaging of 

two others by submarine landslides during Hurricane Camille in 1969 (Sterling and Strohbeck, 1973; Bea and others, 

1975) demonstrates the practical importance of understanding slope-failure mechanisms. 

After Hurricane Camille, considerable research on 

the offshore part of the Mississippi Delta was initiated by 

the petroleum industry, consulting firms, governmental 

agencies, and universities, including a cooperative study 

involving the U.S. Geological Survey (USGS); the Coastal 

Studies Institute, Louisiana State University (LSU); and 

Texas A&M University. The focus of this cooperative 

project was to (1) establish the regional geologic framework 

of the delta, (2) map the distribution and describe the 

variety of types of submarine slope instabilities, (3) characterize the sediment properties and their behavior under 

various stresses, and (4) determine the mechanisms responsible for the submarine slope failures. This section describes 

the results of part of this project. Specifically, it describes 

the extent and morphology of slope failures in the area, 

includes quantitative information on the rates and styles of 

movement, and explores the causes of the failures. 

MISSISSIPPI RIVER DELTA SIETTING 

The Mississippi, the largest river system in North 

America, drains an area of 3.4 million square kilometers 

(km2

) or over 40 percent of the area of the coterminous 

United States. Average water discharge of the river at its 

delta is 15,400 cubic meters per second (m3

/sec), and can 

be as high as 59,500 m3

/sec; the annual sediment discharge 

Tex. 

1 Louisiana State University, Baton Rouge, La. 2 Geological Survey of Canada, Dartmouth, Nova Scotia, Canada. 3 Ocean Drilling Program, Texas A&M University, College Station, 

is estit:nated at 210 to 680 million tons (Coleman and others, 

1980; Milliman and Meade, ·1983). The bedload consists of 

90 percent fine sand, and the s~spended load is characterized by 65 percent clay and 35 percent silt and very fine 

sand. Thus, the Mississippi River carries a substantial 

sediment load annually, a high percentage of which consists 

of fine-grained clay and silt transported as suspended load. 

The coarser material is deposited 'at or near the distributary 

mouths of the modem subaerial delta. The modem bird-foot 

delta is the youngest of the del~a lobes of the Mississippi 

River; radiocarbon dates indicate that it formed within the 

past 600 to 800 years (Fisk and others, 1954). The average 

thickness of this delta lobe is 90 to 105 meters (m); seaward 

progradation rates of the dis.tributary mouths vary from 50 

to 100 m per year. The fine-grained sediment remains in 

suspension and spreads laterally; often as a visible plume, 

far beyond the immediate mou~hs of the distributaries. 

Sedimentation rates seaward of the distributaries are 

extremely high, averaging 0.6 to 1.2 m per year. In adjacent 

interdistributary bays, accumulation rates rarely exceed a 

few centimeters (em) per year, and in some places, the bay 

bottom is being eroded (Coleman and others, 1980). 

Deposition of the fine-grained sediment builds a 

platform fronting the bird-foot delta that consists of rapidly 

deposited clay which has an extremely high water content 

and, because of abundant fine-grained organic matter, 

includes large concentrations of gases (primarily methane 

and carbon dioxide). Rapid deposition does not allow pore 

fluids to migrate through the low permeability clays and 

silts, and thus, the pore fluids remain trapped. As sediment 

accumulates, loading increases, and the pore fluids bear the 

weight of the overlying load; consequently, pore-fluid 

pressures increase to high levels. In-place measurements in 

shallowly buried delta sediment (less than 25 m beneath the 

sea floor) show pore-fluid pressures exceeding the pressure 

that would be produced by the weight of overlying water 

(hydrostatic pressure), and some pore pressures approach 

the pressure that would be produced by the weight of both 

overlying water and overlying sediment (geostatic pressure). Such a condition of elevated pore fluid pressures is 

termed underconsolidation. 

Offshore of the Mississippi River Delta, the sea floor 

is nearly flat. Slopes of the delta-front rarely exceed 1.5°; 

and in the interdistributary bays, bottom slopes are generally less than 0.5° and are rarely greater than 0.2°. At the 

shelf break, which generally is in water depths of 150 to 200 

m, the slopes increase slightly, averaging 1. 7° to 2. 2°. In 

Slope Failures in an Area of High Sedimentation Rate: Offshore Mississippi River Delta 79 
general, the bottom is characterized by many radial submarine gullies in water 10 to 75 m deep and by broad, flat 

terraces seaward to water depths as great as 180 m. These 

submarine gullies were first described and illustrated from 

the Mississippi Delta by Shepard (1955). At the shelr'edge 

and on the upper continental slope, abrupt scarps occur on 

the sea floor; some localized scarps are as high as 45 m with 

slopes that approach 2.5° to 3.0°. 

The major characteristics of the Mississippi River 

Delta and of its continental shelf influence the stability of 

marine sediment. These characteristics include (1) high 

rates of sedimentation, which result in excessive sedimentary loading; (2) deposition of dense sand and silt over weak 

clay, causing differential loading of the underlying sediment; (3):high water contents, generally low strengths, and 

underconsolidation of the deltaic deposits; (4) biochemical 

degradation of organic material in the deposits, which 

results in. formation of large quantities of gases in the 

sediment; and (5) annual passage of winter storms or 

hurricanes, which result in cyclic wave-loading processes. 

Notably, the sea-floor slopes are very gentle. 

TYPES AND SOURCES OF DATA 

Maps depicting various aspects of the near-surface 

marine geology of a part of the delta region were prepared 

from high-resolution geophysical and sidescan-sonar surveys, which were supplemented by many bottom samples 

and sediment borings (location of study area in fig. 1; see 

Coleman and others, 1980, for a complete set of the maps). 

Most of the geophysical surveys used high-frequency (110 

kilohertz (kHz)) acoustic sources for bathymetry, 3. 5 to 12 

kHz sources for near-surface (less than 60 m) subbottom 

penetration, and lower frequency (50 to 100kHz) sparkers 

or other acoustic sources for deeper-subsurface penetration 

(generally on the order of 150-250 m). Sidescan-sonar data 

were. acquire~ with both standard analog systems and a 

digital system. Digital data were processed for corrections 

of ship speed and slant range; conventional sidescan-sonar 

data were distorted in both ship speed and slant range, and 

manual corrections were applied to these data. 

MEASURED CHANGES IN BATHYMETRY 

Sediment-accumulation rates are an important aspect 

of interpreting the frequency of sediment mass movements; 

and the delta region, because of the importance of navigation at the mouths of Southwest and South passes, has been 

the subject of many bathymetric surveys. Although many 

maps are available, only a few cover the entire region 

offshore of the Mississippi River Delta. Three accurate 

bathymetric charts have been prepared from data obtained 

in 1872 to 1874, 1940, and 1977 to 1979 (Coleman and 

others, 1980). 

Data extracted from these maps can be used to draw 

regional cross sections that show the magnitude of mass 

movement that has taken place during a relatively short 

time. Figure 2 illustrates two such cross sections off South 

Pass, Mississippi River Delta. In profile A-A', large 

amounts of sediment can be seen to have accumulated 

between 1872 and 1979 in water depths of 90 to 180 m. The 

accumulation of such large quantities of sediment in these 

water depths cannot be explained by settlement out of the 

water column and, thus, probably represents deposition by 

submarine mass movement. In profile A-A' off South Pass, 

some 30 m of sediment has accumulated in 120 m of water 

during the 100-year span. In addition, ·the shelf-edge break 

in slope has built seaward or prograded some 2.5 1an: 

Profile B-B' (fig. 2), which is west of South Pass, 

shows still another type of phenomenon revealed by comparisons of the historic maps. Prior to 1874, a large mass of 

sediment, some 33m thick, accumulated in water depths of 

90 to 120m. During the period 1872 to 1940, either current 

erosion or, more likely, slope failure resulted in removal of 

this sediment. If the removal was by slope failure, a large 

volume of sediment probably moved "en masse" well 

beyond the shelf-edge break before 1940. Deeper water 

surveys are not available to confirm this interpretation. 

Since 1940, sediment has again begun to accumulate in 

these water depths by mass-movement processes. 

Even more specific information on accumulation 

rates can be obtained in those few areas that have been 

mapped more frequently. Figure 3 illustrates such examples 

in South Pass Block 30. On the earliest map (1875), points 

of reference were chosen, in a topographic gully and on an 

adjacent ridge. When these points are then transferred to 

each· subsequent map, a profile can be drawn that shows 

how the water depths have changed with time; Two 

examples are shown (fig. 3). Water-depth changes on the 

ridges indicate nearly continuous shoaling or accumulation 

with time. The average rate is approximately 0.6 m per 

· year. In the gullies, however, periods of rapid infilling were 

followed by peri'ods of sediment loss. Accumulation rates 

were as much as 5 m per year. Usually the sediment loss 

takes place in less than a year's time. This episodic infilling 

and excavation of the gullies indicates the frequency of 

mass-movement processes. Comparison of many such sites 

around the delta indicates that gully evacuation takes place 

primarily after the gully totally infills and is nearly level 

with the adjacent ridge. Gully excavation (slope failure) 

usually follows major floods on the river. Sediment filling 

the gullies has accumulated extremely rapidly and, therefore, possesses h~gh pore-water pressures and requires little 

additional stress to cause failure. Accordingly, once a 

landslide gully forms, it maintains episodic activity for long 

periods, whereas adjacent ridges maintain stability during 

the same time interval. 

80 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
Figure 1. Modern "bird's foot" of the Mississippi River 

Delta and the extent of the survey area offshore of the 

delta. "Complete coverage" indicates total overlapping 

sidescan-sonar imagery areas; "Incomplete coverage" 

designates areas where survey lines were spaced more 

than 700 meters apart, requiring considerable interpretation; "No data" indicates those areas having no survey 

coverage or areas where survey grids were so widely 

spaced that spatial interpretation was impossible. Profiles 

8-8' and A-A' are given in figure 2. 

Slope Failures in an Area of High Sedimentation Rate: Offshore Mississippi River Delta 81 
A AI 

Or===.~~~~---------.----------.---------.---------~--------~------

---...:::::.:.·~·.:·~·~~··········· ........ . 

50 

E 

I 100 1-

a.. 

w 

0 

150 

' ' .......... ..... __ ----....... 

--

._.::=-------oo 

0.2° 

1977-1979 

1940 

1872-1874 

2.0° DISTANCE (meters) 200 

0 1000 2000 3000 4000 5000 6000 

8 8 1 

Or----------,----------.-----------.----------.----------.-----------~---------, 

.5 

I 

I50 

~ 100 0 

------------

1977-1979 

1940 

1872-1874 

-................... 

...... -... 

.... .... 

' ' ' 150 ' DISTANCE (meters) ' ' ' ' ·· .... 

0 1000 1000 3000 4000 5000 6000 7000 

Figure 2. Cross sections showing bathymetric changes between 1872 and 1979. Profile locations shown in figure 1. 

SEA-FLOOR MORPHOLOGY 

Sidescan-sonar data obtained from late 1977 through 

early 1979 show a wide variety of submarine landslide 

features and other sea-floor irregularities (fig. 4). Because 

of the dynamic nature of the ill.~ss-movement processes, 

additional changes will undoubteJly take place. 

The following principal features are found on 

sidescan-sonar mosaics: 

1. Collapse depressions and bottleneck slides. Collapse 

depressions and bottleneck slides (a specific type of 

mudflow illustrated in fig. 5) are present primarily in the 

shallow-water areas of the interdistributary bays and 

slightly beyond the bays to water depths of 9 to 15 m. 

They are most commonly associated with slopes ranging 

from less than 0.1 o to 0.4 ° and show a spectrum from 

small rounded collapse features on lower slopes to more 

elongate bottleneck slides on the .steeper slopes. In areas 

where they are most common, sedimentation rates are 

slower than rates elsewhere in the· delta. Collapse 

depressions range in diameter from 35 m to more than 

150 m and have length-to-width ratios of 1.0 to 1.5. 

Typically, the depressions are bounded by curved or 

near-circular scarps as much as 3m high, within which 

the bottom is depressed and filled with irregular blocks 

or clasts of sediment (fig. 6). 

Bottleneck slides (figs. 5B and 6) occur on slightly 

steeper (0. 2° to 0.4 °) slopes within the interdistributary 

regions and on the continental slope. These features are 

similar morphologically to collapse depressions, but the 

boundary scarps do not form a totally closed perimeter 

around the instability. Rather, they have narrow openings at the downslope margin, through which flowing 

sediment is discharged over surrounding intact sea floor. 

2. Peripheral slumps. Downslope movement of large sediment masses commonly begins near the distributary 

mouths of the river, where bottom slopes range from 

0.2° to 1.0°. In many places, major scarps having 

distinctive curved or curvilinear plan views scar the sea 

floor. The scarps range in height from 3 to 6 m and have 

local slopes of 1° to 4°. In many areas, these scarps give 

the sea floor a stairstepped appearance in profile view 

82 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
WATER DEPTH (fi1) 

35 30 25 

• 

1950 

• 

I 

20 15 

Gully ......._ ~, / / 

/ 

10 

/ 

/ 

/ 

3 35 30 25 20 .15 

/ . / 

/ Gully-.,: 1 

.... I _ -.., 

10 

1940 • • •\ Ridge . ..... . I 

1930 • 

• 

• 1920 

w 

t- • 

<( 

0 

1910 

I I 

I 

I 

I 

I 

I 

I 

I 

• I 

I 

\ 

\ 

\ 

I 

•J 

I 

I 

I 

'? 

\:, . 

'} """' . 

i \ Formation of 

! \ new gully I \ I \ 

I ' I \ 1900 t---------7~---+--------- I" ' 

I . 

-,:..,' 

1890 

/ 

/ / 

.()' 

<. ..... ...... 

,..--

1!./ / 

I / 

I 

./ 

/ 

'---, 

--· 

/ 

1880 • Major river flood r_ 

1870~----~--~----~--~----~----~ 

Figure 3. Sediment accumulation in mudflow gullies and adjacent ridges in South Pass Block 30 during the period from 

1874 to 1953. A dot at the left-hand margin denotes that a map is available for .that peri_od. Squares represent time~ of 

major flood. 

(figs. 5C and 7). Although movement rates are difficult 

to document, repeated surveys suggest that blocks can 

move downslope more than 1.5 km in a 1-year period. 

3. Mudjlow gullies. Extending radially seaward from each 

of the distributaries, in water depths of 6 to 90 m, are 

major elongate systems of sediment instabilities referred 

to as mudflow gullies (fig. 5D). Sidescan-sonar records 

(fig. 8) from an extremely disturbed area of slumped 

topography high on the submarine delta. Each gully has 

a clearly recognizable area of rotational instability or 

slumping at its upslope margin. Mudflow gullies are the 

most common type of sediment instability fronting the 

Mississippi River Delta. 

Each mudflow gully possesses a long, sinuous, 

narrow chute or channel that links a depressed, hummocky source area on the upslope margin to composite 

overlapping dt?positi9ii'al mudflow lobes on the seaward . ., ,I . 

end (fig. 5D). The slopes along the sides of the gullies 

range from less .than 1 o to as high as 1 ~o. ·Most of ~he 

gullies extend downslope approximately at right angles 

to the depth contours and may be more than 75 km long. 

Th~ widths of th~ individual gullies range fro~· 20 m to 

550 m. 

Elongate chutes of this type are similar morphologically to subae~al debris flows and sqme types of 

subaerial mudflows. The chutes or channels generally 

emanate from upslope slump zones and constitute transport conduits for flowing sediment, together with dis:-

placed blocks of yarious sizes. 

4. Mudjlow lobes .. At the seaward or downslope ends of t))e 

mudflow gullies, extensive areas of irregular bottom. 

topography are c9mposed of blocky disturbed sedirrierit. 

Slope Failures in an Area of High Sedimentation Rate: Offshore Mississippi Rive.r Delta . · 83 
COLLAPSE DEPRESSION 

MUD DIAPIR 

INCIPIENT MUD DIAPIR 

OVERLAPPING TOE LOBES 

UNDISTURBED SEA FLOOR 

PRE~URE RIDGES I DEEP NO~L FAULT 

I 

MUD/GAS VENTS 

l 

CONTINENTAL SLOPE 

MUD DIAPIR 

l 

-300m 

l 

Figure 4. Schematic block diagram showing the relationship of the various types of submarine sediment instabilities. 

In plan view, this discharged sediment is arranged into 

widespread overlapping lobes or fans (fig. 9). Each lobe 

is composed of two major morphological features: an 

almost flat or gently inclined surface (less than 0.5°) and 

an abrupt distal slope representing the downslope toe (or 

seaward limit) of the displaced sediment. The toes range 

in height from only a few meters to more than 20 m and 

have slope as great as 7° to 10°. In plan view, the toes 

are generally curved, and adjacent lobes are separated 

from one another by reentrants. Because of the large 

number of gullies that front the present delta of the 

Mississippi Ri ver, the displaced sediment from adjacent 

gullies may coalesce, providing an almost continuous 

sinuous frontal toe 25 to 30 km seaward of the delta. The 

mudflow Jobes advance downslope at varying rates and 

probably move rapidly but episodically. Although no 

actual instantaneous rates have been measured, repeated 

surveys have shown as much as 5 km per year seaward 

advance of some mudflows. 

CAUSES OF SLOPE FAILURE 

One of the best documented examples of the occurrence of sediment instability in the Mississippi Delta was 

the failure of bottom-supported structures in South Pass 

Block 70 associated with the passage of Hurricane Camille 

in 1969 (Bea, 1971; Sterling and Strohbeck, 1973; Bea and 

others, 1983). Three of the eight platforms in the area 

failed; platform "B," in 100m of water, fell to the west and 

moved downslope about 30 m. Investigations included a 

comparison of bathymetric surveys obtained before and 

after the hurricane, a detailed sidescan sonar, diving, and 

underwater TV inspection of the fallen platform "B," and an 

above-water inspection of several adjacent platforms. The 

studies showed that waves with heights of 21 to 23 m 

occurred near the area accompanied by bottom relief 

changes of up to 12 m. At the site of platform "B," the sea 

floor dropped over 1.5 m (Bea and others, 1983). Major 

reductions in sediment strength occurred to depths in the 

sediment of 24 m (Sterling and Strohbeck, 1973). Consideration of these factors, along with the observation that piles 

failed in bending, rather than by pullout, led investigators to 

believe that sediment mass movements were a primary 

cause of the toppling of platform "B" and major damage to 

two nearby platforms. 

The suggested trigger for these movements is cyclic 

loading of the sea floor by passing waves (Henkel, 1970). 

As large waves pass, the crests produce elevated pressure 

on the sea floor, while the troughs produce lowered pressure. The passage of many waves loads the sea floor 

cyclically and gradually lowers the strength of the seabed 

sediment. Ultimately, a combination of intense wave loading , pre-existing low strength owing to underconsolidation 

and gas charging, and further strength reduction caused by 

cyclic loading causes the sediment to fail. On these gentle 

slopes there is a small component of gravitational stress that 

is directed downslope. A combination of this gravitational 

stress and the wave-induced stresses causes a part of the sea 

floor to move downslope. 

84 Submarine landslides: Selected Studies in the U.S. Exclusive Economic Zone 
A MAIN HEAD 

SCARP REVERSE SLOPE 

HUMMOCKY I g~~~~ B~OCK SURFACE 

8 

HEAD SCARP 

c CROWN CRACKS GRABEN 

0 

MULTIPLE 

HEAD SCARPS 

MULTIPLE 

CROWN CRACKS 

SHEAR PLANE HUMMOCKY 

BLOCK SURFACE 

I i 

MARGINAL 

SHEARS 

COMPOSITE TOE FANS/LOBES 

ELONGATE CHUTE WITH STEPPED PROFILE 

Figure 5. Schematic diagrams illustrating the morphology of (A) collapse depressions, (8) bottleneck slides, (0 

peripheral slumps, and (0) mudflow gullies and depositional mudflow lobes. 

Slope Failures in an Area of High Sedimentation Rate: Offshore Mississippi River Delta . 85 
Figure 6. Sidescan-sonar mosaic, illustrating several collapse depressions and a 

bottleneck slide. The grids are 25 meters apart and the mosaic covers a region of 

1.3X2.2 kilometers. (A) collapse depressions; (B) crown cracks; (C) bottleneck 

slide; (D) depositional lobe of bottleneck slide. 
· Figure 7. Conventional sidescan-sonar record, showing several slumps. Navigational fixes are 152 meters apart, 

and lateral timing lines are 25 meters apart. (A) shear planes showing up as scarps on the sea floor; (B) rotated block; 

(C) shrimp trawler's drag mark. 

There is also evidence that collapse depressions, 

bottleneck slides, and mudflow gullies have been active 

when there were no hurricanes (Prior and others, 1979). In 

particular, the creation of new mudflow gullies frequently 

accompanies major flood and sediment discharges from the 

Mississippi River. In the South Pass area, 15 new chutes 

followed the 1922 flood, 7 in 1927, and 10 were observed 

after the 1945 flood. Recent annual surveys have also 

shown large mudflow lobe advances; in the period 1975 to 

197 6, the toe of one lobe advanced a distance of 1. 9 km 

(Prior and Coleman, 1978). Repeated annual sidescan-sonar 

mapping since 1974 of particular chute areas has also shown 

bottom sediment changes associated with instability. Feature enlargement, offset of anchor scars and trawler drag 

marks on the bottom, as well as comparisons of identifiable 

blocks of sediment, indicate rates of movement up to 100m 

per year (Coleman and Garrison, 1977). 

SUMMARY 

Owing to high sedimentation rates, fine grain-size, 

and abundance of organic matter and gas, sediment deposited on the sea floor offshore of the Mississippi River Delta 

is extremely underconsolidated. Although the bottom slopes 

are very gentle, ranging from less than 0.5° to somewhat 

over 2° beyond the shelf-edge break, the occurrence of 

sediment instability features is widespread. In some secSlope Failures in an Area of High Sedimentation Rate: Offshore Mississippi River Delta 87 
Figure 8. Sidescan-sonar mosaic showing several landslide gullies. Grid is 25 X25 meters, and 

the mosaic is 1.1 X1.8 kilometers. Water depths are approximately 20 meters at the top of the 

figure and 35 meters at the lower end. (A) source areas; (B) mudflow gully; (C) narrow incised 

gully; (D) sidewall-instabi lity slides; (E) large erratic blocks in gully floor. 

88 Submarine landslides: Selected Studies in the U.S. Exclusive Economic Zone 
Figure 9. Sidescan-sonar mosaic showing multiple overlapping mudflow depositional lobes. Grid tick marks are 

25 meters apart. (A, B, and C) mudflow lobes; (D) erratic blocks; (E) pressure ridges. 

Slope Failures in an Area of High Sedimentation Rate: Offshore Mississippi River Delta 89 
J 

Ze•o d;•tu•bance I ~Slide scar 

/'

() t ..... -- 0 Undisturbed 0 ·~g.~} ! Reef 

0 5 

I I I 

KILOMETERS 

Figure 10. Map illustrating the distribution of slope-failure features on the delta-front of the Mississippi River Delta (from 

Coleman, 1988). 

tions, most of the sea floor is covered by these features (fig. 

10). The slope failures have varying morphologies. Collapse depressions and bottleneck slides occur in water 

depths shallower than 15 m on slopes less than 0. 5°. On 

steeper slopes near the distributary mouths of the river are 

peripheral slumps, which give these locations a stairstepped 

appearance. Farther seaward are major elongate systems of 

mudflow gullies that carry sediment downslope into mudflow lobes. These gully-lobe systems are the most common 

type of sediment instability in the Mississippi River Delta. 

The mass-movement features of the Mississippi River 

Delta are very dynamic and have shown considerable 

change over the last century. The gullies infill slowly and 

then evacuate rapidly, often during or after major floods 

(fig. 3). Hurricanes can also cause sea-floor slope failures, 

as shown dramatically during Hurricane Camille in 1969. 

Documented major changes to the sea floor occurred and 

caused considerable damage and destruction to offshore 

drilling structures. 

ACKNOWLEDGMENTS 

This section is a direct product of the Mississippi 

Delta Project, a joint research program between LSU and 

the USGS, but is also, in a broader sense, a result of the 

cooperation of government agencies, academic institutions, 

and the offshore petroleum industry. We extend our thanks 

to members of the Minerals Management Service in Metairie, La., (formerly part of the USGS) who supplied the 

major part of the data for this study, especially to Dick 

Scrivener and Bill Sweet of that office, who assisted greatly 

90 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
in the project, and to U.S. Bureau of Land Management 

personnel in New Orleans, where Jack Rebman and Doug 

Elvers were most helpful. Special thanks go to the many oil 

company geologists, engineers, and consulting firms who, 

through the contribution of data and interpretations, helped 

to fill in some of the blank areas in our coverage. The 

authors thank Monty Hampton and James Booth for helpful 

reviews of this section. 

REFERENCES CITED 

Bea, R.G., 1971, How sea floor slides affect offshore structures: 

Oil and Gas Journal, v. 69, p. 88-92. 

Bea, R.G., Bernard, H.A., Arnold, Peter, and Doyle, E.H., 

1975, Soil movements and forces developed by waveinduced slides in the Mississippi Delta: Journal Petroleum 

Technology, v. 28, p. 500-514. 

Bea, R.G., Wright, S.G., Sircar, Partha, and Niedoroda, A.W., 

1983, Wave-induced slides in South Pass Block 70, Mississippi Delta: Journal of Geotechnical Engineering, v. 109, p. 

619-644. 

Coleman, J .M., 1988, Dynamic changes and processes in the 

Mississippi River delta: Geological Society of America Bulletin, v. 100, p. 999-1015. 

Coleman, J.M., and Garrison, L.E., 1977, Geological aspects of 

marine slope stability, northwestern Gulf of Mexico: Marine 

Geotechnology, v. 2, p. 9-44. 

Coleman, J.M., Prior, D.B., and Garrison, L.E., 1980, Subaqueous sediment instabilities in the offshore Mississippi River 

delta: Bureau of Laad Management, Open-File Report 

80--0 i, 60 p. 

Fisk, H.N., McFarlan, E.H., Jr., Kolb, C.R., and Wilbert, L.J., 

Jr., 1954, Sedimentary framework of the modem Mississippi 

Delta: Journal of Sedimentary Petrology, v. 24, p. 76-99. 

Henkel, D.J., 1970, The role of waves in causing submarine 

landslides: Geotechnique, v. 20, p. 75-80. 

Milliman, J.D. and Meade, R.H., 1983, World-wide delivery of 

river. sediment to the oceans: Journal of Geology, v. 91, p. 

1-21. 

Prior, D.B., and Coleman, J.M., 1978, Disintegrating, retrogressive landslides on very low angle subaqueous slopes, Mississippi Delta: Marine Geotechnology, v. 3, p. 37-60. 

Prior, D.B., Coleman, J.M., Suhayda, J.N., and Garrison, L.E., 

1979, Subaqueous landslides as they affect bottom structures: 

Proceedings, Conference on Port and Ocean Engineering 

under Arctic Conditions, Trondheim, Norway, v. 2, p. 

921-933. 

Shepard, F.P., 1955, Delta front valleys bordering the Mississippi 

distributarie: Geological Society of America Bulletin, v. 66, 

p. 1489-1498. 

Sterling, G.H., and Strohbeck, E.E., 1973, The failure of the 

South Pass 70 "B" platform in Hurricane Camille: Proceedings, 6th Offshore Technology Conference, Paper No. 1898, 

p. 719-730. 

Slope Failures in an Area of High Sedimentation Rate: Offshore Mississippi River Delta 91 
Salt Tectonics and .Slope F~i,lure i.n an Area of Salt 

Domes in the Northwestern :Gulf of Mexico 

By B.A. McGregor1

, R.G. Rot~well , N.H. Kenyon2, and D.C .. Twichell 

INTRODUCTION AND REGIONAL GEOLOGIC 

SETTING 

The northwestern part of the Gulf of Mexico (fig. 1) 

is an ~ea where an enormous amount of sediment has been 

depo.sited during the last 63 million years (the Cenozoic 

Era). The sedimentary deposits consist of a series of deltas 

on the continental shelf with associated submarine fans in 

the deep Gulf of Mexico basin. The centers of sediment 

deposition in this area migrated eastward through the 

Cenozoic from the Rio Grande River area of southern Texas 

to the presently active Mississippi River in the north central 

Gulf of Mexico (Humphris, 1984). 

The sediment of the outer continental shelf and 

continental slope in the northweste~ Gulf of Mexico has 

been subject to a unique form of deformation due to a thick 

layer of salt that is buried by this Cenozoic sediment. The 

thick layer of salt was deposited during the Jurassic period 

(205 to 138 million years ago) when the Gulf of Mexico 

ocean basin was forming (Martin, 1978). The loading of the 

Cenozoic sediment on the salt caused the salt to deform and 

to flow plastically. As the salt flows under the load of the 

overlying sediment, numerous fingers of salt (diapirs) move 

upward throJ.Igh the sediments. When these salt fingers 

reach the sea floor, they create mounds known as salt 

domes . .The morphology of the continental slope in ·the 

northwestern Gulf of Mexico is extremely comple~ as a 

consequence of the numerous salt domes that have 

deformed the sea floor (fig. 2). 

Mass wasting is often associated with areas of rapjd 

sedimentation (Lee, .Schwab, and Booth, this report; Coleman and others, this report; Schwab and Lee, this report) 

.and \\:'ith areas subject to active salt deformation (Popenoe, 

Schmuck, and Dillon, this report). A major landslide, the 

East Breaks submarine landslide, has been identified on the 

continental slope in the northwestern Gulf of Mexico using 

seismic-reflection profiles (Lerner, 1969), but these seismic 

data do not define the extent of this landslide.deposit nor the 

relative importance of rapid sedimentation and salt deformation in the formation of this landslide. More control on 

the sea-floor morphology and sea-floor geology was needed 

to address this question. A detailed understanding of the 

surficial geology ~f this morphologically complex area 

became available with the collec~ion of GLORIA (Geolog1 U.S. Geological Survey, Reston, Va. 2 Institute of Oceanographic Sciences, Godalming, Surrey, U.K. 

ical LOng-Range Inclined Asdic) sidescan-sonar data by the 

U.S. Geological Survey (USGS) (EEZ-SCAN 85 Scientific 

Staff, 1987). The GLORIA data in concert with bathy'metric 

measurements collected along closely spaced ship tracklines 

provide~ a clearer view of the surface expression of this 

landslide ·deposit and it~ association with the sea-floor 

morphology. ·. . _ 

The continental slope in the northwestern Gulf of 

Mexico is thought to contain significant amounts of oil and 

gas resources and is a major deep-water exploration frontier. Slope failures are known to destroy or damage oil rigs 

and pipelines placed on the sea floor with ~ potential for an 

accompanying loss of human life (Lee, Schwab, and Booth, 

this report). The ability to identify and predict slope failure 

is- an important aspect in locating and designing sea-floor 

structures. As interest in exploring the deeper water areas of 

the Gulf of Mexico increases, a corresponding need to 

understand the geologic hazards of the area must be 

addressed. The GLORIA sidescan-sonar images are proving 

useful as a means to target potentially dangerous areas for 

more detailed study and to predict how geohazards such ~s 

landslides and salt deformation might affect man's activities 

on the sea floor. 

EAST BREAKS SUBMARINE LANDSLIDE 

A bathymetric map was compiled using the GLORIA 

sidescan-sonar images. in concert with bathymetric information collected along the survey tracklines. A perspective 

view of the bat~ymetry of the study area is shown in figure 

2. The sea floor has a regional gradient of 1 o to 3° on the 

upper continental slope (water depths of 200 to 1,000 

meters (m)) that decreases to about 0.5° on the middle slope 

(water depths of 1,000 to about 1,500 m). The gradient 

again becomes steeper on the lower continental . slope 

between 2,000- and 3,000-m depths where the Sigsbee 

Escarpment is crossed (fig. 1). The Sigsbee Escarpment 

represents the seaward limit of the salt deformation. 

Salt domes are numerous on the upper and middle 

sections of the continental slope, have as much as 150-m 

relief, and disrupt the regional sea-floor gradient (fig. 2). 

Locally, on the ,flanks· of salt domes the sea-floor gradient 

exceeds 10°. The salt domes do not have a systematic 

distribution and give the continental slope a mottled appearance of domes and depressions· (fig. 2). Submarine canyons, which are common on most continental slopes such as 

off the Eastern or Western United States (for example, 

92 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
0 

0 

200 

I 

I 

100 

400 

I I 

200 

I 

600 KILOMETERS 

I 

300 NAUTICAL MILES 

Figure 1. Index map of the Gulf of Mexico. Contours are in meters. The box outlines the study area. 

Booth and others, this report; O'Leary, this report; Carlson 

and others, this report), are absent or so completely disrupted by the salt deformation as to be unrecognized in this 

area. 

Combining the GLORIA data with the bathymetric 

data shows the relationship between the acoustic backscatter 

of the sea floor and the sea-floor relief (fig. 3). A particularly notable feature on the GLORIA image is a region of 

high backscatter (bright area) that correlates with the East 

Breaks submarine landslide that was initially identified by 

Lerner (1969) based on seismic-reflection profiles. The 

high-backscatter area corresponds with an area of hyperbolic echoes on seismic-reflection profiles (fig. 4). These 

hyperbolae indicate a rough, blocky sea floor. GLORIA 

images show that the landslide covers an area of 2,250 

square kilometers (km2

) on the upper and middle continental slope (fig. 3). The head of the landslide is located in 

approximately 200m of water near the shelf edge. The body 

of the landslide can be divided into two lobes. The eastern 

lobe extends 70 km downslope to a water depth of 1,350 m 

and covers an area of 650 km2 . It is separated from the 

western lobe by a topographic high-a salt dome. The 

western lobe covers 1,600 krri2 and extends 110 km 

downslope to a water depth of approximately 1 ,300 m 

where it is channeled between two salt domes. Downslope 

of these salt domes, the landslide continues as fingerlike 

projections to a maximum water depth of 1,600 m (fig. 3). 

The thickness of the landslide deposit, based on seismicreflection studies varies from an estimated 20 m for the 

eastern lobe to· over 100 m for the western lobe (Lerner, 

1969; Woodbury, 1977; EEZ-SCAN 85 Scientific Staff, 

1987). A maximum thickness of 180 m occurs on the mid 

slope where the salt diapirs have constricted movement of 

the slide and caused ponding. 

The East Breaks landslide fills in bathymetric lows 

around the salt domes. The ·deposit does not have bathymetric expression that can be resolved by available bathymetric data (fig. 2). Thus, it is only by combining the 

bathymetry and GLORIA image (fig. 3) that the extent of 

this deposit can be clearly defined. The high backscatter 

that corresponds to the area of the landslide debris is 

believed to be caused by fine-scale surface irregularities as 

shown by the hyperbolae on the seismic-reflection profiles 

(fig. 4). This surface roughness presumably was formed by 

deformation during the downslope movement. Sediment 

cores in the landslide show contorted beds of sand and mud 

(Woodbury and others, 1978). Sand and mud layers are 

typical of deltaic sediments, and imply an original shallowwater source for these sediments. 

ACE OF SLOPE FAILURE 

The East Breaks landslide is classified as a mass flow 

because :of the degree of disintegration shown by the 

Salt Tectonics and Slope Failure in an Area of Salt Domes in the Northwestern Gulf of Mexico 93 
50 km 

Figure 2. Perspective view of the bathymetry of the study area on the continental slope in the northwestern Gulf of 

Mexico (see fig. 1 for location). The bathymetry was compiled using the GLORIA sidescan-sonar images in concert with 

bathymetric information collected along the survey tracklines. 

deposits. Based on examination of core samples, the mass 

movement can be further classified as a sandy debris flow. 

Interpretation of seismic-reflection profile data from the 

East Breaks landslide shows that slope failure has occurred 

repeatedly in the area (Lerner, 1969). During geologic 

periods when sea level was lower and the shelf was 

subaerially exposed, large deltas built the shelf edge seaward and sediment was deposited on the upper continental 

slope. The source area for the East Breaks landslide is the 

seaward edge of a delta that was constructed at the shelf 

edge during the last low stand of sea level 15,000 to 20,000 

years ago (fig. 5; Suter and Berryhill, 1985). Cores from the 

East Breaks landslide recover sediment that is interpreted to 

be deltaic in origin (Woodbury and others, 1978) and 

indicate that the landslide is younger than 15,000 to 20,000 

years. We speculate that this land lide occurred 15,000 to 

20,000 years ago when sea level was lower. The delta likely 

would have contained underconsolidated sediment subjected to cyclic loading by storm waves. Cyclic loading by 

storm waves on modern deltas is an important triggering 

mechanism for slope failures (Lee, Schwab, and Booth, this 

report; Coleman and others, this report; Schwab and Lee, 

this report). This slope failure probably is not younger than 

the la t period of low sea level because similar types of 

sediment on the steep flanks of salt domes do not show any 

evidence of mass movement. 

IMPLICATIONS 

The combination of the GLORIA and bathymetric 

data reveals that the salt domes do not appear to have played 

a major role in the formation of this landslide. Scarps on the 

sides of the salt dome and landslide depo its originating 

from the salt domes generally cannot be resolved by the 

GLORIA images and are presumed to be either small or 

nonexistent. Accordingly, we suggest that the salt domes 

have not been a significant source of failed sediment. 

Instead, the salt domes appear to have contributed only to 

haping the pathway of the landslide. 

The flanks of salt domes have proven to be effective 

petroleum traps because the sedimentary layers are bent up 

and truncated as the salt penetrates the overlying sediment. 

Landslides do transport coarse-grained sediment to the 

continental slope, and these coarse-grained deposits could 

prove to be effective reservoirs for petroleum. Thus, understanding the distribution of these deposits, in addition to 

showing the geologic hazards of the area, may also prove to 

94 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
Figure 3. GLORIA sidescan-sonar data are draped over the bathymetry data (fig. 2). Areas of high backscatter (light areas) 

are the surface of the East Breaks submarine landslide. 

800 

E 

~ 1000 n. 

w 

0 

0:: 

w 

~ 

<( 

~ 

1200 

1400 

6 

- I 

~----,!LANDSLIDE -...;:----~ 

Figure 4. A high-resolution subbottom profile (10-ki lohertz frequency) showing surface irregularity (hyperbolic echoes) 

on the western lobe of East Breaks landslide. The topographic high on right side of profile is the flank of a salt dome (see 

fig. 5 for location of the profile). 

Salt Tectonics and Slope Failure in an Area of Salt Domes in the Northwestern Gulf of Mexico 95 
EJ Submarine moss-flow deposits (probably sandy debris flows) 

~ Blanket mud deposits (mud tubidites + hemipelagic mud) 

.(f"T' Scarps 

H Location of profile shown in Figure 4 

Figure 5. Sketch of the major geologic features in the northwestern Gulf of Mexico. 

be a useful depositional model in the exploration for 

petroleum in this large salt-dome province. 

REFERENCES CITED 

EEZ-SCAN 85 Scientific Staff, 1987, Atlas of the U.S. Exclusive 

Economic Zone, Gulf of Mexico, U.S. Geological Survey 

Miscellaneous Investigations Series I-1864--A, 104 p. 

Humphris, C.C., Jr., 1984, Interrelations of Tertiary deposition, 

growth faulting, and salt movement, northern Gulf of Mexico, in Characteristics of gulf basin deep-water sediments and 

their exploration potential: Fifth Annual Research Conference 

Gulf Coast Section of Society of Paleontologists and Mineralogists, p. 50-51. 

Lerner, Peter, 1969, Salt tectonics and Pleistocene stratigraphy on 

continental slope of northern Gulf of Mexico: American 

Association of Petroleum Geologists Bulletin, v. 53, p. 

2431-2479. 

Martin, R.G., 1978, Northern and eastern Gulf of Mexico 

continental margin: stratigraphic and structural framework, in 

Bouma A. H., Moore G.T., and Coleman, J.M., eds., Framework, facies, and oil-trapping characteristics of the upper 

continental margin: American Association of Petroleum 

Geologists, Studies in Geology No. 7, p. 21-42. 

Suter, J.R., and Berryhill, H.L., 1985, Late Quaternary shelfmargin deltas, northwestern Gulf of Mexico: American Association of Petroleum Geologists Bulletin, v. 69, p. 77-91. 

Woodbury, H.O., 1977, Movement of sediment on the Gulf of 

Mexico continental slope and upper continental shelf: Marine 

Geotechnology, v. 2, p. 263-273. 

Woodbury, H.O., Spotts, J.H., and Akers, W.H., 1978, Gulf of 

Mexico continental-slope sediments and sedimentation, in 

Bouma A.H., Moore G.T., and Coleman, J.M., eds., Framework, facies, and oil-trapping characteristics of the upper 

continental margin: American Association of Petroleum 

Geologists, Studies in Geology No.7, p. 117-137. 

96 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
Slope Stability in Regions of Sea-Floor Gas 

Hydrate: Beaufort Sea Continental Slope 

By R. E. Kay en and H .J. Lee 

INTRODUCTION 

The continental slope of the Alaskan Beaufort Sea is 

disrupted by an apparently continuous area of submarine 

landslides (Grantz and Dinter, 1980). The zone of slope 

instability extends from water depths of 200 to 400 meters 

(m) at the shoreward edge, to depths in excess of 2,000 m 

(fig. 1 ). The thickness of displaced sediment typically is 

100 to 400 m. The area of slope failure largely lies in a 

region underlain by marine gas hydrate (sediment bonded 

by a solid mixture of water and methane). Gas hydrate has 

156' 154' 150' 

72' 

been interpreted from seismic-reflection profiles (Grantz 

and Greenberg, 1981; Grantz and others, 1981), and 

extends down from near the sea floor to at least 700 m 

beneath the sea floor between the 400- and 2,800-m water 

depths (fig. 2). For many of the landslides, the basal shear 

surface corresponds approximately with the base of the gas 

hydrate. For example, the mass-movement deposit in figure 

3, profiled in the eastern Alaskan Beaufort Sea, is bounded 

upslope by a scarp and appears to share its basal shear 

surface with a strong gas-hydrate reflector. Many of the 

71' 

70' 

71' 

158' ~~~~~~~~~~~~~~~~~~ 

Study 

<t 

Figure 1. The zone of large landslides on the continental margin of the Beaufort Sea, interpreted from seismic-reflection 

profiles (from Grantz and Dinter, 1980). 

Slope Stability in Regions of Sea-Floor Gas Hydrate: Beaufort Sea Continental Slope 97 
72' 

71' 

158' 

71' 

70' 

Figure 2. The region of sea-floor gas hydrate on the continental slope extends from approximately 400 meters water depth 

near shore to over 2,000 meters. The presence of gas hydrate within sea-floor sediment is interpreted from a strong 

bottom-simulating reflector in seismic-reflection profiles (from Grantz and Dinter, 1980). 

observed slope failures probably were triggered during 

Pleistocene sea-level fall in response to partial disassociation of the gas hydrate. 

THE FORMATION OF GAS HYDRATE 

Solid-solution gas hydrate forms beneath the sea floor 

from natural gas and water within a stability field determined by temperature and pressure (fig. 4). Formation can 

occur in both polar and nonpolar regions due to the 

normally cold bottom-water conditions (typically 2° to 5°C) 

found on continental slopes throughout the world. Hence, 

most slopes found in water deeper than 400 to 500 m have 

a sea-floor pressure and temperature environment within the 

stable phase for gas-hydrate formation. To produce a 

marine gas hydrate, water molecules bond to form a cubic 

lattice structure within which individual gas molecules, 

primarily methane, are caged (Kvenvolden and McMenamin, 1980). The resulting structure densely packs natural 

gas molecules in a configuration that is far tighter than 

would exist in free gas at the same pressure and temperature. For example, at standard temperature and pressure 

approximately 170 volumetric units of free methane gas are 

compressed and stored in a single volumetric unit of gas 

hydrate. 

Conditions conducive to formation of gas hydrate lie 

below a phase boundary (left side of fig. 4). The actual 

depth and temperature conditions at a representative location on the Beaufort Sea slope (profile A-A') intersect the 

phase boundary diagram and show where gas hydrate is 

stable beneath the sea floor. Because temperature increases 

relatively rapidly with depth in the sediment due to the 

geothermal gradient, gas hydrate can form only within the 

upper section beneath the sea floor. The depth beneath the 

sea floor at which the pressure and temperature conditions 

reach the phase boundary determines the location of the 

base of the gas-hydrate zone, which can be seen readily 

in seismic-reflection profiles (fig. 3, also see Carpenter, 

1981). The reflection from the base of the gas hydrate 

approximately mimics the bottom bathymetry and commonly is termed a bottom-simulating reflector or BSR 

(fig. 3). 

EFFECT OF SEA-LEVEL FLUCTUATIONS ON 

GAS HYDRATE 

During ice ages, a worldwide drop in sea level that 

occurs in response to the formation of massive glaciers on 

land causes reduced total pressure within sediment beneath 

the sea floor. Normally, pore pressures within the hydratefree sediment dissipate very rapidly during a drop in sea 

level to maintain essentially hydrostatic conditions (fig. 

SA). In regions of gas hydrate, however, the lowering of sea 

level reduces pressure along the gas-hydrate base, initiating 

disassociation (melting) and consequently releasing large 

volumes of bubble-phase gas into the sediment directly 

beneath the hydrate base. The dense concentration and large 

quantity of natural gas within a gas hydrate is such that 

disassociation of only a small part of the base elevates 

pressures back into equilibrium phase-boundary conditions. 

That is, as sea level drops, gas-augmented pore-water 

pressures near the phase boundary remain almost as great as 

they were when sea level was high. 

Therefore, as sea level falls, the gas-hydrate base 

tends to disassociate and retreat upward in the sediment 

column to a new position of lower equilibrium pressure and 

temperature (Field and Kvenvolden, 1985; 1986), but 

elevated pore-water pressures at the base, trapped beneath 

98 Submarine landslides: Selected Studies in the U.S. Exclusive Economic Zone 
N s U.U 

MUL TIC~ANNEL SEISMIC-REFLECTION PROFILE 

CJ) 1.0 a: 

w ,..... ...... ()W 

(1)~ 

cno 

'-' l() 

w~'--

~~ 2 0 -w I- .... 

<t: 

....J~ 

w-

>~ <(a: 

a:& 

l-<t: 30 

>~ <(<t: 

~6 .w 

Oo 

~6 '-t J f-0 

w 

CJ) 

5.0 

Figure 3. The landslide revealed on this seism ic-reflection 

profile in the eastern Beaufort Sea displays a 100-meterhigh headwall scarp at the shelf-edge break that defines 

the landward edge of the slope failure (data from Grantz 

and Greenberg, 1981). Seaward, the hummocky irregular 

topography, characteristic of the disrupted surface of a 

the largely impermeable gas hydrate above, oppose further 

disassociation. As a result, excess pressures preserve a base 

that is deeper in the sediment column than the base would 

be if these pressures did not exist. 

A predicted pore-water pressure profile through the 

sediment column at the completion of a large drop in sea 

level is presented in figure 58. Because of disassociation, 

hydrodynamic pressures are elevated at the gas-hydrate base 

with respect to the hydro tatic profile developed in unhydrated sediment as seen in figure SA. These excess pressures compromise the stability of the slope by reducing the 

normal stress between individual sediment grains and, 

mass-movement deposit, extends to a water depth of at 

least 1,900 meters. The basal shear surface of the slide 

appears to closely follow the strong bottom-simulating 

reflector that marks the base of the gas hydrate. Note: 

Vertical exaggeration (VE) is 3 times. 

consequently, the sediment's resistance (sediment strength) 

to shear stresses that tend to move the sediment downslope . 

Because the shear stresses acting downslope in this region 

are primarily caused by gravity acting on a sea-floor slope, 

the excess pore-water pressure may cause the slope under 

static conditions to become un table (Kayen, 1988). 

SlOPE STABiliTY DURING GAS-HYDRATE 

DISASSOCIA liON 

Evaluating the effect of gas-hydrate disassociation on 

slope stability requires quantifying the process of excess 

Slope Stability in Regions of Sea-Floor Gas Hydrate: Beaufort Sea Continental Slope 99 
Stability Field For Occurrence Of Methane Hydrate 

Methane Hydrate On The Continental Slope 

A A 

0 0 

2000 200 

Water 

400 

Vi 

co Vi (.) 

(/) 6000 600 ro 

a. Gas Hydrate And ~ .2 Methane Sediment <l> 

:s2 800 .s ~ Hydrate ..c 

Phase a. ::I <l> (/) 

(/) 10000 1000 0 <l> Boundary a: 

Geothermal 12000 1200 

Gradient Sediment 

Base Of Gas 

14000 1400 

Hydrate Base Of Gas 

Hydrate 

16000 20 A' A' 1600 

-30 -20 -10 0 10 

Temperature c Downslope Profile Of Continental Slope 

Figure 4. On the left is a gas hydrate pressure-temperature 

stability field (phase diagram) and on the right a schematic 

diagram of the continental slope. Formation of gas hydrate 

can occur only below the methane-hydrate phase boundary. Pressure (in kilopascals) and temperature (in degrees 

Celsius) conditions of profile A-A' through the sediment 

pore-pressure generation and dissipation. Numerical solutions were obtained by Kay en ( 1988) for the temporal and 

spatial distribution of excess pore-water pressures developed during gas-hydrate disassociation for a variety of 

possible sediment types, from coarse sand to fine-grained 

deposits of clay, beneath a gas-hydrate base. The numerical 

approach, a finite-difference model, was formulated around 

an averaged and linearized sea-level fall of 100 m, occurring over 9,100 years ago, representative of the last Pleistocene lowering of sea level as based on the work of 

Matthews (1973), Steinem and others (1973), Bloom and 

others (1974), and Chappell (1974). The model for predicting excess pore-water pressures beneath the base of the 

hydrate was constrained by previously determined gas 

hydrate thermodynamic data (K venvolden and others, 1984; 

Makogon, 1981; and Sloan, 1990). 

The model shows that a continuous fall of sea level 

causes a slow degradation of the gas-hydrate base. The level 

of excess pore-water pressure that develops near the base 

depends largely on the ability of the sub-base sediment to 

dissipate excess pressures. For example, clayey deposits 

have a low pore-fluid dissipation capability: excess pressures do not dissipate and, therefore, suppress further 

disassociation (fig. 6). Accordingly, at the completion of a 

sea-level fall, a large excess pressure is present. If little 

slope on the right are drawn on the phase diagram. 

Environmental conditions are well within the stability field 

at the sea-floor surface but intersect the phase boundary 

with burial, which determines the thickness of the hydrate 

zone in sediment. 

disassociation occurs, the excess pressure at the end of the 

drop in sea level will be nearly equal to the total hydrostatic 

pressure change, from high stand of sea level to low stand. 

In contrast, sand deposits can readily dissipate excess 

pressures, so the rate of disassociation along the base is 

relatively high and largely controlled by heat flow into the 

base. At the end of a large drop in sea level, a marked 

upward retreat of the base and relatively low excess 

pressure are expected. The numerical solution for a sandy 

sediment supports this prediction (fig. 7). The two sediment 

types, sandy and clayey, can be considered end-members 

with regard to pore-fluid dissipation. Thus, a sub-base 

clayey sediment will tend to develop greater excess porewater pressure than sandy sediment during gas-hydrate 

disassociation and would be more prone to slope failure. 

SLOPE STABILITY OF THE BEAUFORT SLOPE: 

CONCLUSIONS 

Studies of sediment sampled from the zone of slope 

failure of the Beaufort Sea continental slope indicate that 

the diffusion and physical properties are more like clayey 

sediment than sandy sediment (Kayen, 1988). Given the 

geometry and sediment physical properties of the contine100 Submarine landslides: Selected Studies in the U.S. Exclusive Economic Zone 
..c 

+-' 

0. 

Q) 

0 

Ctl 

+-' 

0 

1-

1.-_c 

Q) +-' 

+-' 0. 

Ctl Q) so 

+-' 

c 

Q) ..c 

+-' E 0. 

""0 Q) 

Q) 0 

CJ) 

B 

Gas Hydrate & Sediment 

Sediment Sediment 

Pore-Water Pressure ____ ..,....,..Pore-Water Pressure 

A) Without Gas Hydrate 

Figure 5. Pressure profiles before and after a 100- meter 

sea-level fall caused by a glaciation cycle are displayed for 

(A) normal sea floor without gas hydrate and (B) sea floor 

bearing gas hydrate. The dashed line marks the hydrostatic pressure profile before sea-level fall. The solid line 

marks the pressure profile at the completion of the 

sea-level fall. Without gas hydrate, sea-floor sediment can 

tal slope, the amount of excess pore-water pressure that will 

cause slope failure can be calculated using basic soil 

mechanics principles (Kay en, 1988). The range of pressures 

likely to trigger slope failure of a 100- and 200-m-thick 

sedimentary deposit on a 5° sea-floor slope, typical of the 

Beaufort margin, is presented in figure 8. Superimposed on 

this critical range of pore pressures are the pressures likely 

to be generated at a gas-hydrate base during disassociation 

induced by sea-level fall. 

For finer-grained sediment, dissipation of excess 

pressures through sediment pore space is probably sufficiently slow to cause slope failure on the continental slope 

of the Beaufort Sea. Stability is maintained only if excess 

pressures are rapidly vented away from a disassociating 

ba e, perhaps along fault planes. Stability may also be 

8) With Gas Hydrate 

fully adjust to the lowering of sea level, and a hydrostatic 

pore-water pressure profile can be maintained. The presence of gas hydrate prevents the normal reduction of 

pressures with sea-level fall because the lowering of 

pressure initiates disassociation along the base. This 

excess pressure reduces slope stability if the sea floor is 

inclined. 

maintained if gas hydrate forms in the pore space of 

strongly cemented sediment or in the joint space (cracks) of 

a consolidated rock mass. In both cases, the materials have 

high inherent cohesive strengths. For many of the unlithified sedimentary deposits on the Beaufort Slope, however, 

excess pressure generation at the base of the gas-hydrate 

zone during Pleistocene drops in sea level was sufficient to 

initiate sea-floor landsliding (Kayen, 1988). This mechanism for triggering mass movement probably also applies to 

the numerous other regions of the world where gas hydrate 

is present within sea-floor sediment (Lee, Schwab, and 

Booth, this report; Popenoe, Schmuck, and Dillon, this 

report). Accordingly, times of lowered sea level during ice 

ages also are times of widespread instability of continental 

slopes that are underlain by gas hydrates. 

Slope Stability in Regions of Sea-Floor Gas Hydrate: Beaufort Sea Continental Slope 101 
""O 

8 

a~ t ? 1 -X ~\ 0 

"uO 

'V '1( ~ 

~VQ 1 

2o J 

, 'o 

0~ 

Clayey Sediment 

Figure 7. In contrast to figure 6, 

excess pore-water pressures (in kilopascals) generated beneath the base 

of a decomposing gas hydrate within 

sandy sediment can more readily dissipate away from the gas-hydrate base 

as seen by the broad fan of isobars. As 

a result, disassociation of the base 

occurs more rapidly than clayey sediment, and the base retreats upward in 

the sediment column where equilibrium pressures are lower. Consequently, relative to slopes of clayey 

sediment, there are lower excess 

pressures within a sandy slope at any 

given time during a sea-level fall. The 

sandy slope would probably be less 

likely to fail. 

Figure 6. This three-dimensional block 

diagram demonstrates the excess porewater pressures (in kilopascals) in 

clayey sediment beneath a gas hydrate 

base during a sea-level drop predicted 

by a finite-difference model. The lower 

left axis is the depth below the hydrate 

base; the lower right axis is time during 

which sea level falls steadily by 100 

meters. The upper left-hand (vertical) 

axis shows the predicted pore-water 

pressures, ue, in excess of hydrostatic. 

Because clayey sediment has low fluiddissipation properties, pressures cannot readily diffuse away from the disassociating base. The excess pressures 

generated at the base are maintained, 

as seen by the tightly grouped isobars. 

Consequently, disassociation of the 

base is severely limited and a condition 

of high excess pressure exists. Such a 

condition can lead to slope instability. 

Sandy Sediment 

102 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
1000 

900 

co 

Q.. 800 : 

w 700 RANGE OF CRITICAL a: 

:::> EXCESS PRESSURE (/) 

(/) 600 TO TRIGGER FAILURE w 

a: 500 OF A 100-200 METER Q.. 

w THICK BLOCK a: 

0 

Q.. 

400 v~ (/) 300 (/) 

w 

u 200 X 

w 100 

0 

0 2000 4000 6000 8000 

YEARS SINCE START 

OF. SEA-LEVEL DROP 

Figure 8. Predicted excess pore-water pressures (in kilopascals) generated at the base of a disassociating gas 

hydrate (diagonal lines) during a 100-meter sea-level fall 

are presented against, and exceed, a range of likely excess 

pressures required to initiate slope failure (horizontal 

lines). For sandy to clayey sediment, slope failure is 

predicted unless excess pore pressures can be rapidly 

vented to the sea floor through alternate conduits. 

REFERENCES CITED 

Bloom, A.L., Broecker, W.S., Chappell, J.M.A., Matthews, 

R.K., and Mesolella, K.J., 1974, ~uaternary sea level 

fluctuations on a tectonic coast: New 23 Th/ 234U dates from 

the Huon Peninsular, New Guinea: Quaternary Research, v. 

4, p. 185-205. 

Carpenter, G., 1981, Coincident sediment slump/gas hydrate 

complexes on the U.S. Atlantic continental' slope: GeoMarine Letters, v. 1, p. 29-32. 

Chappell, J.M.A., 1974, Geology of coral terraces, Huon Peninsula, New Guinea: a study of Quaternary tectonic movements 

and sea level changes: Geological Society of America Bulletin, v. 89, p. 356. 

Field, M.E., and Kvenvolden, K.A., 1985, Gas hydrates on the 

northern California continental margin: Geology, v. 13, p. 

517-520. 

---1986, Reply on "Gas hydrates on the northern California 

continental margin": Geology, v. 14, p. 537-538. 

Grantz, Arthur, and Dinter, D.A., 1980, Constraints of geologic 

processes on western Beaufort oil developments: Oil and Gas 

Journal, v. 78, p. 304-319. 

Grantz, Arthur, Eittreim, S.L., and Whitney, O.J., 1981, Geology and physiography of the continental margin north of 

Alaska and implications for the origin of the Canada Basin, in 

Nairn, A.S., Chamber, M.T., and Stehli, F.A., eds., The 

ocean basins and margins: Plenum Publishing Corp., p. 

439-492. 

Grantz, Arthur, and Greenberg, A.L., 1981, High-resolution 

uniboom seismic reflection profiles from the Beaufort Sea, 

1977: U.S. Geological Survey Open-File Report 81-34. 

Kayen, R.E., 1988, Arctic Ocean landslides and sea-level fall 

induced gas hydrate decomposition: M.S. Thesis, California 

State University, Hayward, 227 p. 

Kvenvolden, K.A., Claypool, G.E., Threlkeld, C.N., and Sloan, 

E.D., Jr., 198( ·Geochemistry of a naturally occurring 

massive marine gas hydrate: Organic Geochemistry, v. 6, p. 

703-713. 

Kvenvolden, K.A., and McMenamin, M.A., 1980, Hydrates of 

natural gas: a review of their geological occurrence: U.S. 

Geological Survey Circular 825, 11 p. 

Makogon, Y.F., 1981, Hydrates of natural gas (translation of: 

Gidraty prirodnykh gazov): PenWell Publishing Company, 

Tulsa, Oklahoma, p. 237. 

Matthews, R.K., 1973, Relative elevation of late Pleistocene high 

sea level stands: Barbados uplift rates and their implications: 

Quaternary Research, v. 3, p. 147-153. 

Sloan, E.D., Jr., 1990, Clathrate hydrates of natural gases: New 

York, Marcel Dekker. 

Steinem, R.P., Harrison, R.S., and Matthews, R.K., 1973, 

Eustatic low stand of sea level between 125,000 and 105,000 

B.P.: Evidence from the sub-surface of Barbados, West 

Indies: Geological Society of America Bulletin, v. 84, 

p. 63-70. 

Slope Stability in Regions of Sea-Floor Gas Hydrate: Beaufort Sea Continental Slope 1 03 
Mass Movement Related to Large Submarine 

Canyons Along the Beringian Margin, Alaska 

By P.R. Carlson, H.A. Karl, B.D. Edwards, J.V. Gardner, and R. Hall 

INTRODUCTION 

The 1 ,400-kilometer-long Beringian continental 

slope extends between the Aleutian Islands and the Soviet 

Far East and divid.es the Bering Sea into the wide, shallow 

Bering Sea shelf and the deep Aleutian Basin (fig. 1). The 

continental slope and . outer shelf of the u.s. part of the 

ALEUTIAN BASIN 

180" 

Beringian margin is dissected by seven large submarine 

canyons, several of which are among the world's largest 

(Carlson and Karl, 1988). Recently collected GLORIA 

sidescan-sonar imagery has documented the importance of 

mass movement in shaping the entire Beringian continental 

margin. 

176"W 

~ St. MattheW Is. Nunivak Is. 

BERING SEA SHELF 

7 

<I prlbiiOI Is. 

100 200 300 

KILOMETERS 

172" 

Figure 1. The Beringian margin, incised by s·everal large canyons, lies between the Bering Sea shelf and the Aleutian basin. 

The seismic-reflection profiles and GLORIA images shown in figures 2 to 14 are'indicated by numbered lines and outlined 

areas. 

104 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
675 

0 3KM 

en 

0::: 825 LLI 

~ 

LLI 

~ 

:I: 

~ 

a.. 

w 

C) 975 

0::: 

LLI 

~ 

~ 

1125 

Mass-Movement 

Deposit 

I 

Figure 2. Seismic-reflection profile (3.5 kilohertz) across ·southeast branch of Navarin Canyon showing slope failure 

scarps, about 15 meters high, on both walls and mass-movement deposits several ten~ of meters thick on the canyon 

floor. 

Soviet scientists were some of the first to study the 

Beringian margin and discov~red several of the canyons 

(Kotenev, 1965). U.S. scientists began to study the area· in 

the late 1960's (Scholl and others, 1968), and petroleum 

lease sales efforts in the 1970's and early 1980's intensified 

scientific interest. In 1986, the U.S. Geological Survey 

conducted three GLORIA (Geological LOng-Range 

Inclined Asdic) sidescan-sonar surveys that covered the 

entire Bering ian slope and provided a reconnaissance image 

of this frontier region. 

This report illustrates the large canyons that are 

incised in the margin and demonstrates that these canyons 

have been shaped by several styles of mass movement, 

including mudflows, debris flows, slumps, and massive 

slides. The illustrations show bathymetric, GLORIA 

sidescan-sonar, and seismic-reflection data. We discuss 

selected mass-movement features of the Beringian margin 

in three segments, beginning in the noJ1h with Navarin 

Canyon, which straddles the U.S.-U.S.S.R. 1867 convention line, then shifting to the central part of the margin, 

which is dominated by Zhemchug Canyon, and ending with 

the southern margin, which includes two large canyons and 

an oceanic plateau (fig. 1). 

NORTHERN MARCIN 

Navarin Canyon, the most northerly (fig. 1) and the 

second largest of the Beringian canyons, is 258 kilometers 

(km) long from its head to the base of the slope and has a 

width and relief, where it cuts the shelf edge, of 100 km and 

1,150 meters (m), respectively (Carlson and Karl, 1988). 

Many of the seismic-reflection profiles across Navarin 

Canyon show slope-failure scarps on both walls and massmovement deposits on the canyon floor (fig. 2). Massmovement features were identified along 830 km of a total 

of 2,570 km of seismic-reflection profiles (32 percent) 

collected across the northern Beringian margin (Carlson and 

Karl, 1984/85). An example of a relatively thick landslide 

mass on the floor of Navarin Canyon is presented in figure 

3. Figure 3A shows a seismic-reflection profile collected in 

1980 that crosses the axis of Navarin Canyon transversely 

Mass Movement Related to Large Submarine Canyons Along the: Beringian Margin, Alaska 1 05 
-en 

"0 

NW 

2.0-

1-

5 2.5-

(.) 

<1) 

en 

-<1) 

E 

t= 

Q) 

~ 3.0-: ·'"-

~ 

>. 

ro 

~ I 

0 

~ 

~ 3.5-

A 

NW 

8 

'' 

'I 

0 

__ ,.: 

0 

I 

r< ·1 . I t 

': l 

.r 

:j . 

I. .. 

Figure 3. Two views of a mass-movement deposit on the 

floor of Navarin Canyon, (A) seismic-reflection profile 

showing hummocky, crumpled sediment constituting the 

toe of a mass-movement deposit. The ship track was run 

transverse to the axis of the canyon (vertical exaggeration 

I. 

L. 

IOKM 

5km 

I SE 

.. •. I ~-~ll~i~ ~+l+:. • . -1500 

. .r ·:-;/~-'1~}---i"i , .. ~·- _.:.:{<(~:: c 

:· .. "'~ . 

. . 'J . ,:~ .. ~~~~-- ·fi~ 

}•4 · .. ,_-_ . j' :· l ' 

' i '?~- -~I'·:·_· ,. -2000 ~ .. 

-2500 

"0 

-

=r 

i 

-.., 

~ 

en 

-

SE 

is 10x ). (B) GLORIA sidescan-sonar image showing same 

hummocky toe (X-Y). The width of this deposit is about 10 

kilometers. The trackline of the GLORIA image is located 

on the canyon wall about 5 kilometers to the left (northwest) of the seismic-reflection profile. 

106 Submarine landslides: Selected Studies in the U.S. Exclusive Economic Zone 
N~ 

0 50KM 

Figure 4. Three-dimensional shaded-relief image of bathymetry showing relatively small canyons (gorges) incised in the 

slope between Navarin and Pervenets canyons. This computer-processed image uses a look-angle of 45° above.. the 

horizon and has a vertical exaggeration of 5 x. 

and shows the hummocky nature of the 100-m-thick toe of 

a landslide (X-Y). In 1986, our GLORIA survey of this 

area imaged this same landslide deposit (fig. 3B). The 

seismic-reflection profile shows that the landslide deposit is 

about 100m thick. By combining the map view provided by 

the GLORIA image and the sediment thickness provided by 

the seismic profile, we can calculate the volume of sediment 

disturbed to be at least 5 km3

. 

Several relatively small canyons, 5 to 10 km wide, 

are incised into the 100-km-long upper slope between 

Navarin and Pervenets canyons (fig. 4). GLORIA imagery 

of the upper slope near the heads of these small canyons, 

suggests that these incisions may have been initiated by 

sliding of large sedimentary blocks (fig. 5). These blocks 

are as large as several kilometers across, thus, nearly the 

width of the small canyons. 

CENTRAL MARCIN 

The central part of the Beringian margin is dominated 

by the massive Zhemchug Canyon (fig . 6). The submarine 

canyon, with a volume at 5,800 km3 is the world's largest. 

It is 168 km long from its head to the base of the slope, and 

at the shelf edge, its incision has a relief of 2.6 km and a 

width of 100 km (Carlson and Karl, 1988). Most seismicreflection profiles across the Zhemchug Canyon system 

show evidence of mass movement. Some mass-movement 

deposits are composites of multiple events that accumulated 

on the floor of the canyon and severely constricted the axis 

of the canyon (fig. 7). Such constrictions along the axis of 

the canyon inhibit water circulation as well as mass flows 

down the canyon. In the development of submarine canyons, as well as subaerial canyons, however, such constric 

tions or dammings are geologically short lived, as long 

Mass Movement Related to large Submarine Canyons Along the Beringian Margin, Alaska 107 
SLOPE 

Figure 5. GLORIA sidescan-sonar image and interpretive 

sketch of upper slope area between Navarin and Pervenets canyons showing a 4-kilometer long, 2-kilometer 

wide block (B) that has slipped away from long irregular 

as the canyon-forming processes are active. Cores collected 

on the basin floor at the mouth of the canyon contain graded 

sand and silt layers and shallow-water organisms, evidence 

0 5 

I I 

Kilometers 

scarps (5). One scarp can be traced for 26 kilometers along 

the slope. Cracks (C) have developed at several locations 

along the scarp. A large block has slipped down at left of 

center of image from evacuated area (X). 

of the transporting power of mass flows that in some 

instances originated at the shelf edge (Carlson and Karl, 

1988). 

108 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
N 

~ 

0 40 KM 

Figure 6. GLORIA sidescan-sonar imagery merged with bathymetry of the central Beringian 

slope including Zhemchug Canyon. The look angle is 45° and vertical exaggeration is 5 x . The 

canyon walls and lower continental slope are intensively gullied. Some of the large mounds 

near the base of the slope are enormous slide masses whose edges are also highly gullied. 

Mass Movement Related to Large Submarine Canyons Along the Beringian Margin, Alaska 109 
0 

I 

NE 

-CIJ 

"0 

c: 

0 

() 

Q) 

CIJ 

-Q) 

E 

i= 

(i) 

1.0-

> 

ro 

~ 

1-

~ 

ro 

~ I 

0 

~ 

1-

2.0-

Figure 7. Seismic-reflection profile across upper Zhemchug Canyon showing the complex mass-movement 

deposit that has accumulated on the floor of the canyon, 

constricting the down-canyon axial flow. A fault scarp 

5km 

I 

Multiple 

sw 

.. / · ': . .:.· 

0 

-750 ~ 

-':J" 

-

3 

<t> 

-

<t> 

.., 

CJ) 

-

{ > A-1500 

forms the southwest wall of the canyon. The southeast 

branch of the canyon was eroded along this scarp after the 

shelf edge was breached by the headward erosion of the 

canyon. 

110 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
0-

1500--------------------------------------------------~ 

(F) 

0: 

w 

I-

~ 3000---

:::t: 

IC... 

w 

0 

0: 4500 w 

~ 

7500 

Figure 8. Seismic-reflection profile at base of slope seaward of Middle Canyon showing a large slide block (greater than 

10 kilometers long and 500 to 800 meters thick). The bottom-simulating reflector (BSR), which has been speculated to be 

a silica diagenetic boundary (Cooper and others, 1987), may have been the slip surface for the large block . 

Other slope failures along the central margin have 

generated large block slides (fig. 8) that have been found at 

the base of the slope near the mouths of several canyons 

(Carlson and Karl, 1984/85; Cooper and others, 1987; Karl 

and others, 1987). Some seismic-reflection profiles collected over this large lope failure show an internal reflector 

that cut acros layered reflectors (indicative of rock/sediment layers) and mimics the sea-floor slope (bottomimulating reflector, BSR). In one example, a BSR coincides with the ba e of the slope-failure zone (fig. 8) and 

may be the glide plane along which the failed sediment 

mass moved. Similarly, a large sediment mass at the base of 

the slope near Zhemchug Canyon, al o associated with a 

BSR (fig. 9), may have been one of the large blocks that 

was dislodged from the margin and slid downslope as 

Zhemchug Canyon was formed. 

We hypothesize that Zhemchug Canyon was formed 

by headward erosion, principally by large block slide (fig. 

lOA, B), until finally the shelf edge was breached and the 

canyon reached a series of outer-shelf parallel faults (fig. 

lOC). Erosion then proceeded along the fault carps nearly 

parallel to the shelf edge (fig. lOD). Subsequently , further 

mass movement has produced the intensively gullied walls 

evident on the GLORIA imagery (fig. 6). 

SOUTHERN MARGIN 

The southern part of the Beringian slope includes 

Pribilof and Bering canyons, but the dominant morphologic 

feature is the large, Umnak Plateau (fig. 1), whose flat 

summit is at a water depth of about 2,000 m (fig. 11). 

Bering Canyon, whose 400-km-long path is structurally 

controlled by the Aleutian Island arc and Umnak Plateau, 

and Pribilof Canyon, whose morphology and probably its 

genesis mimic that of Zhemchug Canyon , are both areas of 

extensive slope failures similar to those in other Beringian 

canyons (Scholl and others, 1970; Cooper and others, 1986; 

Stevenson and others, 1988). At the northeastern margin of 

Umnak Plateau, GLORIA imagery (fig. 12) shows slope 

failure, apparently resulting in mass flows that have moved 

down Bering Canyon and undercut the base of the slope. 

An even larger slope failure, at the western edge of 

the plateau, can be observed on the GLORIA imagery (fig. 

13). This lope failure is about 55 km long and 35 km wide. 

Nearby imagery (fig. 14) shows a small slope failure. Both 

of these slope failures are apparently associated with emergent diapirs (Cooper and others, 1986); a diapir is a fold in 

which a mobile core has risen and broken through brittle 

overlying rock and sediment (McGregor and others, this 

Mass Movement Related to large Submarine Canyons Along the Beringian Margin, Alaska 111 
N 

3.oI 

·, 

~ _; 

I 

5km 

s 

-2500 ~ 

A) 

~ 

I 

-3000 -i 

~ en 

-

-3500 

Figure 9. Seismic-reflection profile across large slide block near the mouth of Zhemchug Canyon. The bottom-simulating 

reflector (BSR) is probably a silica diagenetic boundary. 

GRABEN 

Figure 10. Hypothetical model of evolutionary development of Zhemchug Canyon. (A) Gullies begin eroding into 

upper slope. Outer shelf is cut by normal faults that are 

parallel to the shelf edge and a slight graben is formed. (B) 

Large blocks tear loose from upper slope initiating the 

breeching of the shelf-slope break. (q Continuing mass 

movement results in headward and lateral canyon excavation of the continental margin. Here, the headward 

growth of the canyon reaches the shelf-edge faults. (0) 

Excavation of canyon progresses laterally along the fault 

scarps and canyon develops its modern morphology. 

112 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
A 

ALEUTIAN BASIN 

0 40KM 

Figure 11. (A) Three-dimensional shaded relief image of Umnak Plateau and adjacent 

continental slope with a look angle of 20° from the horizontal and a vertical exaggeration of 

10X. 

8 N~ 

ALEUTIAN BASIN 

0 40KM 

Figure 11. (B) GLORIA image of same area merged with bathymetry. 

Mass Movement Related to large Submarine Canyons Along the Beringian Margin, Alaska 113 
Figure 12. GLORIA sidescan-sonar image of a slope failure on eastern edge of Umnak Plateau, adjacent to Bering Canyon. 

Note that much of the debris from the slope failure has been removed from the channel. Based on the size of the slopefailure scarp, more than 18 cubic kilometers of sediment appears to have been incorporated in this mass movement. 

114 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
Figure 13. GLORIA sidescan-sonar image of a large slope failure on the western edge of Umnak Plateau. This 60 cubickilometer mass-movement deposit averages 150 meters in thickness and covers an area of 400 square kilometers. 

report). This association suggests that the emplacement of 

the e diapirs contributed to these failures. 

SUMMARY 

We have een from seismic-reflection profiles and 

sidescan-sonar imagery that mass movement is a dominant 

and apparently on-going process that is sculpting the Beringian margin. The large canyons appear to have been 

excavated principally by mass-movement processes. The 

canyons grow head ward by a combination of processes, but 

sliding of large blocks is volumetrically the most important. 

Slumps, block slides, and mass flows also provide the 

principal mechanisms for lateral and vertical excavation. In 

the case of Zhemchug Canyon, submarine erosion resulted 

in an incision 2.6 km deep and 100 km wide at the edge of 

the shelf along the central Beringian margin. 

REFERENCES CITED 

Carlson , P.R. , and Karl , H.A., 1984/85, Mass movement of 

fine-grained sediment to the basin floor, Bering Sea, Alaska: 

Geo-Marine Letters, v. 4, p. 221- 225. 

---1988, Development of large submarine canyons in the 

Bering Sea, indicated by morphologic, seismic, and sedimentologic characteristics: Geological Society of America Bulletin , v. 100, p. 1594-1615. 

Cooper, A.K., Scholl, D.W., and Marlow, M.S., 1987, Structural framework , sedimentary sequences, and hydrocarbon 

potential of the Aleutian and Bowers Basins, Bering Sea, in 

Scholl, D.W., Grantz , Arthur, and Vedder, J.G. , eds., 

Geology and resource potential of the continental margin of 

western North America and adjacent ocean basins- Beaufort 

Sea to Baja, California: Circum-Pacific Council for Energy 

and Mineral Resources Earth Science Series, v. 6, p. 

473- 502. 

Cooper, A.K., Stevenson, A.J., Kenyon, Neil, and Bishop, 

Derek, 1986, GLORIA study of the Exclusive Economic 

Zone off Alaska-southern Bering Sea: Initial report for 

cruise F4-86-BS: U.S. Geological Survey Open-File Report 

86-596, 19 p. 

Karl, H.A., Gardner, J.V., and Huggett, Quentin, 1987, GLORIA images of Zhemchug Canyon and Bering Channel-Fan 

system, Bering Sea, in Hamilton, T.D., and Galloway, J.P., 

eds., Geologic studies in Alaska by the U.S. Geological 

Survey during 1986: U.S. Geological Survey Circular 998, p. 

147- 151. 

Kotenev, B.N., 1965, Submarine valleys in the zone of the 

continental slope in the Bering Sea: All-Union Research 

Institute Marine Fishery Oceanography Transactions, U.S. 

Mass Movement Related to large Submarine Canyons Along the Beringian Margin, Alaska 115 
IOkm ' 

Figure 14. GLORIA sidescan-sonar image of an exposed diapir and associated small slope failures along the western edge 

of Umnak Plateau. 

Naval Electronics Laboratory, San Diego, California, NEL 

Translations 112, v. 58, p. 35--44. 

Scholl, D.W., Buffington, E.C., and Hopkins, D.M., 1968, 

Geologic history of the continental margin of North America 

in the Bering Sea: Marine Geology, v. 6, p. 297-330. 

Scholl, D.W., Buffington, E.C., Hopkins, D.M., and Alpha, 

T.R., 1970, The structure and origin of the large submarine 

canyons of the Bering Sea: Marine Geology, v. 8, p. 

187-210. 

Stevenson, A.J., Karl, H.A., Gardner, J.V., Cooper, A.K., 

Carlson, P.R., and Edwards, B.D., 1988, Slope failure and 

sediment mass movement on the margins of the Aleutian 

Basin, styles and causes: Geological Society of America, 

Abstracts with Program, v. 20, no. 7, p. Al29. 

116 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
Comparison of Tectonic and Stratigraphic Control of 

Submarine Landslides on .the Kodiak Upper 

Continental Slope, Alaska 

By M.A. Hampton 

DESCRIPTION 

The upper continental slope southeast of the Kodiak 

Island group, Alaska, contains two distinct types of submarine landslides (slope failures) whose locations apparently 

are controlled by different geologic factors (Hampton and 

Bouma, 1977). The first type, large slumps, appears in 

seismic-reflection profiles from two areas, one off southernmiddle Albatross Bank and the other off Portlock Bank (fig. 

1), where the geologic structure indicates that the sea floor 

is being deformed in response to tectonic forces. In contrast, the large slumps are absent in seismic-reflection 

profiles from the intervening area off northern Albatross 

Bank where the structure indicates relatively little deformation, and the sea floor is less steep. The second type, 

shallow slides, appears 'ln seismic-reflection profiles from 

all areas of the upper continental slope. Their location 

probably is related to local stratigraphy; that is, the presence 

of a buried weak sediment layer or an abrupt sedimentstrength increase at some depth below the sea floor. Strong 

earthquakes occasionally affect the entire region and may be 

responsible for actually triggering the slides. Slides are 

virtually absent from the adjacent continental shelf, probably because the inclined areas of the sea floor ar~ underlain 

by coarse-grained sediment that is strong enough to withstand the forces that cause the slope failures on the 

continental slope. 

The best examples of large slumps show a steep 

headwall scarp upslope from a slide mass that is displaced 

a relatively short distance along a curved slide surface, or 

slip plane (figs. 2 and 3). The height of the headwall scarp 

and the thickness of the slide mass range up to a few 

hundred meters. The best examples of shallow slides show 

a headwall scarp and sevefal displaced blocks above a 

distinct, undeformed acoustic reflector no deeper than about 

50 meters (m) beneath the sea floor (fig. 4). 

The appearance of both types of slides varies widely 

on the seismic-refle~tion profiles, from a clear display of 

the distinctive features described above to equivocal indications such as hummocky sea floor that might indicate the 

presence of erosional submarine canyons rather than 

landslide-disrupted sea floor (for example, compare figs. 2 

to 4 with figs. 5 and 6). 

CAUSES 

The Kodiak continental margin is located at the edge 

of the North American lithospheric plate where it is being 

underthrust from the south by the oceanic crust of the 

Pacific plate. The junction of the two plates occurs at the 

Aleutian Trench. A collisional setting such as this creates 

strong tectonic forces that deform the margin. The deformation is spatially nonuniform; in particular, it is strongest 

where the large slumps occur. For example, the seismicreflection profile in figure 2 shows a large, fault-bounded, 

downdropped block (graben) on the continental shelf, and 

the profiles in figures 5 and 6 show a relatively steep-limbed 

anticlinal fold near the edge of the shelf and a similar, but 

smaller, fold seaward. Growth of the faults and folds might 

have occurred recently, as judged by their deformation of 

the sea floor. Indications of slumps on the continental slope 

appear in seismic-reflection profiles in each of the deformed 

areas. In contrast, the shelf-edge. anticline is subdued and 

major faults are absent along the seaward edge of northern 

Albatross Bank (fig. 7; von Huene and others, 1980). Here, 

the sea floor of the upper continental slope is more gently 

inclined, and there is no indication of large slumps. Therefore, this regional overview indicates that large slumps 

occur only in areas where the sea floor has been tectonically 

deformed in the recent past. 

On the other hand, the shallow slides occur throughout the region, regardless of the degree of deformation. The 

displaced blocks disrupt acoustic reflectors on seismicreflection profiles (indicative of sedimentary bedding) only 

down to a certain, shallow subbottom depth (fig. 4). 

Beneath the displaced blocks is a continuous acoustic 

reflector that defines the deepest extent of the slide. By 

analogy with studied examples on land (Varnes, 1978), the 

surface along which the slide initiated probably lies at or 

just above this reflector. That is, slope failure might have 

initiated along a relatively thin, weak sediment layer that 

could not resist the downslope forces acting upon it. 

Alternatively, slope failure might have occurred just above 

a surface of abrupt strength increase, such as at an erosional 

unconformity, where forces of sufficient magnitude would 

tend to make an overlying, relatively homogeneous unit fail 

(Lee and Edwards, 1986). Movement would be translaTectonic and Stratigraphic Control of Submarine Landslides, Kodiak Upper Continental Slope, Alaska 117 
Figure 1. Location of the Kodiak upper continental slope seaward of the Kodiak shelf. Numbered dashed lines indicate 

location of the seismic-reflection pr?files in figures 2 to 7. 

118 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
0 

-

C/) 

-

w 

0 2 NMI 

I 't 

~ 0 2 KM 

1-

_, 

w 

> 

c( 

a: _, 1-

> ·:" "'!:~~.;. ..... 

< 

~ 

.. bl• ...... 

. -~·-

I ~-

0 

~ 

1-

2 

Figure 2. Seismic-reflection profile across the outer continental shelf and upper continental slope of Portlock 

Bank that displays the features of a large slump: Headwall 

scarp at the top of the slump is the main separation of the 

failed and unfailed slope sediment. The vertical relief of 

the scarp is about 150 meters. Slide mass is the displaced, 

failed body of sediment; that is, the slide proper. The slide 

mass is about 120 meters thick. Slide surface is the curved 

continuation of the headwall scarp beneath the slide 

tional along such a surface, and the presence of several 

distinct slide blocks may indicate episodes of retrogressive 

slope failure (in the upslope direction) after the initial event. 

Strong earthquakes occur frequently throughout the 

Kodiak region; it is one of the most seismically active areas 

on Earth (Jacob, 1986). The great Alaska earthquake of 

~3.4° 

0 

C/) 

w 

~ 

1-

....1 

w 

> 

c( 

a: 

1-

>-

c( 

~ I 

0 

~ 

1-

2 

I 

'~.:·:· •. ~.~~~ . ''::.. .... i ::;;... I 

I 

I 

·' .. I 

·' . 'I 

t 

I 

J. 

Figure 3. Slump with the same clearly displayed features 

as shown in figure 2. The slump occurs adjacent to a 

sediment-filled basin that is disrupted by a fault. Slumping 

probably is controlled by movement of the fault, which 

.~ 

mass. Slope failure occurred along the headwall scarp and 

the slide surface. The slump occurred on an initial seafloor slope of about 5.2°. Notice the large downdropped 

graben that offsets the sea floor. The tectonic forces that 

caused this structural feature also may be responsible for 

the slump, by steepening the sea floor and by causing 

large earthquakes. (One second of two-way travel time 

indicates a water depth of approximately 750 meters.) 

1964 triggered many landslides, some of which were 

initiated offshore (Coulter and Migliaccio, 1966; Lemke, 

1967; Kachadoorian, 1969; Hampton, Lemke, and Coulter, 

this report). Earthquakes can trigger slope failures in several 

ways (Lee, Schwab, and Booth, this report). Th~ earthquake accelerations create a direct force that can augment 

0 2 NMI 

0 2 KM 

.·~. ·;:::: 

removes support from the base of the slope and also 

causes earthquake shaking. (One second of two-way 

travel time indicates a water depth of approximately 750 

meters.) 

Tectonic and Stratigraphic Control of Submarine Landslides, Kodiak Upper Continental Slope, Alaska 119 
0 

w 

~ 

1-

..J 

w 

> 

< 

a: 

1-

> 

<( 

~ I 

0 

~ 

1-

0 . 2 NMI 

I 0 2 KM 

_. ,---- --~------~---

1 

................. ~--- --------------------

-~~-

.. , ..•.,.,_ 

~ . , -,-- __ :::::...:-;:::-_---------------.. ---------· ---. - ·-----···-·-- --

·~ ........ 

0.5 

I 

CONTINUOUS 'REFLECTOR --- ,---- ---------:--·--"-·---------------·.·---·------- . r-'-----:---.:-----,--'----.---+~-

1 I 

Figure 4. Shallow slide consisting of several slide blocks, 

less than 20 meters thick, that extend down to a continuous acoustic reflector. The features of this slide suggest 

that initial failure occurred either within a relatively thin, 

weak sediment layer at or slightly above the continuous 

reflector or at the base of a relatively homogeneous 

deposit in contact at the reflector with an underlying 

the force of gravity and cause failure of an otherwise stable 

slope (Morgenstern, 1967; Lee and Edwards, 198q). In 

addition, the repeated jostling of grains within a deposit can 

cause buildup of internal pore-water pressure that weakens 

stronger unit. Earthquakes may trigger this kind of shallow 

slide, but tectonic deformation of the sea floor (slope 

steepening and/or faulting) does not appear to be important because this type of slide occurs off northern Albatross Bank, where the tectonic deformation is relatively 

minor. (One se.cond of two-way travel time indicates a 

water depth of approximately 750 meters.) 

the sediment, sometimes to the point of failure (Poulos and 

others, 1985). Stability analysis based on a few measurements of geotechnical (civil engineering) properties of 

sediment from tl).e Kodiak upper continental slope indicate 

CONTINENTAl SHELF CONTINENTAL SLOPE 

LANDWARD ANTICLINE 0 

I 

2 NMI 

~ 0 

UJ 

~ 1-

..J 

UJ 

> 

< 

a: 

1-

> 

< 

~ 

I 

0 

~ 1.0 1-

·~: :: .. :~. 

-..... ' 

.. .:.. ' ... ' -· ' 

... ·.' 

...... ~ .... -

0 2 KM 

SEAWARD 

ANTICLINE 

~5.2° 

Figure 5. A large anticlinal fold (an arch of strata in which the sediment beds bend downward in opposite directions) is 

succeeded seaward by a smaller and probably younger anticline whose recent growth may have contributed to slumping 

on the steepening seaward limb. (One second of two-way travel time indicates a water depth of approximately 750 

meters.) 

120 Submarine landslides: Selected Studies in the U.S. Exclusive Economic Zone 
0 5 NMI 

I 

0 5 KM 

CONTINENTAL SHELF 

LANDWARD ANTICLINE 

I CONTINENTAL 

SLOPE 

0 

w 0 ~ 

~ 

...J 

w 

> 

4( 

a: 

~ 

> 

4( 

3: 

0 1.0 .:~~:. . .-... : ·)· 

3: 

~ 

~e-. 

. . .~ ... 

.. '_· )!~~~s1:ffp1;!f\ ~ ..... ~ .. ,..,.~,_,,_. 

SEAWARD 

ANTICLINE 

Figure 6. A more mature example of the multiple anticlinal folds shown in figure 5, where the seaward anticline has 

grown sufficiently to extend the shelf seaward and define a new shelf-slope break. (One second of two-way travel time 

indicates a water depth of approximately 750 meters.) 

0 

I 

2 NMI 

0 2 KM CONTINENTAL SHELF CONTINENTAL SLOPE· 

...J 

w 

~ 0 

a: 

1-Ci) 

>--

<( w ~::E 

I -

01-

~ 1 ·IBROAD ANTICLINE 

Figure 7. Relatively mildly deformed outer shelf and upper slope on northern Albatross Bank. The subsurface strata are 

broadly folded, the inclination of the upper continental slope sea floor is gentle, and no large slumps occur, at least to 

the seaward extent of the seismic profile. (One second of two-way travel time indicates a water depth of approximately 

750 meters.) 

that all but the steepest areas are stable under static, 

gravitatioJrlal loading (downslope- directed forces of gravity 

acting on the sedimentary deposit due to the sea-floor slope 

angle), but that earthquake loading can cause slope failure 

(Hampton. and others, 1978; Hampton, 1989). Earthquakes 

probably triggered many of the slides on the Kodiak upper 

continental slope. 

Another factor associated with tectonism is removal 

of the buttressing support of slopes by earthquakeassociated! faulting. Such an example is shown on the 

seismic-re:flection profile in figure 3. The slump occurs on 

the landward slope of a basin that is filled by a thick 

sequence of well-stratifie~ sediment. Offset of the sea floor 

within the basin, just seaward of the toe of the slide mass, 

and discontinuity of acoustic reflectors beneath the offset 

suggest the presence of a recently active fault. Hampton and 

others ( 1978) proposed that repeated movement of the fault 

removes support from the toe of the slide mass and, 

.. thereby, promotes sliding.· 

SUMMARY 

Geologic analysis of seismic-reflection profiles indicates that large slumps on the Kodiak upper continental 

slope occur only where tectonic deformation has occurred 

recently'· with the implication that sea-floor steepening and 

removal of support by faulting control the location of the 

slumps. Smaller, shallow slides <?Ccur in all areas of the 

upper continental slope~ implying that their location is 

independent of sea-floor. deformation. By analogy with 

similar slides studied on land, their location probably is 

Tectonic and Stratigraphic Control of Submarine Landslides, Kodiak Upper Continental Slope, Alaska 121 
determined by the local presence of a buried weak sedimentary layer or an abrupt strength increase at the depth of 

failure. Limited geotechnical data support the contention 

t~at e~hquakes actually trigger slides in the region. 

REFERENCES CITED 

Coulter, H.W., and Migliaccio, R.P., 1966, Effects of the 

earthquake _Of March 27, 1964, at Valdez, Alaska: U.S. 

Geological Survey Professional Paper 542 C, p. C1-C36. 

Hampton, M.A., 1989, Geotechnical properties of unconsolidated 

sediment on the Kodiak continental shelf and upper slope, 

Gulf of Alaska: Marine Geotechnology, v. 8, p. 159-180. 

Hampton, M.A., and Bouma, A.H., 1977, Slope instability near 

the shelf break, western Gulf of Alaska: Marine Geotechnology, v. 2, p. 309-331. 

Hampton, M.A., Bouma, A.H., Carlson, P.R., Molnia, B.F., 

· Clukey, E.C., and Sangrey, D.A., 1978, Quantitative study 

of slope instability in the Gulf of Alaska: Proceedings of the 

lOth Offshore Technology Conference, p. 2307-2318. 

Jacob, K.H., 1986, Seismicity, tectonics, and geohazards of the 

Gulf of Alaska regions, in Hood, D.W., and Zimmerman, 

S.T., eds., The Gulf of Alaska-Physical environment and 

biological resources: U.S. Department of the Interior, Min-

. eral Management Service, Washington, D.C., p. 145-186. 

Kachadoorian, Reuben, 1969, Effects of the earthquake of March 

27, 1964, at Whittier, Alaska: U.S. Geological Survey 

Professional Paper 542 B, p. B1-B21. 

Lee, H.J., and Edwards, B.D., 1986, Regional method to assess 

offshore slope stability: Journal of Geotechnical Engineering, 

American Association of Civil Engineers, v. 112, p. 

489-509. 

Lemke, R. W., 1967, Effects of the earthquake of March 27, 1964, 

at Seward, Alaska: U.S. Geological Survey Professional 

Paper 542 E, p. E1-E43. 

Morgenstern, N .R., 1967, Submarine slumping and the initiation 

of turbidity currents, in Richards, A.F., ed., Marine Geotechnique, University of Illinois Press, Urbana, Ill., p. 

189-210. 

Poulos, S.J., Castro, Gonzalo, and France, J.W., 1985, Liquefaction evaluation procedure: Journal of Geotechnical Engineering, American Association of Civil Engineers, v. 111, p. 

722-791. 

Varnes, D.J., 1978, Slope movement types and processes, in 

Schuster, R.L., and Krizek, R.J., eds., LandslidesAnalysis and control: Washington, D.C., National Academy 

of Sciences, p. 12-33. 

von Huene, R., Hampton, M.A., Fisher, M.A., Varchol, D.J., 

and Cochrane, G.R., 1980, Map showing near-surface geologic structures of Kodiak shelf, Alaska: U.S. Geological 

Survey Miscellaneous Field Studies Map MF-1200, 

1:500,000 Scale. 

122 · Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
Subrnarine Landslides That Had a Significant Impact 

on J\1an and His Activities: Seward and Valdez, 

Alasl'a 

By M.A .. Hampton, R.W. Lemke1

, and H.W. Coulter2 

INTRODUCTION 

On the evening of March 27, 1964, the largest 

earthquake: to occur in North America during this century 

struck southern Alaska. The epicenter of the magnitude 

(Mw) 9.2 shock"was at the head of Prince William Sound 

near the foot of the Chugach Mountains (fig. 1). The 

duration o:f strong ground motion lasted an incredibly long 

3 to 4 minutes, with tectonically induced elevation changes 

1 Consulting Engineering Geologist, Lakewood, Colo. 2 Retimd, Washington, D.C. 

0 

occurring over a land and sea-floor area exceeding 180,000 

square kilometers (km2). Of the several destructive 

earthquake-related effects, slope failure (landslides) probably caused the most property damage, and large sea waves 

(tsunami), most of which were generated by submarine 

landslides, took the most l~ves (Hansen and Eckel, 1966). 

These two phenomena were particularly devastating to the 

communities of Seward and Valdez (Lemke, 1967; Coulter 

and Migliaccio, 1966). Large landslides originated below 

sea level and retrogressed landward to destroy the waterfront of both towns, sinking the dock and harbor facilities 

that were their economic lifeblood. Sea waves generated by 

25 50 MILES 

I I I 

0 25 50 KILOMETERS 

. 

·····~ .............. _.,. CHUGACH MOUNTAINS 

GUlF OF AlASKA 

Figure 1. Location of the epicenter of the Alaskan earthquake of 1964 and the towns of Seward and Valdez. 

Submarine Landslides That Had a Significant Impact on Man and His Activities: Seward and Valdez, Alaska 123 
60° 

05' 

0 

o. 

2 MILES 

2 KILOMETERS 

Mineral Creek 

Townsite 

PORT VALDEZ 

<11111· figure 2. Northern part of Resurrection Bay showing 

Seward, Alaska, and vicinity. (Contours are in feet.) 

the landslides caused further destruction, both by flooding 

and by spreading burning oil from local storage depots. 

The geologic settings of Seward and Valdez are 

similar, a fact that explains the similarity of earthquake 

effects. Seward is located near the head of Resurrection Bay 

and Valdez at the head of Port Valdez, both steep-sided 

fjords (figs. 2 and 3). The landslide-affected areas are on 

deltas constructed of stream-transported sediment that 

derives from a nearby steep, glaciated mountain front. This 

type of sediment, especially when composed of sand- and 

silt-size grains,· is particularly susceptible to liquefaction 

during earthquakes (Committee on Earthquake Engineering, 1985; Lee, Schwab, and Booth, this report; Schwab 

and Lee, this report). Landslide displacement of water in 

the restricted bays creates an ideal situation for the generation of destructive waves that reflect back and forth across 

the narrow fjords and repeatedly inundate the coast. 

0 2 MILES 

I I 

0 2 KILOMETERS 

Figure 3. Eastern part of Port Valdez showing the town of Valdez, Alaska, and vicinity. Note the Mineral Creek townsite, 

a safer location where the town of Valdez was moved after the earthquake. (Contours are in feet.) 

124 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
Figure 4. Seward before the earthquake (from Lemke, 1967). (Scale is in feet.) 

<o,..V..t"-c!DytJS4._C••Piolt"'f•_, 

-~..--,.VlG..lO<f•(CIIS..,._, 

I . 

.t.Pf'MOlllM.lTtSCll(I"'Ul 

~ • ., ~.....,.,...o~, .... ,.c..,....~~dl....,.._ 

Figure 5. Seward 1 day after the earthquake (from Lemke, 1967). Note the white line that indicates the landward limit of 

wave run up. The most obvious indication of the landslide effects at the scale of the photograph is the missing docks and 

other coastal facilities (see fig. 4). 

THE EARTHQUAKE DAMAGE AT SEWARD 

Seward is located principally on the Seward delta of 

Lowell Creek (fig. 2), about 150 km south southwest of the 

earthquake's epicenter (fig. 1). After about 35 to 40 seconds 

of earthquake shaking on March 27, 1964, a strip of 

waterfront 1,200 meters (m) long and 15 to 150 m wide 

started to subside slice by slice and eventually disappeared 

into the bay (figs. 4 and 5). This was a consequence of 

landward retrogression of a slope failure that initiated on the 

steep (20° to 35° to a water depth of 50 m) submerged 

delta-front. By the time shaking stopped, a zone of incipient 

slope failure marked by cracked ground had formed for a 

distance up to 250 m behind the landslide (figs. 6-8). 

Nearshore water depths increased more than 30 m in some 

places (fig. 9). Delta sediment in the northwest corner of the 

bay, deposited from the Resurrection River, also failed, and 

the sea surface deformed, presumably in response to the 

sudden underwater sediment movement. Within 30 seconds 

after the slope failure began, the first of three landslidegenerated sea waves attacked the shoreline, continuing for 

about 15 minutes and surging to elevations of approximately 10 m above sea level. Wave runup resumed when 

another train of waves, generated by tectonic displacement 

of the sea floor outside Resurrection Bay, rather than by the 

landslides, hit the coast about 30 minutes after the earthquake. 

The land that slid into Resurrection Bay took docks, 

railroad equipment, and oil storage tanks with it (fig. 10). 

Fire erupted when storage tanks overturned and pipes 

Submarine landslides That Had a Significant Impact on Man and His Activities: Seward and Valdez, Alaska 125 
Figure 6. Southern part of the Seward waterfront showing the limit of fractured ground associated with landslides (from 

Lemke, 1967). 

Figure 7. Northern part of the Seward waterfront showing the limit of fractured ground associated with landslides (from 

Lemke, 1967). Index map shows relation between figures 5 and 6. 
Figure 8. An example of ground displacement (approximately 1.5 meters) and fractures associated with landslides, 30 

meters from the shoreline (from Lemke, 1967). 

zl 

Ql. 

A ~:: 8' 

0' ~------~M~e~a~n~lo~w~er~l~ow~w~al~e~r-------~~IL---4 0' 

Poslearthquake prollle 

Mar~ne sed1men1s (clay , Sill, and sand) 

,_ -~Dnfl (1111 and outwash) 500' -'---....:::....:0~----------------------...::.::.==...::.::.=-------L---L 500' ....L....::..:....:.:~~=~~~~~=~~------':..!~~~~~--_L __ __l... 500' 

c 

0' 

500' 

2400 FEET 

r-----~----,--L--.---~ 

800 1600 

600 "'ETERS 

c· 

Postearthquake prof1le 

~==~""-"=-:""!-~-~-""-""-~--=-=:...:.:..~-:__-=--:_:..:-=-:_:...:.:_~_:_~_--7"'-=-_"-_~-:::-:::':l- 500' 

-- _ Manne sed1ments (clay. Sill and sand) Landslide 

- debns 

Figure 9. Cross sections showing pre- and postearthquake profiles of the land surface in Resurrection Bay (from Lemke, 

1967). Note that the sea floor has been lowered up to 30 meters by the displacement of sediment by the landslides. The 

locations of these cross sections are shown on figure 2. 

ruptured. Water that receded from the shore as a result of 

the initial landslide displacement carried the fire offshore, 

then sea waves returned it with each subsequent runup. The 

waves lifted railroad cars off the tracks, smashed earthmoving equipment, lifted boats over a breakwater, displaced houses from their foundation, and dumped a cargo 

Submarine Landslides That Had a Significant Impact on Man and His Activities: Seward and Valdez, Alaska 127 
Figure 10. Landslide destruction of railroad and petroleum-storage facilities on the Seward waterfront (from Lemke, 

1967). 

dock into the bay (fig. 11). The death toll at Seward was 13 

people, mostly as a result of wave runup. Damage to 

property was estimated at $22,363,349. 

Geologic and engineering studies conducted after the 

earthquake identified several factors that contributed to the 

landslides at Seward (Shannon and Wilson, Inc., 1964a; 

Lemke, 1967). Onshore and offshore subsurface borings 

were made to identify sediment types, and physical properties were measured on sediment core samples. Offshore 

seismic-reflection profiling provided acoustic data for 

description of sea-floor morphology and subbottom stratigraphy. Obviously, the slope failures were related to the 

earthquake. Strong ground motion initiated the slope failures, and the long duration of the earthquake led to the great 

extent of mass movement. Some of the deltaic sediment in 

Resurrection Bay, such as rapidly deposited, interbedded 

fine- and coarse-grained layers, is known to build up 

pore-water pressures and liquefy under repeated, 

earthquake-type loading, even if the packing of the sediment grains is medium to high density (Committee of 

Earthquake Engineering, 1985). A large component of 

gravitational force acted downslope because of the steep 

delta fronts, and the weakness along sedimentary bedding 

planes that dip up to 30° seaward probably lessened the 

applied force necessary for failure. 

The less obvious factors concern static pore-water 

pressure conditions within the sedimentary deposit (see 

Lee, Schwab, and Booth, this report). In particular, sediment borings revealed the presence of high artesian pressure 

in some confined, buried sedimentary beds, which 

decreased the stabilizing frictional stress ("effective stress") 

that acts between grains, thereby weakening the sediment 

irrespective of earthquake loading. Moreover, the low tidal 

level at the time of the earthquake, along with the rapid 

drawdown of water in the bay that accompanied the initial 

slope failure, also decreased the stability because the pore 

water could not drain from the sediment quickly enough to 

maintain hydrostatic conditions during the removal of 

external water pressure. Submarine landslides on fjord 

deltas elsewhere have occurred in conjunction with tidal 

drawdown, without earthquake loading (Terzaghi, 1956; 

Prior and others, 1981; Prior and others, 1982). 

Man-induced loading in the form of artificial fill, 

dock facilities, oil storage tanks, and railroad cars aided the 

slope failure, as did the added surcharge due to wave runup. 

The magnitude of these man-induced effects relative to 

others is uncertain, however. 

Seismic-reflection profiles point out that postearthquake sea-floor slopes are as steep as those before. A 

stability analysis based on measured solid properties implies 

that pre- and postearthquake slopes are stable under static 

conditions but could readily fail under earthquake accelerations of 0.15 g (Shannon and Wilson, Inc., 1964a). 

Contrary to normal expectations, the overall stability of the 

offshore sediment was not improved because of the slope 

failure. 

A scientific and engineering task force was formed to 

study the earthquake effects and, thereby, aid the reconstruction effort. On July 24, 1964, the task force made 

recommendations to Seward city officials regarding the 

128 Submarine landslides: Selected Studies in the U.S. Exclusive Economic Zone 
Figure 11. Houses and other debris carried by earthquake-related waves into the lagoon at the north end of Seward (from 

Lemke, 1967). 

stability of the Seward delta. The recommendations were 

based on the geologic studies of Lemke ( 1967), the geotechnical engineering studies of Shannon and Wilson, Inc. 

(1964a), and visits by the task force to Seward (Hansen and 

others, 1966). The delta area was divided into two categories: (1) nominal risk and (2) high risk (fig. 12). The 

high-risk category included much of the waterfront area and 

was essentially coincident with the area of fractured ground 

shown in figures 6 and 7. On the basis of these recommendations, the city officials restricted the high-risk area, 

formerly used for railroad marshalling and by the oil tank 

farms, to parkland use. There was little doubt that under 

resumed dynamic conditions of severe earthquake shaking , 

the area of fractured ground would again fail and begin 

sliding into Resurrection Bay. 

THE EARTHQUAKE DAMAGE AT VALDEZ 

At Valdez, located about 70 km east of the earthquake's epicenter (figs. 1 and 3), the greatest damage was 

associated with a delta-front landslide involving an estimated 75 million cubic meters of sediment. The landslide 

was first observed indirectly when, shortly after shaking 

began, a cargo ship unloading at the town dock began to 

toss violently, with vertical motion in excess of 10 m and 

rolls of 50°. The motion was caused by landslide-generated 

sea waves. Shoreward retrogression of the landslide soon 

reached the docks, causing them to vanish instantly into the 

turbulent water. The entire waterfront was lost, along with 

warehouses, a cannery, heavy equipment, and 30 people 

(figs. 13-15). The initial 10-m-high wave inundated the 

Submarine landslides That Had a Significant Impact on Man and His Activities: Seward and Valdez, Alaska 129 
THE ALASKA EARTHQUAKE, MARCH 2 7 , I 9 6 4 

EXPLANATION 

Nominal-risk area High-risk area Mean lower low water line 

Little li k ehltood of landtlidu except for 

1m.all •lu m.pt, la rgd~ in artificial fill 

In all other r upecte riaka are no gr eater 

than it norma.llv e. z~cted in the con~ rue~ 

tion induJLTlf C-urren t Un\form Build-

\ 1lQ' Code for Seitmic Zone 3 a pplU• both 

to n t w buildino• and to plant f or rehabilitation of earthquab-damaqt.d ltrucLand cont idtred u?u tablt, parhc ularlv in 

event of fu ture earthquake11, no economicallJI featib le mean• of ltabiltzation 

kno'&(·n No repair, rehabilttation, or 

ntu.• conetrurtion involving uae of Federal 

f unds i1 rtcomm en ded, except f or grading a1td lio Itt fi ll 

Figure 12. High- and nominal-risk areas of a part of Seward, as concluded by the scientific and engineering task force 

(from Hansen and Eckel, 1966). 

waterfront and propagated westward down the bay where it 

surged to 50 m above sea level. A wave of similar size, 

probably reflected from the far end of the bay, struck 

Valdez about 10 minutes after the earthquake. Later that 

night, at 11:45 p.m. and 1:45 a.m., tidally augmented 

seiche waves (an oscillation of the surface of the bay) again 

advanced into the town. Waves surged a distance exceeding 

500 m inland at Valdez. Parts of the shore area subsided 

because support was removed from the face of the delta. 

Subsidence continued for several months after the earthquake . 

The sediment along the waterfront consists of a 

surficial 7- to 10-m-thick layer of sandy gravel fill that is 

underlain by an unknown thickness of gravelly sand outwash with interbedded layers of silt. Both sedimentary units 

have loose to medium density. Scientists and engineers who 

studied the slope failure at Valdez speculated that the 

delta-front liquefied in response to the earthquake and the 

drop in water level, then the landslide material transformed 

into a mobile, low-density turbidity current that spread 

much of the sediment as a thin deposit away from the delta. 

At least five previous seismic events in Valdez, 

between 1899 and 1925, were accompanied by submarine 

landslides, as evidenced by the occurrence of phenomena 

such as communication cable breaks and sudden waterdepth increases (Coulter and Migliaccio, 1966). There are 

reports of three small submarine landslides (in the early and 

late 1920's and in the early 1940's) that damaged parts of 

the dock facilities but were not associated with earthquakes. 

Instead , they probably were due to loading by the docks 

themselves or by localized sedimentation consequent to 

river-control projects. 

As at Seward, the stability of the delta at Valdez did 

not seem to be increased by the landslide, and similar large 

slope failures m.·e expected in future earthquakes. This threat 

prompted the decision to move the entire town of Valdez to 

a more stable site 5.5 km to the west on the Mineral Creek 

fan (fig. 3). An engineering and geological study of the fan 

pointed out the comparative desirability of this site (Shannon and Wilson , Inc. 1964b). The area is underlain by 

complexly bedded, medium to very dense sand and gravel 

that is buttressed by bedrock ridges and apparently was 

stable during the earthquake. The nearshore sea-floor slope 

is relatively gentle to a distance of 100 to 300 m offshore, 

where it steepens to about 25°. Offshore sediment in part of 

the area is a loose, fine sand of the type that is prone to 

slope failure when shaken, and comparison of pre- and 

postearthquake water depths indicates that a small mass of 

this sediment about 100 m offshore failed during the 

earthquake . The overall stability of the Mineral Creek fan 

during the earthquake indicates that it is a safer location for 

the town of Valdez, however. 

CONCLUSION 

The destruction at Seward and Valdez clearly demonstrated the devastating effects that offshore landslides can 

have on man and his activities. Most submarine slope 

130 Submarine landslides: Selected Studies in the U.S. Exclusive Economic Zone 
Figure 13. Submarine landslide area at Valdez (from Coulter and Miliaccio, 1966). The dashed lines ind icate the 

dock area destroyed by the landslide. 

Submarine landslides That Had a Significant Impact on Man and His Activities: Seward and Valdez, Alaska 131 
Figure 14. Valdez before the earthquake. Note the intact dock facilities extending out into Port Valdez (from Coulter and 

Migliaccio, 1966). 

failures are benign in this regard; they occur far offshore 

and the range of their influence does not extend to the 

coastline. Moreover, the geologic conditions (coarse sediment, gentle sea-floor slope) in most nearshore areas do not 

permit slope failure even during the strongest seismic 

events; however, in those places where sandy and silty 

sediment is deposited rapidly on a steep sea-floor gradient, 

especially if earthquakes occur or if the tidal range is high, 

offshore landslides must be regarded as a potential hazard. 

This situation exists in nearly all Alaskan fjords. 

132 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
Figure 15. Valdez about 3 months after the earthquake (from Coulter and Migliaccio, 1966). Note the disappearance of 

the dock facilities and the devastation along the waterfront. 

REFERENCES CITED 

Committee on Earthquake Engineering, 1985, Liquefaction of 

soils during earthquakes: Washington, D.C., National 

Research Council. National Academy Press, 240 p. 

Coulter, H.W., and Migliaccio, R.P., 1966, Effects of the 

earthquake of March 27, 1964, at Valdez, Alaska: U.S. 

Geological Survey Professional Paper 542-C, p. Cl- C36. 

Hansen, W.R., and Eckel. E.B., 1966, A summary description of 

the Alaska earthquake-Its setting and effects, in Hansen, 

Submarine landslides That Had a Significant Impact on Man and His Activities: Seward and Valdez, Alaska 133 
W.R., Eckel, E.B., Schaem, W.E., Lyle, R.E., George, W., 

and Chance, Genie., eds., U.S. Geological Survey Professional Paper 541, p. 1-37. 

Hansen, W.R., Eckel, E.B., Schaem, W.E., Lyle, R.E., George, 

Warren, and Chance, Genie, 1966, The Alaska earthquake of 

March 27, 1964: field investigations and reconstruction 

effort: U.S. Geological Survey Professional Paper 541, 

111 p. 

Lemke, R. W., 1967, Effects of the earthquake of March 27, 1964, 

at Seward, Alaska: U.S. Geological Survey Professional 

Paper 542-E, p. El-E43. 

Prior, D.B., Coleman, J.M., and Bornhold, B.D., 1982, Results 

of a known seafloor instability event: Gee-Marine Letters, v. 

2, p. 117-122. 

Prior, D.B., Wiseman, W.J., and Gilbert, R.A., 1981, Submarine 

slope processes on a fan delta, Howe Sound British Columbia: Gee-Marine Letters, v. 1, p. 85-90. 

Shannon and Wilson, Inc., 1964a, Report on subsurface investigation for city of Seward, Alaska and vicinity: Seattle, 

Wash., Shannon and Wilson, Inc., 26 p. 

---1964b, Report on subsurface investigation for Mineral 

Creek townsite, city of Valdez, Alaska: Seattle, Wash., 

Shannon and Wilson, Inc., 12 p. 

Terzaghi, Karl, 1956, Varieties of submarine slope failures: 

Proceedings of the 8th Texas Conference on Soil Mechanics 

and Foundation Engineering, p. 1-41. 

134 Submarine landslides: Selected Studies in the U.S. Exclusive Economic Zone 
Processes Controlling the Style of Mass Movement 

in Glaciomarine Sediment: Northeastern Gulf of 

Alasl<a 

By W.C. Schwab and H.j. Lee 

INTRODUCTION 

The type of mass movement that occurs following 

slope failure can range from rigid block motion to fluidlike 

flow. As use of sea-floor resources and development of the 

sea floor continues to expand, it becomes increasingly 

desirable to develop methods that allow the ocean engineer 

to predict not only the relative stability of a slope but also 

, the amount of sea-floor deformation that follows slope 

failure. w·e investigated several large areas of slope instability on the continental shelf of the northeastern Gulf of 

Alaska in an attempt to explain the different types of slope 

failure encountered (Schwab and Lee, 1983, 1988; Lee and 

Edwards, 1986; Schwab and others, 1987). Results of these 

studies are applicable to the development of a methodology 

that can predict the consequences of slope failure (also see 

Edwards, Lee, and Field, this report). 

580 

HOLOCENE !Bill GLACIOMARINE 

SEDIMENT 

.• •• .• 200m ISOBATH 

@ SLOPE FAILURE 

0 

I 

IOOKM 

I 

Glaciation is the dominant process contributing sediment to the northeastern Gulf of Alaska continental shelf 

(Molnia, 1983). Just as in the case of streams of water, 

glaciers perform erosion, transportation, and deposition of 

mineral matter. Blocks of rock being carried within the 

glacial ice are scraped and dragged along the rock floor, 

gouging and grooving the bedrock and chipping out fragments of rock. Much of the rock is ground by the glacier 

into extremely fine particles of silt- and clay-sized sediment; termed rock flour. Glaciers in the Chugach-St. Elias 

Mountains discharge their sediment into lakes, streams, and 

bays, and in tum, much of this material is transported by 

currents into the open-marine environment. This Holocene 

(deposited since the last ice age) glaciomarine sediment 

blankets most of the inner continental shelf (fig. 1), 

reaching a thickness of 200 meters (m) seaward of Icy Bay 

and Yakutat Bay (Carlson and Molnia, 1975) and 260 m 

.. 

··· ... •. . ALSEK VA·~·~.E~·;· :: •••• 

.. .. <'oo.. : .... 

Figure 1. Northeastern Gulf of Alaska depicting the Holocene (deposited since the last ice age) glaciomarine sedimentary 

deposit and the location of areas of slope failure on the continental shelf, including (A) the Icy Bay-Malaspina slump, 

(B) the Yatkutat slump, and (C) the Alsek prodelta slope instability area (from Schwab and Lee, 1988). 

Processes Controlling the Style of Mass Movement in Glaciomarine Sediment: Northeastern Gulf of Alaska 135 
0 

NORTH SOUTH 

40 

0 0.5 1KM 

80 

_120 

~ 

:r: 160 tCL .''] 

w 

·1.... ..··· ........ .. · \. .. . -·· --·· .. J 

0 

a:: 

w 

t- 0 <( 

3: 

40 

80 

120 BLOCK 

160 

Figure 2. A 3.5-kilohertz seismic-reflection profile and interpretive sketch collected over part of the Icy Bay-Malaspina 

slump (modified from Lee and Edwards, 1986). 

seaward of Dry Bay (Schwab and Lee, 1983). Slope failures 

on the continental shelf of the northeastern Gulf of Alaska 

occur entirely within this Holocene glaciomarine sedimentary deposit. 

CASE STUDIES 

The Icy Bay-Malaspina slump, the Yakutat slump, 

and mass flows on the Alsek prodelta (fig. 1) typify the two 

types of slope failures found on the continental shelf of the 

northeastern Gulf of Alaska. The region offshore from Icy 

Bay and the Malaspina Glacier is an area where the 

glaciomarine clayey silt has failed over an area of about 

1,080 square kilometers (km2

) in water depths of 70 to 150 

m on a sea-floor slope less than 0.5° (Carlson, 1978). 

Seismic-reflection profiles (for example, fig. 2) collected 

over the area of slope failure show broken acoustic reflectors in the subsurface (reflectors are indicative of sedimentary bedding) and scarplike surficial forms, suggesting that 

discrete slump blocks have undergone a slight backward 

tilting while moving approximately 18m downslope. These 

slump blocks extend over the entire failure area and are 

about 0.5 km loNg and offset the sea floor from 2 to 5 m. 

136 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
SOUTH NORTH 

0 ·-···-·-·- --·-----· -··- -· ----

•• -~· ... ·~· '•, 1° .!' ..,:;.- · .•.• "'~.I.~ .• -~~·. 

-~~~~~==~~==~~9r==F. ==~~~~~~~~~~~~ 

40 0 0.5 I KM -· ------------- -------r·--:~·------ .. ····-

1 

'........Jit ... ~ _ ...

80 

-

~ _ . .,.,. .. 

...._ . L:~- . ·"'· . _ ... _ . ·. /'" ::::c 

t- 120 

a_ 

w 

Q 

a:: 

10 

w 

t-

<l' 

3: 

410 

SHEAR PLANES 

Figure 3. A 3.5-kilohertz seismic-reflection profile and interpretive sketch collected over the Yakutat slump (Schwab and 

Lee, 198B). 

The individual failure planes (shear planes) extend down 

below the sea floor to a depth of 15 to 40 m, where they 

form a Wt::ll-defined glide plane parallel to the sea floor. 

The Yakutat slump is characterized by backwardtilted blocks of glaciomarine clayey silt that have undergone 

about 13 m of downslope movement in water depths of 65 

to 90 m (Carlson and others, 1980). Seismic-reflection 

profiles show that the slump blocks, although not as well 

developed as in the Icy Bay-Malaspina slump, extend over 

an area of about 260 km2 and are about 100m long and have 

sea-floor relief of 3 to 4 m (fig. 3). The failure planes 

extend 10 m below the sea floor. The nearshore part of the 

Yakutat slump has a slope angle of about 1 o. The sea-floor 

slope decreases to about 0.5° at the seaward edge of the 

failure area. 

Slope failures (fig. 4) cover an area of at least 150 

km2 on the Alsek prodelta (Molnia, 1982). These s1ope 

failures originate in a water depth of about 35 m on a 

Processes Controlling the Style of Mass Movement in Glaciomarine Sediment: Northeastern Gulf of Alaska 137 
SOUTH NORTH 

0 

.. ·; . - . . ... _,. t • '~~~· ....... ,. ; .:. ., 4 · .. .,.. 

40 0 0.5 IKM -- -------- --·- ----,------:-;·· ----.-

I 

80 

-

~ 

- ;~~~~4

:r: 1~-. 

~ ... - _, "C" ~ t . . -·-· ,,. 

/ " 

..._ 120 

Cl... 

w 

0 

a:: 0 

w 

..._ 

<{ 

~ 

40 

80 

PLANES 

120 

Figure 3. A 3.5-kilohertz seismic-reflection profile and interpretive sketch collected over the Yakutat slump (Schwab and 

Lee, 1988). 

The individual failure planes (shear planes) extend down 

below the sea floor to a depth of 15 to 40 m, where they 

form a well-defined glide plane parallel to the sea floor. 

The Yakutat slump is characterized by backwardtilted blocks of glaciomarine clayey silt that have undergone 

about 13 m of downslope movement in water depths of 65 

to 90 m (Carlson and others, 1980). Seismic-reflection 

profiles show that the slump blocks, although not as well 

developed as in the Icy Bay-Malaspina slump, extend over 

an area of about 260 km2 and are about 100 m long and have 

sea-floor relief of 3 to 4 m (fig. 3). The failure planes 

extend 10 m below the sea floor. The nearshore part of the 

Yakutat slump has a slope angle of about 1°. The sea-floor 

slope decreases to about 0. 5° at the seaward edge of the 

failure area. 

Slope failures (fig. 4) cover an area of at least 150 

km2 on the Alsek prodelta (Molnia, 1982). These slope 

failures originate in a water depth of about 35 m on a 

Processes Controlling the Style of Mass Movement in Glaciomarine Sediment: Northeastern Gulf of Alaska 137 
precludes determination of the maximum depth below the 

sea floor that is affec.ted by slope failure; however, it is 

estimated to be less than 20m (Carlson and others, 1980). 

Using engineering an(l.lyses of the strength characteristics of the glaciomarine sediment and the environmental 

stresses affecting this sedimentary deposit, Schwab and Lee 

( L 983) determined that earthquake-shaking and the passage 

of storm waves are the two triggering forces that caused the 

slopes to fail. Downslope stress resulting from gravitational 

forces (that is, sea-floor slope angle) are a less important 

factor, although they determine the ultimate direction of 

movement. Some uniformity in the style of mass movement 

following slope failure on the continental shelf of the 

northeastern Gulf of Alaska might be expected because the 

sediment :in the failure areas is all of the same type, 

glaciomarine clayey silt. However, variation in the morphology of the mass movement deposits implies that there 

are major differences in the styles of mass movement. 

ANALYSIS 

To determine what caused the sediment found on the 

northeastern Gulf of Alaska continental shelf to deform in 

two differe:nt ways, we compared the environmental factors 

affecting the slumps and mass flows (Schwab and Lee, 

1988). Our three study areas are suited for this type of 

comparison because the seismic and storm-wave environments, as well as the sediment, are similar in each. For 

example: 

1. Due to its tectonic setting (Bruns, 1985), the northeastern Gulf of Alaska is the most seismically active region 

in the United States, apart from the Aleutian Islands. All 

three study areas are located in a region suggested as 

having a uniform ground-shaking intensity; that is, each 

study area will, over a period of time be subjected to 

earthquake movements having about the same maximum 

intensity (Stephens and Page, 1982). 

2. All three areas are influenced by the same storm-wave 

climate. Large storm waves, which commonly have a 

wave height of at least 15 m, roll across the continental 

shelf throughout the winter (Royer, 1975). The probable 

maximum height of winter storm waves in the area is 37 

m (Quayle and Fulbright, 1975; Bea, 1976). 

Table 1. Sediment textural data for the three slope failure 

study areas 

Sand Clay 

(>0.063 mm) Silt (<0.004 mm) 

(%) (%) (%) 

Icy-Bay-Malaspina 1.23± 1.94 66.15±9.94 32.60± 10.11 

slump 

Yakutat slump 7.74±6.09 54.32±6.59 37.93±6.20 

Alsek prodelta 1.70±2.41 59.26±7.33 39.16±6.83 

3. The grain-size distribution of samples collected in the 

three study areas varies locally but similarly in each area 

(table 1). The glaciomarine sediment on the continental 

shelf of the northeastern Gulf of Alaska is dominantly 

clayey silt. 

4. The mineralogy of the sediment from the three study 

areas is similar (Molnia and Hein, 1982; Schwab and 

Lee, 1988). 

5. Sediment cores in the three study areas that contain a 

higher proportion of sediment with water content in the 

range of 35 to 45 percent of the dry sediment weight 

correlate with the locations of submarine slope failure 

(Schwab and Lee, 1983). The water content of this 

sediment is a simple index property that correlates with 

more advanced engineering properties and also more 

basic sediment properties such as grain-size distribution. 

6. Sediment pore-water pressures in the three study areas 

~o not appear to be in excess of hydrostatic pressure, an 

Important condition that relates to slope stability 

(Schwab and Lee, 1983; Lee, Schwab, and Booth, this 

report). 

Our slope-stability analysis (Schwab and Lee, 1983) 

described the influence of storm waves and earthquake 

forces that lead to slope failure of the glaciomarine sediment. These forces were defined by laboratory testing in 

which only a relatively small amount of sediment deformation was found to occur. This level of deformation 

although small, is sufficient to be seen in high-resolutio~ seismic-reflection profiles collected over the Icy BayMalaspina and Yakutat slumps (fig. 2). The slope failures 

on the Alsek prodelta (fig. 4), however, include elements of 

flow that extend well beyond the level of deformation that 

typically is observed in a laboratory-test-based slope stability analysis. Therefore, although our initial slope-stability 

analysis most likely predicts the initiation of slope failure, it 

provides little information regarding its consequences. 

Two end-member consequences of slope failure have 

been described: disintegration and nondisintegration (Whitman, 1985; Schwab and others, 1987). Disintegrative 

failure is a condition in which a sediment mass is so weak 

following slope failure that it deforms continuously under 

downslope gravitational stresses even after the stress causing th~ slope failure has been removed (for example, 

followmg an earthquake). This deformation ceases only 

~fter large displacement has occurred or the slope gradient 

IS reduced. Nondisintegrative failure involves permanent 

displacement during slope failure, but the sedimentary 

deposit stops deforming after the triggering stress is 

removed. The Icy Bay-Malaspina slump and the Yakutat 

slump are nondisintegrative failures whereas the mass flows 

in the Alsek prodelta are disintegrative failures. 

To achieve a disintegrative failure, the sediment must 

lose a good deal of its strength during a transient loading 

event (earthquake or major storm). In the case of the 

continental shelf in the northeastern Gulf of Alaska, the 

Processes Controlling the Style of Mass Movement in Glaciomarine Sediment: Northeastern Gulf of Alaska 139 
sediment strength must be reduced to the point that contin-. 

ued downslope sediment movement can occur on slopes that 

are less than 1 o. Our laboratory testing of sediment samples 

from the area of the slope failures show that the strength of 

the sediment cannot be reduced sufficiently, as long as the 

natural water content of the sediment remains constant. 

That is, the sediment in place has a water content that is too 

low to allow the development of a mass flow. If the water 

content were increased during a transient loading event, 

however, the sediment's strength could be lowered enough 

to allow a mass flow to develop (Schwab and others, 1987; 

Schwa~ and Lee, 1988). 

How can sediment water content be increased during 

an earthquake or major storm? One mechanism proposed by 

Whitman (1985) involves local pore-water movement 

'Within the sediment column; that is, pore· water is redistributed within the sediment but does not flow out of the overall 

sediment system. A unique characteristic of the response of 

fine-grained sediment to cyclic stresses (the stresses 

induced by storm waves or earthquakes) is the tendency to 

develop sediment pore-water pressures that equal lithospheric stress (the stress induced by the overlying sediment 

which, in tum, increases with increasing depth in the 

deposit). If the level of cyclic stress is high enough, 

pore-water pressures that are equal to the lithospheric stress 

develop over a substantial thickness of the sedimentary 

deposit. The resulting pore-pressure gradient leads to an 

upward flow of pore water; from the area of high pressure 

to the area of relatively lower pressure (fig. 5). The base of 

the part of the sediment column affected by the cyclic 

stresses becomes more dense while the upper sections 

loosen, resulting in a continually decreasing strength of the 

upper sedimentary section. Once the sediment strength falls 

below a critical level, a disintegrative failure occurs. 

Another mechanism for converting a nondisintegrative failure into a disintegrative failure was described by 

Hampton ( 1972). He suggested that water is incorporated 

from the overlying sea water into a submarine slump by the 

jostling and agitation it received as it begins to move. 

Sediment deformation during a transient loading event can 

become large. Some of the sea water may be incorporated 

into the sediment simply as a result of these rapid movements and agitations, thus significantly reducing the 

strength of the sediment. 

On the continental shelf in the northeastern Gulf of 

Alaska, we suggest that these two mechanisms are most 

effective during storm-wave loading and that storms are the 

principal cause of mass flows in this glaciomarine sediment. 

Sediment framework expansion (and strength reduction) 

during a major storm event, enhanced relative to what 

would occur in an earthquake, can be justified in three 

ways. 

First, a long series of loading cycles causes the 

sediment strength to fall to a low level. Our experimental 

results on the behavior of Gulf of Alaska glaciomarine 

c.layey silt suggest that the strength of sediment samples is 

about 73 percent lower after 1 ,000 stress cycles are applied 

than after 10 (Schwab and Lee, 1983, 1988). Major 

earthquakes produce from 10 to 25 representative loading 

cycles (Seed and Peacock, 1971), whereas ~ major storm 

would likely produce 1,000 cycles or more. With such large 

reductions of sediment strength during major storms, the 

level of deformation experienced during each cycle of 

storm-wave loading should be greatly enhanced. The mechanism proposed by Hampton ( 1972) could then come into 

play; the increased level of jostling would cause increased 

amounts of sea water to be incorporated into the sediment. 

Second, the long duration of stress application during 

a major storm allows increased pore-water redistribution. 

Thus, by Whitman's (1985) mechanism, water begins to 

flow from deeper strata into shallower strata (fig. 5). If the 

duration of cyclic stress application is short, the process is 

limited, and pore-water pressure drops in the deeper strata, 

thus, halting the upward flow. With a long period of cyclic 

stress application, however, the pore-water pressures in the 

deeper strata are renewed continually. A steady pumping 

action, drawing water from deeper sediment toward steadily 

expanding sediment near the sea floor occurs. 

Third, storm waves generate stresses that propagate 

from the sea floor down, whereas earthquakes generate 

stresses that propagate from the bedrock up. Because 

weakening and failure of sediment occurs after application 

of a number of cycles of stress, the ability of the failed 

sediment to transmit stresses is degraded. In the case of an 

earthquake, the slump blocks, once they have formed, 

become partially isolated from the source of the stress 

application below. Significant stresses cannot propagate 

upward across the failure plane (Lee, 1976). On the other 

hand, with storm-wave-induced failures, it is the zone under 

the failure blocks that become partly isolated from the 

source of stress. The blocks themselves are still openly 

exposed to wave-induced stresses propagating down from 

the sea floor. The processes leading to framework expansion are free to proceed, unhindered by the development of 

a barrier to stress propagation. 

The suggestion that storm waves are more likely to 

cause mass flows than are earthquakes in this environment 

is also supported by field and laboratory evidence. We 

determined the water depths in which major earthquakes are 

most likely to cause a slope failure and those in which peak 

storm waves are the most critical factor (Schwab and Lee, 

1983). We found that earthquake shaking dominates in 

water depths greater than 76 m and storm-wave loading 

dominates in water depths less than 35 m. Sediment in 

depths between 35 and 76 m have an equal chance of failure 

by either mechanism. Using these criteria, we suggested 

that the Icy Bay-Malaspina slump, in water depths between 

70 and 150 m, was caused by earthquake shaking. The 

Yakutat slump, with water depths between 65 and 90 m, 

was also probably seismically induced. However, the Alsek 

140 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
8 

A 

Figure 5. Illustration of the possible redistribution of pore 

water during the application of cyclic stresses: (A) the 

sediment column at rest, (8) pore water flowing upward 

due to th~~ pore-water pressure gradient induced by. the 

prodelta failures, in water depths of 35 to 80 m, occurred in 

a depth range where storm waves could cause slope failure 

as readily as earthquakes. 

Disintegrative failures on the Alsek prodelta are 

found within an environment in which either winter storm 

waves or earthquakes can cause failure. On the other hand, 

nondisintegrative failures in the Icy Bay-Malaspina slump 

and Yakutat slump are associated with environments in 

which storm waves are probably insufficient to cause 

application of cyclic stresses, and (0 the base of the 

sediment column affected by the cyclic stresses contracts 

due to the redistribution of pore water while the upper 

section expands. 

failure, but in which earthquake shaking is clearly capable 

of producing the slumps. We suggest that storm waves, 

which represent the application of cyclic stresses for a long 

duration, were responsible for generating disintegrative failures on the northeastern Gulf of Alaska continental shelf, and that earthquake shaking, which is characterized by the application of cyclic stresses for a relatively 

short duration, led to nondisintegrative failures in this 

environment. 

Processes Controlling the Style of Mass Movement in Glaciomarine Sediment: Northeastern Gulf of Alaska 141 
CONCLUSIONS 

Three differences-(!) enhanced jostling, (2) 

enhanced redistribution of pore water, and (3) the absence 

of a partial barrier to stress propagation-separate the 

application of cyclic stresses related to storm waves from 

those related to earthquakes. These factors are sufficient to 

allow disintegrative failures to form during storms. On the 

other hand, only nondisintegrative failures are likely to 

occur during earthquakes on the northeastern Gulf of Alaska 

continental shelf. These differences explain the varied types 

of slope failure found in the region. 

These findings regarding the cause of mass flows are 

not universally applicable to other sedimentary deposits. 

That is, we cannot state that slumps are caused by earthquakes and disintegrative failures by pore-water redistribution during major storms. For example, earthquake-induced 

mass flows may be common wherever relatively loose 

(low-density) sedimentary deposits are found. Recent 

extensive liquefaction on a 0.25° slope following an earthquake off northern California (Field, this report) illustrates 

such a situation. A mudflow on a 4° slope in Santa Barbara 

Basin, Calif., is another illustration (Edwards, Lee, and 

Field, this report). Likewise, disintegrative failures in 

fine-grained deltaic deposits can probably occur without a 

significant redistribution of pore water during major stormwave-loading events. Disintegrative failures on very gentle 

slopes (0.01 o to 0.45°) of the Mississippi Delta (Coleman 

and others, this report) illustrate the behavior of sediment 

that flowed without needing to expand. In each new 

situation, therefore, a full evaluation of the factors relating 

to slope failure is needed. For the case of the northeastern 

Gulf of Alaska however, we have shown that the unique 

loading environment, sediment type, and sediment conditions lead to limited possibilities in terms of resulting failure 

types. 

ACKNOWLEDGMENTS 

Initial phases of this study were supported by the 

U.S. Geological Survey and the U.S. Bureau of Land 

Management (BLM). Support from BLM was through an 

interagency agreement with the National Oceanic and 

Atmospheric Administration, under which a multi-year 

program responding to needs of petroleum development of 

the Alaskan continental shelf, was managed by the Outer 

Continental Shelf Environmental Assessment Program 

Office. Reviews of the manuscript by W. W. Danforth and 

D.C. Twichell were extremely helpful. 

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Hydrocarbon resource report for proposed OCS lease sale 88: 

southeastern Alaska, northern Gulf of Alaska, Cook Inlet, 

and ShelikoffStrait, Alaska: U.S. Geological Survey OpenFile Report 82-928, p. 115-121. 

Whitman, R.V., 1985, On liquefaction: Proceedings of the 11th 

International Conference on Soil Mechanics and Foundation 

Engineering, San Francisco, v. 4, p. 1923-1926. 

142 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
Liquefaction of Continental Shelf Sediment: The 

Northern California Earthquake of "1980 

By M.E. Field 

INTRODUCTION 

Earthquakes have long been recognized as a major 

cause of submarine landslides. Much of this recognition has 

been based on ( 1) the assumption that earthquakes alone 

posse s the force required to cause abrupt slope failures 

and, to a degree, on (2) comparisons with onshore observations of mass movement during earthquakes. Our understanding of exactly how an earthquake affects the sea floor 

has been hampered by a lack of observations. Because 

sea-floor landslide deposits often consist of multiple events, 

it is difficult and often impossible to determine exactly what 

happened during a particular earthquake. 

In 1980, following a large (Richter magnitude 

M =7 .0) earthquake off northern California, a unique 

opportunity existed to survey a landslide caused by 

earthquake-induced liquefaction of sediment on the continental shelf (for a definition of liquefaction see Lee, 

Schwab, and Booth, this report). Many aspects of the 

earthquake and the landslide were well documented-an 

unusual circumstance for marine scientists unraveling geologic events for which most evidence is thousands to 

millions of years old. The timing, size, magnitude, and 

location of the earthquake were accurately known, and, 

because the sea floor had been surveyed 1 year prior to the 

earthquake, all changes to the sea floor could be linked 

directly to this particular earthquake. Within a month of the 

earthquake, marine surveys were initiated to map the sea 

floor in an area where local fishermen had reported the 

sudden presence of scarps and ridges. What follows is a 

synthesis of those surveys, emphasizing the landslide patterns and processes observed and identifying the implications this study has for development activities (for example, 

mining, disposal, pipelines) on the continental shelf. This 

paper draws heavily on previously published results (Field 

and others, 1982; Field and Hall, 1982; Field, 1984) as well 

as on research that is still in progress. 

The November 8, 1980 Earthquake 

High levels of seismic activity are commonplace in 

northern California due to overthrusting of the Gorda 

lithospheric plate by the North American plate and rightlateral slip along the Gorda Escarpment (fig. 1; Couch, 

1980). Since 1920, for example, an average of one earthquake of M~6.0 per decade has occurred (Couch and 

others, 197 4). Before the 1980 earthquake, two other events 

of M~7 .0 were known to have occurred in the area in the 

past century (Real and others, 1978). 

At 2:27 a.m. (Pacific Standard Time), on November 

8, 1980, a large-magnitude earthquake occurred 60 kilometers (km) west of Trinidad, Calif., on the continental 

margin. The earthquake had an estimated magnitude of 

M=7.0. Some damage occurred onshore, but for the most 

part, the effects were relatively minor in comparison with 

onshore earthquakes of comparable size (such as the Lorna 

Prieta California earthquake of October 17, 1989). Rockfalls and sand boils (caused by the liquefaction of sand 

below the ground) were reported along the coast, and, south 

of Eureka, an overpass on U.S. Highway 101 collapsed 

(Lajoie and Keefer, 1981). No other immediate reports of 

damage were received. 

Several days after the earthquake, Crescent City 

commercial fishermen, who frequently travel the coastal 

waters, reported the presence of one or more northwestOREGON ------- Crescent CALIFORNIA 

City 

GORDA Klamath River 

PLATE 

400 Mendocino Fracture Zone 

NORTH 

PACIFIC AMERICAN 

PLATE 

0 50 100 km 

Figure 1. This map shows the complex interaction of 

tectonic plates near the coast of northern California. 

These interactions result in a high number of earthquakes 

in the region. On November 8, 1980, an earthquake of 

magnitude 7.0 (simi lar to the Lorna Prieta earthquake of 

October 17, 1989) occurred offshore, and the seismic 

motion caused liquefaction and ground failure onshore 

and offshore along the coast. 

liquefaction of Continental Shelf Sediment: The Northern California Earthquake of 1980 143 
trending scarps offshore of the Klamath River. This area is 

a smooth, featureless depositional environment, and the 

sudden appearance of scarps suggested a causal relation to 

the earthquake. A survey of the area was conducted in· 

mid-December 1980 aboard the research vessel S.P. Lee, 

using high-resolution seismic-reflection equipment. As a 

part of the expedition, several tracklines that had been 

surveyed 1 year before the earthquake were resurveyed. The 

entire area was surveyed again in May and October 1981 , 

~using high-resolution seismic-reflection equipment, sidescan sonar, and bottom cameras. 

Setting of the Northern California Continental 

'Shelf 

The northern California continental shelf exhibits 

both active tectonism and relatively high rates of sedimentation. Off the Klamath River, the shelf is composed of a 

complex sequence of folded and faulted rocks that were 

deposited prior to several hundred thousand years ago. 

These strata are underlain by older rocks of the Franciscan 

Complex, ~nd they are overlain by undeformed seawarddipping sediment deposited during the past 20,000 years. 

Offshore of the Klamath River, these young deposits lie on 

an erosional surface and attain a thickness of 20 to 50 

meters (m) in water depths of 30 to 70 m (figs. 2 and 3; 

Field and others, 1980). This lobe-shaped sediment body 

presumably is derived largely from sediment delivered to 

the coast by the Klam~th River, a major contributor of 

sediment to the northwestern U.S. continental shelf. Two 

main types of surficial sediment occur on the shelf in this 

area: muddy sand inshore of the 60-m isobath, and sandyclayey silt from the 60-m isobath seaward (Field and others, 

1980; Welday and Williams, 1975). 

NATURE OF THE LANDSLIDE 

Geometry of the Landslide Deposit 

The submarine landslide is highly complex in shape, 

and this complexity indicates that several different processes occurred during or shortly after the earthquake. The 

part of the shelf that failed forms a 1- to 4-km-wide 

horizontal terrace (termedfailure terrace) lying in 60 m of 

water. The seaward edge of this mass-movement deposit is 

defined by a 1- to 2-m-high toe ridge that is nearly 

continuous for a distance of 10 to 15 km along the shelf (fig. 

3). Comparison of high-resolution seismic-reflection profiles obtained immediately after the earthquake with profiles 

collected a year before illustrates the sudden and distinct 

changes that occurred as a direct result of the earthquake. 

The slope of the sea floor in the landslide area is approximately 0. 25°. Although the subsurface depth of failure is 

not precisely known, seismic-reflection data indicate that it 

is at least 5 m and no more than about 15 m. 

In map view, the landslide zone is irregular, measures 

about 2 km by 20 km, and extends northwestward along the 

shelf, parallel to the coastline and shelf bathymetric contours (fig. 2). 11he landward side of the zone is marked in 

some places by a subtle scarp or a change in slope angle. 

Some of the features within and seaward of the landslide 

provide clues as. to the mechanisms and processes involved 

in slope failure (figs. 4 and 5). Specific features include: toe 

ridges, pressure ridges, sand boils, collapse depressions, 

sediment mass flows, and gas seeps. A prominent toe ridge 

is evident along the seaward edge of much of the landslide, 

particularly in the southern part (figs. 4 and. 5C). Seaward 

of the mass flow and toe ridge is a zone of compressional 

ridges up to 100m wide and lying parallel or subparallel to 

the orientation ·of the toe ridge (figs. 4, 5C, and 6). 

Sidescan-sonar images collected in ·May 1981 (6 months 

after the earthquake) show ·evidence of the eruption of sand 

boils onto the failure terrace (see sketch in fig. 6). Discrete 

rings of sand, some superimposed on one another, measure 

from a few metefs to as much as 25 m in diameter (Field and 

others, 1982). Sand boils are commonly observed on land 

after large earthquakes and show a similar range in size 

(Herd and others .• 1981). In many areas, the failure terrace 

has a mottled appearance that likely results from modification of the sand boils and other features by waves and 

currents. 

Depressions of the sea floor on the failure terrace, 

indicative of settling within the sediment and collapse of 

evacuated voids, occurred throughout the area at a broad 

range of scales (fig. 6). Individual pockets of settled 

sediment coalesced to form a cluster of depression features, 

often bounded by small scarps generally less than 0.5 m 

high. Some collapse depressions measure less than several 

meters across; others are approximately 25 m across and 

roughly circular to elliptical in shape. Sidescan sonographs 

and bottom photographs from the .north half of the landslide 

zone show evidence of deposits that formed by the flow of 

liquefied sediment for short lateral distances into overlapping lobes measuring as much as several hundred meters 

across. As many as five successive overlapping lobes were 

identified at one locality. Most pf these deposits are 

interpreted to result from lateral spreading of sediment, are 

generally of low relief (less than 0.5 m), show only subtle 

variations in sediment texture, and do not show evidence of 

having moved more than 100m (fig. 5D). 

In the general vicinity of the landslide zone, there is 

also evidence of seepage of hydrocarbon gases. Gas is 

evident from the presence of bubbles dispersed in the water 

column, small vents, and several large active seeps. These 

indications appear on high-resolution seismic-reflection 

profiles and on sonographs obtained at intervals of 1, 6, and 

11 months after. the earthquake. The origin of the gas 

emanating from the seeps is surmised to be thermogenic 

(Field and Jennings, 1987). 

144 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
l24 (' 30 

. . -- -- -- -- -.- ... - - . . . - ---- -- ---c;::·.--·- .. - =---~--=-----·~·::~-~ Pt. St. George 

---- ~·.·.:·:·::_.·_. Crescent 

41" 20' 

Fine to 

medium sand 

~ Muddy sand 

~ 1 Mud 

0 5 10 KM 

Figure 2. Northern California continental shelf showing 

the general location of the landslide zone in relation to 

the nature and thickness of surface sediment. The landslide occurred in sandy deposits derived from the Klamath River and did not spread into the mud deposits that 

characterize the mid-shelf; thus, it appears that grain size 

city 

was an important control on slope failure (the sediment 

distribution is from Welday and Williams, 1975). The 

thickness contours show that where the landslide 

occurred, the deltaic deposits were only 20 to 30 meters 

thick and that failure did not occur in the thickest 

sediment deposits. 

Liquefaction of Continental Shelf Sediment: The Northern California Earthquake of 1980 i 45 
WEST EAST 

AFTER 

L _j - __ L __ l 

- l -

t., !' '; I I ·:""···,.-r,-,1-. ..:~..: .·.v:- :.;;':" ~: :-: · ' 

- -----~ ~' ·,-- - ~---~-.·~-::.:-:-:-.--. ~- :-r., -=-:.· . ::.-:-:-~~;-;-.- T..,_.--.....-*-.......,_~,..., . ...;,·~· -·-· ·..:..\i_ .. ;._,-:,., 

•• :;"· ... .J •• - .. ··,· 

,. .. ··.· .... -_.. ~~·,-_.···_..·r_........._ __ ...._L __ _ 

--~ - --.. -- . 

_ _L ______ j. __L_-- r ••·,, I I 

Figure 3. Two seismic-reflection profiles collected along 

the same line, one before the earthquake, and one after 

(note that the folded strata below the sea floor are the 

same in each profile). In the "before" profile, the sea floor 

is smooth and gently inclined; in the "after" profile, the 

Styles of Mass Movement 

The complex and varied features in the landslide zone 

indicate that the progression of failure was complicated and 

included several modes of mass movement (fig. 6). Lateral 

spreading is inferred from the presence of a nearly continuous toe ridge that bounds the seaward edge of the flat-lying 

failure terrace, and the presence of a zone of compression 

(pressure ridges) lying seaward of the zone of mass movement. Liquefaction, the primary mode of slope failure, 

resulted in lateral spreading of approximately 800 million 

cubic meters of sediment and ejection of sand onto the sea 

floor (Field, 1984). A map of surface sediment types, with 

the landslide zone superimposed, shows that failure 

occurred in predominantly sandy deposits (fig. 2). The 

T l r - ------- T LINE ..:r -4 

sea floor has changed dramatically. The unconformity is a 

surface of erosion separating two sediment sequences. 

These profiles are best viewed from the side at a low 

angle. The vertical scale is based on the speed of sound in 

water. 

presence of overlapping sediment lobes indicates that the 

sediment liquefied and flowed as part of the failure process. 

As Lee, Schwab, and Booth (this report) point out, liquefaction occurs when a loosely packed sediment collapses 

under a force. Here, the grains temporarily lose contact with 

one another, and the particle weight is temporarily transferred to the pore fluid. The liquefied sediment may flow 

downslope or spread laterally. A summary of the processes 

involved in the Klamath landslide is given in figures 6 and 

7. Subsurface liquefaction is inferred at shallow depths as a 

result of excess pore-fluid pressures generated in sand by 

the earthquake and, possibly, enhanced by the presence of 

gas. Following liquefaction, or as a part of the process, 

reorganization of the subsurface sand into a somewhat 

146 Submarine landslides: Selected Studies in the U.S. Exclusive Economic Zone 
~~ , MOVEMENT 

. :E 

-- -'- ~ 1',' 

·o . 

0 ' 

100. M 

Figure 4. Sidescan-sonar images of the sea floor showing 

the complexity of the landslide. (A) The failure terrace has 

a mottled appearance distinctly different than that of the 

unfailed area of the sea floor. Note the well-defined toe 

ridge that marks the seaward edge of the landslide and the 

parallel pressure ridges formed by movement from upper 

denser packing arrangement and expulsion of pore fluids 

and gases occurred and led to collapse and flow of surface 

sediment. 

Importance of the 1980 Earthquake-Induced 

Landslide 

Because none of the factors, including sediment 

thickness, earthquake magnitude, and sediment type, 

'\ 

IOE RJDGE ~ 

right of the sonograph toward the lower left. (B) Similar to 

A but shows more complexity near the terminus. The 

boundary between the landslide zone and the undisturbed sea floor is marked by a series of discrete blocks 

measuring 10 to 50 meters on each side. 

involved in the 1980 liquefaction of sediment off the 

Klamath River were extraordinary, the landslide is important for appreciating the potential for greater effects here 

and on other shelves. Some of the aspects that are of 

particular importance are 

1. Mass movement occurred off the Klamath River on a 

nearly flat (0. 25° slope) area of the sea floor. Much of 

the continental margin has greater slopes (1.0° to 4.0°) 

and. thus, may be more susceptible to liquefaction by 

earthquake shocks. 

liquefaction of Continental Shelf Sediment: The Northern California Earthquake of 1980 147 
Figure 5. Photographs of the sea floor made by towing an 

underwater camera sled. (A) Photograph of a 70-meterdeep area of the sea floor that was not disturbed by the 

earthquake. Note the large number of burrows and tracks 

formed by biologic activity. (8) Highly disturbed sediment 

2. The earthquake had a magnitude of about 7.0 and 

occurred 60 km from the submarine landslide. Earthquakes of greater than 30 times that energy and much 

closer to the California shelf have occurred in the past 

and will occur in the future; thus, the probability of 

similar or greater landslides due to liquefaction is high. 

3. Upper Quaternary sediment (deposited during the past 

20,000 years) is only 20 to 30 m thick at the landslide 

site; thus, thick accumulations of modern sediment are 

not requisite for slope failure. This thickness of upper 

Quaternary sediment is equaled or exceeded in many 

areas of the California shelf and slope, so many areas are 

sites of potential landsliding. 

4. The landslide occurred in Holocene terrigenous sediment on the prodelta of a medium-size river. Many other 

such prodeltas exist in seismically active areas of the 

U.S. Pacific coast, and these are probable sites of 

submarine landslides or liquefaction. 

near the toe ridge showing complete disruption of the sea 

floor. (C) Example of a small pressure ridge formed 

seaward of the landslide zone. (D) Residual mass-flow 

deposits, with sediment appearing remolded as a result of 

liquefaction and flow. 

5. The surface manifestations of the landslide deposit are 

small (maximum relief of 2 m) and are difficult to detect 

with conventional geophysical techniques. Furthermore, 

the landslide occurred in relatively shallow water (60 m) 

that is subject to smoothing and obliteration of diagnostic features by waves and currents and by subsequent 

deposition of sediment. Therefore, absence of surficial 

expression does not necessarily indicate that mass movement has not taken place in the past; it merely indicates 

that it has not occurred or been observed within the past 

several years or decades. 

ACKNOWLEDGMENTS 

Brian Edwards, James Gardner, Robert Hall, and 

Ann Jennings were all valuable contributors to earlier 

reports used in this summary. Paul Carlson and Robert 

Kayen provided careful and thoughtful reviews of the 

manuscript. 

148 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
A 

8 

1/4° 

c 

PROGRESSION OF FAILURE 

SANDY MUD 

-

PRESSURE 

RIDGES 

I 

I 

I 

MUDDY SAND 

LIQUEFACTION 

LATERAL 

SPREADING 

/COLLAPSE FLOWS ........,__ /_ \ "''f ... SETTLEMENT and 

SECONDARY LATERAL SPREADING 

Figure 6. This series of diagrams shows the supposed 

progression of landsliding, based on the observations of 

features in the failure zone. (A) Initiation of slope failure 

started with earthquake-induced liquefaction of sandy 

sediment, probably as deep as 5 meters below the sea 

floor. (8) As the near-surface deposits liquefied, they 

spread laterally and erupted vertically onto the sea floor. 

(0 In the final stage, loose sediment flowed across the 

terrace and surface deposits collapsed and settled. 

Liquefaction of Continental Shelf Sediment: The Northern California Earthquake of 1980 149 
Figure 7. This three-dimensional diagram shows the subsurface geology and the overall nature of the landslide 

zone on the shelf. Note the irregular nature of the toe 

ridge along the length of the slope failure. This diagram is 

REFERENCES CITED 

Couch, R.W., 1980, Seismicity and crustal structures near the 

north end of San Andreas fault system, in Streitz, Robert, and 

Sherburne, Roger W., eds., Studies of the San Andreas Fault 

Zone in Northern California: California Division of Mines 

and Geology Special Report 140, p. 13"9-151. 

Couch, R.W., Victor, L.P., and Keeling, K.M., 1974, Coastal 

and offshore earthquakes of the Pacific Northwest between 

39° and 49° N latitude and 123° and 131° W longitude: 

Corvallis, Oreg., Oregon State University Press, School of 

Oceanography, 67 p. 

Field, M.E., 1984, The submarine landslide of 1980 off northern 

California, in Clarke, S. H. , Jr. , ed. , Highlights in Marine 

Research of the U.S. Geological Survey: U.S. Geological 

Survey Circular 938, p. 65-72. 

Field, M.E., Clarke, S.H., Jr., and White, M.E., 1980, Geology 

and geologic hazards of offshore Eel River Basin, northern 

California continental margin: U.S. Geological Survey OpenFile Report 80-1080, 80 p. 

Field, M.E., Gardner, J.V., Jennings, A.E., and Edwards, B.D., 

1982, Earthquake-induced sediment failures on a 0.25° slope, 

Klamath River delta, California: Geology, v. 10, p. 542-546. 

an accurate portrayal of the features and geometry of the 

landslide zone that were observed using sidescan sonar, 

underwater cameras, and acoustic profilers. 

Field, M.E., and Hall, R.K., 1982, Sonographs of submarine 

sediment failure caused by the 1980 earthquake off northern 

California: Geo-Marine Letters, v. 2, p. 135-142. 

Field, M.E., and Jennings, A.E., 1987, Seafloor gas seeps 

triggered by a northern California earthquake: Marine Geology, v. 77, p. 39-51. 

Herd, D.G., Youd, T.L., Meyer, Hansji.irgen, Arango, J.L., 

Person, W.J., and Mendoza, Carlos, 1981, The great 

Tumaco, Colombia, earthquake of 12 December 1979: 

Science, v. 211, p. 441-445. 

Lajoie, K.R., arid Keefer, O.K., 1981, Investigations of the 

November 1980 earthquake in Humboldt County, California: 

U.S. Geological Survey Open-File Report 81-397, 30 p. 

Real, C.R., Toppozada, T.R., and Parke, D.L., 1978, Preliminary earthquake epicenter map of California, showing events 

from 1900-1974 equal to or greater than magnitude 4.0 or 

intensity V: California Department of Conservation, California Resources Agency, Sacramento, Open-File Report 78-4 

SAC, scale 1:1,000,000. 

Welday, E.E., and Williams, J.W., compilers, 1975, Offshore 

surficial geology of California: California Division of Mines 

and Geology Map Sheet 2, scale 1:500,000. 

150 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
A Submarine Landslide Associated with Shallow SeaFloor Gas and Gas Hydrates off Northern California 

By M.E. Field and j.H. Barber, Jr. 

INTRODUCTION 

Submarine Landslides and Their Causes 

Submarine mass movements of sedimentary material 

occur in a remarkable variety of geologic settings around 

the world (Lee, Schwab, and Booth, this report). They 

range in size from a few meters to hundreds of kilometers 

along a side and involve materials that range from thin 

layers of weak, recently deposited sediment to great thicknesses of sediment or lithified strata. The factors that cause 

sediment slope failure to occur, however, have not been as 

thoroughly studied as the shape and character of the failures 

themselves. Discussions by Prior and Coleman (1982), 

Field (1987); and Lee (1989) point out that setting and 

sediment mass are important characteristics to evaluate. 

Scientists now recognize that earthquakes, storm waves, 

and locally high rates of sedimentation are associated with 

sediment slope failure. Specific reasons why one area 

undergoes slope failure while an adjacent area does not, 

however, are elusive. Most likely, these reasons are related 

to both the heterogeneous nature of the sediment mass and 

the variations in the local environmental setting (Schwab 

and Lee, Khis report; Edwards, Lee, and Field, this report). 

The natural variations in grain size, mineralogy, porosity, 

and bedding surfaces which occur in sedimentary deposits 

produce variations in sediment strength; those variations 

may prodJUce planes of weakness along which slope failure 

can occur. Locally, variations in the slope geometry, 

deposition rate, current activity, earthquake ground-motion 

responses, and the presence of excess pore-water pressures 

(in exces!; of hydrostatic pressure) and gaseous hydrocarbons change the environmental stress acting on a sediment 

mass and affect the strength of the sediment (Lee, Schwab, 

and Booth, this report). 

Gassy Sediment and Submarine Landslides 

Gaseous hydrocarbons occur naturally in marine sediment. They are commonly classified as either biogenic gas 

(chiefly methane), which forms by biologic decay of 

organic material near the sea floor, or thermogenic gas, 

which forms by exposure of organic material to high 

temperatures from deep burial and includes many complex 

hydrocarbons. Gaseous hydrocarbons also occur in the 

shallow subsurface in a solid, icelike structure called a gas 

hydrate. Changes in temperature, pore pressure, or depth of 

burial can cause the hydrate to become unstable (melt) and 

release gas into the sediment (Popenoe, Schmuck, and 

Dillon, this report; Kayen and Lee, this report). 

Gas concentrated in seeps and disseminated in sediment is not unusual in areas of sediment slope failures, for 

example, the organic-rich fluvial deposits that form the 

Mississippi Delta (Sieck, 1973; Whelan and others, 1977). 

Dissolved gas is relevant to considerations of sediment 

slope stability because it can abruptly change phase out of 

solution to a gas in response to rapid decreases in pressure 

(for example, from storm waves in shallow water, extreme 

tidal fluctuations, or earthquake accelerations). Although its 

exact role in slope failure is unclear, the presence of 

bubble-phase gas in pore spaces is thought by some workers 

to decrease sediment strength by generating excess porefluid pressures and reducing effective confining stresses, 

thereby increasing the likelihood of failure (Sangrey, 1977; 

Whelan and others, 1977). Those authors point out, however, that the presence of bubble-phase gas may absorb 

pressure changes from an external source and, thereby, 

strengthen the deposit. More clear is the role of exsolution 

of dissolved gas in muddy sediment, where it almost 

certainly disturbs the sediment fabric and leads to a reduced 

sediment strength. Thus, when bubble-phase gas is present, 

regardless of its source, a reduction of sediment strength is 

possible due to an increase in excess pore pressures. 

Although our study focuses on the northern California 

continental margin, there are at least two other locations 

where subsurface gas thought to be derived from degrading 

gas hydrates has been suggested as the principal cause of 

slope failure. Off the Southeastern United States, Carpenter 

(1981) and Popenoe, Schmuck, and Dillon (this report) 

suggest that the decay of gas hydrates and subsequent 

release of gas into the sea floor led to failure of sediment. 

On the north slope of Alaska, recent studies by Kayen 

(1988) and Kayen and Lee (this report) document the 

relationship of large sediment slides to the release of gas 

from degradation of gas hydrate beneath the sea floor. 

THE NORTHERN CALIFORNIA CONTINENTAL 

MARGIN: ABUNDANT SLOPE FAILURES AND 

GASSY SEDIMENT 

The continental slope off Eureka, northern California, lies within the Eel River Basin, an active forearc basin 

that contains over 4,000 meters (m) of Pliocene- and 

A Submarine Landslide Associated with Shallow Sea-Floor Gas and Gas Hydrates off Northern California 151 
126° 124° 122° 

42° 

OREGON .............. --· ........... ---- .................... CALIFORNIA 

GORDA 

PLATE 

Humboldt Slide Zone 

40° 

NORTH 

PACIFIC AMERICAN 

PLATE 

38° 

0 50 100 km 

Figure 1. Northern California continental margin. The 

study area is on the continental slope west of Eureka, just 

north of where the Pacific, Gorda, and North American 

crustal plates meet. 

Quaternary-age sediment (deposited throughout the past 5 

million years) (Field and others, 1980; Bachman and 

Crouch, 1986). The study area lies just north of the 

Mendocino triple junction, the intersection of the Gorda, 

Pacific, and North American crustal plates (fig. 1). Active 

tectonic forces have generated numerous large-magnitude 

earthquakes in the region (fig. 2; Couch, 1980), and these 

no doubt have been influential in causing substantial slope 

failure. Another result of the regional tectonic activity has 

been uplift of onshore regions, which has in tum led to 

substantial erosion and sediment transport to the continental 

shelf and slope. Local rivers deliver more than 20 million 

metric tons of organic-rich sediment to the coast annually 

(Field and others, 1980). Biogenic gas derived from these 

deposits, combined with the upward migration of thermogenic gas from sources deeper in the basin deposits, 

provides an abundance of gas in the near-surface deposits. 

All of these factors make the northern California continental 

margin between Eureka and Crescent City (fig. 1) an ideal 

location to examine the relationship between sediment mass 

movement and sea-floor gas. 

Through repeated research cruises to the area, much 

information about sediment character and processes has 

been acquired. The entire slope region is dominated by the 

presence of areas of mass movement, and one of these, the 

Humboldt Slide Zone, has been studied in detail. Other 

studies in the area (for example, Field and Kvenvolden, 

1987) have mapped subsurface acoustic patterns (on 

seismic-reflection profiles) indicative of shallow sea-floor 

gas and of gas hydrates; follow-up sampling cruises recovered sediment enriched in both. This pool of information, 

along with published studies from other regions, provides a 

solid background for examining the inferred role of gas in 

decreasing sediment strength and increasing the possibility 

of slope instability. 

Humboldt Slide Zone 

The continental margin off California north of Cape 

Mendocino contains more landslides than any other comparable region along the west coast of the United States. 

One of the more prominent areas of slope instability is the 

Humboldt Slide Zone, which lies west of Eureka on a 4° 

slope at water depths of 250 to 500 m. The Humboldt Slide 

Zone consists of large slump blocks that failed in a 

retrogressive (failure retrogrades upslope) manner (fig. 3). 

In cross section, individual slump blocks are up to 500 m 

wide and extend 50 m below the sea floor. The Humboldt 

Slide Zone is young, because it creates surface relief and is 

unburied , even though local sediment deposition rates are 

high relative to most continental margins (Lee and others, 

1981; EEZ-SCAN 84 Scientific Staff, 1986; Field and 

others, 1987). 

Studies of the physical properties and stratigraphic 

relationships within the upper several meters of the Humboldt Slide Zone, which consist of late Pleistocene age 

(deposited in the last 100,000 years) thin-bedded turbidites 

and silt interbeds, indicate that the sediment apparently 

failed due to earthquake-generated accelerations (Lee and 

others, 1981). Adjacent areas also have high earthquake 

incidence and a rapid sedimentation rate but have not failed. 

Therefore, some additional factor or factors must predispose certain areas, such as the Humboldt Slide Zone, to 

slope instability. Our work shows that gas is abundant in the 

shallow subsurface and may be important in reducing the 

strength of the sediment and allowing failure to occur. 

Evidence of Shallow Gas and Gas Hydrates 

The evidence for shallow gas beneath the northern 

California slope and outer shelf is abundant. Seismic 

reflections from below the sea floor are affected by the 

presence of gas both by masking (blanking out) and by 

enhancement (brightening) of reflectors from bedded deposits. Both of these effects are common throughout the study 

area, particularly in the slide zone (fig. 3; Field and others, 

1980; Richmond and Burdick, 1981). Hundreds of pockmarks (craters formed by seeping gas) have been mapped 

along two sidescan-sonar images that were collected parallel to the slope through the slide zone (pockmarks are shown 

152 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
.. 

• 

•• 

• 

• 

• 

• 

• 

~·. 

EARTHQUAKE e< 4 MAGNITUDE 

• 

• 

• 

• • 

• >6. 

Figure 2. Study area showing. the location of the Humboldt Slide Zone and the·many 

historical earthquakes that have occurred in the area. The epicenters represent 

historical observations and instrument measurements from 1860 to 1980 compiled 

mainly by Couch (1980). Also shown are the locations ·of the seismic profiles in figures 

3 and 4. The bathymetric contours are in meters. 

in fig. 4 and their location in fig. 5). The pockmarks 

average about 4 m in diameter and range up to a maximum 

of 25 m. S1tudies from a host of continental margin settings 

around the world have shown that such pockmarks are 

diagnostic of seepage of gas or fluid from· the sea floor 

(Curzi and Yeggiani, 1985; Hovland and Judd, 1988). 

Cores collected from the northern California margin 

were routinely analyzed for dissolved gas content. In one 

area just upslope from the Humboldt Slide Zone, these 

analyses indicate elevated levels (> 10,000 microliters per 

liter (~L/L)) of methane gas in the upper 2 m of sediment 

(fig. 5; Field and Kvenvolden, 1987). Normal background 

levels of dissolved methane in this area are less than 1000 

~L/L. The high-methane samples were recovered from the 

area of gas-charged sediment that includes the slide zone. 

The presence of gas hydrates in the study area is well 

documented. Gas hydrates were initially inferred to be 

present off northern California on the basis of a BottomSimulating Reflector (BSR). that underlies the continental 

margin at a subsurface· depth of about 250 m (Field and 

Kvenvolden, 1985). A BSR is a subbottom reflector that 

mimics the sea . floor and appears to cut across other 

A Submarine Landslide Associated with Shallow Sea-Floor Gas and Gas Hydrates.off Northern California 153 
w 

PROFILE 114 

LANDSLIDE ZONE 

PROFILE 115 LANDSLIDE ZONE 

+ 

-~':'::~1 •. J. ,.1 . L ~ .... I r '. .. I 

-----GASSY SEDIMENT 

Figure 3. High-resolution acoustic profiles crossing the 

Humboldt Slide Zone (see fig. 2 for locations). Note the 

hummocky surface of the sea floor in the slide zone (best 

seen on profile 115). Also evident in these profiles are 

numerous individual shear planes and slump blocks of 

Figure 4. Sidescan-sonar image of the sea floor along the 

slope (see figs. 2 and 3 for location). The sonar vehicle was 

towed from left to right along the center of the image. 

Dark patterns are areas of high-energy return (strong 

E 

--~ ·· 

--~·] 

'.i_;,;::;..; 

sediment. Beneath the slide zone, acoustic reflectors are 

locally enhanced or "brightened," and are indicators of 

subsurface gas. The arrows show the location of the 

sidescan-sonar image shown in figure 4. Vertical scale bar 

is based on speed of sound in water . 

. ,_.;...:. ~::;;~ 

'"'""" 

backscatter); light patterns are areas of low backscatter of 

energy, or shadows. Thus, the circular features viewed in 

this image, shadows succeeded by strong returns, are 

depressions on the sea floor. 

154 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
Figure 5. Study area showing the relationship between the 

location of the Humboldt Slide Zone and observations of 

shallow gas and gas hydrates. The arrows indicate the 

direction of mass movement. Note that (1) gas hydrates 

were observed (acoustically on seismic-reflection profiles) 

and sampled in surface sediment just downslope from the 

slide zone, (2) acoustic anomalies on seismic-reflection 

profiles indicative of shallow gas have been observed 

throughout the area, and (3) cores containing high conreflectors, thus indicating that it is not a sedimentary layer. 

BSR's apparently follow the gas-hydrate phase boundary 

(the pressure-temperature boundary that separates the gaseous phase below from the solid-hydrate phase above); 

thus, they mark the base of an extensive zone of gas hydrate 

(Kvenvolden, 1988). The landward extent of gas hydrate 

off northern California (fig. 5) is not accurately known 

owing to the difficulty in recognizing the BSR in flat-lying 

strata. The presence of gas hydrate off northern California 

was recently confirmed with samples obtained from the 

upper 1 m of the sea floor (Brooks and others, 1991). The 

hydrate samples occur as solid layers, lumps, and crystals 

separated by normal unhydrated terrigenous mud. The gas 

hydrates begin to sublimate (degrade) as soon as the core 

samples are withdrawn from the sea floor. As they decay, 

they yield gas volumes (at sea level) that are many times 

larger than their volume in the hydrated state. Thus, the 

hydrates, which cover an estimated area of 3,000 square 

kilometers (km2

) (Field and K venvolden, 1985), represent a 

potentially enormous source of sea-floor gas. The volume 

of gas released in the sea floor, where it is still under 

pressure and at low temperature, is significantly less. 

KILOMETERS 

10 20 

X Hydrate Sample 

0 Methane Sample 

:>/< Seep Crater 

centrations of dissolved methane-rich gas were collected 

from within and upslope of the slide zone. Sources of 

information are as follows: gas hydrate zone (Field and 

Kvenvolden, 1985); gas hydrate samples (Brooks and others, 1991); gas anomalies (Field and others, 1980; Field and 

Kvenvolden, unpublished data); gas measurements (Field 

and Kvenvolden, 1987); and Humboldt Slide Zone (Field, 

Clarke, and White, 1980; Field, Gardner, and others, 1987; 

Lee and others, 1981). 

Nonetheless, if the hydrate is in a state of disequilibrium, 

perhaps due to changing geothermal gradients, then bubblephase gas may be released into the sediment. 

THE ROLE OF CAS IN CAUSING SLOPE 

FAilURE IN THE HUMBOLDT SL.IDE ZONE 

Factors contributing to the abundance of landslides 

along this region of the Pacific continental margin are high 

levels of seismicity (historically, one event of magnitude 6 

or greater per decade), ongoing tectonic uplift and deformation, and the large quantities of sediment delivered to the 

margin each year from rivers. More recently, interstitial gas 

(derived from biogenic, thermogenic, and possibly from 

degraded gas hydrate sources) has been recognized as a 

potentially important factor contributing to the landslides. 

Gas in bubble phase can markedly increase the pore-water 

pressure and decrease the effective strength of the sediment, 

thereby reducing stability. 

A strong correlation exists between the location of the 

gas hydrate, gas-charged sediment, and the Humboldt Slide 

A Submarine landslide Associated with Shallow Sea-Floor Gas and Gas Hydrates off Northern California 155 
A 

8 

GAS VENTING THROUGH 

SLOPE AND SHELF EDGE SEDIMENT 

INCREASED 

VENTING 

GASSY SEDIMENT 

SEA FLOOR 

SEDIMENT FAILURE ~---...., 

GASSY SEDIMENT 

Figure 6. Schematic diagram showing the process by which subsurface gas, perhaps derived 

from a decaying gas hydrate, might exacerbate slope instability. In the top diagram (A), shallow 

gas seeps up in the sediment column and migrates updip beneath a partially impervious 

sediment layer. Due to the migrating gas, acoustic anomalies are present beneath the slope 

and outer shelf. The elevated gas content of pore waters decreases the strength of the 

sediment and increases the potential for slope failure, as shown in (8). Slope failure of the 

weakened sediment likely occurs in response to accelerations experienced from nearby 

earthquakes. 

156 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
Zone. Gas hydrates contain enormous quantities of gas, and 

their presence, along with abundant free gas in the slide 

zone, indicates a possible link between the gas hydrates and 

the slope failure. Gas migrating updip beneath the slope 

seeps through slope and outer shelf sedimentary deposits 

(fig. 6A). The resulting increase in bubble-phase gas content, in turn, leads to an overall reduction in strength and an 

increased tendency toward slope failure (fig. 6B). On some 

continental margins, this increased weakness may not result 

in slope failure without an external driving force. In Eel 

River Basi111, however, the frequency of large-magnitude 

earthquakes (fig. 2) is probably more than sufficient to 

induce failure of the weakened slope deposits. 

REFEREN~CES CITED 

Bachman, S.B., and Crouch, J.K., 1986, Geology and Cenozoic 

history of the northern California margin: Point Arena to Eel 

River, in lngersoll, R.V., ed., Cenozoic Basin Development 

of Coastal California: Englewood Cliffs, N.J., Prentice Hall, 

p. 124-·145. 

Brooks, J.M., Field, M.E., and Kenicutt, M.C., II, 1991, 

Observations of gas hydrates offshore northern California: 

Marine Geology, v. 96, p. 103-109. 

Carpenter, George, 1981 , Coincident sediment slump-clathrate 

complexes on the U.S. Atlantic continental slope: GeeMarine Letters, v. 1, p. 29-32. 

Couch, R. W., 1980, Seismicity and crustal structure near the 

north end of the San Andreas fault system, in Streitz, Robert, 

and Sherburne, Roger, eds., Studies of the San Andreas Fault 

Zone in Northern California: California Division of Mines 

and Geology Special Report 140, 187 p. 

Curzi, P. V., and Veggiani, A., 1985, I pockmarks nel mare 

Adriati~::o centrale: Acta Naturalia de l'Ateneo Parmense, v. 

21' p. 79-90. 

EEZ-SCAN 84 Scientific Staff, 1986, Atlas of the Exclusive 

Economic Zone, Western Conterminous United States: U.S. 

Geological Survey Miscellaneous Investigations Series 

1-1792, 152 p., scale 1:500,000. 

Field, M.E., 1987, Sediment mass-transport in basins: controls 

and patterns, in Gorsline, D.S., ed., Depositional Systems in 

Active Margin Basins: SEPM Pacific Section, v. 54, p. 1-23. 

Field, M.E., Clarke, S.H., Jr., and White, M.E., 1980, Geology 

and geologic hazards of offshore Eel River Basin, northern 

California continental margin: U.S. Geological Survey OpenFile Report 80-1080, 80 p. 

Field, M.E., Gardner, J.V., Drake, D.A., and Cacchione, D.A., 

1987, Tectonic morphology of offshore Eel River Basin, 

California, in Schymiczek, Herman, and Suchsland, Reinhard, eds., Tectonics, sedimentation, and evolution of the Eel 

River and other coastal basins of Northern California: San 

Joaquin Geological Society Miscellaneous Publication 37, p. 

41-48. 

Field, M.E., and Kvenvolden, K.A., 1985, Gas hydrates.on the 

northern California continental margin: Geology, v. 13, p. 

517-520. 

---1987, Preliminary report on gaseous hydrocarbons in 

sediment and seeps, offshore Eel River Basin, California, in 

Schymiczek, Herman, and Suchsland, Reinhard, eds., Tectonics, sedimentation, and evolution of the Eel River and 

other coastal basins of Northern California: San Joaquin 

Geological Society Miscellaneous Publication 37, p. 55-60. 

Hovland, Martin, and Judd, A.G., 1988, Seabed pockmarks and 

seepages: Graham and Totman, London, 293 p. 

Kayen, R.E., 1988, The mobilization of Arctic ·ocean landslides 

by sea level fall-induced gas hydrate decomposition: Unpublished M.S. Thesis, California State University, Hayward, 

227 p. . 

Kvenvolden, K.A., 1988, Methane hydrates and global climate: 

Global Biogeochemical Cycles, v. 2, p. 221-229. 

Lee, H.J., 1989, Undersea landslides: extent and significance in 

the Pacific Ocean, in Brabb, E.E., and Harrod, B.L., eds., 

Landslides: Extent and Economic Significance: Balkema, 

Rotterdam, p. 367-379. 

Lee, H.J., Edwards, B.D., and Field, M.E., 1981, Geotechnical 

analysis of a submarine slump, Eureka, California: Proceedings of the Offshore Technology Conference, Houston, Tex., 

OTC 4121, p. 53-59. 

Prior, D.B.,.and Coleman, J.M., 1982, Active slides and flows in 

underconsolidated marine sediments on the slopes of the 

Mississippi delta, in Saxov, Svend, and Nieuwenhuis, J.K., 

eds., Marine Slides and Other Mass Movements: Plenum, 

New York, p. 21-49. 

Richmond, W.C., and Burdick, D.J., 1981, Geologic hazards and 

constraints of offshore northern and central California: Proceedings of the Offshore Technology Conference, Houston, 

Tex., OTC 4117, p. 9-12. 

Sangrey, D.A., 1977, Marine geotechnology-state of the art: 

Marine Geotechnology, v. 2, p. 45-80. 

Sieck, H.C., 1973, Gas-charged. sediment cores pose possible 

hazard to offshore drilling: Oil and Gas Journal, v. 71, p. 

148-163. 

Whelan, Thomas, III, Coleman, J.M., Suhayda, J.N., and Roberts, H.H., 1977, Acoustical penetration and shear strength in 

gas-charged sediment: Marine Geotechnology, v. 2, p. 

147-159.· 

A Submarine Landslide Associated with Shallow Sea-Floor Gas and Gas Hydrates off Northern California 157 
Sur Submarine Landslide, a Deep-Water Sediment 

Slope Failure 

By C.E. Gutmacher and W.R. Normark 

INTRODUCTION 

Sur submarine landslide originated on the continental 

slope west of Point Sur, central California (fig. 1). The 

failed material moved downslope onto the surface of the 

adjacent Monterey Fan (Hess and others, 1979) and came to 

rest between 3,200 and 3,750 meters (m) water depth (fig. 

2). The resulting mass movement deposit is 70 kilometers 

(km) long, 5 to 25 km wide, and covers an area slightly less 

than 1,000 square kilometers (km2

). It ranges in thickness 

from a thin veneer (centimeters) to approximately 75 m and 

has an estimated volume of 35 cubic kilometers (km3

). Our 

study of Sur landslide is unusual because we have used 

many methods of remote-sensing, plus physical cores of the 

mass-movement debris, on a deep-water deposit. Many 

similar studies have been done in shallow water on smaller 

deposits where it is much easier and faster to gather data. 

Our work helps bridge a gap in the understanding of 

underwater slope failures, because Sur landslide is intermediate in size between the landslides studied in shallow water 

and the commonly much larger deep-water sediment slope 

failures. A full presentation of the data and comparisons 

with modem smaller-scale fjord and delta slope failures is 

given by Normark and Gutmacher (1988). 

Mass-movement debris, a product of the Sur submarine landslide, is one of the most recent depositional units 

on the Monterey Fan, a large turbidite system on the central 

California continental margin (fig. 1). Monterey Fan is fed 

by two major submarine canyon systems, the Monterey and 

the Ascension. Coarse-grained terrigenous sediment commonly reached the heads of these canyons during periods of 

lowered sea-level, but during sea-level high-stands (interglacial times such as today), only the Monterey canyon was 

an active pathway for sediment en route to the deep-ocean 

floor (Normark, 1970; Normark and others, 1985). 

The Sur landslide deposit was initially recognized and 

mapped during a cruise aboard the research vessel S.P. Lee 

in 1977. Subsequent cruises aboard the research vessel Sea 

Sounder in 1978 and 1979 completed our data base of 

3.5-kilohertz (kHz) bathymetric profiles, high-resolution 

seismic-reflection profiles, and sediment cores. We compiled the bathymetric map of figure 2 using depth measurements along our survey tracklines together with limited 

multibeam sounding data (Eittreim and others, 1983). The 

limit of the landslide deposit shown in figure 2 was 

interpreted from our 3.5-kHz profiles. GLORIA (Geological LOng-Range Inclined Asdic) sidescan-sonar imagery 

collected in 1984 provides a map of the near-surface part of 

the deposit and more detailed definition of the margins (fig. 

3; EEZ-SCAN 84 Scientific Staff, 1986). 

MORPHOLOGY 

The Sur landslide deposit extends southwest approximately 70 km from the base of the continental slope, 

crossing the main Monterey Fan valley at about 35°45'N 

latitude (figs. 2 and 3). The landslide deposit is elongate 

with irregular margins. It occupies, and locally spreads 

laterally from, a roughly rectilinear, sloping depression that 

is 8 to 15 km wide. The shape of the deposit is influenced 

by the continental slope to the east, local topographic highs 

near the southern distal margins, and by the levee of the 

Monterey Fan valley. The northwest side of the deposit 

onlaps the eastern levee of Monterey East Fan valley 

(MEFV in fig. 1). Except where the landslide deposit abuts 

a steeply dipping surface, the surfaces of Monterey Fan can 

be followed under the edge of the landslide deposit (fig. 4). 

The present surface of the landslide deposit has a relatively 

uniform slope in the direction of movement, averaging 

0.46°. 

The Sur landslide may have originated in 2,100 m 

water depth where a scarp can be seen on seismic-reflection 

profiles and the GLORIA imagery (fig. 3). Above the 

scarp, seismic-reflection profiles show gently folded sedimentary units of the lower continental slope mantled by a . 

layer of unlithified sediment with slope-parallel internal 

structure. This sediment layer is missing downslope from 

the scarp and presumably now forms much of the landslide 

deposit. The mobilized sediment apparently moved onto the 

fan near "X" in figure 2. Part of the landslide material 

moved to the northwest, where it ponded between the 

continental slope and the fan-valley levee, but most of the 

material moved to the southwest where it crossed the 

Monterey Fan valley (see arrows in fig. 2). The GLORIA 

imagery (fig. 3) confirms that the landslide debris extends 

past the Monterey Fan valley. It also shows the landslide 

deposit area to be generally wider than can be recognized on 

3.5-kHz profiles alone, especially along the northwestern 

margin. 

SUR LANDSLIDE STRUCTURE AND DEPOSITS 

The Sur landslide deposit exhibits several major 

topographic and structural features, best seen in the 3.5-kHz 

158 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
36° 

Figure 1. Location of Sur landslide deposit in relation to 

the outlint:! of Monterey Fan (Inset A; modified from 

Normark and others, 1985); inset 8 shows location of figure 

1 in relation to the State of California. The heavy dashed 

lines show major fan valleys as follows: Monterey fan 

valley (MFV), conduit of sediment from Monterey Canyon 

and, thus, from the rivers that empty into Monterey Bay; 

Ascension fan valley (AFV), a hanging tributary to Monterey 

fan valley that receives sediment that moves along the 

profiles, that distinguish it from the turbidite sediment of 

the Monterey Fan. The presence of landslide material is 

further supported by the chaotic nature of sediment in cores 

50 KM 

CALIFORNI.A 

(j~ 

~~ON ~ su~ N 

I <JP'-~'{ON 

\..uc'P.. 

outer continental shelf; Monterey East fan valley (MEFV), a 

hanging distributary from Monterey fan valley. "Hanging" 

means that the Monterey fan valley floor is lower than the 

floors of AFV and MEFV at their junctions with MFV. The 

dashed line encloses area shown in figure 2. The dotted 

outline shows the area covered by the GLORIA image of 

figure 3. Figure modified from Normark and Gutmacher 

(1988). 

recovered from the deposit, as opposed to the orderly layers 

of turbidite sediment seen in cores from the immediately 

adjacent fan and continental slope. 

Sur Submarine Landslide, a Deep-Water Sediment Slope Failure 159 
123°30' 122°30' 

36° r-----------~-------.-----.~~-.~rr-rr~-.~----------------~-.--------------------~ 

30' 

0 

~ headwall scarp 

/ direction of movement 

10 

KILOMETERS 

Bathymetric contour 

interval 25 meters 

20 

35° 

30'~------~----~--~~~------------~------------------~----------------~ 

Figure 2. Detailed bathymetry (topography of the ocean 

floor) of Sur slide and nearby fan valleys (see location in 

fig. 1). The outline of the Sur landslide deposit is derived 

from interpretation of 3.5-kilohertz records, and dashed 

where the edge of the landslide deposit is poorly known. 

Depths, taken from acoustic soundings (measurements 

of travel time of reflected sound energy), are corrected 

for variation of the speed of sound in water using correction tables by Carter (1980). The location of seismicreflection promes and cores shown in figures 4 and 5, 

respectively, are labeled. X marks the location where 

landslide material moved onto Monterey Fan. Z marks 

where the landslide debris crossed the MFV. The portion 

of headwall scarp that falls within this map boundary is 

shown to help orient the reader when comparing figures 

2 and 3. Figure modified from Normark and Gutmacher 

(1988). 
Figure 3. GLORIA sidescan-sonar imagery of the Sur landslide area (EEZ-SCAN 84 Scientific Staff, 1986). See Lee, 

Schwab, and Booth (this report) for a description of 

sidescan-sonar techniques. The arrow in the northeast 

corner points to the headwall scarp (breakaway area) of 

Sur landslide; this scarp appears as a black shadow perpendicular to the tip of the arrow. The other arrows point 

to the irregular boundary of the landslide deposit, which 

Acoustic Characteristics of the Sur landslide 

Deposit 

The acoustic characteristics of the Sur landslide 

deposit are generally similar to those seen on echo-sounding 

and seismic-reflection records from larger deep-water landslide deposits on passive margins, for example, Embley 

SHIP'S NADIR 

SHIP'S NADIR 

25 km 

is slightly lighter (grayer) on these GLORIA images. The 

boundary is distinct in some places but is difficult to 

recognize in areas of local relief on the fan (for example, 

northwest side of the landslide north of latitude 36°). 

Monterey Fan valley (MFV) can be discerned easily along 

the north and west sides of the image. As in figure 2, Z 

marks where the landslide debris crossed the MFV. Figure 

modified from Normark and Gutmacher (1988). 

(1976) and Damuth and Embley (1981). The deep water, 

however, often prevents sufficient resolution of detail 

needed for interpretation of the processes involved. Therefore, to make better use of detailed data from our modestsized Sur landslide deposit, we have also looked to studies 

of shallow-water ( < 500 m) landslide deposits to help 

interpret the acoustic character in conjunction with the core 

Sur Submarine landslide, a Deep-Water Sediment Slope Failure 161 
E 3100! ~: - 8 3400 

Figure 4. Selected 3.5-kilohertz acoustic-reflection profiles across the Sur landslide deposit. Line drawings show 

interpretation of reflecting surfaces to indicate structure. 

See figure 2 for locations. Wand E indicate west and east 

orientations, respectively, for the individual profiles. Vertical exaggeration of the profiles is between 9 and 14 

times. This means that the slope of the sea floor and of any 

subbottom reflecting surfaces will appear grossly exaggerated (up to 14 times actual slope). Areas of different 

acoustic character are indicated as follows: (1) hummocky 

data presented later. Only detailed studies of shallow-water 

deposits have led to definitive correlation of type of 

sea-floor bottom relief with given acoustic character, for 

example, Prior, Bornhold, Coleman, and Bryant (1982), 

Prior, Bornhold, and Johns (1984), and Coleman and others 

(this report). 

Hummocky-surfaced deposits occur over large areas 

of the northern half of the Sur landslide deposit. Where 

observed with a wide-beam profiling system (that is, 3.5 

kHz) from a sea-surface source, edges of depressions and 

the summits of individual blocks or ridges of sediment 

within the landslide mass act as point reflectors. Such point 

reflectors produce hyperbolic echoes on the 3.5-kHz profiles, resulting in a generally hummocky appearance. Profiles from the northern part of the Sur landslide deposit 

show numerous hyperbolic echoes (fig. 4), with a few 

individual sea-floor reflection sources standing more than 

10 to 20 meters above adjacent hummock apices. The 

hyperbolic echoes are too numerous to allow an estimate of 

0 

I 2m l '_r:l 

w 

returns, (hyperbolic echoes) thought to be from blocks of 

landslide debris, (2) smooth sea-floor reflection overlying 

acoustically transparent sediment, thought to be debrisflow deposits r¢sulting from Sur landslide, and (3) diffuse 

returns, thought to be from surface roughness on the 

order of tens of centimeters to a few meters high. Under 

the acoustically,transparent layers in the smooth areas, the 

pre-existing surfaces of Monterey Fan and the lower 

continental slope can be observed to a depth of more than 

50 meters. 

the size or height of individual blocks. We interpret the 

hyperbolic echoes to be from areas of blocky debris flow. 

The original sea 'floor is observed to dip beneath the margins 

of the landslide mass but is not observed beneath the 

strongly reflective hummocky terrane (fig. 4). 

Smooth-surfaced deposits without internal reflectors 

occur as ponded sediment units both within the main 

landslide deposit and perched on its margins (fig. 4, profile 

A); this type of deposit also abuts most hummocky areas 

(fig. 4, profile B). We interpret the strongly reflective, 

smooth sea floor overlying the acoustically transparent 

sediment to be areas of debris-flow deposits following the 

interpretations by Damuth and Embley (1981). On Sur 

landslide, the smooth-surfaced debris-flow deposits are 

mostly around the margins of the northern hummocky area 

and near the distal margin adjacent to Monterey Fan valley. 

Zones of small-scale hummocky sea-floor relief and 

areas of smooth sea floor both exhibit a weak (diffuse) 

sea-floor acoustic return that tends to obscure subsurface 

162 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
acoustic character. These zones are distinct from both the 

areas of s1rong hummocky and smooth reflectors described 

above but are not always distinct from each other. Zones of 

diffuse, weak surface reflectivity (fig. 4, line A) on Sur 

landslide appear to be an intermediate stage between blocky 

(hummocky) debris flow and smooth-surfaced debris flow, 

and are common in the southern half of the mass movement 

deposit. \Ve interpret the zones of diffuse reflectivity to 

result from fine-scale relief on the surface of the massmovemem deposit. Embley (1976) suggested small-scale 

folding or other relief causes the prolonged echo character 

in his deep-water study areas (we use the term diffuse to 

describe d:1e same echo character). 

There is a fringing zone where the mass-movement 

deposits are too thin ( <3 m) to be resolved on 3.5-kHz 

profiles but where the character of the sea-floor reflector is 

nevertheless distinct from that of the adjacent fan or 

continentaJ-slope sediment surface. The acoustic backscatter pattern on GLORIA imagery shows this fringing zone 

locally has a greater extent than can be detected on our 

3.5-kHz profiles (compare the landslide outlines in figs. 2 

and 3). 

Fan sediment, characteristic of overbank deposition 

(parallel-bedded, locally showing the "dune" shape of 

sediment waves), derived from the MEFV is buried by the 

mass-movement deposit without apparent disruption (see 

west end of profile fig. 4B). Most margins of the landslide 

deposit show this onlapping onto prelandslide sediment. 

Small, isolated, topographic highs and lows (fig. 2) 

lie along a band of darker gray backscatter within the lighter 

gray landslide area seen on the GLORIA image (fig. 3). 

This darker band might mark a continuous channellike 

depression which might have partly channelized the flow or 

been created by it. 

We know that Sur landslide is relatively young, 

because no layer of sediment thick enough to be resolved on 

the 3.5-kHz profiles is observed to bury the landslide 

deposit on any of the profiles. 

Cores From the Sur landslide Deposit 

It is important to remember that the thickness of the 

landslide mass (up to 75 m) is far greater than the length of 

any available core. The cores show us the composition of 

only the top of the deposit and the thin postlandslide 

deposits that the 3.5-kHz system cannot resolve. The six 

cores we obtained from the study area are illustrated in 

figure 5; their locations are shown in figure 2. Core 130 is 

immediate:ly adjacent to the landslide deposit, and contains 

undisturbed turbidites of the Monterey Fan. 

Cores l P and 12G are from the area of hummocky 

acoustic character and contain a muddy matrix with mud 

clasts (mud balls, mud chips, and large rounded to subangular fragments) that range in size from a centimeter across 

to some that are wider than the core barrel. Indeed, some of 

the thicker "layers" in the cores may be fully penetrated 

large clasts (for example, see "clast" in lP, fig. 5). The 

mud-clast rich intervals include both matrix- and clastsupported intervals (see 1P). In both cores, the mud-clast 

units are overlain by a sand and muddy turbidite unit. We 

think the mud-clast layers and overlying turbidite units pond 

in the low areas between the large slide blocks of the 

hummocky areas. As the blocks move and break ·apart, 

various sized clasts would be continuously generated. 

Color can be used to distinguish turbidite mud from 

hemipelagic mud (sediment derived mainly from particles 

sinking through the water column) on Monterey Fan (Brunner and Normark, 1985; Brunner and Ledbetter, 1987). 

Mud in cores 120 and 1P, however, includes mixt.ures of 

both of these distinctive sediment types indicating a postdepositional mixing of sediment such as by involvement in 

the Sur landslide event. There is also a variety of other mud 

types not seen in cores from the Monterey Fan, indicating 

the sediment probably is from a nonfan source area. Further 

support for a nonfan source is the fact that the benthic 

foraminifers taken from both the clasts and matrix in these 

cores indicate the original site of deposition was on the 

continental slope (oral comm., J.C. Ingle, Jr., 1979). 

Cores 2P and 4P from areas of diffuse sea-floor 

reflectivity (interpreted from acoustic character to be debris 

flows) consist mainly of intensely deformed turbidites, 

overlain by thicker units of undisturbed turbidite mud and 

sand. A layer of mud with some upper-slope component 

separates the deformed turbidites below from the undisturbed turbidite at the surface of both cores. Much of core 

2P has vertical or steeply inclined sections of contorted and 

sheared turbidite sediment (fig. 5). Other indications of 

disturbance within these two cores are overturned folds or 

refolded tabular rip-up clasts (see F, fig. 5), mud-clast 

debris-flow units, mudballs with the appearance of sandy 

armor (see middle of E, fig. 5), and irregularly shaped sand 

blebs in mud matrix (see lower F and mid-lower 0, fig. 5). 

These cores (especially 2P) may have penetrated a zone of 

small blocks of deformed sediment or fold ridges similar to 

those observed in a fjord and described by Prior and others 

(1984). The deformed turbidites in cores 2P and 4P suggest 

either incorporation of Monterey Fan sediment into the 

debris flow or deformation of the fan surface by its passage. 

These details have been adequately described only from the 

fjord (shallow water) studies. Their sedimentologic characters are consistent with an area of fine-scale topographic 

relief on the order of tens of centimeters to a few meters 

high. This fine-scale relief would contribute to a diffusely 

reflective acoustic character as observed in our profiles of 

the area. 

Core 110 (fig. 5) is from the middle of the landslide 

area, where the mass-movement deposits, as interpreted 

from acoustic character, are locally thin or nonexistent 

(similar to the fringe zone). The core contains sections of 

Sur Submarine Landslide, a Deep-Water Sediment Slope Failure 163 
o Hue;£ REFLECT y 

r 1\..c 

0 4P·~ 4P·3 4P·<1 0 p 2P·t 2P·2 

G F E 00 

Figure 5. Schematic logs (drawings) of sediment cores 

from Sur landslide deposit, with selected associated photographs. All scales are in centimeters: 100-centimeter 

interval for the drawings and 50-centimeter interval for 

photographic sections. Lines and letters A through G next 

to the core logs indicate section of core shown in photographs A through G. The core logs are horizontally exagmixed-source mud, but the undisturbed appearance of the 

sandy turbidite units makes it look like a typical fan core. 

The corer seems to have penetrated through a thin veneer of 

debris-flow mud (represented by a sandy mixed-source mud 

from 0 to 12 centimeters (em)) and recovered underlying 

preslide fan turbidites. 

We have no cores from any area of strong, smooth 

reflectors. More complete core descriptions are given by 

Normark and Gutmacher (1988). 

"'\r 

)ZG, 

• 

HUMMOCKY ACOUS IC 

CHARAC ER __/'\._ 

1P 1P·l 

I 

A 

\ 

I 

\ 

B 

u DISTURBED ONTEREY 

fAN TURB'OI ES VE EER 

' ,_J....___, ~~ 

A 

DEPTH • CE METERS 

' xi!O mud 

Sand 

···~Sandy mud 

Mudd· Is 

4 Top ol land tide 

depo tt 

110 

gerated (2 times) to make it easier to show important 

features; a side effect of this exaggeration is a noticeable 

difference in the shape of some features between the log 

and the photograph (for example, the armored mudball 

(half-circle with dotted rim) in section E appears rounder 

in the drawing). Core locations are given in figure 2. 

Figure modified from Normark and Gutmacher (1988). 

VOLUME 

When considering an underwater landslide deposit 

that covers an area roughly equal to that of the entire 

Monterey Bay, it is interesting to look also at the volume of 

material that moved. Hess and others ( 1979) calculated the 

volume of the landslide deposit to be 35 km3 based on the 

thickness of disturbed sediment shown by the 3. 5-kHz 

profiles and assuming a maximum thickness of 75 m for the 

164 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
axis of the deposit, where the base of the landslide could not 

be resolved by the profiles. This is the greatest thickness 

observed in any 3.5-kHz profile from the landslide deposit 

and is a reasonable estimate considering that the thickness 

of the deposit cannot be resolved on many of the available 

single-channel seismic-reflection profiles that cross it and 

that can generally be used to map layers 50 to 60 m or 

thicker. A crude estimate of the missing lower-slope section 

described above (see fig. 5 in Normark and Gutmacher, 

1988), however, is only about 10.5 km3 (derived from an 

observed average slab thickness of 100m and a "denuded" 

area of 105 km2 based on an approximation from the 

GLORIA :image). These two volumes are a factor of three 

apart, suggesting either that much of the landslide debris 

came from other areas of the slope or that a large component 

of fan sediment has been incorporated into the landslide. 

AGE OF MOVEMENT 

The timing of submarine mass movement is cc;>mmonly difficult to establish. In rare cases, the age of a 

landslide iis known, as when it moves manmade objects 

from their known locations (Hampton, Lemke, and Coulter, 

this report; Normark, Moore, and Torresan; this report) ·or 

when pre- and postlandslide data are available in a given 

area (Field, this report). In most cases, we measure the 

thickness of undisturbed, overlying sediment and estimate 

its rate of deposition. The age of the Sur landslide is 

difficult to determine from the overlying sediment cover, 

but the following lines of reasoning suggest a geologically 

recent event: 

1 . The four cores from the landslide deposit (12G, 1 P, 2P, 

and 4P) have between 23 and 87 em of apparently 

undisturbed hemipelagic and turbiditic sediment at th~ 

top. The closest data we have are from. a paleoenvi.ron~ 

mental study of a levee core taken on Monterey Fan 50 

km northwest of the landslide (Brunner and Normru:k, 

1985; JvlcGann, 1986), showing the Holocene hemipelagic sedimentation rate is about 15 emil ,000 years. If 

all the hemipelagic sediment veneer were deposited at 

this rate, the Sur landslide's age is 1,500 to 6,000 years. 

2. Sand and mud turbidites occur in the top parts of the 

same four cores (fig. 5). Because turbidites are deposited almost instantaneously relative to hemipelagic mud, 

we use only the hemipelagic intervals to calculate a .time 

of landslide movement of about 1 ,500 years befor~ 

present. If, however, the surficial turbidite layers were 

generated at the time of a landslide event, the youngest 

phase of the Sur landslide may be just several decades 

old, and the failure must have occurred in more.than one 

event. Until accurate dates are obtaine~ from the 

deposit, however, the age estimates are speculative. 

SUR LANDSLIDE SCENARIO 

The source area for the Sur landslide is on a sedimentologically inactive section of lower continental slope. It is 

bypassed by Monterey Canyon and its large load of terrigeno~s sediment. It may receive small amounts of sediment 

~rom downslope transport, but we believe the primary 

sediment input to be the slow rain of hemipelagic material. 

The source area is deep .enough (about 2,200 m), to be 

beyond the deepest effect of storm waves. Its slope of 5° 

and slow gradual loading by hemipelagic sedimentation 

point to a statically stable environment. In their study of a 

mud flow in Santa Barbara Basin, Edwards, Lee, and Field 

(this report) analyze a somewhat more gentle slope with a 

source area of similar although smaller features, and conclude that an earthquake was· needed to trigger such a 

landslide. Likewise, one or more earthquakes likely caused 

the Sur landslide. Such an earthquake could occur along one 

of the many faults in. this seismically active region of 

northern California. As an example of the importance of 

earthquakes in· causing landslides in this area, researchers 

suspect that new slope failures took place along this part of 

the California margin as a result of the Lorna Prieta 

Earthquake of October 17, 1989 (Greene and others, 1991). 

· We envision that the Sur landslide deposit formed 

from breakup and downslope. transport of large slabs of 

slope sediment that were destabilized by one or more 

earthquakes. It is not possible with the available data to 

discern w~ether this landslide was the result of a single 

event or is a multistage slope failure. In either case, some 

intact sections of slope sediment as large as office buildings 

( :525 m high) moved 5 ·km down the continental slope and 

as much as 20 km across the gentle 0. 5° incline of Monterey 

Fan. Smaller·, Jlouse-sized (:51 0 m high) blocks were 

transported as much as 30 km farther. Movement of these 

blocks incorporated prelandslide fan sediment within the 

main mass-movement. deposit. Breakup of the partially 

consolidated sediment blocks and incorporation of water 

gave rise to debris 'flow(s) and turbidity current(s). Deposits 

from the latter events both formed the cap or "icing" on the 

Sur landslide bloc~s and also travelled another 15 km to 

cross the 50:-m deep. Monterey Fan valley before finally 

coming. to rest. 

ACKNOWLEDGMENTS 

We thank G.R. Hess for his assistance during the 

initial phase of this study. C.A. Brunner helped us prepare 

detailed core descriptions, and T.E. Chase provided 

continuous-strip 3. 5-kHz profiles and color photographs of 

the core sections shown in figures 4 and 5. We thank J.V. 

Gardner, for providing the GLORIA data for figure 3. The 

manuscript was 'improved through reviews by Mike Field 

and Monty Hampton. 

Sur Submarine Landslide, a Deep~Water Sediment Slope Failure 165 
REFERENCES CITED 

Brunner, C. A., and Ledbetter, M.T., 1987, Sedimentological and 

micropaleontological detection of turbidite muds in hemipelagic sequences: an example from the late Pleistocene levee 

of Monterey Fan, central California continental margin: 

Marine Micropaleontology, v. 12, p. 223-239. 

Brunner, C.A., and Normark, W.R., 1985, Biostratigraphic 

implications for turbidite depositional processes on the Monterey deep-sea fan, central California: Journal of Sedimentary 

Petrology, v. 55, p. 495-505. 

Carter, D.J.T., 1980, Echo-sounding correction tables: Hydrographic Department, Ministry of Defence, Taunton, United 

Kingdom, 150 p. 

Damuth, J .E., and Embley, R. W., 1981, Mass-transport processes on Amazon Cone: western equ~totial Atlantic: Bulletin 

American Association of Petroleum Geologists, v. 65, p. 

629-643. 

EEZ-SCAN 84 Scientific Staff, 1986, Atlas of the Exclusive 

Economic Zone, western conterminous United States: U. S. 

Geological Survey Miscellaneous Investigations Series, 

1-1792, 152 p., scale 1:500,000. · 

Eittreim, S.L., Embley, R.W., and Normark, W.R., 1983, 

Monterey sea-valley meander revisited: EOS, Transactions 

American Geophysical Union, v. 64, p. 730. 

Embley, R. W., 1976, New evidence for occurrence of debris-flow 

deposits in the deep sea: Geology, v. 4, p. 371-374. 

Greene, H.G., Gardner-Taggart, Joan, Ledbetter, M.T., Barrninski, Robert, Chase, T.E., Hicks, K.R., and Baxter, Charles, 

1991, Offshore and onshore liquefaction at Moss Landing 

spit, central California-Result of the October 17, 1989, 

Lorna Prieta earthquake: Geology, v. 19, p. 945-949. 

Hess, G.R., Normark, W.R., and Gutmacher, C.E., 1979, Sur 

submarine slide, Monterey Fan, central California: Geological Society of America Abstracts with Programs, v. 11, p. 

83-84. 

McGann, M.L., 1986, Paleoenvironmental analysis of latest 

Quaternary levee deposits of Monterey Fan, Central California continental margin: foraminifers and pollen: Unpublished 

M.S. thesis, University of California, Berkeley, 155 p. 

Normark, W.R., 1970, Channel piracy on Monterey deep-sea fan: 

Deep-Sea Research, v. 17, p. 837-846. 

Normark, W.R., and Gutmacher, C.E., 1988, Sur submarine 

slide, Monterey Fan, central California: Sedimentology, v. 

35, p. 629-647. 

Normark, W.R., Gutmacher, C.E., Chase, T.E., and Wilde, Pat, 

1985, Monterey Fan, Pacific Ocean, in Bouma, A.H., 

Barnes, N.E., and Normark, W.R., eds., Submarine Fans 

and Related Turbidite Systems: Springer-Verlag, New York, 

p. 79-86. 

Prior, D.B., Bornhold, B.D., Coleman, J.M., and Bryant, W.R., 

1982, Morphology of a submarine slide, Kitimat Arm, 

British Columbia: Geology, v. 10, p. 588-592. 

Prior, D.B., Bornhold, B.D., and Johns, M.W., 1984, Depositional characteristics of a submarine debris-flow: Journal of 

Geology, v. 92, p. 707-727. 

166 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
Seisrnically Induced Mudflow in Santa Barbara Basin, 

California 

By B.D. Edwards, H.j. Lee, and M.E. Field 

INTRODUCTION 

The California Continental Borderland is a seismically active region of complex topography located offshore 

of southern California. This submerged margin is characterized by slope failures and mass-movement deposits of 

many scales, ranging from features covering less than 100 

square meters (m2

) to those covering more than 100 square 

kilometers (km2

) (Thornton, 1986; Field and Edwards, this 

report). This section summarizes a detailed study of one of 

these features, a 6- to 15-m-thick mass flow covering nearly 

4 km2 of the sea floor in Santa Barbara Basin (fig. 1). This 

study allows us to evaluate a number of driving mechanisms 

that might have initiated the slope failure as well as to 

provide an understanding of why the failure developed into 

a mud flow. 

Mass flows involve some amount of continuous 

deformation throughout the failed sediment mass, in contrast to slumps which exhibit discontinuous deformational 

.._. __ ..&.....a------· Holocene Undifferentiated Quaternary 

behavior (distinct shear surfaces) along the failure plane and 

within the failed mass (Lee, Schwab, and Booth, this 

report). A mudflow is a special type of mass flow that 

involves predominantly muddy sediment. Most mudflows 

that have been studied in the marine environment are 

located at active delta-fronts (Coleman and Garrison, 1977; 

Roberts, 1980; Prior and <?thers, 1984; Coleman and others, 

this report; Schwab and Lee, this report). The deposit we 

describe here lies on a smooth section of the continental 

slope removed from major river sediment sources. 

The muddy sediment blanketing the floor and flanks 

of Santa Barbara Basin is derived from the Ventura and 

Santa Clara River systems which carry detritus eroded from 

coastal mountains into the eastern end of the basin (fig. 1). 

At present, no delta system exists at the river mouths; 

river-derived sediment is transported southeastward along 

the coast by longshore currents where most of the sand 

component is shunted into the adjacent Santa Monica Basin 

0 KM 

119°30' 

25 

Barbs on upthrown side of reverse or thrust fault; 

dashed where location uncertain; dotted where buried 

lsobaths in meters 

Transverse Mercator projection 

Figure 1. Santa Barbara Basin showing the location of the slope failure. The location of the Pitas Point-Ventura fault is 

from Yerkes (1985). Bathymetry is in meters. 

Seismically Induced Mudflow in Santa Barbara Basin, California 167 
• • 

EXPLANATION 120o

00 . Transverse Mercator projection 119o 30 . 

• • • 0 25 KM 

• lsobaths in meters 

• • 

• • • • • ~4..~ 

+ • ~ 

~·~~ .. • ~~ • ':J'&: . : .· . 

• 

• 

+ 

• 

.. 

Magnitude: 

• 5.0-5.9 

• 4.0-4.9 

• 3.0-3.9 

"\:71812: M=7.0-7.5 

V Epicentral Region 

i:11925: M=6.3 

.· ·.···.··•··•····. · ... • . =!i: .:::."(' .• .... ~·"' 

Figure 2. Seismicity of the Santa Barbara Basin region for the period 1932 to 1981. Also shown are the estimated locations 

for selected events: 1925 magnitude=6.3 and 1812 magnitude=7.0 to 7.5 (Yerkes and others, 1981). The zone shown for 

the 1812 event indicates the uncertainty in interpreting historic records. 

through submarine canyon systems (Malo uta and others, 

1981). In contrast, the fine-grained suspended sediment 

load is transported westward into Santa Barbara Basin by 

the Anacapa Current entering the basin from the east 

(Thornton, 1984). 

TECTONIC SETTING 

Santa Barbara Basin began forming about 15 million 

years ago by lateral shear at the boundary between the North 

American and Pacific plates (Crowell, 1974; Howell, 

197 6). This coastal region is cut by a large number of faults, 

many of them active in Pleistocene and Holocene time (2 

million years ago to present; Yerkes, 1985). Most of the 

faults trend parallel or sub-parallel to the coastline and 

regional bathymetry. One of the major faults, the Pitas 

Point-Ventura fault, cuts near-surface sediment just upslope 

from the area of slope failure (fig. 1). Continuing tectonic 

disturbance is indicated by strong seismic activity recorded 

throughout historic time (fig. 2). Notably, the Richter 

magnitude 6.3 Santa Barbara earthquake of 1925 was 

centered roughly 20 km from the site of slope failure. The 

even stronger 1812 earthquake, which did considerable 

damage to Spanish miSSions located in the area, had an 

estimated magnitude of 7.0 to 7.5; estimates of the epicenter location range between 25 to 40 km from the site of the 

slope failure (fig. 2; Yerkes and other , 1981). A magnitude 

7.5 event is thought to be the largest credible earthquake for 

the Santa Barbara Basin area (Greensfelder, 1974). 

MORPHOLOGY OF THE MUDFLOW 

More than 100 trackline-km of high- and intermediate-resolution seismic-reflection profiles document a 

well-bedded, prograding sequence of marine sediment that 

has developed on the mainland flank of Santa Barbara Basin 

since the late Plei tocene (700,000 years ago; Vedder and 

others, 1969). Deformed sediment and faults that cut this 

prograded sequence at depth attest to tectonic disturbance of 

the sediment throughout this period. Only the uppermost 

part of this sedimentary section, however, has been disrupted by the shallow-seated (6- to 15-m-thick) mudflow 

described here (fig. 3). 

Major characteristics of the zone of slope failure (as 

shown in figs. 4, 5, and 6) include the following: 

1. A main scarp at the head of the slope failure that marks 

the point of detachment from the upslope , undisturbed, 

168 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
PROFILE S-2 

---SOOm 

Figure 3. Seismic-reflection profile (40-cubic inch airgun 

source) of the Santa Barbara Basin mainland slope. The 

record shows a prograding sequence of Pleistocene and 

Holocene sediment. Note the deformed sediment and 

shallow-seated fault just upslope from the head scarp of 

the failure . Trackline location is shown in figure 70. One 

millisecond (ms) of two-way travel time is approximately 

equal to 0.75 meter water depth. 

• 

intact sediment on the basin flank. This scarp, termed 

the head scarp, extends from water depths of 374 to 400 

m. 

2. An arcuate, amphitheater-shaped region, termed the 

evacuation zone, is exposed downslope of the head 

scarp . Sediment that originally filled this zone was 

dislodged and removed from the area during slope 

failure and now constitutes the main body of the mudflow deposit located farther downslope. The head scarp 

continues laterally around the upper part of the evacuation zone to become side scarps. 

3. In the western part of the slope failure, the evacuation 

zone has not been covered by sediment as viewed on 

high-resolution seismic-reflection profiles (figs. SA and 

7). Here the slope-failure surface remains exposed or 

only thinly mantled ( < 1 m) and is termed the scar. 

4. The eastern part of the slope failure has been enlarged by 

the degradation of the head scarp, a process referred to 

as retrogressive failure (figs. 5B and 7). The resulting 

coherent blocks of sediment and secondary mudflow 

deposits are found in the evacuation zone below the head 

scarp. In places, secondary and tertiary mudflow deposits rest on top of older deposits located farther downslope (figs. 4, 5B, and 7). 

5. The main body of the mudflow is tongue-shaped and has 

roughly parallel flanks. The main body is about 1 km 

long, has up to 12 m of relief relative to the adjacent 

Main Body 

of 

Mudflow 

• Core Locations 

Figure 4. Interpretive block diagram showing important morphologic features of the mudflow 

site and sampling locations. 

Seismically Induced Mudflow in Santa Barbara Basin, California 169 
-

500m 

--

PROFILE S -2 

~ E 

0 

0 Ln 

UNDISTURBED 

SEDIMENT 

.. ·I · 

' I 

] 

.I 

I 

I. 

I ' ,. 

A 

500m PROFILE S-3 UNDISTURBED 

SEDIMENT 

UNDISTURBED 

SEDIMENT 

Figure 5. (A) High-resolution 3.5-kilohertz seismicreflection profile of part of the record shown in figure 

3 showing the general morphology of the slope failure 

zone. Note the broad expanse of the exposed scar 

surface, the secondary flow deposits onlapping the 

main mudflow body, and evidence of older massmovement deposits deeper in the section. The location of the seismic profile of figure 6 is shown as a 

B 

circled dot. Trackline locations are shown in figure 70. 

(8) High-resolution 3.5-kilohertz frequency seismicreflection profile of the eastern part of the slope 

failure zone. Note the coherent blocks and secondary 

mass-movement deposits resting on the scar surface 

near the head scarp. Also note evidence of older mass 

movement deposits deeper in the section. Trackline 

locations are shown in figure 70. 
PROFILE S- 9 

-= - - PROFILE S-9 

500m 

§~~ EVACUATION ZONE 

Undisturbed Sediment Undisturbed Sediment 

-:-----~ I Secondary Deposits Side Scarp \ / I"""' 

~ " y -

- -... 

-------........ ;I ~ ---- ~ - Original Failure ------ Surface - - ~ - - ,, 

-- - Figure 6. Slope-parallel high-resolution 3.5-kilohertz frequency seismic-reflection profile and interpretive line drawing 

across the evacuation zone. Note the infilling of the initial failure surface by secondary deposition. 

undisturbed sea floor, and represents the deposit derived 

from the head scarp. The main body terminates as a 

bulging toe at a water depth of about 510 m where the 

regional slope angle decreases from 4° to less than 1°. 

Slope failure occurred in several stages. Initially, 

muddy sediment failed along a 6- to 10-m-high scarp and 

disintegratc::d into a mudflow which transported material to 

the base of the slope (stage 1, fig. 7A). Next, retrogressive 

failure along the eastern part of the head scarp resulted in 

deposition both on the exposed, initial scar surface and on 

the initial mudflow deposit (stage 2, fig. 7B). Subsequent 

retrogressive failure resulted in emplacement of coherent 

blocks and additional mudflow deposits (stage 3, fig. 7C). 

This entire sequence of slope failures may have occurred 

rapidly or the individual stages of failure may have been 

separated in time by days, weeks, or years; we cannot 

resolve the time scale with our data. 

SEDIMENT CHARACTERISTICS 

A series of gravity cores was collected from sites 

within and near the slope failure in order to characterize the 

near-surface sediment and to analyze the cause of the slope 

failure (fig. 4). From our analyses of the cores, sediment 

physical properties vary with increasing distance from 

shore. For example, sediment mean grain size, which is 

relatively constant downcore except in the cores from the 

scar, decreases from about 12 microns to less than 8 

microns with increasing distance from shore. Sediment 

water content correlates with mean grain size and increases 

offshore as the sediment becomes finer grained. There are 

no visual differences between the cores within and outside 

the mudflow, but geotechnical (engineering) measurements 

indicate that distinct differences do exist. For example, 

cores recovered from the scar have unusually high strength 

Seismically Induced Mudflow in Santa Barbara Basin, California 171 
A Stage 1 

I 

I 

Jj I 

Stage 2 

S-9 

Figure 7. Interpretive model showing developmental stages of the slope failure zone (A-C). D 

shows trackline coverage (bold lines show the locations of the seismic-reflection profiles 

presented in figs. 3, 5, and 6). 

below about 0.50 m, the same level at which the core 

becomes significantly finer grained. These variations document the location of the failure surface and the amount of 

sedimentation that has occurred since the slope failure. 

Reported sedimentation rates for this area range from 

0.8 m/1,000 yrs (Marks and others, 1980) to 1.4 m/1,000 

yrs (Duncan and others, 1971). Duncan and others ( 1971) 

recovered only 0.15 m of postfailure sediment atop the scar 

and report that slope failure occurred about 100 years ago. 

Applying these rates to the 0.50 m of sediment we recovered overlying the scar indicates that failure occurred 

between 350 and 625 years ago. Thus, slope failure likely 

occurred between 100 and 625 years ago and may be 

younger if a significant part of the sediment overlying the 

scar is associated with retrogressive failure deposits and not 

uniform accumulation of sediment. 

172 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
GEOTECHNICAL ANALYSIS 

Slope failure occurs when the driving stresses exceed 

the capacity of the sediment to resist the stress (Lee, 

Schwab, and Booth, this report). Three agents commonly 

produce the downslope driving stress that can trigger slope 

failure: gravity, ocean waves, and earthquakes. Gravity acts 

continuously and has a downslope component that increases 

with the steepness of the slope. Waves produce cyclically 

varying shear stresses. As a result of the oscillatory loads, 

internal pore-water pressure is developed which reduces 

sediment r•esistive strength. The magnitude of such waveinduced stresses varies with wave length, wave height, 

water depth, sediment properties, and location in the 

affected sediment column. Earthquakes also generate cyclically varying stresses within the sediment. These stresses 

vary with earthquake magnitude, distance from the zone of 

rupture (earthquake epicenter), slope geometry, position 

within the sediment column, total thickness of the sediment 

body, and the properties of the affected sediment and rock 

mass. 

The capacity of the sediment to resist driving stresses 

(that is, the sediment strength) is a complicated quantity that 

typically varies with depth in the sediment column, sediment grain characteristics (for example, mineralogy, grain 

size, and plasticity), sediment history, intensity and rate of 

loading, type of loading (cyclic or static), duration of 

loading, and the overall configuration of the slope and its 

stratigraphy (Lee, 1985; Lee and Edwards, 1986; Kaye~ 

and Lee, this report; Lee, Schwab, and Booth, this report; 

Schwab and Lee, this report). For example, the strength 

developed in sediment to resist earthquake shaking is a 

function of the sediment static and cyclic stress history and 

sediment physical characteristics. As a consequence of 

earthquake loading, shear stresses acting on a slope are 

typically increased while the resistive strength of the sediment is reduced due to the generation of excess pore 

pressures (Lee, Schwab, and Booth, this report). 

We performed a geotechnical engineering analysis on · 

selected cores to evaluate each of the driving stresses 

mentioned above and to determine how the slope failure 

was initiated. Additional analyses were performed to deter~ 

mine how the slope failure developed into a mudflow and to 

evaluate why retrogressive failure occurred. Analysis 

showed that the slope is stable under conditions of static 

gravitational loading and that storm waves generate negligible stress·es at these water depths. The analysis showed, 

however, that a strong earthquake can easily generate 

stresses great enough to fail the slope. 

We used the procedures of Lee and Edwards (1986) 

to reconstruct and estimate, theoretically, the strength 

properties of the sediment that likely existed above the 

present scar prior to failure. These procedures assume that 

deposition ·iin the area has been fairly· steady and uniform 

and that sediment at depth is comparable (except for shear 

strength and consolidation properties) to the sediment that 

was removed during the slope failure. We. compared the 

strength of this theoretically reconstructed sediment with 

driving stresses (Seed and ldriss, 1971; Joyner and Boore, 

1981) that would likely be produced by a magnitude 7. 5 

earthquake occurring 25 km away (probably comparable to 

the 1812 event). Slope failure can occur when the driving 

stresses exceed the sediment resistive strength, that is when 

the ratio.of sediment strength to driving stress is less than 1. 

In the study area, this condition was met for a subbottom 

depth range of about 5 m to ·greater than 20 m with the 

lowest value (about 0. 75) occurring at a subbottom depth of 

10 m (fig. 8). Based on the degree of compaction of 

sediment in the present scar, we believe that slope failure 

occurred to a depth of about 8 m in the sediment column. 

Thus, earthquake loading is the only one of the three 

primary agents capable of producing downslope driving 

stresses large enough to trigger the slope failure. This slope 

failure almost certainly was caused py seismic shaking and 

the depth of failure was constrained by the physical properties of the local sediment column. Sediment from other 

locations near the site of the slope failure also shows ratios 

of sediment strength to driving stress that are less than 1, 

but none are as low as that for the sediment that existed 

above the present scar (fig. 8). Our conclusion is that much 

of this basin slope is marginally stable to unstable during 

strong earthquake motion. The occurrence of the slope 

failure at this exact site was likely controlled by local 

ground motion and sediment behavior effects not considered in this analysis. 

To determine why the failed sediment mobilized into 

a mudflow, we estimated the sediment strength that would 

remain after the initial failure caused by seismic loading 

(see Schwab and Lee, this report). We found that this 

residual or "steady-state" strength (Poulos and others, 1985) 

was less than the gravitational shear stress on the 4° basin 

slope but greater than the gravitational shear stress available 

near the edge of the basin floor (about a 1° slope). 

Accordingly, following the earthquake, the weakened sediment was able to flow down the 4° slope under the 

influence of gravity alone. The mudflow left behind a steep 

scarp at the head of the evacuation zone and stopped at the 

base of the slope because gravitational stresses on the basin 

floor were not great enough to cause further movement. 

Why was the failure retrogressive? Either additional 

seismic shaking or high static stress concentrations within 

the head scarp after the earthquake led to further slope 

failure and movement of sediment away from the scarp. 

Failure propagated upslope until the static sediment strength 

was great enough to prevent further disintegration of the 

head scarp. The head scarp that exists today likely has a low 

resistance against static gravitational failure and probably 

will fail again during strong earthquakes in the future. 

Seismically Induced Mudflow in Santa Barbara Basin, California 173 
(I) 1.5~~~--------------------------------------------~ (I) 

Q) 

.. .... 

Cl) 

0) 

c 

·:;: 

·.: ...... 

.............. ...... Q 

0 

.... 

...... .... 1 .. 0 ...... -------"'iilh:-"-.. -................. -.. -. -•• ----------_-_-_-_ -.. -..... ~'-------. -•• -.-. -:_.-... ~. ":'"": ~~'--1 

.... ' ...... :.·~ .... ~----------- ...... ·~:-.- . ..c 

.... 

0) ............ :~: ~·:~: ~·:~: ~·:~: ~·:~: ~·:~: ~ ...... . c 

Q) 

.. 

.... 

Cl) 

.... 

c 

Q) 

.•.•.•.•.•.•.. to side of failure zone 

e 

:a 

0.5 ................. above failure zone 

Q) 

Cl) 

'+oo ------in failure zone before failure (estimated) 

0 

.9 

.... below failure zone 

ca 

=: 

0 10 

Subbottom Depth (m) 

20 

Figure 8. Plot of the ratio of sediment strength to driving stress versus subbottom depth for 

selected sediment sites in and near the failure zone. Sediment strength is the estimated cyclic 

shear strength; driving stress is the estimated average peak downslope shear stress developed 

during a magnitude 7.5 earthquake located 25 kilometers from the slope failure. Slope failure is 

likely when this ratio has a value that is less than 1. Also shown is the estimated ratio for a 

theoretically reconstructed, intact sediment profile located at the site of the scar at a time prior 

to slope failure. Note that this presumed condition (bold line) has the lowest ratio of all the 

surrounding cores and documents why failure occurred at that site. 

SUMMARY 

The slope failure likely occurred in response to 

shaking from a large magnitude (Richter magnitude of 

about 7 .5) earthquake occurring within Santa Barbara Basin 

within the last 1 ,000 years and possibly within the last few 

hundred years. A mudflow developed in the western part of 

the study area involving approximately 0.01 to 0.02 km3 of 

slope sediment. Because the residual strength of the sediment mass was low relative to downslope gravitational 

stress, slope failure developed into a mudflow leaving a 

steep 6- to 10-m-high scarp at the head of the evacuated 

area. Either continued earthquake shaking or high residual 

static shear stresses within the head wall caused subsequent 

slope failure and upslope erosion of the headwall scarp with 

deposition of secondary mudflow deposits on the exposed 

scar region. 

REFERENCES CITED 

Coleman, J.M., and Garrison, L.E., 1977, Geological aspects of 

marine slope stability, northwestern Gulf of Mexico: Marine 

Geotechnology, v. 2, p. 9-41. 

Crowell, J.C., 1974, Origin of late Cenozoic basins in southern 

California, in Dickinson, W .R., ed., Tectonics and Sedimentation: Society of Economic Paleontologists and Mineralogists Special Publication 22, p. 190-204. 

Duncan, J.R., Hoover, R.A., Pflum, C.E., Widmier, J.M., and 

Daetwyler, C.C., 1971, Near surface geology of Santa Ynez 

Unit, Santa Barbara Channel, California: Esso Production 

Research Company, EPR.49ES.71, 40 p. 

Greensfelder, R.W., 1974, Maximum credible rock acceleration 

from earthquakes in California: California Division of Mines 

and Geology Map Sheet 23, scale 1:2,500,000, 12 p. 

Howell, D.O., ed., 1976, Aspects of the Geologic History of the 

California Continental Borderland: American Association of 

Petroleum Geologists Miscellaneous Publication 24, 561 p. 

Joyner, W.B., and Boore, D.M., 1981, Peak horizontal acceleration and velocity from strong-motion records including 

records from the 1979 Imperial Valley, California, earthquake: Bulletin of the Seismological Society of America, v. 

71, p. 2011-2038. 

Lee, H.J., 1985, State of the art: Laboratory determination of the 

strength of marine soils, in Chaney, R.C., and Demars, 

K.R., eds., Strength testing of marine sediment: laboratory 

and in situ measurements, ASTM STP883: American Society 

for Testing and Materials, Philadelphia, Pa., p. 181-250. 

174 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
Lee, H.J., and Edwards, B.D., 1986, Regional method to assess 

offshore slope stability: Journal of Geotechnical Engineering, 

Ameriican Society of Civil Engineers, v. 112, p. 489-509. 

Malouta, D.N., Gorsline, D.S., and Thornton, S.E., 1981, 

Processes and rates of Recent (Holocene) basin filling in an 

active transform margin: Santa Monica Basin, California 

Continental Borderland: Journal of Sedimentary Petrology, v. 

51' p. 1077-1096. 

Marks, J.G., Marianos, A.W., Gonzaga, F.R., and Pflum, C.E., 

1980, Foraminiferal correlation of Quaternary sediments in 

the Santa Barbara Channel, California: Cushman Foundation 

Special Publication No. 19, Memorial to Orville L. Bandy, p. 

127-133. 

Poulos, S..l., Castro, Gonzalo, and France, J.W., 1985, Liquefaction evaluation procedure: Journal of Geotechnical Engineering, American Society of Civil Engineers, v. 111, p. 

722-791. 

Prior, D.B., Bornhold, B.D., and Johns, M.W., 1984, Depositional characteristics of a submarine debris flow: Journal of 

Geology, v. 92, p. 707-727. 

Roberts, H.H., 1980, Sediment characteristics of Mississippi 

River delta-front mudflow deposits: Gulf Coast Association 

of Geological Societies Transactions, v. 30, p. 485-496. 

Seed, H.B., and ldriss, I.M., 1971, Simplified procedure for 

evaluating soil liquefaction potential: Journal of Soil Meehanics and Foundation Engineering Division, American Society 

of Civil Engineers, v. 97, p. 1249-1273. 

Thornton, S.E., 1984, Basin model for hemipelagic sedimentation 

in a tectonically active continental margin: Santa Barbara 

Basin, California Continental Borderland, in Stow, D.A.V., 

and Piper, D.J.W., eds., Fine-Grained Sediments: Deep 

Water Processes and Facies, Blackwell Scientific Publications, Oxford, p. 377-394. 

---1986, Origin of mass flow sedimentary structures in 

hemipelagic basin deposits: Santa Barbara Basin, California 

borderland: Geo-Marine Letters, v. 6, p. 15-19. 

Vedder, J.G., Wagner, H.C., and Schoellhamer, J.E., 1969, 

Geologic framework of the Santa Barbara Channel Region, in 

Geology, Petroleum Development, and Seismicity of the 

Santa Barbara Channel Region, California: U.S. Geological 

Survey Professional Paper 679, p. 1-11. 

Yerkes, R.F., 1985, Geologic and seismologic setting, in Ziony, 

J .1., ed., Evaluating earthquake hazards in the Los Angeles 

Region-An earth-science perspective: U.S. Geological Survey Professional Paper 1360, p. 25-41. 

Yerkes, R.F., Greene, H.G., Tinsley, J.C., and Lajoie, D.R., 

1981, Seismotectonic setting of the Santa Barbara Channel 

Area, southern California: U.S. Geological Survey Miscellaneous Field Studies Map MF-1169, 1 sheet, 25 p. 

Seismically Induced Mudflow in Santa Barbara Basin, California 175 
Submarine Landslides in a Basin and Ridge Setting, 

Southern California 

By M.E. Field and B.D. Edwards 

INTRODUCTION 

One of the hallmarks of the California continental 

borderland is the extraordinary topographic and geologic 

complexity of this type of continental margin. Unlike the 

continental shelf-slope-rise sequence that characterizes 

most continental margins of the world, the continental 

borderland of southern California is a repeating pattern of 

extremes. The relatively narrow mainland shelf. (average 

width of 8 kilometers (km)) gives way to steep slopes that 

lead, in tum, to flat-floored basins and troughs; nearshore 

basins are bo!Jnded on the west by slopes that rise to island 

platforms, bank tops, and ridge crests. This pattern continues for over 200 km to the west (fig. 1). Between Point 

~onception and the latitude of the U.S.-Mexican· border, 

there are 11 major basins; numerous smaller basins and 

sediment ponds are present throughout the area (Emery, 

1960). 

, A simplified contour map of the southern California 

borderland (fig. 1) shows the distinctive general pattern of 

a predominantly sloping terrane containing isolated shelves, 

banks and basins. 'Emery (1960) estimated that 63 percent 

of the borderland area is composed of slopes and deep 

irregular areas, 20 percent is shelves and bank tops, and 17 

percent is basins and trough floors. Neither these data nor 

the generalized map of figure 1, however, can fully convey 

the regional extremes and local complexities in slope 

geometry that characterize the borderland. 

The borderland was formed by tectonic forces active 

throughout the past 30 million years, and its morphology 

reflects this influence. Many present-day slopes are controlled by the dip of large-scale flexures of sedimentary 

strata, some are shaped by variations in resistance of 

interbedded sedimentary and volcanic rocks, and others are 

formed by large faults that have offset the sea floor. For 

example, seismic-reflection profiles from the east slope of 

the Santa Rosa-Cortes Ridge (fig. 2) illustrate that the dip of 

folded subsurface strata defines the dip of that slope. 

Additionally, minor flexures and faults create local changes 

in slope morphology. 

Although the overall configuration of borderland 

slopes is controlled by these large-scale geologic structures, 

secondary modifications to slope geometry result from 

sedimentary processes. These processes include erosioninduced formation of canyons and scarps, removal and 

deposition of sediment by slope failure and mass ·movement, and local sediment buildup from surface and deep 

currents. 

SIZES OF LANDSLIDES ON BORDERLAND 

SLOPES 

Numerous geologic studies conducted in the borderland show that landslide deposits vary widely in size from a 

few meters to_ tens of kilometers on a side. Because of this 

lar~e variation in size, the ability to identify and describe 

accurately the morphology of a particular landslide deposit 

depends on the technique of observation. The tools 

employed in our studies include (1) high-resolution seismicreflection systems; (2) sediment cores; (3) bottom cameras; 

(4) near-bottom towed geophysical systems; at?-d (5) observations from a manned submersible. Each tool has a unique 

resolution capability and, hence, is limited in the size of 

feature that it can identify. Our observations on different 

scales of mass movement reflect this constraint. 

Large-Scale Landslides 

. Large-scale landslides measuring hundreds of meters 

to kilometers along a side are commonly recognized on 

borderland slopes, and they appear to be more common than 

smaller ones. This apparent greater abundance of large 

slope failures may be due to the large number of seismicreflection profiles available, the limited resolution of 

seismic-reflection systems operated from the sea surface, 

and the limited number of camera and submersible observations that would be more suitable for identifying· small 

failures. Of the~e large landslides, most do not exceed 5 

km, and few exceed 10 km, on a side. They are commonly 

found at a significant break in slope gradient or at the base 

of slopes (fig. 2). In contrast, those landslide deposits found 

resting on the mid-slope are generally less than 5 km on a 

side. Many of the larger landslides (>5 km) have a chaotic, 

hummocky appearance, and in many places show evidence 

of a complex failure history (figs. 2 and 3). 

Small-Scale Landslides 

Smaller-scale landslides, those measuring meters to 

hundreds of meters on a side, are also common on slopes of 

the borderland.· In ancient slope sequences that are now 

subaerially exposed, small slope failures measuring a few 

meters to a few tens of meters are common and are more 

easily delineated in outcrop than are large-scale failures. 

For example, studies by Cook (1979) identified numerous 

s!llall-scale deposits of slides and mass flows in slope rocks 

176 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
(/) 

ex: 

UJ 

1-

0 

:g 1000 

z 

£2000 

IC... 

UJ 

0 3000 

Santa Ynez 

A' 

Figure 1. Generalized bathymetric map of the offshore 

region of southern California, known as the California 

continental borderland. Depths are shown at 500-meter 

increments; along the mainland coast the 100-meter 

contour is also shown. The three patterns divide the 

offshore region into three types of terrain: (1) shelves and 

bank tops, which are relatively shallow and flat; (2) slopes 

and irregular areas that are generally steep and lead from 

of Cambrian age (570 to 500 million years old). On most 

modem continental slopes and deep-sea basins, however, 

slides and mass flows of this scale have received scant 

attention relative to the larger slope failures because they 

are too small to delineate with conventional, surfaceEXPLANATION 

CJ Shelves and bank tops 

CJ Slopes and irregular areas 

mJ Basins and trough floors 

0 20 40 KM 

I •' •' 

0 10 20NM 

Contours in meters 

.}\: ;~~~~ &!ach 

. '·:: · · Newport 

··<·:·Beach 

.. :::· .: 

SANTA MONICA 

BASIN 

:_.:'::J~S.:. 

.~t')(. 

·.:.:·. 

A 

shelves to deeper water; and (3) basins and trough floors, 

which are the deepest areas but relatively flat. Note the 

abundance of steep slope areas throughout the borderland. The cross section A-A' shows the irregular nature of 

sea-floor topography off southern California and the 

repeating nature of the shelf-slope-basin sequence across 

the entire borderland. 

operated acoustic systems. Instruments capable of resolving 

landslides of this scale do exist, however. Field and Clarke 

(1979) were able to identify the presence of small-scale 

landslide features on the slope off San Nicolas Island by 

using a deep-towed geophysical package containing up-, 

Submarine Landslides in a Basin and Ridge Setting, Southern California 177 
t~\~~mF~~ . ~-~

. ~ .. ·~ ~:-.:::~·-, -.· ··-=~:·: ::.--~~~'w\ ~.~ <.::;. ~· ~-~~·~;-~=~. .. ·:r~ ,. - ~.-.. ~-;, .... ~ ._._ ·)·; .;:·.~ ~----~~1~ 

::,;~i~~\1~~:~~~~~~ . ... -1- .·. ~···-· .. 11 . ' . '~ ,, .. ,. ~ 

w 

SANTA ROSA- CORTES RIDGE 

5 KM 

.5 KM 

Figure 2. Geophysical profile ("sparker" record) and a 

line-drawing interpretation of a large landslide zone in 

western Santa Cruz Basin at the base of the slope of the 

Santa Rosa-Cortes Ridge. Mapped in the mid-1970's (Nardin and others, 1979; Field and Richmond, 1980), this is 

one of the largest landslides known off southern California and probably resulted from a series of slope failures 

rather than a single event. The slope is relatively steep 

compared to other continental-margin or borderland 

~~: .. ~ t.:,-~s~'~ .~

. 

._·, 

·-'·~ .;t·;_.:~,, ......... ,.,, 

:~,.~~- \i' 

:<{C.. ·~ ~-; 

.. :~.-~. .~, :~.: .. -:-,.,~ 

,~~§:l'~~~7"'---<--- ~=)-.a:;._~ .. :~~=- ·~.~- ..... ~ 

-~.:&-.~·~~'3;.$; .. ·~~.., ... ~,. . ~~ ~~~ ~-........~· 

E 

slopes and locally exceeds 10°, a factor which may have 

contributed to slope-sediment failure. The flat, eroded 

surface of the ridge top results from migration of the 

coastline back and forth across the ridge top during 

glacial epochs (as sea level falls and rises); this produces 

an abundance of loose sediment at the ridge edge, 

perhaps another contributing factor to the slope failures. 

The vertical bar scale is based on the speed of sound in 

water. 

178 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
.I . 74T"'·:,,..·v>', .. . ;;', >;'• ,, .... ·· 

t •' '.T: .. ••'•· • •·;·' · •·• ' ·• •'.·'·"'' 

., · .. · ,u .. · •.• ·, f",. •" I ' :;, •' ~ '• f' .1..' ' '• J • ' ~ ,i I 

: .. '.' ~ ~ '. ' 

. . .. 1 ~'~( · - , .. 

i' · .... . ¢ ,., . . . • '.l.~ • .. ' : .• ; . 

•• 0 , ~ ' ~oo m 

25m 

~ ·:F 

,. __. 

• 1,\' 

• r \' 

•,· 

Figure 3. A high-resolution seismic-reflection profile ("uniboom") showing a typical large-scale sediment landslide lying 

on the mainland slope (Edwards and others, 1980). 

down-, forward-, and side-looking sonars, bottom cameras, 

and a 4-kilohertz (kHz) frequency seismic-reflection system. Within a small ( ~ 150 km2

) intercanyon slope region, 

they identified six landslides that vary in size, geometry, 

age, and probable mechanism of failure. One of the 

landslides is shown by the seismic-reflection profile and 

interpreted line drawing in figure 4. This slump deposit is 

less than 400 meters (m) long and about 15 to 20 m thick. 

The lateral extension is unknown, for although a scarp can 

be traced more than 150 m laterally from the line of the 

profile, it appears to be discontinuous. Because of its small 

size, the slump was not evident on the shipboard seismicreflection profiler and would not have been recorded by 

most surface-towed instruments. 

Even though many small-scale landslides have been 

located in modern environments, the identification of their 

character and origin is difficult because of the resolving 

power of most surface-towed acoustic systems. The smaller 

landslides of the borderland tend to be simple in shape 

compared to large-scale landslides, which may be a reflection of the inability of marine techniques to resolve the fine 

details of the sea floor. The small landslides commonly are 

characterized by a smooth upper surface and either an 

acoustically transparent interior or rotated internal reflectors. The chaotic internal fabric and hummocky or chaotic 

upper surfaces that are common to larger landslide deposits 

are rare. Bottom photography has been an aid in identification of small landslide deposits, but often the field of 

view is too limited to allow coverage of the entire deposit. 

Photographs from Field and Clarke ( 1979) show some of 

the characteristics of sediment in a small landslide zor.e. 

Clasts are as large as 1 m and some of them show evidence 

(sag fractures and rounded noses) of downslope transport. 

Photographs in figure 5 show a comparison of the smooth 

and burrowed nature of the sea floor in an undisturbed area 

(fig. SA) with the eroded and slablike character of the sea 

floor in a recent landslide area (fig. 5B). 

Small-scale landslides on slopes of the California 

borderland are recognized but poorly understood. They 

have been found only in those regions where detailed work 

has been conducted and then only through careful study 

using special instruments. Direct observations from manned 

submersibles, such as Sea Cliff, in addition to bottom 

photography and deeply towed instrument packages, are 

typically the only means of identification of small landslides 

simply because the scales of high-resolution geophysics and 

wide-swath sidescan sonar are not ideally suited for 

their size. 

Submarine landslides in a Basin and Ridge Setting, Southern California 179 
A 

-----~]5om 4 kHz Bottom record 500m 

B ~arp 

Head\\ ~--~ 

50m 

500m 

Figure 4. Seismic-reflection profile (4-kilohertz frequency) obtained with a bottom-towed system and line-drawing 

interpretation of a small-scale landslide. This feature, which lies on the slope east of San Nicolas Island (figs. 1 and 6) 

could not be identified using most surface-towed acoustic systems (from Field and Clarke, 1979). 

Very Small-Scale Landslides 

Some deposits derived from sediment landslides, and 

more commonly from sediment mass flows, have little 

lateral extent and can be identified and measured only in 

terms of their thickness. The deposits are centimeters to 

meters thick and frequently are observed only in individual 

cores. Edwards (1979) and Nardin and others (1979) 

presented examples of thin zones of deformation observable 

on radiographs of box and piston cores collected from the 

west slope of Santa Cruz Basin. These types of deposits are 

commonly characterized by an abrupt or erosional lower 

contact, rotated or chaotic internal beds, and occasional 

rip-up clasts. 

Basinward transport of sediment by mass flows also 

produces anomalies in sediment particle sizes, mineral 

content, and animal trails in surface or near-surface sediment. The anomalies may or may not be associated with 

slope failures that can be defined acoustically. Photographs 

of the sea floor near an acoustically recognized land lide 

area off San Nicolas Island revealed that the activity of 

benthic fauna was markedly different inside and outside of 

180 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
Figure 5. (A) Sea floor east of San Nicolas Island at a 

water depth of about 800 meters. The photograph is 

typical of these depths and shows soft, fine-grained 

sediment shaped by animals into burrows, trails, 

and mounds; the white spots are sea-urchin shells. 

There is no sign of disturbance by slope failure. (B) 

Sea floor east of San Nicolas Island in the vicinity of 

the photograph in SA above. The sea floor here 

shows the effects of slope failure; the surface is 

blocky and cracked, and no burrows are evident. 

Submarine landslides in a Basin and Ridge Setting, Southern California 181 
119°W 

Figure 6. Southern California borderland showing the 

location of sediment slides and mass flows (modified 

from Field and Edwards, 1980). Sediment landslides are 

common on the slopes of the borderland, and they 

occur at various scales. Slides and mass flows on the 

offshore slope from south of San Nicolas Island to Santa 

Rosa Island and on the mainland slope from San Diego 

and west of Santa Barbara are relatively abundant, and 

this may simply reflect the large effort invested in identifying slope failures in those two areas. With only two 

exceptions, the slope-sediment failures in the mainland 

EXPLANATION 

c::::z::D Sediment Slides and Mass Flows 

0 10 20 30 Kilometers 

0 5 10 15 Nautical m1les 

Contours in meters 

0 Los 0 

AnQeles 

11 8°W 

slope are larger than those on the offshore slope. Most 

failures mapped on borderland slopes are seen only in a 

single crossing; some of the larger ones were identified 

on multiple crossings. Our understanding of the distribution and the sizes of mass-transport deposits seems to 

depend largely on the density and kinds of available 

geophysical data. Landslides shown on the map are from 

Greene and others (1975), Haner and Gorsline (1978), 

Nardin and others (1979), Field and Edwards (1980), and 

Richmond and others (1981)0 
the landslide zone (fig. 5). Within the landslide zone, tracks 

and trails of surface grazing organisms (holothurians and 

echinoids) predominated, whereas out of the landslide zone 

the bottom was heavily burrowed and traces of surface 

feeding organisms were not common. Presumably this 

difference resulted from the repopulation of the recently 

disturbed landslide zone by mobile fauna which ingest 

surface sediment, whereas the undisturbed regions were 

heavily populated by infaunal deposit feeders. 

DISTRIBUTION OF LANDSLIDES ON 

BORDERLAND SLOPES 

Detailed studies of · selected areas by a host of 

investigators have contributed measurably to our knowledge 

of slope-sediment failures on slopes of the southern California borderland. By collating information from our studies and those of our colleagues onto a map of the borderland, we can examine the distribution of sediment 

landslides throughout the entire region. The map is not 

complete, because many areas have not been explored or 

have been studied only in a reconnaissance mode; the map, 

however, i1s representative of the slopes of the mainland and 

the Santa Rosa-Cortes Ridge (fig. 1). The density of 

mapped landslides shown in figure 6 is strongly biased by 

the amount and kinds of data collected. For example, few 

slope failures are mapped on the slopes of the Catalina 

Ridge or off San Clemente Island, probably because of the 

paucity of data from those areas, not necessarily because of 

a .scarcity of slope failures. In areas where 'high-resolution 

seismic-reflection data were collected specifically to identify the scale and geometry of slides and mass flows, many 

such failures are identified (for example, off San Nicolas 

Island and the central Santa Rosa-Cortes Ridge). 

Because of the imbalance in areal data collection, it is 

not possible to infer causes of slope failure based on their 

apparent distribution. Nevertheless, several general trends 

and relations can be identified: 

1. Slope failure has occurred on practically every type of 

slope, :regardless of steepness, orientation, or smoothness. 

2. Slope failures of all sizes have been mapped but most of 

those that have been identified measure on the order of 

a few hundreds to thousands of meters on a side. 

3. Very large zones of slope failure, measuring tens of 

kilometers on a side, occur in selected localities and 

perhap~: result from a series of slope failures rather than 

from a single event. 

4. The presence of active submarine canyons does not 

inhibit slope failures. Indeed, some canyons derive part 

of their transport load from failures on the canyon walls 

or adjacent slopes. 

5. The large number of slope failures along the mainland 

slope of the borderland correlates with relatively high 

sedimentation rates on this slope that result from fluvial 

transport from upland regions. 

6. A source of sediment, either active or formerly active at 

a time of lower sea level, is usually evident for slopes 

having numerous failures. 

7. Slopes having no apparent source of material except 

hemipelagic deposition have few large landslides, and 

most slides and mass flows are relatively thin. 

REFERENCES CITED 

Cook, H.E., 1979, Ancient continental slope sequences and their 

value in understanding modem slope development: Society of 

Economic Paleontologists and Mineralogists, Special Publication 27, p. 287-305. 

Edwards, B.D., 1979, Animal-sediment relationships in dysaerobic bathyal environments, California continental borderland: 

Unpublished Ph.D. Dissertation, University of Southern 

California, Los Angeles, Calif., 291 p. 

Edwards, B.D., Field, M.E., and Clukey, E.C., 1980, Geological 

and geotechnical analysis of a submarine slump, California 

borderlands: Proceedings, 12th Annual Offshore Technology 

Conference, Houston, Tex., v. 1, OTC 3726. 

Emery, K.O., 1960, The sea off southern California, a modem 

habitat for oil: New York, N.Y., John Wiley and Sons, 

366 p. 

Field, M.E., and Clarke, S.H., Jr., 1979, Small-scale sbmps and 

slides and their significance for basin slope processes, southem California Borderland: Society of Economic Paleontologists and Mineralogists, Special Publication 27, p. 223-230. 

Field, M.E., and Edwards, B.D., 1980, Slopes of the southern 

California borderland: a regime of mass transport, in Field, 

M.E., Bouma, A.H., Colboum, I.P., Douglas, R.G., and 

Ingle, J .C., eds., Pacific Section SEPM, Pacific Coast 

Paleogeography Symposium No. 4, Los Angeles, Calif., 

p. 169-184. 

Field, M.E., and Richmond, W.A., 1980, Sedimentary and 

structural patterns on the northern Santa Rosa-Cortes Ridge, 

southern California: Marine Geology, v. 34, p. 79-98. 

Greene, H.G., Clarke, S.H., Jr., Field. M.E., Linker, F.l., and 

Wagner, H.C., 1975, Preliminary report on the environmental geology of selected areas of the southern California 

continental borderland: U.S. Geological Survey Open-File 

Report 75-596, 69 p. 

Haner, B.E., and Gorsline, D.S., 1978, Processes and morphology of continental slope between Santa Monica and Dume 

submarine canyons, southern California: Marine Geology, v. 

28, 

p. 77-87. 

Nardin, T.R., Edwards, B.D., and Gorsline, D.Scott, 1979, Santa 

· 1 Cruz basin, California borderland: dominance of slope processes in basin sedimentation: Society of Economic Paleontol-

. ogists and Mineralogists, Special Publication 27, p. 209-221. 

Riclimond, W.C., Cummings, L.J., Hamlin, S., and Nagaty, 

M.E., 1981, Geologic hazards and constraints in the area of 

proposed OCS oil and gas lease sale 48, southern California: 

U.S. Geological Survey Open-File Report 81-307, 37 p. 

Submarine Landslides in a Basin and Ridge Setting, Southern California 183 
Giant Volcano-Related Landslides and the 

Development of the Hawaiian Islands 

By W.R. Normark, j.G. Moore, and M.E. Torresan 

INTRODUCTION 

Using the sidescan-sonar system GLORIA (Geological LOng-Range Inclined Asdic), recent investigations of 

the U.S. Exclusive Economic Zone (EEZ) that surrounds 

Hawaii have defined 37 major landslides '(figs. 1 and 2; 

table 1). The astonishingly large size of the area covered by 

such massive submarine mas$-movement deposits was not 

anticipated before these GLORIA surveys. This section 

reviews the characteristic features of some of these landslides and attempts to relate them to the processes that built, 

and modified, the volcanic systems of the Hawaiian Ridge. 

The role of major landsliding in shaping Hawaiian 

volcanoes has long been a subject of debate. The subaerial 

parts of the volcanoes are fairly well known, but the 

submarine parts, which constitute the great bulk of these 

volcanoes, are poorly understood because only limited 

submarine mapping existed until recently. An early controversy concerned the relative importance of faulting versus 

marine erosion in producing high cliffs along the coasts of 

some of the islands. The high cliffs of northeast Oahu 

(Nuuanu Pali), east Niihau, the Na Pali coast of Kauai, and 

northern Molokai were interpreted by Dana (1890) as 

resulting from "profound fracturing of the mountain dome" 

and "downdrop" of the seaward part. beneath the sea. 

Dana's faulting model was rejected by Hitchcock (1900), 

who believed that erosion alone was sufficient to form the 

cliffs without the additional complexity of faulting. Wentworth (1927), comparing estimates of marine and fluvial 

erosion rates, concluded that marine erosion alone was 

insufficient to produce some of the larger cliffs (for example, those of north Molokai), and that they were probably 

produced by faulting. 

The idea of faulting of the flanks of the volcano led to 

the concept that the faults bound the headwalls of giant 

landslides. Steams and Clark (1930) and Steams and 

Macdonald ( 1946) suggested that the 50-kilometer (km) 

long Hilina fault system on the south flank of Kilauea 

volcano, located on the island of Hawaii, owes its origin to, 

and is at the head of, a major landslide.·Macdon~ld (1956) 

later discounted landsliding, suggesting that large-volume 

intrusion beneath the volcano center .forcibly uplifted the 

summit causing faulting on the volcano flank. Moore and 

Krivoy (1964)·revived the landslide concept for Kilauea's 

south flank and suggested that not only. is the Hilina fault 

system related to landsliding, but also that the east rift.zone· 

of the volcano forms the upper margin of a large slump. 

Lipman and others (1985) assessed the deformation associated with a ~agnitude 7.2 earthquake, which occurred in 

· 1975 on Kilauea's south flank, and concluded that a 

deep-seated slump, partly related to forcible intrusion of 

magma into the rift zone at its head, could explain the 

geologic and geophysical observations. They suggested that 

the base of the mass movement was the pre-Hawaiian sea 

floor and, hence, that the landslide was as much as 10 km 

thick. 

Irregular blocky ridges extending downslope from 

giant amphitheaters on the submarine north flanks of 

Molokai and Oahu were interpreted as representing giant 

landslides by Moore (1964). Macdonald and Abbott (1970), 

· however, questioned the immense size of these features and 

proposed that blocky edifices located seaward of these giant 

amphitheaters are in-place volcanoes. Irregular bathymetry 

suggesting submarine landslides on the west flank of Mauna 

Loa on the island of Hawaii was investigated by the U.S. 

Geological Survey in 1976 and 1978. This work showed 

that a large region (including the south Kona slump and 

Alika debris avalanche complex; fig. 2) between the previously enigmatic, subaerially exposed Kealakekua and 

Kahuku faults was involved in landsliding that reached 80 

knl offshore (Normark and others, 1979). Analysis of 

bathymetry and newly acquired SeaMARC II sidescansonar images of small areas (Fomari and Campbell, 1987) 

further emphasizes the importance of mass movement on 

the flanks of the Hawaiian Ridge. The ubiquitous nature of 

large landslides from the volcanoes of the Hawaiian Ridge 

was not fully realized, however, until the systematic EEZ 

GLORIA surveys began in 1986. 

METHODS 

Regional mapping of the U.S. EEZ around Hawaii 

was conducted aboard the research vessel Parnella beginning in 1986 by the U.S. Geological Survey and the U.K.· 

Institute of Oceanographic Sciences using the GLORIA 

sidescan-sonar system. The GLORIA images, which extend 

as much as 22.5 km to each side of the ship track, can 

resolve features as small as a few hundred meters (Somers 

and others, 1978). The systematic and uniform coverage by 

the GLORIA surveys is well suited to compare the more 

than 30 large landslide deposits (for further discussion of 

methods used to recognize and map submarine landslides 

see Lee, Schwab, and Booth, this report). The surficial 

parameters (relief and shape) of the slide deposits and other 

acoustically sensed characteristics can be used to help 

184 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
25° 

Figure 1. Hawaiian Ridge showing major landslides 

mapped as of the spring of 1990 by the GLORIA sidescan 

sonar system. The main islands are shown in black and the 

landslides around them are shown with dotted lines; see 

figure 2 for details. Landslides to the northwest are shown 

with dashed lines because the position is approximate and 

distinguish between different parts of the same landslide or 

to compare widely separated slides of different age that 

have a common mode of origin. 

LANDSLIDE TYPE 

Along the 1 ,400-km reach of the Hawaiian Ridge 

from Hawaii to Gardner Pinnacles, well-exposed massmovement deposits involve roughly one-half of the flanks 

of the Ridge (figs. 1 and 2). Most of these mass movements 

have been known only since the recent GLORIA surveys, 

and until more detailed studies are carried out, our understanding of the processes of landslide formation will be 

limited by the small scale of available sonar images. Finer 

blocky facies and related turbidite deposits, both of which 

are generally unresolvable with GLORIA data, doubtless 

extend beyond the presently identified landslide margins 

(fig. 2). The general characteristics and distribution of the 

giant landslides, with a detailed review of three of the larger 

and younger debris avalanches, are the focus of this report. 

The large Hawaiian slope failures are divided into 

two types based on the classification system of V ames 

(1978) that has been adapted for use in submarine landslides 

(Lee, Schwab, and Booth, .this report). We distinguish 

slumps (for example, Hilina slump, fig. 2) and debris 

~ =~- "' c ..,"" ~ go ·. 

based on unprocessed data. Mapping of the north side of 

the Hawaiian Ridge west of 16rW. longitude is incomplete, and the distribution of slide deposits is poorly 

known. Contours are the 2,000- and 4,000-meter isobaths 

and are adapted from U.S. Naval Oceanographic Office 

(1973). 

avalanches (for example, Nuuanu debris avalanche, figs. 2 

and 3) while the term landslide is retained as a general term 

for all mass movements. A slump is a movement of 

essentially rigid, internally undeformed masses along discrete slip planes in which the blocks of failed material rotate 

along curved slip surfaces. Giant landslides that involve the 

failure of thousands of cubic kilometers of rock and 

sediment that have disintegrated into relatively smaller 

pieces (compared to the large slump blocks but commonly 

include blocks of many cubic kilometers) and have clearly 

moved rapidly are referred to as debris avalanches. Each 

debris avalanche is thought to represent a single episode of 

catastrophic slope failure. 

The slumps on the Hawaiian Ridge are particularly 

steep and thick (up to 10 km) and are wide at their heads 

relative to their lengths. The overall sea-floor gradient in the 

area of slumping is generally greater than 3°, and the 

width/length ratio exceeds one. The slumps maintain physical continuity, that is, the bulk of the failed material is still 

on the flank of the volcano and only a relatively limited 

volume of debris has moved far from the source. Several 

slumps lack well-defined amphitheaters at their heads, 

probably because the morphology was continually modified 

by volcanic material throughout the episodic development 

of the slumps. The surfaces of the slump deposits are 

commonly marked by transverse ridges and scarps that are 

Giant Volcano-Related Landslides and the Development of the Hawaiian Islands 185 
+ 

·-·-- + .......... """" + 

+ 

0 

............ , 

+ 

100 

........... ............... __ ·-..... 

+ 

200 KM 

Figure 2. Southeastern Hawaiian Ridge from the 

island Kauai to the island of Hawaii showing the 

outline of major landslides (dashed lines) identified 

by name and number (see table 1 for landslide 

characteristics). Dotted pattern indicates hummocky 

deposits observed on seismic-reflection profiles and 

GLORIA images; more widely spaced dot pattern 

indicates subdued relief. Hachured lines indicate 

generally bounded by faults but perhaps in some places 

represent bulges or folds (Borgia and others, 1990). The 

toes of the slumps commonly form a distinct scarp where 

they meet the pre-slide sea floor. 

Using the Hilina slump on the flank of Kilauea 

volcano (fig. 2 and table 1) as a model, scientists have 

determined that these slumps apparently develop during the 

active stage of shield-building. A shield volcano is built 

from lava erupted quietly from a central vent or from two or 

more rift zones that meet at the central vent. The subaerial 

slopes of shield volcanoes seldom exceed 5°, but submarine 

slopes may exceed 10°. When rift zones are well estabmajor fault scarps. Thin, downslope-directed, irregular lines indicate submarine canyons and their subaerial extensions. Heavy dashed line is the axis of the 

Hawaiian Trough, and the dash-dotted line is the 

crest of the Hawaiian Arch. Alika-1 and Alika-2 are 

separate depositional deposits described as phase-1 

and phase-2 Alika failures in text and table 1. 

lished, mass movement may result from the gravitational 

instability of the growing and widening volcanic shield, the 

stresses caused by injection of magma into the rift zones 

(Lipman and others, 1985), and the reduction in strength as 

magma is injected along possible failure planes. The slumps 

along the Hawaiian Ridge probably moved intermittently at 

a rate that was controlled by the accumulation of lava flows 

on their upper reaches, as well as by magma intrusion in the 

rift zones. Earthquakes probably have contributed to the 

development of these slope failures and also are likely a 

result of their movement. Earthquake focal depths for the 

1975 7.2-magnitude Kilauea earthquake and its aftershocks 

186 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
Table 1. Hawaiian submarine landslides 

[Landslides are located in figure 2] 

Area length2 Width3 Estimated Age4 

No. Name Type1 location (km2) (km) (km) (thousand yrs.) 

1 ......................... N Kauai A N Kauai 14,000 140 100 <5,000 v 

2 ......................... S Kauai A S Kauai 6,800 100 50 <5,000 v 

3 ......................... Kaena A NE Oahu 3,900 80 45 <3,700 v 

4 ......................... Waianae s SW Oahu 6,100 50 80 <3,700 v 

5 ......................... Nuuanu A NE Oahu 23,000 235 35 2,600-1,400 v 

6 ......................... Wailau A N Molokai >13,000 <195 40 1,400 V, S 

7 ......................... Han a s NE Maui 4,900 85 110 <750 v 

8 ......................... Clark A SW Maui 6,100 150 30 >650 R 

9 ......................... Pololu A N Hawaii 3,500 130 20 370 R, S 

10 ......................... S Kona s W Hawaii 4,600 80 80 300-105 V, T 

11 ......................... Alika-1 A W Hawaii 2,300 88 15 300-105 V, T 

12 ......................... Alika-2 A W Hawaii 1,700 95 15 105T 

13 ......................... W Ka Lae A S Hawaii 850 85 10 <105 T5 

14 ......................... E Ka Lae A S Hawaii 950 75 10 <105 T5 

15 ......................... Hilina s S Hawaii 5,200 40 100 A 

16 ......................... Papa'u u S Hawaii 200 20 6 5V 

17 ......................... Loihi u S Hawaii 500 15 10-30 A 

1 Typ1~ of slide is: A, debris avalanche; S, slump; or U, undifferentiated. 2 Length of the Waianae and South Kona slumps omits indistinct irregular topography beyond the steep toe. 3 Width at the head of the landslide. 4 Age constraints: V, age of host volcano lavas; R, coral reef date; S, inferred from terrace depth and assumed rate of submergence of the Hawaiian 

Ridge; T, age of tsunami deposit tentatively correlated with the landslide; A, intermittently active. 5 Based on a qualitative estimate comparing the degree of acoustic backscattering observed on the GLORIA images between these landslides and 

the Alika phase-2 event. 

indicate tJhat the base of the Hilina slump is close to the 

10-km-de~:!p, pre-Hawaiian sea-floor surface (Lipman and 

others, 1985). Geodetic measurements show that the flank 

of Kilauea volcano near sea level subsided as much as 3.5 

meters (m) and locally moved seaward more than 8 m 

during th{: earthquake. 

The debris avalanches typically are longer and not as 

thick as the slumps (fig. 2 and table 1). The amphitheaters 

at their heads mark the source areas from which the 

avalanche material was derived. The debris avalanches 

commonly include a long middle and distal train of hummocky de:bris (figs. 3, 4, and 5). Blocks up to tens of 

kilometen; in horizontal dimension (for example, Tuscaloosa Seamount; fig. 3) may be present immediately 

below the amphitheater, and blocks 1 km across are 

common in the distal parts of the deposits. The deposits are 

of variable thickness along their course, but the greatest 

thickness in the blocky and hummocky sectors is probably 

0.4 to 2 km, and the average thickness may be as little as 50 

m in the smaller avalanches. Debris blocks are relatively 

scarce immediately below the amphitheaters; the debris 

layer in the distal part is thin and probably includes deposits 

that are transitional with, and include, turbidite units. Some 

of the deb:ris avalanches may develop by partial or complete 

failure of a volcano slope already oversteepened by a 

slump. Unlike the slumps, most debris avalanches probably 

move at a high velocity in a single episode, as shown by the 

uniform mound size and spacing in the distal hummocky 

facies, and the fact that some debris avalanches have moved 

upslope several hundred meters in their distal reaches 

(Moore and others, 1989). 

RELATIVE AGES OF LANDSLIDES 

Proceeding northwest along the Hawaiian Ridge, the 

volcanoes become progressively older. Kure Island, which 

is 2,600 km northwest from the active volcanoes on the 

island of Hawaii, was built by volcanoes about 30 million 

years ago (Clague and Dalrymple, 1987). Along this span, 

GLORIA images and seismic-reflection profiles show a 

progression from youthful- to older-appearing slope-failure 

features (Torresan and others, 1990). The Hilina slump and 

Alika debris avalanches are some of the youngest examples 

on the slopes of Hawaii's active volcanoes (Moore and 

others, 1989), and they are covered by less than 1 m of 

sediment. GLORIA images, SeaBeam bathymetric data, 

and seismic-reflection profiles of the Alika debris avalanches depict rugged, hummocky topography (figs. 4 and 

5; Normark, Lipman, and Moore, 1979). 

In contrast, the older slumps and debris avalanches 

along the Hawaiian Ridge west of Niihau are covered by as 

much as 30m of sediment that drapes, subdues, and locally 

masks many of the features commonly observed on the 

younger landslides. The hummocky topography of older 

debris avalanches is more gently undulating because of the 

thicker sediment cover. Back-rotated blocks of older 

Giant Volcano-Related Landslides and the Development of the Hawaiian Islands· 1'87 
Figure 3. Mosaic of GLORIA sonographs 

showing the Nuuanu and Wailau debris avalanches; see figure 2 for location and outline 

of deposits. Lighter shades designate regions 

of greater acoustic backscatter either because 

of more reflective bottom material or because 

of a steep slope facing the incoming sonar 

signal; this type of acoustic return characterizes the blocky terrain of debris-avalanche 

deposits. The debris blocks range in size from 

Tuscaloosa Seamount, the large (greater than 

20 km across) tabular block extending northwest from 22°N. , 157°W ., down to talus below 

the resolution of the GLORIA images (about 

100 meters). The apparent elongation of hummocks parallel to ship tracks (observable on 

the sonar images as straight line swaths of 

poor data quality) is an artifact caused by 

horizontal beam spreading and yaw of the 

towed GLORIA vehicle. The relatively thin, 

irregular, downslope trending light returns 

immediately offshore of the islands are from 

the numerous submerged canyons. 

188 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
0 

0 

0 5 

L--.......L..'----' 

KILOMETERS 

Figure 4. Alika phase-2 debris avalanche along the west 

side of the island of Hawaii outlined by the heavy dashed 

line. Ridges and levees are shown by dotted lines, and 

discrete hummocks, or large debris blocks are shown in 

black and are identified as those features on the SeaBeam bathymetry data that are more than 35m high and 

250 m wide. Contour interval is 100 m. Bathymetry from 

NOAA SeaBeam charts. 

Giant Volcano-Related Landslides and the Development of the Hawaiian Islands 189 
0 10 20 30 40 50 KILOMETERS 

A 

Figure 5. (A) Mosaic of GLORIA images with (B) interpretation showing both phases of the Alika debris avalanche. 

Compare with the interpretation on figure 4, which focuses on the debris lobe and leveed pathway for the Alika 

phase-2 event, and the mosaic of the Nuuanu and Wailau debris avalanche on figure 3 for explanation of the acoustic 

patterns. 

190 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
................................... 

. ::::::::::::::::: :~(j~~::::::::: . 

··················~~ . . . . . . . . . . . . . . . . . . . " ········· ........ . . ··················· v"/:'- ······· 

:::::::::::::::::::: ~- /.:::::::. 

0 

KILOMETERS 

B 

Figure 5. -Continued. 

Giant Volcano-Related Landslides and the Development of the Hawaiian Islands 191 
slumps, such as that on the southeast side of St. Rogatien 

Bank, pond sediment as much as 25 m thick (Torresan and 

others, 1990). The GLORIA images adjacent to Necker 

Island show two debris avalanches on the north side of the 

Hawaiian Ridge that appear smaller and display apparently 

fewer blocks with a larger lower size limit and a smaller 

upper size limit than those observed in younger, more 

southeasterly debris avalanches depicted in figure 2. These 

differences can all be attributed to more extensive postslide 

sediment cover. 

·Several lines of geologic evidence provide constraints 

on the absolute ages of the giant slumps and debris 

avalanches (table 1), but much careful study is needed to 

assign precise ages (Moore and others, 1989). Direct dating 

of the Hawaiian slope failures is difficult, and our main 

control on the periodicity of the failures comes from 

volcano growth rates. The current output of lava from the 

Hawaiian hot spot is about 0.1 cubic kilometers (km3

) per 

year (Swanson, 1972); hence, 10,000 years is required to 

produce the 1 ,000 km3 of the average large debris avalanche. Not all eruptive material can be involved in landsliding, however, or the volcanoes would never" grow. We 

estimate that as much as 10 percent of the erupted material 

might be involved in the landslides. If correct, then this 

approximation suggests an average periodicity of 100,000 

years for the major landslide events. 

Most of the larger, older landslides that have been 

recognized seem to have occurred late in the shield-building 

phase of the host volcano (Moore and others, 1989). Hence, 

the ages of lavas (Clague and Dalrymple, 1987) of this 

phase of volcano development provide a rough estimate of 

ages of the youngest landslide. At the older volcanoes, 

northwest of Kauai, insufficient evidence exists to confirm 

whether landsliding occurred past the end of shield building. The most active volcanoes, Mauna Loa and Kilauea, 

which have not yet completed their shield-building phase, 

show ample evidence of major landslides; this indicates that 

slope failure is a common process through much of the 

history of volcano growth and is not restricted to the end of 

shield building. . 

In addition to the dating of the volcanic sequences 

that have become involved in landsliding, or that postdate 

landslides, constraints on landslide age can be deduced 

from the dating of coral reefs that appear to overlie 

landslides. Landslide ages also can be tied to ~ates of 

coral-reef fragments that apparently were deposited on 

subaerial slopes of adjacent islands by giant waves initiated 

by landslide movement. Moore and Moore (1984) report 

coral debris as high as 326 m above sea level on the island 

of Lanai that is probably related to the Alika phase-2 debris 

avalanche. Additional information on landslide ages may 

come from the deep-sea sedimentary record, both by 

examination of sediment atop the landslide debris and by 

identifying perturbations in the sediment stratigraphy 

beyond the landslide toes that may reflect currents initiated 

by the mass failures. 

Because of the generally long recurrence interval for 

the giant ·landslides (perhaps 100,000 years), they are not 

considered to pose a serious threat despite the enormity of 

the tsunami (large. sea waves). that might be produced 

(Moore and Moore, 1984) and the potential loss of land. 

Both_small mass failures, however, such as those caused by 

the collapse of lava delta.s formed where lava from sustained 

eruptions flows into the sea, and mid-size slope failures, 

such as the Papa'u and Loihi landslides (fig. 2), probably 

occur ,far more frequently than the giant ones. The collapse 

of the small lava deltas have proquced waves as high as 20 

em that have been recorded on tide gauges several tens of 

ki_lometers away. The mid-size slope failures could produce 

tsunami of intermediate size; that is, far larger than the 

tsunami generated by earthquakes around the margins of the 

Pacific Ocean that have devastated Hawaiian coastal settlements in the recent past. Our knowledge of these nearshore, 

mid-size landslides is incomplete because the relatively 

shallow water on steep, highly irregular slopes has hampen~d detailed investigation. 

DESCRIPTIONS OF THREE DEBRIS 

AVALANCHES 

Nuuanu Debris Avalanche 

The Nuuanu debris avalanche covers about 23,000 

square kilometers (km2

) (the largest landslide mapped), on 

the northeast flank of Oahu (figs. 2 and 3). It extends about 

230 km from its headwall at Nuuanu Pali on Oahu to its toe, 

which is found half-way up the southwest-sloping flank of 

the Hawaiian Arch. 

The amphitheater above the Nuuanu debris avalanche 

deposits is defined by bathymetric contours that are 

deflected as much as 10 km shoreward from the projected 

original shape of the volcanic shield (Moore and others, 

1989). The thickness of material removed to form this 

amphitheater is 700 to 1,100 m. Below the amphitheater is 

a tongue-shaped mass studded with giant blocks that are 

tens of kilometers in maximum dimension (fig. 3) and rise 

0.5 to 1.8 km above the regional slope. These blocks 

commonly have back-tilted flat summits; they originated in 

the amphitheater, and some have moved more than 50 km to 

their present position. The largest block, Tuscaloosa Seamount, 90 km northeast of Oahu, is 30 km long, 17 km 

wide· and has a broad flat summit about 1. 8 km above its 

base. 

- After crossing the axis of the Hawaiian Trough, the 

Nuuanu debris avalanche traveled uphill a distance of about 

140 km from a present water depth of 4,600 m in the 

Trough to 4,300 m on the Hawaiian Arch where the debris 

avalanche ends. Thus, the upslope transport is at least 

192 Submarine landslides: Selected Studies in the U.S. Exclusive Economic Zone 
300 m. The momentum necessary for the debris·to ride up 

such a slope requires rapid movement. A fast-moving 

submarine. landslide of this size would displace enormous 

volumes of sea water and probably produced large surface 

waves even more destructive than the wave recorded on 

Lanai (Moore and Moore, 1984) that' was presumably 

caused by the smaller Alika phase-2 event (table 1). 

The distal 80 km of the Nuuanu debris-avalanche 

deposit, which lies on the Hawaiian Arch, is well defined 

on the GLORIA images as scattered blocks ran~ing up to 

about one kilometer in size (fig. 3). Insights 'on the 

thicknesses of debris avalanches are obtained by comparison of longitudinal profiles down the axis of the avalanche 

with parallel profiles to either side of it. These profiles 

suggest a minimum thickness of the cel)tral part of the 

avalanche of about 400 mandan average.of perhaps 200m 

over the entire landslide area. A conservative estimate of 

200-m thickness yields a volume of about 5,000 km3 for 

this debris-avalanche deposit. 

Several submarine canyons incise the headwall area 

of the Nu.uanu debris avalanche (figs. 2 and 3). These 

submarine canyons continue to supply sediment that is 

covering tlhe blocky terrain of the upper landslide deposit. 

Because these submarine canyons are clearly a~sociated 

with land canyons, Andrews and Bainbridge (1972) and 

Coulbourn and others (1974) concluded that the canyons 

were subat!rially carved and subsequently drowned during 

subsidence: of the Hawaiian Ridge. This subsidence had 

been as much as 1, 800 m because the canyon morphology 

is well-developed to that depth (Andrews and Bainbridge, 

1972). Because the canyons are cut in the upper part of the 

slide scar, we infer that the Hawaiian. Ridge stood .at least 

1 ,800 m higher at the time of slope failure. 

Wailau Debris Avalanche 

The Wailau debris avalanche has removed the north 

part of East Molokai Volcano and left a well-defined, 

40-km-wide amphitheater at its head. The debris avalanche 

apparently overrode the south side of the older Nuuanu 

debris avalanche an unknown distance. The largest block 

(Tuscaloosa Seamount) in this crossover zone, however, is 

not a part of the Wailau debris avalanche but rather belongs 

to the Nuuanu debris avalanche, on the basis of the 

composition of lava dredged from it. 

A regional terrace that abruptly steepens below 1 ,300 

m water de:pth provides some constraints on the age of the 

debris avalanche. This terrace extends completely across 

the headwall amphitheater of the Wailau landslide (Shepard 

and Dill, 1'966). The nearly horizontal nature of the terrace 

indicates that it is probably a feature produced at or close to 

sea level (Mathewson, 1970) and that it postdates the 

Wailau debris avalanche. Thus, the slope failure occurred 

when the volcano stood at least 1 ,300 m higher than at 

present. 

The 1 ,300-m-deep terrace is about 800 m deeper than 

a 500-m-deep terrace that formed about 850,000 years ago 

(Moore and Campbell, 1987). If the Molokai subsidence 

rate was comparable to that of northern Hawaii (2.5 

millimeters per year (mm/yr)) during the period between 

formation of the 1 ,300-m-deep terrace and the 500-m-deep 

terrace, then the estimated age of the 1 ,300-m-deep terrace 

is about 1.17 million years. Radiometric dating of lavas that 

mark the end of the shield-building stage of the East 

Molokai Volcano indicates an age of 1. 50 million years 

(Clague and Dalrymple, 1987). Hence, the Wailau avalanche event cannot be much older than the 1 ,300-m-deep 

terrace (1.17 million years) and is younger than 1.50 

million years old because postfailure volcano growth has 

been primarily limited to the small Kalaupapa volcano, 

which forms the small peninsula on the north side of 

Molokai and has been dated from radiometric dates of lavas 

at -570,000 to 340,000 years old (Clague and others, 1982). 

Kalaupapa volcano grew after both the landsliding and 

much of the observed subsidence (nearly 1,300 m) of the 

island of Molokai. Moore and others ( 1989) have, thus, 

estimated that the age of the Wailau debris avalanche is 

about 1.4 million years. 

In the region where the Nuuanu debris avalanche and 

the neighboring Wailau avalanche meet (fig. 2), the major 

elongate blocks are more nearly normal to the long axis of 

the Nuuanu debris avalanche. Moore (1964) suggested that 

the Nuuanu debris avalanche was younger than the Wailau 

debris avalanche, because the long axes of landslide blocks 

are more commonly perpendicular to the movement direction of the Nuuanu debris avalanche. GLORIA images, 

however, (fig. 3) show a field of hummocky terrain typical 

of distal debris avalanche facies east of the upper Wailau 

debris avalanche. This material is probably not a part of the 

Wailau debris avalanche because of its location to the side 

and outside of a levee on the east side of the Wail au debris 

avalanche; hence, it is more likely a part of the great distal 

debris apron of the Nuuanui debris avalanche. If the Wailau 

debris avalanche was emplaced first, it is unlikely that finer 

material from the Nuuanu avalanche could have crossed it. 

For this reason, we consider the Wail au debris avalanche to 

be younger than the Nuuanu debris avalanche (table 1). The 

younger relative age inferred for the Wailau debris avalanche parallels the relative ages of the two volcanoes that 

spawned these slope failures. 

Alika Debris Avalanches 

The Alika debris avalanches are probably among 

Hawaii's youngest (table 1; Lipman and others, 1988). 

These two partly overlapping landslides, designated Alika 

phases 1 and 2, cover 2,300 and 1,700 km2 , respectively 

Giant Volcano-Related Landslides and the Development of the Hawaiian Islands 193 
E 

.c -

C) 

C1) 

J: 

1000 1:-

100 1:-

10 

Hummocks 

ie=• 

•• •• 

X 

Mx 

d:IP 

xxx.:x 

0 

x Alika Phase 2 

• Mt. St. Helens 

o Nuuanu 

1L-~~~w··~~~~~~~~~~~~~ 

. 0 1 • 1 1 0 100 

Diameter, km 

Figure 6. Comparison of diameter and height of hummocks (large blocks) in the Alika phase-2 debris-avalanche 

deposit with blocks from the submarine Nuuanu debrisavalanche deposit (number 5 in table 1) and the subaerial 

deposit from Mount St. Helens that was catastrophically 

formed in 1980. The size relation forms a continuum for 

both submarine and subaerial mass movements (with 

scale varying by as much as two orders of magnitude). The 

submarine examples off Hawaii emphasize the tremendous energy associated with catastrophic, rapid downslope movement of these debris avalanches. 

(figs. 2, 4, and 5). They extend nearly 100 km from their 

source near the present shoreline to depths of 4,800 min the 

Hawaiian Trough. The estimated thickness of the debris 

avalanches is 50 to 200 m based on the size of the blocks 

and the height of natural levees that border the main 

transport paths. ·volume estimates for both failures together 

range from 200 to 800 km3

. 

The lower part of the Alika debris-avalanches 

deposit, below about 4 km water depth, is marked by 

closely spaced hummocky mounds (figs. 4 and 5). The 

individual mounds are commonly a few hundred meters 

across and 25 to 75 m high, but some are several kilometers 

across and several hundred meters high (fig. 6). Over much 

of this terrain, the hummocks are closely spaced and similar 

in size and height, suggesting that they were all produced by 

the same processes in the same episode. Downslope from 

the zone of closely spaced hummocks is a fringing zone 

from 5 to 20 km wide of isolated blocks or mounds. Where 

this zone on the phase-2 Alika debris avalanche was 

examined with a high-resolution sidescan-sonar system and 

cameras towed near the sea floor, blocks and mounds are 

found that range from 500 m in diameter (large enough to 

identify on GLORIA images) to as small as cobble-size 

pieces. Toward the distal margin of this zone, the larger 

resolvable mounds on the GLORIA imagery are increasingly separated, commonly by 0.5-1 km. 

The hummocky topography and marginal levees of 

the Alika landslides resemble the 1980 Mount St. Helens 

subaerial debris avalanche and suggest that a similar high 

emplacement velocity prevailed for the Alika features (fig. 

6). Rapid avalanching of rock volumes of this size could 

produce a large tsunami. The general youth and location of 

the Alika phase-2 debris avalanche (fig. 2) make this 

mass-movement event the most likely to have caused the 

extensive 105,000-year-old wave deposits that have been 

documented on Lanai and nearby islands up to heights of 

326 m above sea level (Lipman and others, 1988; Moore 

and Moore, 1984, 1988). 

LANDSLIDING AND VOLCANO 

DEVELOPMENT 

Major landsliding has occurred throughout the growth 

of Hawaiian volCanoes, and may continue at a reduced rate 

long after dormancy. More than half the surface area of 

Loihi, the youngest volcano in the ·chain (fig. 2), is 

extensively modified by landsliding even though its summit 

is still 1 km below sea level and the volcano is perhaps a 

hundredth the size of a mature shield volcano (Moore and 

others, 1989). We expect the largest and most far traveled 

debris avalanches to occur during the final stages of the 

shield-building cycle when not only is the volcanic production still significant but also the volcanoes' stand is highest 

above sea level. Before and after that time, smaller landslides that travel shorter distances could be expected. 

Large-scale landsliding probably occurs, however, intermittently throughout most of the shield-building stage with the 

slope failure deposits being incorporated into the edifice as 

it grows so that little surface evidence for gliding remains. 

The Nuuanu, Wailau,. and Pololu debris avalanches, 

and the Waianae, Hilina, and South Kona slumps (figs. 1, 

2, and table 1) all head at, and have moved perpendicular 

to, the primary rift zones of their host volcanoes. Other 

slope failures, such as the Kaena debris avalanche and the 

Hana slump, head on the sides of long, shallow submarine 

ridges that apparently represent submerged rift zones. This 

rift-zone/slope-failure relationship is part of the cause-effect 

relation reflecting the gravitational instability of these 

volcanoes. Rift-zone trends can be strongly influenced by 

the position of tensional zones at the head of landslides, but 

landsliding can be induced by the lateral shoving caused by 

magma injection into the rift ·zones (Lipman and others, 

1988). 

CONCLUSIONS 

A primary result of GLORIA sidescan-sonar surveys 

of the Hawaiian Exclusive Economic Zone was to document 

the enormous scale of submarine slope failure along the 

Hawaiian Ridge. Slumps and debris avalanches were found 

194 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
to cover nearly 100,000 km2 of the ridge and adjacent sea 

floor just within the segment from Kauai to Hawaii; 

100,000 km2 represents an area more than 5 times the 

combined land areas of the islands. About 20 additional 

major landslides have been imaged from Kauai Island 

northwest to Kure Island on the Hawaiian Ridge. Some of 

the individual debris avalanches are more than 200 km long 

and about 5,000 km3 in volume, ranking them among the 

largest on Earth (Lipman and others, 1988). These slope 

failures bt:!gin early in the history of individual volcanoes 

when they are small submarine seamounts, culminate near 

the end of subaerial shield-building, and may continue long 

after dormancy. Thus, mass-movement deposits and structures are an important element in the composition and 

evolution of the volcanoes. 

The landslides are of two general types: slumps and 

debris avalanches. Slumps are wide (up to 110 km) and 

thick (about 10 km) with transverse blocky ridges and steep 

toes. Debris avalanches are fast-moving, long (up to 230 

km) compared to their width (commonly less than 50 km 

wide in the proximal part but many produce broad depositional lobes more than 100 km in diameter at their distal 

reaches), and comparatively thin (0.05 to 2 km); they 

commonly have well-defined amphitheaters at their heads 

and hummocky terrain in their lower parts. 

Most of the landslides moved into, and locally across, 

the axis of the Hawaiian Trough, which flanks the Hawaiian 

Ridge (fig. 2). Thus, mass movement is a primary mechanism for filling the Trough and widening the base of the 

ridge. Tbe eruptive behavior of the volcanoes can be 

modulated by landsliding, and the subaerial geometry of the 

slope failures is related to the structural features of the 

volcanoes including rift zones and fault systems. Water 

disturbanc:es caused by the rapid movement of the debris 

avalanches probably produced the high waves responsible 

for elevated reef deposits on Lanai and adjacent islands. 

Most submarine canyons offshore of the Hawaiian volcanoes were: carved subaerially in the upper amphitheaters of 

debris avalanches. Subaerial canyon cutting in these amphitheaters is probably enhanced by the resulting oversteepened slop,es. 

The: widespread development of landslides along the 

submarine slopes of the Hawaiian Ridge suggests that 

analogous. slides should be present on other oceanic volcanoes cons.tructed in deep water. Isle de Ia Reunion in the 

Indian Ocean, for example, has large-scale submarine 

landslides on the east flank of the active volcano, Piton de 

Ia Fournaise. The landslide was inferred by Duffield and 

others (1982), and recent SeaBeam bathymetric mapping, 

high-resolution sidescan-sonar imaging, sea-floor photography, and dredging along the submarine east flank of Piton 

de Ia Fournaise has confirmed the widespread presence of 

submarine landslide material (Lenat and others, 1989). 

Hummocky terrain, similar to the Hawaiian debrisavalanche deposits, extends at least 40 km offshore to 

2,500 m depth; the full dimensions of the large submarine 

slope failures on the east flank of La Reunion, however, 

have not yet been delimited. Steep submarine slopes along 

older parts of La Reunion (Piton de Neiges) that grade down 

into a broad gently sloping apron on the lower slopes 

suggest that mass failure is widespread around the entire 

island. 

The recurrence interval of landslides on oceanic 

volcanoes is long, from 1 to 100 thousand years, in part 

being limited by the large ratio of landslide volume to 

annual eruptive volume especially for the largest failures. 

Because large landslides have also been identified at other 

oceanic volcanoes rising from abyssal depths, however, and 

because their effects can be catastrophic, a survey of 

landslide-prone oceanic volcanoes should be initiated. 

ACKNOWLEDGMENTS 

We are indebted to the crews and technical staffs of 

the R. V. Farnella for their outstanding work during the 

·cruises in 1986 and 1988, and we have benefited from 

discussions with our colleagues of the U.K. Institute of 

Oceanographic Sciences, R. Belderson, J. Campbell, M. 

Harris, R.G. Rothwell, R.C. Searle, M.L. Somers, and 

J.B. Wilson. R.M. Carter, D.A. Clague, R.P. Denlinger, 

R.T. Holcomb, M.L. Holmes, P.W. Lipman, K. Hon, and 

A.N. Shor help'ed with the data collection and provided 

important geologic insights. We appreciate thoughtful 

reviews by D.E. Drake and D.W. Peterson. 

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2646. 

Moore, J.G., Clague, D.A., Holcomb, R.T., Lipman, P.W., 

Normark, W.R., and Torresan, M.E., 1989, Prodigious 

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Normark, :W.R., Lipman, P.W., and Moore, J.G., 1979, 

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Intraplate Volcanism and Submarine. Volcanism, International Association of Volcanology and Chemistry of the 

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U.S. Naval Oceanpgraphic Office, 1973, Bathymetric atlas of the 

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176, Washington, D.C., p. 11-33. 

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196 Submarine Landslides: Selected Studies in the U.S. Exclusive EconomiC Zone 
Subrnarin~ Slope Failures Initiated by Hurricane lwa, 

Kahe~ Point, Oahu, Hawaii \. 

By ~.R. Normark, Pat Wi~de\ J.F. Caf'!lpbell2 , T.E. Chase, and Bruce Tsutsui3 

INTRODUCTION 

Rea·l-time "monitoring" of submarine slope failure (or 

landslide) events has generally been limited to visual 

observation of the collapse of shoreline features into the 

ocean and associated de~truction of manmade coastal structures and (or) the appearance of muddy turbulent surface 

waters in the shallow waters of fjords, off large deltas or in 

the heads of submarine canyons. In deeper water settings, 

temporal observations have generally been limited· to a 

knowledge of the time of disruption/destruction of manmade stru·:tures, especially submarine telecommunication 

cables (see Menard, 1964, and Lee, 1989, for· a review of 

the types of observations). The 1.929 Grand Banks earthquake, which generated mass flows on the upper continental 

slope off eastern Canada, has long been a classic example of 

submarine slope failure and subsequent generation of turbidity currents. The timing of sediment movement and areal 

extent of the failure during the 1929 earthquake have been 

deduced from the sequential disruption of a series of 

submarine telecommunication cables. The limitation of this 

type of indirect observation of mass-movement events 

however, has become evident as recent data from the Grand 

Banks margin, including high-resolution ·sidescan-sonar 

imagery and inspection of the sea floor using manned 

submersiblles, have resulted in a substantial revision of our 

thinking about both the nature of the slope failure and the 

character of the sediment mass flows that were generated 

(Hughes Clarke, 1988; Hughes Clarke and others, 1990). 

A se:ries of sea-floor surveys and instrument deployments to monitor oceanographic conditions offshore from 

Kahe Poi111t, Oahu, as part of a feasibility study for the 

Ocean The:rmal Energy Conversion (OTEC) project (Noda, 

/ 1983_; Tsutsui and others, 1987; Dengler and Wilde, 1987) 

provtded a rare opportunity to observe mass movement of 

sediment on submarine slopes more directly. An array of 

self-contained current meters that were displaced during the 

passage of a major hurricane in 1982 provided measurements of the timing, velocity of movement, and change of 

water depth as sediment was displaced to deeper water. 

Detailed s.urveys of the sea floor in the area of the 

~ Marine Sciences Group, University of California, Berkeley, Calif. 

Seafloor Surveys International, Inc., Honolulu Hawaii 3 . ' . 

School of Ocean and Earth Sctence and Technology, University of 

Hawaii, Honolulu, Hawaii. 

current-meter array both before and after the hurricane 

provide additional constraints on the nature of the mass 

movement. 

GEOLOGIC SETTING 

The study area lies off Kahe Point, which is on the 

southwest coast of the island of Oahu (fig. 1). Immediately 

offshore from Kahe Point, the shelf is very narrow, slightly 

more than 1 kilometer (km) wide, and a steep (30° to 40° 

average) up·per slope drops abruptly to 350 meters (m) water 

depth (fig. 2). Farther seaward the slope gradually 

decreases, forming a broad terrace that terminates at about 

1',500 m, where the sea-floor slope increases to about 20°. 

The section of narrow shelf corresponds to a reentrant in the 

upper slope that is about 4 km wide and matched directly 

downslope by a convex-seaward bulge in the outer edge of 

the terrace (figs. 2 and 3). These-relations led Normark and 

others (1982) to suggest that this morphology might represent a small bedrock slump feature; such slumps and more 

catastrophic slope failures have since been documented 

offshore of all major volcanoes of the southeastern Hawaiian Ridge (Moore and others, 1989; Normark, Moore, and 

Torresari, this report). 

A detailed, precision-navigated survey conducted in 

1981 obtained both 3.5-kilohertz (kHz) and minisparker 

seismic-reflection profiles along an orthogonal grid with 0.5 

to 1 km line-spacings allowing Normark and others (1982) 

to prepare a map of estimated sediment character based on 

acoustic reflectivity. Irregular local topography, together 

with an internally chaotic or structureless subbottom character, were interpreted by them to indicate that much of the 

upper slope and inner terrace off Kahe Point is underlain by 

sediment that has undergone mass movement. The physical 

properties of carbonate sediment samples from the study 

area are consistent with this interpretation (Winters and 

Lee, 1982). Posthurricane surveys of the inner part of the 

study area using sidescan sonar and additional core data 

show direct evidence for downslope movement of sediment 

(Tsutsui and others, 1987). 

SEDIMENT MOVEMENT AND ASSOCIATED 

BOTTOM-WATER CURRENTS DURING 

HURRICANE IWA 

Hurricane Iwa passed just west of the island of Kauai 

on November 23, 1982, causing extensive damage on that 

Submarine Slope Failu~es Initiated by Hurricane lwa, Kahe Point, Oahu, Hawaii 197 
QKauai 

~Niihau Oahu~ BEST ESTIMATE OF TRACK '\..,() HAw All 

NORTH 

OF HURRICANE IWA \ ~~ 

v 0 5 10 15 

KM 

MAILI POINT 

I KAHE P~I~T 

Figure 1. Location of the Kahe Point, Oahu, study area, showing the path of Hurricane 

lwa west of the main Hawaiian Islands (modified from Tsutsui and others, 1987). 

island and disrupting a series of telecommunication cables 

in water depths greater than 1 , 100 m between the islands of 

Oahu and Kauai (figs. 1 and 3; Chui and others, 1983; 

Noda, 1983). Additional temporal evidence for submarine 

mass movement was provided from disruption of an array of 

moored instruments that had been deployed west of Kahe 

Point (Noda, 1983; Dengler, Wilde, and others, 1984a; 

Dengler, Noda, and others, 1984b). These moorings 

included self-contained units that recorded water depth, 

temperature, and speed and direction of currents. 

Figure 4 shows a smoothed bathymetric profile for 

the four moorings with current meters that could be recovered after the storm passed. The deduced downslope displacement of the recovered meters is as much as 2.4 km and 

is based on depth changes recorded by the meters assuming 

that movement was perpendicular to the isobaths, which are 

well determined from the prehurricane survey (Normark 

and others, 1982). The displacement .of the current meters 

to deeper water occurred in a series of four episodes with 

recorded water-current speeds of as much as 220 centimeters per second (em/sec). In general, downslope water flow 

during these episodes was in the range of 70 to 135 em/sec 

(Dengler, Wilde, and others, 1984a). The velocity of the 

flow front of one of the episodes was about 300 em/sec; that 

is, the time of passage of the leading edge of the downslopemoving event between moorings was faster than the peak 

recorded water-current speeds within the main body of the 

event. The initiation of each displacement event of the 

meters corresponded with the occurrence of high-velocity 

water flow. The water temperatures recorded at the meters 

198 Submarine landslides: Selected Studies in the U.S. Exclusive Economic Zone 
0 5 

19°15' N KILOMETERS 

Figure 2. Bathymetry of the Oahu slope off Kahe Point (adapted from Normark and 

others, 1982). This bathymetry has been used to create the perspective relief map of 

figure 3. 

during the episodes were generally 2° to 4°C above the 

ambient Kemperature at the mooring site before displacement (Dengler, Wilde, and others, 1984a). 

The data provided by the recovered current meters 

were supplemented by the following observations that 

provide important clues to the processes that displaced the 

moorings (Dengler, Wilde, and others, 1984a): (1) there 

were no ~:ignificant differences between pre- and posthurricane bathymetry of the slope below the reef front indicated 

by later detailed surveys; (2) the shallowest mooring had 

two meters (fig. 4) and the upper meter showed onshore 

flow while the lower meter showed offshore (downslope) 

flow suggesting fluid-flow phenomena in addition to possible mass flow of the sediment at the mooring sites; and (3) 

both the float housings and flat surfaces on the currentmeter housings had accumulated significant amounts of 

sediment:. something not observed in any moorings 

deployed in the Kahe Point area before and after those 

affected by Hurricane Iwa. 

POSTHURRICANE STUDIES 

A SeaMARC II sidescan-sonar mosaic of the upper 

slope and inner terrace provide good insight to slope 

processes during the passage of Hurricane Iwa (fig. 5; 

Tsutsui, 1987). Interpretation of the mosaic is augmented 

by analyses of 19 core samples collected in 1983 and 1984, 

sea-floor photographs, and by observations from a manned 

submersible (Foq1ari, 1983). The most striking patterns of 

the mosaic are 

• the subparallel returns that follow the coastline shape 

representing individual scarps within the steep upperslope area. These are the fronts of a series of coral reefs 

drowned during sea-level changes over the past several 

hundred thousand years. 

• the westward converging dendritic pattern of darker 

sinuous, and locally braided, high-backscatter returns 

that terminate upslope at the base of the upper-slope 

scarps. These sinuous features locally show positive 

relief and trend generally perpendicular to the isobaths. 

A schematic interpretation of part of the sidescansonar mosaic (fig. 6) shows that the displaced current 

meters were deployed along a set of these sinuous features. 

A few core samples recovered from these features contained 

medium sand-sized material. 

Tsutsui and others (1987) emphasize several large, 

broad swaths of uneven intermediate-to-high-backscatter 

Submarine Slope Failures Initiated by Hurricane lwa, Kahe Point, Oahu, Hawaii 199 
0 

-400 

-800 

<J) -1200 a:: 

l.t.J 

1-

l.t.J 

~ -1600 

-2000 

-2400 

-2800 

Limit of SeaMARC II mosaic 

Figure 3. The upper slope off Kahe Point, Oahu, based on 

detailed bathymetry compiled by Normark and others 

(1982) prior to the mass movement events (fig. 2). The 

drawing is constructed from an elevation of 10° and with a 

vertical exaggeration of about 12. The initial mooring 

position (solid circle) and inferred displacement path of 

current-meter moorings (numbered 2, 4, 5, and 6) are 

returns that are best expressed along the north side of the 

reentrant in the upper slope (for example, sand pattern in 

fig. 6 that terminates upslope from the cable break). These 

broad swaths also trend downslope, and core samples from 

the largest, northern example contain very poorly sorted, 

angular, coarse sand and gravel. A minimum thickness of 

2.5 m of the coarse material is indicated from the available 

cores. There is a uniformly high percentage of shal1ow 

water benthic foraminifera in these samples as well. 

Samples from much of the rest of the SeaMARC II 

image area (intermediate-to-low-backscatter background 

returns) are predominantly silt- and clay-sized sediment 

containing very high abundances of exclusively deep-water 

pelagic foraminifera. The sharp contrast in both the acoustic 

backscatter and in the grain size of sediment samples 

between this background pattern and the broad swaths and 

sinuous return suggest that the latter features are zones of 

downslope sediment transport. Evidence for downslope 

transport of the sediment in these swaths is further supported by the common occurrence of displaced shallowwater foraminifera. Because the shallowest telecommunication cable was broken and partially buried in the area 

immediately downslope from these converging zones of 

coarser sediment (fig. 6), it is likely that some, or perhaps 

all, of this coarser sediment was emplaced during Hurricane 

Iwa. 

projected as solid lines. The shallowest telecommunication cable that was damaged as a result of sediment 

movement caused by Hurricane lwa is shown as a dashed 

line with vertical tic marks in the section that was damaged. The irregular polygon (solid line) on the upper slope 

and inner terrace denotes the area covered by the SeaMARC II image of figure 5. 

DISCUSSION 

The deep-water effects of the passage of Hurricane 

Iwa provided a rare opportunity to measure some of the 

sediment-transport parameters that can result from slope 

failure of submarine sediment. Such a glimpse at the 

processes of slope failure events is not possible for geologically old examples; that is, for most of those presented in 

this report. Despite the relative wealth of observations 

before, during, and after the slope failure episodes, however, it is still not possible to deduce unequivocally the 

cause and mechanism of failure, a fact that emphasizes the 

great handicap to scientific observation provided by the 

oblique nature of ocean water itself. 

The available observations are consistent with several 

possible mechanisms that can explain downslope movement 

of both the near-bottom water and the slope sediment. 

Dengler, Wilde, and others ( 1984a) suggested that the most 

likely cause for movement of the current meters was a series 

of discrete turbidity-current flow episodes generated by 

successive sediment slope failures on the upper island slope 

and reef front. A turbidity current is a turbulent body of 

sediment and water that is pulled downslope by gravity 

because the density of the water/sediment mixture is slightly 

greater that the density of the surrounding seawater (Lee, 

Schwab, and Booth, this report). During Hurricane Iwa, 

200 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
0 

#6 (TWO SENSORS) 

200 45 em /sec 

85/-200(m) 

....... 

<X:~ 

(J') 400 ww 0:: (J')o 

w <lo ....... w~ w ......10... #5 75-80 em/sec 

::E E ww 175/1000 (m) 

~ a::.,__ 

I 600 

u....a:: ....... ......1<{ 

c.. wr- w (J') (J') #4 220 em/sec 0 170/1250{m) 

a:: 

w 

....... 

<l 800 ~ 300 70 em/sec 

230/2250(m) 

200 

::E 

0 

a:: 100 

1000 u.... 10 

50 CABLE w BROKEN u 

:z: 

<{ em/sec / ....... 

(J') V.E.= 10:1 

0 

1200 

0 2 4 6 8 10 

DISTANCE FROM INNER REEF (KILOMETERS) 

Figure 4. Summary of the displacement experienced by the four current-meter moorings that were recovered after the passage of Hurricane lwa (from Normark and Piper, 

1991). Smoothed profile along the line of the moorings constructed from the detailed 

bathymetry of Normark and others (1982) with initial (open symbols) and reconstructed 

final positions (blackened symbols) of the moorings. Peak recorded current speeds and 

ratio of total downslope/horizontal displacement given for each mooring. Inset shows 

frontal speed of three of the displacement episodes correlated between adjacent 

meters for the lower three moorings; numbers 2, 4, and 5 (based on Dengler, Wilde, 

and others, 1984a; Dengler, Noda, and others, 1984b). 

current measurements show that warm (less dense) seawater 

was moving downslope; thus, the entrainment of sediment 

is the only plausible cause for the downslope flow (for 

producing denser water). The sediment found on the current 

meters, which were mounted 10 m above the sea floor on 

the moored cables, is good evidence that sediment-laden 

water in a mass flow at least 10 m thick moved past the 

meters. 

The missing observation related to a turbidity-current 

mechanism, however, is how the sediment was mixed with 

the overlying seawater to form the turbidity current. Dengler, Noda, and others (1984b) extend their earlier interpretation by suggesting that the moorings could have been 

moved directly by successive sediment failures (that is, the 

anchors for the mooring rode downslope on sediment blocks 

in submarine slides or were carried with debris flows) that 

were accompanied by turbidity currents generated simultaneously with the slope failures and recorded by the current 

meters. A debris flow is generated as a sediment body 

involved in a submarine slope failure becomes disaggregated and mixed with seawater during downslope transport 

(Lee, Schwab, and Booth, this report). The sediment 

character of the coarser material sampled is consistent with 

debris flow transport that is locally channelized and could 

Submarine Slope Failures Initiated by Hurricane lwa, Kahe Point, Oahu, Hawaii 201 
21 ' 24' 

22' 

21 ' 20' 

0 2 

KILOMETERS 

158'12' 

Figure 5. SeaMARC II sidescan-sonar imagery of the 

Kahe Point area. The darker tones on the imagery indicate a greater level of acoustic backscattering intensity, 

which is a function of slope, local surface texture, and 

sediment characteristics (see Lee, Schwab, and Booth, 

this report); refer to Tsutsui and others (1987) for 

detailed interpretation of the imagery. The areas of the 

sea floor with steeper gradients, more local roughness 

10' 

l · . 

Composite Sea MARC II 

Side-Scan Sonar 

Mosaic 

Kahe Point, Oahu 

May 1984 

158'8' 

(such as unsedimented volcanic rock or piles of rubble 

from steep cliffs), and (or) coarser grain size of the 

sediment covering the sea floor all tend to produce 

higher backscatter (darker areas in the image). The lower 

backscatter (lighter gray) areas are thought to represent 

the undisturbed areas covered by fine-grained pelagic 

sediment. 

202 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone 
0 

btHJ sand ~Boulder 

~ Zones 

D Silts and~ Carbonate 

Cloys ~ Reef 

KM 

Figure 6. Interpretation of SeaMARC II imagery (fig. 

5) showing sediment transport paths, superimposed 

with initial positions of the current-meter moorings 

and location of the shallowest telecommunication 

cable break (from Tsutsui and others, 1987). The 

wide sand swaths on the carbonate shelf represent 

sand-filled channels and sand bodies. The broad 

sand swaths perpendicular to the shelf are areas 

where the SeaMARC II images and core sampling 

produce the high-backscatter patterns observed on the 

SeaMARC II image (Tsutsui and others, 1987). If we 

accept slope failures as the ultimate cause of the displacement of the moorings (with or without the concurrent 

generation of turbidity currents), a key question remains: 

what caused the slope to fail? Rapid loading and unloading 

of the slope sediment as the large hurricane-induced storm 

waves moved through the area might have caused a loss of 

strength within the sediment and would also have generated 

downslope-directed forces to move the sediment (see Lee, 

Schwab, and Booth, this report). Noda (1983) calculated 

that minimum wave heights of 9 m were likely in the area 

offshore of Kahe Point. 

The turbidity currents also could have resulted from 

storm-induced urges without significant associated slope 

failure (Dengler, Wilde, and others, 1984a; Dengler, Noda, 

and others, 1984b). Noda (1983) reconstructed the wind 

and wave condition that probably existed off Kahe Point as 

Hurricane Iwa passed across the islands. His models sugNORTH 

indicate downslope transport of coarse-grained sediment. The darker, narrower, more sinuous bands 

recorded on the SeaMARC II image, shown here as 

narrow, braided or branching patterns, indicate 

other paths of downslope transport of sediment. 

Arrows represent inferred path and estimated displacement of moorings during Hurricane lwa (after 

Noda, 1983). The numbered squares and diamonds 

are current-meter stations. 

gest that strong currents across the reef resulting from 

seawater pushed shoreward from the advancing storm could 

have eroded and put into suspension sufficient ediment to 

generate a turbidity current directly. 

Dengler and Wilde ( 1987) use the data from the 

current-meter array in a later analysis to suggest that a slope 

failure model based on snow avalanches adequately 

accounts for the available observations. For now, we will 

have to await an even more direct observation of modern 

slope failure to improve our understanding of the processes 

involved. 

CONCLUSIONS 

The reentrant in the upper slope and shelf contours off 

Kahe Point has helped to focus downslope sediment transport in this area. The focusing effect is reflected in both the 

general sediment structure and in the surficial relief and 

Submarine Slope Failures Initiated by Hurricane lwa, Kahe Point, Oahu, Hawaii 203 
acoustic backscatter pattern observed on SeaMARC II 

images (Normark and others, 1982; Tsutsui and others, 

1987). The main episodes of transport during the passage of 

Hurricane lwa were of high velocity, suggesting that 

sediment slope failure by cyclic loading during storm surges 

is the probable triggering mechanism for these mass flow 

events. We cannot rule out, however, a mechanism that 

involves sediment suspension resulting from the downslope 

flow of water pushed onto the reef area by storm surges and 

the resultant formation of turbidity currents. Available core 

data suggest that substantial amounts of shelf sediment were 

transported to deeper water in several discrete episodes 

during the passage of the hurricane (Tsutsui and others, 

1987). Much of the sediment traveled along well defined, 

relatively narrow pathways across the terrace before moving 

down the steeper flank of the volcano and disrupting the 

deeper water telecommunication cables. 

ACKNOWLEDGMENTS 

We thank Carolyn Degnan and Chris Gutmacher for 

their assistance in preparing figure 3. The manuscript was 

improved by reviews by H.J. Lee and R.E. Hunter. This is 

contribution MSG 90--010 of the Marine Sciences Group, 

University of California, Berkeley. 

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204 Submarine Landslides: Selected Studies in the U.S. Exclusive Economic Zone