An Introduction to the «c

; Geology and Mineral 

.. Resources of the 

" Continental Shelves 

of the Americas

_* An Introduction to the

 Geology and Mineral 

I Resources of the 

- Continental Shelves

. *

of the Americas

% By JAMES TRUMBULL, JOHN LYMAN, J. F. PEPPER, and E. M. THOMASSON

> ___^___^_ ____ _ _______ _^__^___^_ __^__________^^___^_^_____________

»-

GEOLOGICAL SURVEY BULLETIN 1067

A description of the continental shelves 

of the Americas, their potential mineral 

resources, and problems of petroleum 

development in the Gulf of Mexico
UNITED STATES DEPARTMENT OF THE INTERIOR 

FRED A. SEATON, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

For sale by the Superintendent of Documents, U. S. Government Printing Office 

Washington 25, D. C.
FOREWORD

The presence of shallow water for varying distances from the 

shores of continents and many islands of the earth has been known to 

mariners, and particularly to fishermen, from time immemorial. 

Many years ago knowledge of the width and configuration of these 

shallow-water shelf areas of the continents was gained in fishing 

areas and along the main shipping and oceanic-cable routes. In 

modern times, oceanographers, geologists, and geophysicists have increased their knowledge of the continental shelves through the use of 

new techniques of hydrographic surveying, geophysical exploration, 

bottom sampling, underwater photography, and core drilling. Nevertheless, reliable information concerning continental shelves and the 

rocks underlying them is scarce, and consequently an understanding 

of the origin and composition of the submerged borderlands of the 

continents is still in its infancy.

Scientists who study the configuration of the earth consider the seaward, or outer, margins of the continental shelves to be located where 

the gently sloping shallow platforms surrounding the continents 

steepen abruptly and slope toward the abyssal depths of the oceanic 

basins. Only in local areas of detailed hydrographic surveying are 

the positions of the outer edges of the shelves well known. Elsewhere, 

water depths, and consequently shelf widths, as shown on maps and 

charts are only approximations and are undoubtedly inaccurate in 

many places.

Thus the limits of the continental shelves of the Americas are not 

accurately known, nor is much known about the topographic character 

of important continental-shelf features. This is not surprising, however, for scientific interest in the nature of continental shelves and the 

means for making accurate and extensive surveys beneath the sea are 

both recent developments. The need for greatly expanding the fund 

of factual data about the continental shelves of the Americas is clearly 

evident.

The current trends toward a rapidly increasing population as well 

as a greatly improved standard of living make the mineral resources 

of the continents of interest and concern to all. Mineral resources 

that remain unknown and undiscovered are of no value. Those that 

are known but undeveloped are of no immediate benefit. Those that

m
IV FOREWORD

have been discovered, have been developed, and are now producing 

commodities for world trade, add to the well-being of the international economy. For centuries man has searched the land areas of the 

world for deposits of precious metals, base metals, fuels, and industrial minerals. Although many valuable mineral deposits undoubtedly remain to be discovered on land, the state of knowledge 

about the mineral resources of land areas can be considered to be 

rather advanced. On the other hand, knowledge of mineral resources 

of the submerged margins of the continents is in its infancy.

Coal has been produced from beneath the ocean floor in Chile and 

Nova Scotia, and iron ore has been similarly produced in Newfoundland, production in all three instances having been from mines begun 

on shore and extended beneath the sea floor as much as several miles. 

In recent years oil fields have been discovered in the Gulf of Mexico 

and off the west coast of the United States, and special techniques 

have been devised to permit development of these fields. Tremendous 

impetus has been given to national an international interest in continental shelves by these discoveries of commercial quantities of petroleum beneath the sea floor. Because knowledge of mineral resources 

of the continental shelves is adequate for only a few localities, the extent and ultimate value of the mineral resources recoverable from all 

the shelf areas of the Americas are unknown. The future will without doubt reveal new occurrences of important mineral commodities 

recoverable from the continental shelves. Man's search for knowledge, his ingenuity, and his fervor to explore the unknown guarantee 

this. Furthermore the increasing need to discover new mineral 

sources to augment supplies from our presently known deposits provides a strong economic stimulus to this search.

Matters pertaining to the resources of the continental shelves and 

overlying waters, including both mineral and living resources, were 

considered in 1954 at Bogota, Colombia, at the Tenth Inter-American 

Conference of the Organization of American States (Pan American 

Union). The Bogota conference recognized the complexities of the 

subject and the amount of background information necessary to an 

adequate understanding of the problems involved, and resolved to call 

a Specialized Inter-American Conference on "Conservation of Natural 

Kesources: The Continental Shelf and Marine Waters." This conference was held at Ciudad Trujillo, Dominican Republic, March 

15-28,1956.

The United States delegation's working group on "The Continental Shelf and its Mineral Resources" prepared the following four 

technical papers for presentation at the specialized conference: "Continents and Ocean Basins and Their Relation to Continental Shelves
FOREWORD V

and Continental Slopes," by James Trumbull, U. S. Geological Survey; "A Review of Present Knowledge Concerning the Continental 

Shelves of the Americas," by John Lyman, U. S. Navy Hydrographic 

Office; "Potential Mineral Resources of the Continental Shelves of 

the Western Hemisphere", by James F. Pepper, U. S. Geological 

Survey; "Problems of Petroleum Development on the Continental 

Shelf of the Gulf of Mexico," by Edwin M. Thomasson, U. S. Geological Survey. Each of these papers represents a compilation and synthesis of the basic information available on the particular subject 

covered. Because each paper provides a rather concise although 

simplified summary of the current state of knowledge, and because 

this information is not readily available from any other single source, 

the Geological Survey has prepared these papers for this publication 

in order to make them generally available.

RALPH L. MILLER,

Chief, Fuels Branch

Geologic Division

U. S. Geological Survey

CONTENTS

Page

Foreword, by Ralph L. Miller.._._ v 

Continents and ocean basins and their relations to continental shelves and

continental slopes, by James Trumbull, U. S. Geological Survey...__ 1 

A review of present knowledge concerning the continental shelves of the

Americas, by John Lyman, U. S. Navy Hydrographic Office.._ ____ 27

Potential mineral resources of the continental shelves of the Western Hemisphere, by James F. Pepper, U. S. Geological Survey..______________ 43

Problems of petroleum development on the continental shelf of the Gulf of

Mexico, by Edwin M. Thomasson, U. S. Geological Survey ___________ 67

Index_____________________________________ 91

vn

CONTINENTS AND OCEAN BASINS AND THEIR RELATION 

TO CONTINENTAL SHELVES AND CONTINENTAL SLOPES

By JAMES JTRUMBULL, U. S. Geological Survey

CONTENTS

Page

Abstract.----_.___________--_-__----_-_. --_---- 2

Introduction. ___ ___ __ _ ___ ___ __ -_ -__- _ .. 3

The floor of the ocean________-_____-___-_-__---_-_-----_-____._ 3

The surface of the land_. _____ _ -.__- _ _ _. 4

Physical nature of the continental and oceanic parts of the earth's crust __ 4

Studies of the distribution of rock types...----- ____ _ 4

Studies of seismograms -.___-___ _-_______---_------ --_---- 5

Studies of gravitation__-_-_--_----__--___-_ ------------------ 6

Isostasy_ ___ _____ _ ___ __- - --- -__ _-_-- 7

Structural framework of the Caribbean area.__________-__--___-.__-_- 7

The shifting location of the edge of the oceans____-__---_-_____- --- 8

Vertical movements of land areas relative to the sea______________ 10

Changes in sea level relative to land.__ .___-----_-____-_____- 10

The present overfullness of the ocean basins.._____________________ 11

The continental shelves___________________________________________ 11

Types of continental shelves as exemplified in the Americas --_.___ 12

Broad shelves off stable lowland coasts_.__---_-------------_- 12

Narrow shelves off young mountain coasts. ----------------- 14

Modifications of the two general shelf types ____ _____-__- 14

Glaciated shelves, --- ---__ --_ ----- ----------- 15

Shelves off large rivers-___ _- _-- __ -- ------- 16

Shelves in areas of coral growth....__---_-_-__----__--_- 17

Loose sediments on the continental shelves__ _-_-_ __ _ ___ 18

Origins of continental shelves.-- - ___ .._ -- - - ------- 18

The continental slopes_____________________._.---___----_-_-_---- 20

Continental-slope characteristics in relation to types of coastal areas. 21

Modifying features of the continental slopes___________ _______ 22

Structure and origin of the continental slopes ________ _ __ 24

Selected references.._________ ___ ___ __ _____ __ _- ___ 26

ILLUSTRATIONS

FIGUEE 1. Curve showing the area and percentage of the earth's solid surface above any given elevation or depth.-_ __ __ _ 5

2. Inferred distribution of land and sea in North America during

the Oligocene epoch____ _ _____________ _ ___- __- 9

3. The continental shelves of the Americas.------------------- 13

4. Generalized profiles of the earth's surface from the continents

to the deep ocean basins at selected places in the Americas _ 15

1
2 CONTINENTAL SHELVES OF THE AMERICAS

ABSTRACT

The rocks underlying the continental segments of the earth's crust are 

characteristically light in color and weight, whereas the rocks underlying the 

oceanic segments are characteristically dark and heavy. These segments of 

different properties can be considered to be floating on a heavy plastic substratum. In addition to being lighter in weight, the continental segments are 

thicker than the oceanic segments, and thus stand higher. The average 

difference in elevation between the continental and oceanic segments is on the 

order of 2 miles (3.2 km). The extra thickness of the continental segments 

compensates for the lighter weight, and the two kinds of segments are in 

approximate equilibrium.

The term "Isostasy" (equal standing) is applied both to this condition of floating equilibrium and to the tendency of blocks of the earth's crust to maintain that 

equilibrium by vertical motion. The resulting relative motion is probably 

concentrated, but by no means entirely localized, at the boundary of the continental and oceanic segments. For most of the world this zone of adjustment 

is thought by many authorities to be in the continental slope, which is a sub' 

merged escarpment that is in many places steeper and higher than the fronts 

of most mountain ranges. Between the steep continental slopes and the present 

shorelines of the continents are the more gently inclined continental shelves 

which although submerged are part of the continental areas both in terms of 

rock structure and topography.

The elevation of the surface of the oceans with respect to land (that is, sea 

level) changes continually, and in the geologically recent past it has been both 

higher and lower than it is at present. During the glacial age so much water 

from the oceans was locked in glacial ice that sea level fell an amount usually 

estimated at 150 to 500 feet (45 to 150 m). At that time the continental shelves 

were largely exposed; that they are now submerged is almost fortuitous.

By definition the continental shelves extend from the mean low-water line to 

the sharp change in inclination that marks the beginning of the continental 

slope (the shelf edge). This change occurs at an average depth of 432 feet 

(72 fathoms, 132 m) below present sea level, though for convenience the 600-foot 

(100-fathom, 183-m) line is more often used. Throughout the world the average 

inclination of the continental shelves is less than one-eighth of 1°. Their 

average width is 42 miles (67 km), but their width ranges from a fraction of 

a mile to many hundreds of miles. Stable lowland coasts usually have wide, 

shallow, uniform continental shelves, whereas the shelves off geologically young 

mountainous coasts usually are narrow or absent, at great depths, and variable. 

The continental shelves have been modified in places by glaciation, large rivers, 

and coral growth. The comparative flatness of the continental shelves seems 

due to a combination of factors, including both downcutting by erosive forces 

and upbuilding by deposition of sediments.

The continental slopes extend from the outer margin of the continental 

shelves to the floors of the deep ocean basins. Their average height above the 

deep ocean floors is 12,000 feet (3,660 m); their average width is 10 to 20 miles 

(16 to 32 km); and their average inclination is slightly more than 4° though 

25° is common. Off stable lowland coasts, and especially off large rivers, they 

are more gently inclined; off young mountain ranges they are steeper. The 

continental slopes are modified by large submarine canyons of unknown origin, 

and by plateau areas at intermediate depths. Accumulation, flowing, and sliding of sediments take place upon their surfaces. Although the continental
CONTINENTS AND OCEAN BASINS 3

slopes are thought to have been formed by upward faulting (offsetting) of the 

rocks of the continents relative to those of the ocean basins, this is unproved, 

and other theories have been advanced.

INTRODUCTION

The oceans and seas cover seven-tenths of the surface of the earth, 

but man's knowledge of the world is largely limited to the threetenths on which he lives. In recent years, however, a small amount 

of information has been accumulated about the shape and nature 

of the ocean basins, and the minor as well as the major characteristics 

of the ocean floor are becoming known.

The framework of the continents and ocean basins is more easily 

considered if for the moment the waters of the oceans are neglected. 

A mental picture of the earth with its oceans and seas gone as if 

evaporated is revealing.

THE FLOOR OF THE OCEAN

If all the waters are ignored, the predominent feature of the earth 

is the floors of the ocean basins. In the overall picture they are low, 

relatively flat areas of little relief. Closer examination, however, 

reveals a great number of features of diverse topographic form 

some are submarine mountains or plateaus, others are marked 

depressions.

Elevated areas on the ocean floor include small but broad and flattopped banks, small isolated peaks called seamounts, broad elevated 

areas called rises, and long narrow elevated areas called ridges. The 

best-known example of the latter is the Mid-Atlantic Ridge, which 

nearly bisects the Atlantic basin for its entire length, from north to 

south, and rises 10,000 feet (3,000 m) above the ocean floor.

Depressions in the ocean floor include rounded basins, elongate 

troughs with gentle slopes, and long, narrow, steep-sided trenches. 

The trenches contain the most depressed areas of the floors; these 

are called deeps and commonly have names given them. The Philippine Deep extends to more than 34,000 feet (10,360 m) below sea 

level, and the Brownson Deep, north of Puerto Rico, to 28,500 feet 

(8,700m).

Although there are a number of each of these diverse features of 

the ocean floors, by far the most notable characteristic is the vast 

uniformity of the floors as a whole.

No less striking is the low level of this tremendously broad and 

generally level area. The average depth below sea level of the ocean 

floors, excluding adjacent shallow seas, is about 13,000 feet (3,960 m). 

About 60 percent of the surface of the earth is more than 7,000 feet 

(2,134 m) below sea level.
THE SURFACE OP THE LAND

Viewing the overall shape of the land areas of the earth, the great 

mountain chains immediately draw attention, but these features, although striking in appearance, occupy only a small portion of the 

land areas. Only about 3 percent of the earth's surface is more than 

6,500 feet (1,980 m) above sea level. The mean elevation of the land 

areas is only about 2,700 feet (823 m). As is true of the ocean floors, 

most of the land surface of the earth lies within a small range of 

elevations.

A striking simplicity in the earth's architecture is apparent. Most 

of the earths' surface falls within two separate general levels of elevation. The continents are high and the ocean basins are low; the 

transition zone is narrow. This relationship is clearly illustrated in 

figure 1, which shows the percentage of the earth's surface lying 

between various levels above and below the surface of the sea. The 

2 bulges formed by the bars indicate the 2 major levels of the earth's 

surface. The short bars separating the two bulges show the area 

of the earth's surface that is at relatively small depth below sea 

level. These areas are made up mostly of the transition areas between the continents and the ocean basins.

The following summary of the physical nature of the continental 

and oceanic segments of the earth's crust will provide an explanation 

for the predominance of two separate levels of the earth's surface.

PHYSICAL NATURE OP THE CONTINENTAL, AND 

OCEANIC PARTS OF THE EARTH'S CRUST

The rocks that form the continents and the rocks under the floors 

of the ocean basins are distinctly different in composition and therefore in most of their other characteristics. As will be seen, the great 

height to which the continents rise above the ocean floors results 

chiefly from these differences.

STUDIES OF THE DISTRIBUTION OF ROCK TYPES

Direct information about the composition of the earth's crust comes 

from study of the rocks of the continents, the oceanic islands, and the 

few actual samples of material that have been recovered from the 

ocean floors. Though there are exceptions, one type of rock predominates on the continents, and another type predominates in the oceanic 

areas.

The differences in continental and oceanic rock types are simply 

stated. The rocks that form the continents are characteristically 

light in weight and color. Granite is the major rock type, and the 

continents are commonly described as being granitic. The rocks un-
CONTINENTS AND OCEAN BASINS

10,000

8,000 

6,000

Area of earth's surface, in 10 square kilometers 

s 4

u 4,000

2,000

2,000

4,000

c 6,000

10,000

 8,000

 6,000

 4,000

 2,000

8,000 

10.000

Mean elevation of_land__ - 

Sea level

 2.000

 4,000

6.000

 8.000

10,000

80 90 100

FIGURE 1. Curve showing the area and percentage of the earth's solid surface above any 

given elevation or deptb. Bars at the left show the frequency distribution of elevations 

and depths for 1,000-meter Intervals. The curved line is a cumulative distribution curve, 

not a profile. Mean sphere depth Is the uniform depth to which water would cover the 

earth If the solid surface were smoothed off. It should be distinguished from the mean 

elevation of the earth's surface (not shown), which is determined by the location of the 

50-percent point of the curve. (After Sverdrup, Johnson, and Fleming, 1942, p. 19.)

der the floors of the ocean basins are characteristically heavy and 

dark in color. Basalt is the major rock type, and the oceanic rocks 

are commonly referred to as being basaltic.

STUDIES OF SEISMOGBAMS

Because the two general rock types that make up the continental and 

oceanic segments of the earth's crust differ in density, they also differ
6 CONTINENTAL SHELVES OF THE AMERICAS

in the speed with which they transmit pressure impulses; that is, they 

have different seismic velocities. The records of shock waves from 

earthquakes and manmade explosions that travel through the rocks are 

thus another source of information; these records are called seismograms. This information is not restricted to rocks at the surface of 

the earth, because vibrations from mechanical shocks travel through 

the earth as well as along its surface. Boundaries of earth layers having different seismic velocities can be accurately located and something 

about the properties of the layers determined, though they lie deep 

in the earth.

The two general rock types granitic on the continents, basaltic in 

oceanic areas make up the crystalline crust of the earth. The thin 

mantle of sedimentary rocks that lies on much of the crystalline crust 

looks so small in the overall picture that it need not be included in this 

classification. Though this layer locally may be several miles thick, 

it results from erosional and depositional processes modifying the surface, and is largely incidental to the study of major features of the 

earth's structure.

The crystalline crust is conventionally divided from the deepseated, 

heavier rocks of the earth's interior by a boundary known as the 

Mohorovicic discontinuity, above and below which the rocks have different seismic properties. The depth from the surface to this boundary, which is about the same thing as the thickness of the crystalline 

crust of the earth, averages roughly 18 to 25 miles (29 to 40 km). 

This boundary is at a greater depth under the continents than under 

the ocean basins, and under high mountain areas such as the Sierra 

Nevada and the Alps it is at nearly twice the depth that it is under 

the oceans. The continents thus extend both higher into the air and 

deeper into the interior of the earth than do the ocean basins. The 

relation between mountain areas and other continental areas is similar : not only do the high mountains project far into the air, but they 

have crystalline roots that extend deep into the substratum.

Thus the earth's crust consists of relatively light but thick continental blocks, and dense but thin oceanic blocks.

STUDIES OF GRAVITATION

Much information concerning the crustal structure of the earth is 

also derived from precise local measurements of the strength of the 

earth's gravitational field. The basis of such determinations is of 

course Newton's fundamental law of gravitation: the attraction between any two bodies increases with mass and decreases with distance.

It might be thought that the larger volume of the thick continental 

blocks would result in higher gravity readings for continents than
CONTINENTS AND OCEAN BASINS 7

for ocean basins, but this is not true. Instead, the excess volume of 

the continents is balanced by the low density of the continental material. Similarly, the deficiency of mass in the water-filled ocean basins 

is nicely balanced both by the high density of the underlying crustal 

material and by the nearness of the still denser subcrustal material.

ISOSTASY

The continents float high because of their lower density and greater 

thickness; the oceanic segments are low because of their higher density 

and relative thinness. This condition of floating equilibrium between 

blocks of the earth's crust resting on a somewhat liquidlike interior is 

called isostasy. The term isostasy also means the tendency of crustal 

blocks to maintain their floating equilibrium.

One aspect of this concept of maintaining floating equilibrium is 

of particular interest. Erosive forces continually remove materials 

from the continents and transport it to the oceans. Over geologically 

long periods of time the lightened continents would be expected to rise, 

the weighted ocean floors to sink. Relative motion at the junction of 

the continents and the deep ocean basins might therefore be expected. 

This topic will be treated in the subsequent discussion of the origins 

and nature of the continental slopes.

STRUCTURAL FRAMEWORK OF THE CARIBBEAN AREA

The Antillean island arc, curving through the West Indies and setting off the Caribbean Sea and the Gulf of Mexico from the Atlantic 

Ocean, exhibits a number of features which merit attention. Close 

counterparts to this island arc, geologically speaking, are found in the 

chains of islands that festoon the Pacific coast of Asia: the Aleutian, 

Kurile, Japanese, Ryukyu, and Philippine island arcs separate the 

shallow Bering, Okhotsk, Japan, East China, and South China Seas, 

respectively, from the Pacific Ocean; and the Bonin and Mariana 

island arcs and undersea ridges separate the Philippine Sea from the 

Pacific Ocean proper. There are numerous other examples.

These island-arc areas have great contrasts in elevation within short 

distances, just as is the case where the continents meet the ocean basins. 

Although study of island-arc areas has been intensive and highly revealing, and forms a fascinating field of interest, it is possible here to 

sketch only the barest outlines of present information and hypotheses 

concerning them.

Island arcs have many features in common. They are all convex 

toward the ocean. On the ocean side of each is a narrow, arcuate, 

extremely deep trench in the ocean bottom. The Puerto Rico Trench 

contains the greatest measured depth in the Atlantic Ocean, yet its
8

deepest parts are no more than 100 miles (160 km) north of the Virgin 

Islands, Puerto Rico, and the eastern end of the Dominican Republic.

Measurements of gravity indicate that the rocks underlying a long 

narrow zone parallel to these trenches have an unexpectedly low 

specific gravity. In the Caribbean this narrow strip of low specific 

gravity extends continuously from eastern Cuba to Trinidad. It coincides with the trench north of the Dominican Republic and Puerto 

Rico, but southward from the Leeward Island^ it coincides with a 

ridge of which the island of Barbados is an exposed portion.

Volcanoes commonly occur in a narrow line on the concave side of 

the arcs, at a distance of about 100 miles (160 km) from the axis of 

the central belt of low gravity. Further, an unusual type of rock 

called peridotite is commonly found in a nearby narrow belt that is 

also parallel to the belt of low gravity. This type of rock is uncommon on the surface of the earth, but is thought to be abundant at a 

deep level within the earth.

The generally accepted explanation of these diverse but apparently 

related phenomena is that a great linear fold of the earth's crust has 

buckled downward into the heavy subcrustal material. The resulting 

deep furrow in the ocean bottom roughly represents the upper surface 

of the downfold. The central belt of low gravity is explained by the 

mass deficiency of the downfold of light crustal material. The development of fractures associated with the bending of the crust accounts 

for the observed volcanism and earthquakes. The fractures have been 

the routes by which the peridotite has flowed upward to the surface.

If there should be a weakening in whatever force caused the downfold of light rocks into heavier ones, isostatic adjustment would cause 

the downfold to rise, and a narrow land area might appear on each 

side of the center of the downfold. Most of the islands of the Caribbean, with certain exceptions including Jamaica, Barbados, and Aves 

Island, are thought to represent this uplift on the concave side of 

the furrow.

THE SHIFTING LOCATION OF THE EDGE OF THE OCEANS

It is common knowledge that much of what is now dry land was 

once covered by the ocean. Assemblages of marine fossils known to 

be uniquely characteristic of particular intervals of past geologic time 

are widespread over present land areas. The study of the present 

distribution of rocks deposited during specific intervals of past time 

enables the construction of maps showing the distribution of land 

and sea during those intervals.

Figure 2 is an example of such a map. It shows the extent of land 

and sea in North America during the time interval extending from 

about 40 million years ago to about 28 million years ago. That time
CONTINENTS AND OCEAN BASINS

interval, termed the Oligocene epoch of the Tertiary period, was not 

long ago in terms of the geologic history of the earth, yet there have 

been many changes in the outline of the land areas from Oligocene 

time to the present.

FIGURE 2. Inferred distribution of land and sea In North America during the Oligocene 

epoch, a geologically recent period of time. (After Schuchert, 1055, p. 81.)

451437 58 2
10 CONTINENTAL SHELVES OF THE AMERICAS

From the geologic standpoint, then, the present location of the edge 

of the sea is only temporary. The oceans have been both higher and 

lower in relation to the land in the recent past.

The several causes of change in sea level relative to land may be divided into two groups. The land in any particular area might rise 

or fall, sea level remaining stationary. Or on the other hand, the 

amount of water in the ocean basins (or the capacity of the basins) 

might change, and sea level rise or fall accordingly, the land areas 

remaining unmoved. Both processes have been operating throughout 

geologic time and are operating now.

VERTICAL MOVEMENTS OF LAND AREAS RELATIVE TO THE SEA

The coasts of Scandinavia and of the northeastern United States 

and Canada are today rising at a geologically rapid rate as an isostatic 

adjustment to the removal of the weight of the glaciers that recently 

covered those areas. Other vertical land movements include the uplifting of mountain and high plateau areas by def ormational processes 

acting within the earth, and the previously mentioned isostatic rise 

resulting from loss of weight by erosion at the surface of the earth.

Erosion does not proceed at the same rate everywhere, and apparently all vertical movements of the land vary in amount from place 

to place. Consequently, any change in sea level found to be uniform 

on a worldwide basis is to be attributed to changes in the volume of 

water in the oceans, or perhaps to changes in the volume of the 

ocean basins.

CHANGES IN SEA LEVEL RELATIVE TO LAND

Ample evidence exists of repeated great advances and retreats of 

glaciers during the Pleistocene epoch, or roughly the last million 

years. The advances are thought by most authorities to have been 

four in number, generally of diminishing intensity. During the 

intervening three interglacial periods the world climate was markedly 

warmer than at present.

These repeated glacial advances cannot have occurred without corresponding reductions of sea level, as all the water bound up in the 

glacial ice was obtained from the oceans. The area once occupied 

by the glaciers is fairly well known, but the thickness of the ice can be 

estimated only roughly. The calculated ice volume is therefore only 

an estimate of the order of magnitude, but it generally corresponds to 

a lowering of sea level of some 150 to 500 feet (45 to 150 m) below 

the present position. Estimates of as much as 3,500 feet (1,070 m) 

have been made, however, in an attempt to explain features on the 

ocean floor at that depth that appear to be the result of stream 

erosion.
CONTINENTS AND OCEAN BASINS 11 

THE PRESENT OVERFULLNESS OF THE OCEAN BASINS

The overall relation of present sea level to the continents is revealed 

by maps showing the shape of the sea bottom outward from land, and 

by cross sections such as those of figure 4. They show that off many 

of the coasts of the world the sea floor is gently inclined for a considerable distance seaward, but then at a fairly well-defined line the 

sea floor steepens rather abruptly, and thereafter the water deepens 

rapidly and continuously until the floor of the deep ocean is reached. 

Geologically, the abrupt change in inclination of the sea bottom, 

rather than the present shoreline, marks the true edge of the continents. 

The water depth at the change in inclination is usually between 350 

and 600 feet (58 and 100 fathoms, 107 and 183 m), and averages 432 

feet (72 fathoms, 132 m).

The ocean basins can thus be characterized as overfull water not 

only fills the ocean basins proper, but extends out over the low margins 

of the continents. The submerged and relatively flat-lying margins 

of the continents inshore of the abrupt change in slope are called 

the continental shelves. The abrupt change in slope is properly referred to as the shelf edge. The more steeply inclined surface that 

lies seaward from the shelf edge is referred to as the continental 

slope.

The shelf edge and the bottom of the continenal slope form boundaries between three major topographic and structural areas of the 

earth. Those three areas are the floors of the ocean basins, occupying 

roughly 50 percent of the earth's surface; the continental slopes, 

occupying about 15 percent; and the continental shelves together with 

the land, occupying the remaining 35 percent.

THE CONTINENTAL SHELVES

The submerged margins of the continents have been intensively 

sounded at some places for the benefit of navigation, but in most places 

knowledge of shelf topography is still meager, and the more difficult 

examination of bedrock and sediments on the shelves has only begun. 

Any but the most general statements about the shelf areas are thus 

founded on extremely incomplete data. On numerous points of information there now exist two or more hypotheses which cannot be 

definitely proved or disproved.

Much of the available information on the shelf areas of the world 

has come from the work of Francis P. Shepard, professor of submarine 

geology at the Scripps Institution of Oceanography of the University 

of California at La Jolla. The discussion of the continental shelves 

and slopes in this report is largely based on the work of Dr. Shepard.
12 CONTINENTAL SHELVES OF THE AMERICAS

The figure of 432 feet (132 m) for the average water depth at the 

shelf edge was determined by Shepard from thousands of charts 

covering all parts of the world. Previous to his work the continental 

shelf was generally considered to extend to a depth of about 600 feet 

(100 fathoms, 183 m). That figure is useful as an approximation, 

and the 100-f athom contour line is found on most available charts of 

the ocean bottom. The 100-f athom line bounds the continental shelf 

of the Americas that is shown in figure 3.

Only a few coastlines lack continental shelves. Such coastlines 

are those formed by major faults, or those on which there has been 

extensive glacial deepening. In some places the shelves are very narrow, ranging from a fraction of a mile to a few miles at most; in 

other places the shelves are hundreds of miles wide. Their average 

width throughout the world is 42 miles (6Y km), and the maximum 

width, along the Arctic coast of Europe, is 750 miles (1,200 km). 

The shelves constitute more than 7 percent of the marine area of the 

earth.

The average inclination of the continental shelves is only about 12 

feet per mile (2.3 m per km; 0°07'; 0.2 percent). They are slightly 

steeper in the inner (nearshore) half than in the outer. In some 

places sediments are being deposited on the shelves, in other places 

they are apparently only being transported across them, and in still 

other places the shelf surface is bare of sediments. The shelf edge 

is usually rather straight, though in some places it is sharply indented 

by submarine canyons that cut deeply into the continental slopes 

farther offshore (see fig. 4).

The continental shelves are a simple underwater continuation of 

the adjacent land, and, as would be expected, their geology is merely an 

extension of the geology of the bordering land areas. The surface 

of the shelves has the many small irregularities that are characteristic 

of low-lying coastal areas on land.

TYPES OF CONTINENTAL SHELVES AS EXEMPLIFIED IN 

THE AMEEICAS

The continental shelves are by no means as uniform as the preceding 

overall description and average measurements might indicate. On 

the contrary, they are highly variable in size, shape, and composition. 

The shelves of the Americas give a wide range of examples, and can 

be grouped in accordance with their characteristics and the forces that 

act upon and modify them.

BROAD SHELVES OFF STABLE LOWIiAND COASTS

Most of the Atlantic coastal area of the Americas is a low-lying 

plain. Even in once-mountainous low coastal areas like New England
CONTINENTS AND OCEAN BASINS 13

FIGDBB 3. The continental shelves of the Amerlcas.
14 CONTINENTAL SHELVES OF THE AMERICAS

and the Brazilian Highlands, the age of mountain building has 

passed, and for a long interval of geologic time crustal stability has 

allowed erosion to proceed.

The continental shelf of these stable lowland areas is commonly 

of great and somewhat constant average width. Though there are 

a number of exceptions, the shelf for long distances along the Atlantic 

coast of the Americas is 50 to 100 miles (80 to 160 km) wide (fig. 3). 

Such areas are in strong contrast with the narrow shelves along the 

Pacific coast of the Americas. This is illustrated by the profiles 

shown in figure 4; the upper two profiles cross the Atlantic coastline, 

the lower crosses the Pacific coastline.

NARROW SHELVES OFF YOUNG MOUNTAIN COASTS

The Pacific coastal area throughout the length of the Americas is 

bordered almost entirely by geologically young mountain ranges 

(fig. 4C). For the most part the mountains are still in the process 

of formation, as evidenced by recurrent earthquakes. Faults along 

which blocks of the earth's crust have slipped great vertical distances 

are commonly located near these coasts, and in places they coincide 

with and determine the location of the coastline.

Along these geologically active coasts the continental shelves are 

narrow or even lacking, and are irregular both in width and depth. 

In some of the fjords along the coast of Chile the water is almost a 

mile deep, and along the Chilean and southern Peruvian coast in general there are only local, narrow shelves, and they are of greater than 

average depth and inclination. The shelf width along the west coast 

of North America over long distances does not exceed 50 miles (80 

km), and is only 10 to 20 miles (16 to 32 km) off the United States 

and negligible off long sections of the west coast of Mexico. The 

continental shelf off the west coast of the United States is at markedly 

greater average depth than that off the east coast, with large areas 

exceeding 300 feet (50 fathoms, 91.5 m), and terminations that are 

close to 600 feet (100 fathoms, 183 m).

From the above relations it can be said that the American continents are bordered by two general types of shelves: the wide, relatively shallow and uniform shelves off the stable lowland coasts; and 

generally narrow, deep, and variable shelves along coastlines of young 

mountain ranges. This correlation of shelf type and coastal type is 

only to be expected, for the shelves are but drowned coastal areas.

MODIFICATIONS OF THE TWO GENERAL SHELF TTPES

Geologic processes of a generally smaller or more local nature than 

mountain building have also had a direct effect on the nature of the 

continental shelves. These processes include glaciation, the building
CONTINENTS AND OCEAN BASINS 15

Shoreline^ ^"Continental 

shelf

METERS 

2000

20.000

W.

FEET 

5000-

A. Profile through New York City, bearing 135° true 

Continental shelf

6000

E.

0-

20,000

Shoreline^ ! I T

Sea level

METERS 

2000

B. Profile through Savannah, Georgia, bearing I25°true

6000

E. w.

FEET 

20,000-

20,000

Shoreline-^ /Continental shelf

METERS 

-6000

C. Profile through Valparaiso, Chile, bearing 280°true

6000

Horizontal scale 

200 100

Kilometers Statute miles

Vertical exaggeration 25x

FIODRB 4. Generalized profiles of the earth's surface from the continents to the deep 

ocean basins at selected places In the Americas. The vertical scale is the same and 

the horizontal scale Is the same In each profile, but the vertical scale Is 25 times the 

horizontal, and thus there is a 25-fold vertical exaggeration In the relief. Note that 

the orientation of the Valparaiso section has been reversed to make comparison easier. 

(Constructed from the 1:5,000,000 scale "Map of the Americas" of the American 

Geographical Society of New York.)

of deltas off the mouths of large rivers, and the formation of coral 

reefs. . GLACIATED SHELVES

Glaciers of the recently concluded Pleistocene ice age are known 

to have affected the coasts of northeastern North America, the Pacific
16 CONTINENTAL SHELVES OF THE AMERICAS

coasts of Canada and Alaska, and part of the coast of South America. 

The great changes in terrestrial topography caused by glaciation are 

well known: trough-shaped valleys are formed and enlarged, large 

lakes come into being, and great quantities of loose rock are picked v 

up, carried, and dumped irregularly at the terminal end of the glacier.

Each of these effects seen on land has its counterpart on the glaciated portions of the continental shelves. The resulting character- i. 

istics of glaciated shelves set them strikingly apart from all others.

Nonglaciated areas of the shelves normally become increasingly 

deep from their inner to their outer margins. In contrast, glaciated ^ 

shelves are characterized by banks and shoals rising offshore, with 

deeper water between them and the coasts. Not all banks and shoals 

occur off glaciated areas, but those at a considerable distance from <" 

shore are much more common there than elsewhere.

The linear chain of banks so famous in the annals of Atlantic 

fishing that extends from far at sea off the coast of Newfoundland > 

southwestward to the waters off Cape Cod is of this type. Sable 

Island off the Nova Scotian coast is simply an emergent part of Sable 

Island Bank. These banks are known to be composed of morainal A 

material, which is made up of heterogeneous rock debris carried in 

tremendous quantity by glaciers. Morainal deposits both on land 

and in glaciated shelf areas have a hummocky topography.

Contour charts showing the shape of the surface of the glaciated 

shelf areas reveal great trough-shaped valleys that in many places 

extend out from the coasts as continuations of glacially formed estuaries or fiords, and cross the continental shelf to a termination on the ^ 

continental slope. Just as on land, these glacially deepened troughs 

are generally straight, wide, and deep. The troughs have reversals in 

their longitudinal profiles formed by large basin-shaped depressions 

in the sea floor. If the seas were to be withdrawn, these basins would 

contain some of the largest lakes in the world, comparable in size and 

perhaps in origin to the Great Lakes. One would be present in the + 

Gulf of St. Lawrence, another in the Gulf of Maine.

A further notable characteristic of the glaciated shelves is that 

their width is almost everywhere great, though the shelves of western / 

South America (an area of young mountains) are a striking exception. Even including that exception, all the world's glaciated shelves * 

average 100 miles (160 km) in width, as compared to the overall aver- w 

age shelf width of 42 miles (67 km).

SHELVES OFF LARGE RIVERS

>.

The tremendous volume of sediments carried to the oceans by large 

rivers might be expected to produce especially wide continental » 

shelves near their mouths. *
CONTINENTS AND OCEAN BASINS 17

Although the average shelf width off the great rivers of the world 

is considerably greater than the average for all shelves, no direct 

genetic connection has been demonstrated. The outer edge of the continental shelf off most large rivers, for example, is found to be straight 

rather than bowed seaward, as might be expected. The tentative conclusion is that rivers do not in general selectively widen the shelves 

directly off their mouths.

Near the mouth of the Amazon Kiver the shelf is almost 200 miles 

(322 km) wide, and at the estuary of the Rio de la Plata (between 

Uruguay and Argentina) its width is 260 miles (416 km). These are 

deceptively large figures, for they are due not a seaward bulge of 

the outer margin of the shelf, but to the unusually great distance that 

the broad river estuaries extend inland into the continent (fig. 3). 

In contrast, the shelf at the mouth of the Mississippi River is very 

narrow. This is another deception, for the great Mississippi delta is 

a ramplike land area about 80 miles (128 km) long that has been built 

across the true shelf almost to the inner edge of the continental slope. 

The process has been much the same as the building of a stone quay 

out from shore into the deep waters of a harbor.

Features directly related to river action are the broad horizontal 

terraces found at various depths on the surface of the shelf offshore 

from the mouths of large rivers. They are especially common at 

depths of 30 to 108 feet (9 to 34 m). They are thus decidedly shallower than similar flat terraces not related to large rivers, although 

the offshore river-deposited terraces are also present to depths of 330 

feet (100 m). Near the mouth of the Amazon one such shallow terrace at a depth of 60 feet (18 m) is 60 miles (96 km) across, and 

another at a depth of 24 feet (7.3 m) is 25 miles (40 km) across.

It is thought that the flat terraces at shallower depths off the mouths 

of large rivers are surfaces of earlier deltas that have been submerged 

by compaction, by outward flow of the underlying sediments, or by 

isostatic sinking due to increased weight. No clear explanation of 

the origin of the flat surfaces in deeper water has yet been found. 

They may be delta surfaces formed at a lower sea level during the ice 

age.

SHELVES IN AREAS OF CORAL GROWTH

Where coral-forming organisms are growing actively, continental 

shelves differ from normal in being at very shallow depths and 

having numerous shoals and reefs. The Bahama Banks and the 

Great Barrier Reef of Australia are good examples. The depth of 

water on many of these shelves is commonly less than 120 feet (36.6 m).
18 CONTINENTAL SHELVES OF THE AMERICAS

LOOSE SEDIMENTS ON THE CONTINENTAL SHELVES

The distribution of sediments of various particle size over the 

continental shelves is highly irregular, showing little overall relation 

either to the depth of water or to the distance from land. Some 

control appears to be exerted by the nature of the bottom topography 

and by the character of the adjoining coastal area. A few generalizations seem valid, although local conditions vary.

One of the most decided differences in sediment grain size is found 

in the sediments of open shelves as compared with those of relatively 

closed bays and gulfs. On the open shelves sand is by far the most 

common type of deposit, whereas mud predominates in the protected 

bays and inland seas. Protected bays without strong currents tend 

to have coarse particles near shore, and finer particles in the deeper 

water. The reverse is more often true in bays with strong currents, 

and is generally true on the open shelves: fine material near shore, 

coarser material seaward. The outer part of the open shelves has on 

the average more coarse material, as well as more bedrock bottom, 

than the middle part, and in many places has more than the part 

nearest land.

The shelves off glaciated coasts have a wide and patternless range 

of particle size, and only a few areas of bedrock are exposed. Mud 

bottom is more common off the mouths of large rivers than anywhere 

else, but sand predominates at the outer margin of such areas. In 

general, narrow constricted areas and the bottom off points of land 

are exposed to strong water movements, and coarse sediment or even 

bare rock is present. Conversely, depressions in the shelf generally 

have finer grained sediments. As would be expected, the bottom off 

long areas of beach is usually sandy.

ORIGINS OF CONTINENTAL SHELVES

Hypotheses of continental-shelf origins can be divided into two 

general types. Some investigators think that the continental margins 

were originally higher and have been eroded down to their present 

position; others think that the shelves have been built up to their 

present position.

In regard to downcutting, the zone of breaking surf is known to 

be an area of highly active erosion. The cutting and grinding are 

limited to the zone of great wave turbulence, and they result in the 

formation of flat floors that mark the lower limit of cutting. The flat 

areas formed in this way are called wave-cut terraces or benches. As 

would be expected, the less resistant the exposed rock, the greater the 

dimensions of the wave-cut terrace.
CONTINENTS AND OCEAN BASINS 19

Some small parts of the continental shelves may have been formed 

by long-continued wave erosion resulting in wide wave-cut platforms. 

The present depth of such platforms below the zone of effective wave 

erosion could be caused by a rise of sea level since their formation. 

The decreasing depth of water on the shelves from their outer margins toward the land might then represent the progressively rising 

zone of erosion of a gradually rising sea.

This explanation cannot be applied to all shelves, for the cutting 

process is extremely slow in areas of resistant rocks. The small hills 

and valleys on nearly all the surfaces of the continental shelves also 

argue against wave cutting as the only means of origin, for in general 

that process would be expected to yield flatter surfaces.

The simplest concept of the formation of the continental shelves 

includes the development of a narrow inshore portion of the shelf as 

a wave-cut terrace, and the building out of the outer portion of the 

shelf by the deltalike deposition of the resulting debris. But other 

lines of evidence tend strongly to negate this concept.

In many places it can be demonstrated that the shelves are not 

being built outward by the accumulation of rock debris resulting from 

erosion. The frequently observed areas of solid rock bottom on the 

outer shelf are the strongest argument that extensive shelf areas are 

not now receiving sediments. The walls of submarine canyons that 

cut into the outer shelf also commonly are of solid rock.

Furthermore, if rock debris of all particle sizes were being carried 

outward to the edge of the shelves, the heavier, larger particles would 

be expected near the shore, and the smaller, lighter, more easily transported particles would be expected near the outer margins. This 

condition apparently exists in only a few shelf areas. Instead, coarse 

material such as cobbles, gravel, and large sand grains are common 

on the outer part of the shelves. It is indeed difficult to explain how 

they could be transported there, and fragile marine plants and animals 

growing on them indicate that they are now stationary. It thus 

seems probable that the larger rock fragments on the outer part of 

the shelves are not now being carried there, but have simply been left 

there by the selective removal of the smaller particles.

The portion of the Gulf of Mexico off the coast of the United States, 

however, is a notable exception to the concept of a widespread lack 

of sedimentation on the outer shelf. Nearly constant sinking of the 

earth's crust under a large part of the Gulf of Mexico is indicated 

by the steady and extremely thick accumulation there of sediments 

apparently representing nearly all recent geologic time intervals, including the present.
20 CONTINENTAL SHELVES OF THE AMERICAS

The prevailing absence of sedimentation on the outer parts of the 

continental shelves does not preclude the effectiveness of sedimentation as a shelf-forming agency farther inshore. In some areas during 

past geologic time there may have been chains of islands along the 

coast that were separated from the mainland by relatively deep basins, 

like the island-bordered seas of the eastern Asiatic coast of today. 

Such islands might have been leveled by wave erosion at low stands 

of sea level, and then deeply submerged at today's higher sea levels. 

The deep areas inside such offshore islands and banks may have been 

filled with sediments, and the result would be similar to the present 

concept of shelf structure in some areas.

One of the merits of this theory is that it explains the presence of 

rock bottom and large rock fragments on the outer shelf edges. 

Another merit is that it is in accord with recent seismic investigation 

of the shelf off the eastern coast of the United States. The results 

of that work to date indicate that at some places the surface of the 

solid rock under the sediments is higher near the shelf's outer margin 

than it is farther inshore.

The most definite knowledge of shelf development applies to the 

glaciated areas. The shelves there appear to have been deepened, 

especially in their middle part, by the movement of glaciers out across 

them. Deposition far offshore of the debris derived from glacial 

erosion on the land may well be the cause of the prevailing wideness 

of such shelf areas.

In summary of the possible origins of the continental shelves, it 

can be said that their present condition is not due to any one process 

alone, but is due to a combination of processes. In the present state 

of inadequate knowledge, it appears that both downcutting and 

deposition have been operating.

THE CONTINENTAL. SLOPES

The sea bottom from the outer edge of the continental shelves to 

the floors of the deep ocean is generally one long unbroken slope, appropriately called the continental slope. This is the zone of transition 

from the continental blocks to the oceanic blocks of the earth's crust, 

and is thus a highly critical part of the earth. It is also by far the 

greatest topographic feature of the earth's surface.

As already noted, knowledge of the shallow continental shelves is 

woefully inadequate, but far less is known of the continental slopes. 

The development of sonic depth finders capable of measuring great 

depths has resulted in a large number of soundings, but data about 

many parts of the world are sparse or entirely lacking. Additional 

information has come from seismic, gravimetric, and other geophys-
CONTINENTS AND OCEAN BASINS 21

ical investigations, and from a small amount of bottom sampling. 

But indirect reasoning and speculation of necessity play a large part 

in any discussion of the continental slopes.

The continental slopes have an average height above the ocean floors 

of about 12,000 feet (3,660 m), and in places they attain the tremendous height of 30,000 feet (9,150 m). For about half of their length 

throughout the world they continue directly into the great ocean deeps. 

Their average inclination is only 4° 17' for the first 6,000 feet 

(1,830 m) of vertical descent. The fronts of most high mountain ranges on land have the same angle with respect to the adjacent 

lowlands as have the continental slopes with respect to the adjacent 

ocean floor. The typical continental slope is steep at the top and more 

gently inclined near the bottom: the transition to the ocean floor is 

more gradual than the transition to the continental shelves. The 

steepest known submarine slope of large magnitude in the world is 

found along the south coast of Cuba, where in the first 6,000 feet 

(1,830 m) of descent it averages about 17° and in some places 

appears to exceed 45°.

The overall lengthwise trend of the continental slopes is generally 

straight or gently curving. A large number of basin-like depressions, 

hills, and ridges have been found on the surface of the slopes, but 

in only a few places does the concentration of depth measurements 

allow precise contouring or the recognition of small topographic 

features. The slope is cut by numerous canyons fully comparable to 

but not as closely spaced as those in mountain ranges on land.

Bedrock is commonly exposed in the walls of the submarine canyons 

and on the upper parts of the continental slope. In the limited areas 

where the bottom has been sampled, mud covers about 60 percent and 

sand about 25 percent of the continental slopes. Rock and gravel are 

reported in about 10 percent of notations of the character of the bottom, and shells and ooze are mentioned in about 5 percent.

CONTINENTAL-SLOPE CHAEACTEBISTICS IN RELATION TO TYPES

OF COASTAL AREAS

Continental slopes off stable lowland coasts lacking large rivers 

have an average inclination of only about 3°. This average includes a number of extremes that are not typical of the group. For 

example, the slope off the Brazilian Highlands, where the coast is of 

the stable lowland type, is one of the steepest in the world. There 

are old low mountains near this coast, and the continental shelf is 

anomalously narrow there.

Off large deltas on stable coasts the average inclination of the continental slope is unusually and uniformly low, averaging only 

1°20'. Bottom sediments there are predominantly mud. So far
22

as is known, the topography of the slope in such areas is irregular, 

and includes plateaus, valleys, and canyons, as well as rounded hills 

and depressions.

Fault coasts in general lack continental shelves, and the continental 

slopes of those coasts are the steepest of any group, averaging 

5°40' from the ocast to a depth of 6,000 feet (1,830m), though 

25° is commonly attained. Rock bottom is more common on these 

than on other slopes, and canyons are rare. In some places the inclination of the slopes off faulted shelves increases with depth.

Continental slopes off unf aulted young mountain-range coasts are 

slightly more gently inclined than those off faulted coasts, and are cut 

by numerous submarine canyons. Continental borderlands are more 

common off young mountain coasts than elsewhere. They are described on page 23.

The continental slopes encircling the Pacific Ocean include most 

of the high and steep slopes in the world. All the known areas where 

the continental slopes steepen below a depth of 6,000 feet (1,830 m) 

are in the Pacific Ocean. This circumstance seems to be directly related to the presence of young mountain ranges on the lands bordering 

nearly all of the Pacific Ocean, and to the occurrence of the long linear 

or arcuate deeps in the ocean floor adjacent to these mountains.

MODIFYING FEATURES OF THE CONTINENTAL SLOPES

The most prominent of the several features modifying the continental slopes are submarine canyons (see fig. 4A). A large number 

of them have been discovered off the relatively intensively sounded 

coasts of the United States, and they are widely distributed throughout the remainder of the world. In general, submarine canyons have 

steep walls with heights of hundreds or thousands of feet, narrow 

floors, V-shaped cross sections, and continuously downward-sloping 

bottom profiles. In these ways, and in their overall dimensions and 

winding courses with entering tributary valleys, they are comparable 

with land canyons.

Their similarities to land canyons strongly suggest that submarine 

canyons were formed by fresh-water stream erosion, but this would 

require a past sea level on the order of 12,000 feet (3,660 m) lower 

than that of the present. Most geologists consider this figure so 

great that neither the withdrawal of sea water as glacier ice nor 

the vertical movement of crustal blocks is adequate to explain it. 

Yet there are no alternatives if the canyons were formed on land. As 

a consequence of this stalemate, such underwater agencies as submarine landslides and erosion by locally strong currents or dense 

turbidity currents of suspended sediment have been mentioned as
CONTINENTS AND OCEAN BASINS 23

possible causes of the canyons. In general, these underwater erosive 

forces, with the possible exception of turbidity currents, do not seem 

to be of sufficient magnitude to form such large-scale features. Their 

origin remains a mystery.

The surface of the continental slopes in areas without submarine 

canyons is not everywhere one unbroken incline from the shallow 

continental shelf to the deep ocean floor. In some parts of the slopes, 

at depths intermediate between the deep ocean floor and the shallow 

continental shelves, are found terraces, plateaus, and areas characterized by the alternation of basins and large elevated areas. They 

are referred to as the continental borderlands. Where their surfaces 

are generally flat and plateaulike they may represent subsided areas 

that were originally part of the continental shelf, or they may represent the base of effective erosion by oceanic currents, perhaps at a 

previously lower level of the sea.

A notable example of a continental borderland is the Blake Plateau, located off the southeastern coast of the United States in the 

area between Cape Hatteras and eastern Florida. (See fig. 4B.) 

The Blake Plateau is nearly flat, is at depths of from 1,800 to 3,000 

feet (550 to 915 m), and is as much as 150 miles (240 km) wide. The 

continuation of the continental slope at the outer margin of the plateau has inclinations of about 15° and is directly in line with the 

continental slope to the north of Cape Hatteras.

The continental borderland off southern California has alternate 

high areas and depressions. The deep basins, elevated areas, and 

long straight escarpments between them are similar to those on the 

land nearby. Because those features on land are known to be the result of slipping of crustal blocks relative to one another (faulting), 

that origin has also been ascribed to the continental borderland of 

that area.

Because the continental slopes extend far below the range of most 

erosive agencies, deposition of sediments on them would seem to have 

been going on at some rate or other ever since their formation. 

Their original shape may thus have been modified to some extent. 

The numerous reports of bare bedrock (indicating no sedimentation) 

on the slopes have been obtained largely from the parts at depths less 

than 3,000 feet (915 m), where the inclination of the bottom is relatively high, and from the commonly precipitous walls of submarine 

canyons.

Fine-grained water-saturated sediments are thus abundant on the 

relatively steep continental slopes. Experimental work in soil mechanics and other fields has shown that fine-grained sediments saturated with water will flow under water on surfaces of extremely gentle
24 CONTINENTAL SHELVES OF THE AMERICAS

inclination. The previously mentioned underwater turbidity currents 

are dense suspensions of sediments in water, and, having a high overall specific gravity, they can attain high velocities on relatively 

gentle slopes and can transport rock fragments of large size. Such 

things as slumps, slides, mudflows, and turbidity currents might thus 

be expected on the continental slopes.

These phenomena have not been directly witnessed, but several observed effects may be attributed to them. On several occasions submarine cables have been broken, mangled, and buried by debris over 

wide areas of the ocean bottom. Where several cables in the same 

general area have been broken within a day's time, the appreciable 

amount of time that elapsed between the breaking of each successive 

cable is thought to represent the traveltime of turbidity currents.

New surveys of water depths over areas of moderately large size 

have sometimes shown consistent changes from the depths recorded by 

previous surveys. Such changes are almost certainly due to slumping 

of the bottom and the pattern of the small-scale hills and basins that 

have been found on parts of the slopes resembles the hummocky topography of landslide areas on land.

At any rate, the lower and more gently inclined parts of the slopes 

are largely mantled with loose sediments. It is not yet known how 

much of whatever is beneath the surface of the slopes is actually made 

up of such material.

STRUCTURE AND ORIGIN OF THE CONTINENTAL SLOPES

As previously mentioned, isostatic or other vertical movements of 

the continental masses with respect to the floors of the ocean 

have almost certainly taken place, and the continental slopes seemingly 

should have been the sites of the relative movement. A number of 

direct and indirect lines of evidence substantiate the view that the 

continental slopes have assumed their present form because of 

such relative movements; that is, that the continental slopes are areas 

of movement along faults between major blocks of the earth's crust. 

Other theories to account for the origin of the continental slopes 

have been advanced, however, and they will next be examined briefly.

The simplest and one of the oldest concepts of slope origin is that 

the continental slope is simply the steep front edge of a great pile 

of sediments derived from the erosion of the continents. There are 

a number of observations for which this theory cannot account. Chief 

of these are the demonstrated absence of deposition on the outer edges 

of large parts of the continental shelves, the absence of decreasing 

grain size of sediments with increasing distance from land where 

there is sedimentation, and the presence of large areas of bedrock on
CONTINENTS AND OCEAN BASINS 25

both the shelves and the slopes. Nor would sediments coming to rest 

under water be expected to form such highly inclined surfaces as those 

of the continental slopes.

Another theory is that the slopes are the downwarped or downwardflexed surfaces of the continental blocks. Objections to the theory of 

downwarping of the continental margins are largely based upon the 

lack of observed downward bending of the rock strata on the continental slopes and at the outer edges of the continental shelves. As mentioned previously, there are indications from seismic work off the east 

coast of the United States that the rock strata underneath the outer 

shelf margins may actually be tilted slightly upward. Furthermore, 

downwarped surfaces should have become buried by sediment, yet 

bedrock is common on the continental slopes. Finally, it would seem 

that the downwarping of portions of the continental margins would 

violate the principle of isostasy. If the light material of the continents 

were bent downward toward the same level as the heavy material of 

the deep ocean areas, instead of floating higher, the resulting disturbance of the earth's gravity field would be considerable. No such disturbance has been observed.

The theory that the continental slopes have achieved their present 

form as a result of faultslippage at the contact of the continental 

blocks with the oceanic blocks of the earth's crust seems to accord with 

more observed facts than do other theories. A certain amount of 

such faulting is to be expected as a result of the rise of the continents 

in reaction to erosional lightening. Earthquakes are caused by fault 

slippage within the earth or at its surface, and the continental slopes 

are among the most active earthquake zones in the world. The trend 

of faults and fault zones on land is generally straight or slightly 

curved, and so is the trend of the continental slopes. Another similarity between the continental slopes and faults on land is that the 

trends of both are often at an angle with the general geologic trend of 

nearby areas. The presence of bedrock at the outer continental-shelf 

margin and high on the continental slopes is also in accord with the 

fault-zone theory.

On the other hand, if this postulated zone of faulting is still active 

as is indicated by earthquakes in many areas, the river deltas of the 

Nile and the Niger that have built outward onto the continental slope 

should have been disturbed, but apparently they have not been. The 

gentle inclination of the continental slopes off most other large rivers 

also fails to reveal the expected fault offsetting. It has further been 

argued that faulting would be expected to result in steep longitudinal 

escarpments on the slopes. No such features have been observed, but 

sedimentary debris would probably round the original sharp profile 

of such escarpments.

451437 58 3
26

Despite a multiplicity of conjecture, the origins of the continental 

slopes remain obscure.

SELECTED REFERENCES

[The following references have been selected because they contain full overall accounts of 

the subjects mentioned in this report and, taken together, represent a synthesis of 

present information, concepts, and points of view in this field of study. An extensive 

bibliography is given in nearly all the references.]

Bentley, C. R., and Worzel, J. L., 1956, Geophysical investigations in the emerged 

and submerged Atlantic Coastal Plain Part 10, Continental slope and continental rise south of the Grand Banks: Geol. Soc. America Bull., v. 67, 

p. 1-18.

Carsey, J. B., 1950, Geology of Gulf coastal area and continental shelf: Am. 

Assoc. Petroleum Geologists Bull., v. 34, p. 361-385.

Deitz, R. S., 1952, Geomorphic evolution of continental terraces (continental 

shelf and slope) : Am. Assoc. Petroleum Geologists Bull., v. 36, p. 1802-1819.

Eardley, A. J., 1951, Structural geology of North America: New York, Harper 

and Brothers, Publishers, 624 p.

Flint, R. F., 1947, Glacial geology and the Pleistocene epoch: New York, John 

Wiley and Sons, 589 p.

Gilluly, James, Waters, A. C., and Woodford, A. O., 1954, Principles of geology: 

San Francisco, W. H. Freeman and Co., 631 p.

Hess, H. H., 1938, Gravity anomalies and island arc structure with particular 

reference to the West Indies: Am. Philos. Soc. Proc., v. 79, p. 71-96.

Katy, Samuel, and Ewing, Maurice, 1956, Seismic-refraction measurements in 

the Atlantic Ocean Part 7, Atlantic Ocean Basin west of Bermuda: Geol. 

Soc. America Bull., v. 67, p. 475-510.

Kuenen, Ph. H., 1950, Marine geology: New York, John Wiley and Sons, Inc., 

568 p.

Poldervaart, Arie (ed.), 1955, Crust of the earth (a symposium): Geol. Soc. 

America Special Paper 62, 762 p.

Schuchert, Charles, 1955, Atlas of paleogeographic maps of North America: New 

York, John Wiley and Sons Inc., 177 p.

Shepard, F. P., 1948, Submarine Geology: New York, Harper and Brothers, Publishers, 348 p.

Shepard, F. P., and Emery, K. O., 1941, Submarine topography off the California 

coast canyons and tectonic interpretation: Geol. Soc. America Special Paper 

31,171 p.

Sverdrup, H. U., Johnson, M. W., and Fleming, R. H., 1942, The oceans, their 

physics, chemistry, and general biology: New York, Prentice-Hall, Inc., 

1087 p.

Trask, P. D. (ed.), 1939, Recent marine sediments, a symposium: Tulsa, Oklahoma, Am. Assoc. Petroleum Geologists, 736 p.

Umbgrove, J. H. F., 1947, The pulse of the earth: The Hague, Martinus Nijhoff, 

179 p.

Veatch, A. C., and Smith, P. A., 1939, Atlantic submarine valleys of the United 

States and the Congo submarine valley: Geol. Soc. America Special Paper 

7,101 p.

Weaver, Paul, 1950, Variations in history of continental shelves: Am. Assoc. 

Petroleum Geologists Bull., v. 34, p. 351-360.
A REVIEW OF PRESENT KNOWLEDGE CONCERNING THE 

CONTINENTAL SHELVES OF THE AMERICAS

By JOHN LYMAN, U. S. Navy Hydrographic Office l

CONTENTS

Abstract___ -..-. __ _. _ . ._ 27

Introduction. _____________________________________________________ 28

Sealevel.. _ . 30

Bathymetry- _-_-_-_-_-_---_--___--_-_-__--_______-___----_____-__ 31

Sediments.__-__---_---_---_----_--_---___-__-_-__--__----_-----__ 34

Structure. ________________________________________________________ 35

Gravity ______-_--__-_---------__---_--__--___--_-_-__--------_--- 35

Geomagnetism. ___________________________________________________ 36

Seismic investigations._____-___-___________________-___---__-__..-__ 36

Improving the knowledge of continental shelves.______________________ 37

Acknowledgment. _________________________________________________ 39

Selected references.-_-_-_-__-______-__________________.--___-______ 39

ILLUSTRATIONS

PLATE 1. U. S. Navy Hydrographic Office hydrographic-chart holdings

as of January 1, 1955___-___-______--_______-_-_-___ In pocket

2. Appraisal of world nautical-chart coverage. _.____-___-._ In pocket

Page 

FIGURE 5. Tide-gage stations of the Western Hemisphere______________ 30

ABSTRACT

The earth's surface is composed in general of continents and ocean basins, 

different in elevation because the rocks underlying the continents are lighter than 

those under the oceans. The volume of water now in the oceans slightly exceeds 

the capacity of the ocean basins, with the result that parts of the continents which 

were a few hundred feet above water in the immediate geologic past are now submerged. These shallow submerged areas are the continental shelves. Changes in 

sea level are still taking place, although for much of the Western Hemisphere 

tide-gage records are not yet available to reveal the details of these changes.

1 Opinions or assertions contained herein are not to be construed as being official or reflecting the view 

of the Navy Department or the Naval establishment.

* 27
28 CONTINENTAL SHELVES OF THE AMERICAS

Although more is known about the topography and sediments of the continental 

shelves than about their other characteristics, owing to the importance of this 

information for navigation, accurate surveys are available for only a small part 

of the shelves of the Western Hemisphere.

Methods have been developed for conducting magnetic, seismic, and gravity 

surveys at sea. The information resulting from such surveys is of great value 

in studying the continental shelves, particularly in locating mineral deposits, 

but such studies are still in their infancy. Modern navies possess the primary 

capability of surveying the continental shelves. Fishery investigations and 

oceanographic institutions can also assist.

INTRODUCTION

Geophysicists today are in general agreement that the composition 

of the crust of the earth beneath the continents is fundamentally 

different from that underlying the ocean basins (Bullard, 1954; 

Ewing and Press, 1955). In recent years the properties of the 

earth's crust have been sampled by several different geophysical 

methods, which will be described later in this paper. A general 

summary of the findings is contained in the following quotation from 

Ewing and Press (1955).

Seismic-refraction measurements have established beyond doubt that the silicic 

crust under ocean basins is no more than one-fifth as thick as that under continents, and corresponds in composition to the more mafic parts of the continental 

crust. This crustal structure, revealed by seismic measurements, confirms the 

isostatic equilibrium determined by gravity measurements. Remarkable uniformity of crustal structure within continents or within ocean basins is revealed 

when obviously anomalous areas such as margins, mountains, and trenches are 

omitted. Magnetic measurements are meager, but those available indicate that 

over certain large oceanic areas the magnetic field is unusually smooth.

In simple terms, the earth's crust is composed of large patches of 

light rocks, surrounded by larger patches of heavy rocks. The 

lighter patches, floating higher on the plastic core of the earth, make 

up the continents, while the surrounding lower areas of heavier rock 

compose the ocean basins. The evidence from seismic, magnetic, 

and gravity measurements seems to support this hypothesis, although 

not all geologists (Lees, 1954; Gilluly, 1955; Stetson, 1955) are in 

full agreement with this view.

It is generally recognized that at the present instant of geologic 

time, the quantity of sea water in the oceans somewhat exceeds the 

capacity of the ocean basins (see fig. 1, p. 5).

Figure 1 shows the statistical distribution of elevations of the 

earth's surface. Two levels are much more frequent than any 

others one slightly above present sea level, representing the characteristic level of the dry land of the continents, and the other 4,000 

to 5,000 meters below present sea level, corresponding to the depth 

of the floor of the ocean basins. Mountains and ocean deeps, al-
REVIEW OF PRESENT KNOWLEDGE 29

though striking features, are shown by this figure to be of uncommon 

occurrence. Also of uncommon occurrence are depths in the range 

1,000 to 2,000 meters below sea level. Although the lower boundary 

of this region of the hypsographic curve passes rather smoothly into 

the region representing the ocean floor, there is a very abrupt transition at the upper edge, at a depth between 100 and 200 meters below 

sea level.

This abrupt transition, which is usually even more pronounced 

when individual sections off the coasts of continents, rather than 

statistical averages, are studied, has been named the shelf edge by 

the International Committee on the Nomenclature of Ocean Bottom 

Features (Wiseman and Ovey, 1953). The area between the shelf 

edge and the low-water line is the continental shelf. The declivity 

from the shelf edge into great depths is the continental slope. Wiseman and Ovey (1953, p. 14) point out that although conventionally 

the edge of the shelf is designated as being at a depth of either 100 

fathoms (600 ft or 183 m) or 200 meters (109 fathoms), areas are 

known where the increase in slope occurs at more than 200 fathoms 

(365 m) or less than 65 fathoms (120 m). The common use of 100 

fathoms or 200 meters results from the fact that international .practice in the construction of ordinary nautical charts does not provide 

for depth contours intermediate to 50 and 100 fathoms or to 100, 

200, and 300 meters (International Hydrographic Bureau, 1953, p. 49). 

Since these charts are the primary source of published information 

on submarine topography, their use has often resulted in a somewhat 

restricted and oversimplified view of the character and location of 

the shelf edge.

The hypsographic curve shows clearly that, as far as the statistical 

distribution of elevations is concerned, no distinction can be made 

between the dry-land portion of the continents and the continental 

shelf. When it is considered further that as recently (geologically 

speaking) as 20,000 years ago, sea level stood about 50 fathoms 

(100 m) lower than at present because of the water that was trapped 

in the glacial ice sheets of the Pleistocene (Kuenen, 1955), the identity 

of at least part of the continental shelf with the adjacent land is at 

once apparent.

Likewise there is no reason to believe that present sea level is a 

permanent feature. On the contrary, some land areas such as Finland 

and Scandinavia are known to be rising as a result of isostatic adjustment after melting of the ice sheets that formerly covered them. 

Elsewhere the continued melting, presumably of the Greenland icecap, has resulted in a rise in sea lev^el.
30 CONTINENTAL SHELVES OF THE AMERICAS

SEA LEVEL

Virtually the only means of detecting sea-level changes is through 

the analysis of tide-gage records, carefully taken over a long period 

of years. The availability of such stations in the Western Hemisphere, as far as can be determined, is indicated in figure 5. For 

much of the coast the records do not yet permit conclusions as to longterm changes in sea level.

Stations established since 1940

4

Stations whose status of operation is unknown

FIGUBB 5. Tide-gage stations of the Western Hemisphere. 

Hydrographic Bureau, 1955.)

(After International

Manner (1949) discussed the United States records, which show 

everywhere, except at Ketchikan, Alaska, a rise in sea level relative 

to the land (or a relative subsidence of the land). Since 1930 this 

rise has amounted to about y^ inch (6 mm) per year for the Atlantic 

Ocean and Gulf of Mexico stations, but only about yi2 inch (2 mm) 

per year for the Pacific Ocean stations. Bigelow (1949) found a
REVIEW OF PRESENT KNOWLEDGE 31

rise of nearly 1/4 inch (6 mm) per year from the record at Cristobal, 

Canal Zone, from 1933 to 1943, while at Balboa, at the Pacific entrance to the Panama Canal, the rise was % inch (3 mm) per year 

from 1908 to 1942.

BATHYMETRY

Nearly all bathymetric investigations of the continental shelves 

that have been carried out in the Western Hemisphere were conducted 

for the purpose of constructing nautical charts and associated sailing directions. Certain basic features of such charts tend to limit 

the amount of information that is available for the whole shelf, 

particularly where it is wide. Plate 1 shows charts of the American 

coasts that were available in 1955.

Practical problems connected with producing, shipping, stowing, 

and handling nautical charts limit their greatest dimension to about 

48 inches (122 cm). At a scale of 1:250,000, which is approximately 

the minimum that provides satisfactory delineation of topography, 

this length is equivalent to a distance of about 160 nautical miles 

(300 km). Therefore, in the many areas of the Western Hemisphere 

where the continental shelf extends farther seaward than 300 kilometers, the outer edge of the shelf will not be properly delineated.

Charts of a scale greater than 1:300,000 out to the 100-fathom 

(183 m) contour are available for only a few areas, the best 

coverage being along the west coast of Greenland, along the Ungava 

Peninsula and Labrador, in the Gulf of St. Lawrence and off Nova 

Scotia, and along the coasts of Cuba, Hispaniola, Puerto Rico, 

Venezuela, Brazil, Panama, Costa Rica, and North America between 

Cabo San Lucas, Mexico, and the Aleutians.

Even as scales between 1:300,000 and 1:1,000,000, there are gaps in 

the chart coverage of the 100-fathom (183 m) isobath, the most obvious being in the Canadian Arctic archipelago and in the Bering Sea. 

Other gaps appear to exist off Patagonia and southern Brazil.

The chart coverage shown by plate 1 may tend to give a misleading 

picture of the density of the actual sounding lines plotted on the chart. 

Plate 2, from Karo (1955), reveals that a great deal of the coverage 

shown in plate 1 is actually based upon scattered track lines rather 

than systematic surveys. According to Karo:

The most serious deficiencies in the Western Hemisphere are along the coasts 

of Mexico and Central America, the northeastern coast of South America, the 

Arctice coat of Canada, the northern half of Greenland, and of course antarctica. Many other areas need more detailed and large-scale coverage than presently exists, but the most pressing need is for reliable medium-scale coverage.

In only a very few areas of the Western Hemisphere have bathymetric surveys been carried out for purposes other than the construe-
32 CONTINENTAL SHELVES OF THE AMERICAS

tion of nautical charts. A notable exception is the survey of the Gulf 

of California made by the E. W. Scripps in 1939 and 1940, results of 

which were published by Shepard (1950) as a chart the gulf on 

the scale of 1:1,300,000.

Two other examples of the publication of charts of areas adjacent 

to the continents, for purposes other than navigation, are the series 

of the area off the coast of California by Shepard and Emery (1941), 

and the charts by Veatch and Smith (1939) of the waters off the 

northeastern United States.

The relatively few regions where adequate surveys have been carried 

out and the results published display great variations in the extent 

and character of the continental shelf. From the scientific point of 

view it may be seriously questioned whether statements as to a "typical" form of a continental shelf, or an "average" depth for its outer 

limit have any general application in nature. Ewing and Press 

(1955), discussing the continental margins, the transition zones between the shores of the continents and the ocean basins, propose the 

following worldwide classification:

1. Stable margins, usually associated with a broad continental shelf, 

such as that off the north Atlantic coast of the United States or the 

west coast of Florida.

2. Active margins bordered by basins and ridges, as off the California coast.

3. Active margins bordered by deep-sea trenches, as off the Pacific 

coast of South America.

The continental shelf off the northeastern United States, where adequate surveys exist, has been described by Veatch and Smith (1939). 

Much of it has a well-defined change in slope at a depth of about 

100 fathoms or 200 meters, to seaward of which is a remarkable series '

of submarine canyons. In the words of Veatch and Smith (1939, 

p. 2):

Beyond a belt of the sea bottom that extends seaward from our northeastern 

coast for a distance of 60 to 125 miles [95 to 200 km] and shows only characteristic sea-made forms, there is another belt which is deeply and intricately 

dissected, shows none of the forms characteristic of sea action but on the contrary many of the things characteristic of subaerial erosion, and which has 

local relief in many places much greater than that found in the Appalachian 

Mountains and in fact is more nearly comparable to that of our western mountains. This break between the shelf and the slope is one of the most striking 

and startling changes in topography to be found anywhere in the world.

In these canyons the 100-f athom or 200-meter contour is many dozen 

miles closer to the land than it is in the zones between the canyons. 

Over a considerable distance to the east of the most prominent of 

these canyons, which lies off the Hudson Eiver, the abrupt change
REVIEW OF PRESENT KNOWLEDGE 33

in slope is at a depth of about 500 fathoms (900 m). Farther east, 

north of Georges Bank, is another type of valley. Here, in the Gulf 

of Maine, a broad, shallow basin apparently was scooped out of the 

ocean floor by Pleistocene glaciers. Many soundings are in excess 

of 140 fathoms (250 m), and the bottom is extremely irregular where 

it has been surveyed in detail.

The continental shelf of the Gulf of Mexico has been described in 

some detail by Price (1954). In general, west of Florida, as north of 

Yucatan, there is a submerged limestone shelf, with the abrupt change 

in slope at depths as shoal as 33 fathoms (60 m) and everywhere less 

than 55 fathoms (100 m) but as far as 60 to 120 nautical miles (100 to 

200 km) offshore. The outer edges of these limestone plains seem to 

lack the canyons found by Veatch and Smith in the Atlantic Ocean, 

as does the northwestern part of the Gulf of Mexico as contoured by 

Gealy (1955). However, just as a prominent canyon exists seaward 

of the Hudson River, so there is a very large canyon off the mouth 

of the Mississippi River.

The basin-and-ridge topography off the coast of California is so 

different from what is commonly understood by the term continental 

shelf that it has been designated a continental borderland (Shepard 

and Emery, 1941, p. 9). This is a region of alternating ridges and 

basins, extending as far as 135 nautical miles (250 km) to sea; many 

of the ridges rise nearly to sea level as banks or above it as islands, 

and the basins have depths as great as 1,000 fathoms (2,000 m).

The third category mentioned by Ewing and Press (1955), the 

active margin bordered by deep-sea trenches, cannot be so well demonstrated by published work. An example off the coast of South 

America was reported, however, by Trask (1956). He found a trough 

500 miles (925 km) long with depths of 4,000 to 4,300 fathoms (7,300 

to 7,900 m) roughly parallel to the coast of Chile. In lat 25° S. it was 

only 40 miles (74 km) from shore.

These examples of variation in characteristics of well-surveyed 

continental shelves, when taken with the vastly greater areas too 

poorly surveyed at the present time to permit any definite statements 

to be made, only serve to emphasize the futility of attempting to 

generalize in any description of the continental shelves as a whole. 

No arbitrarily selected depth or distance from shore can, from the 

scientific point of view, distinguish all the submerged land areas of 

the continents from the adjacent floors of the ocean basins.

On one of the most recent charts of a continental margin (Rosfelder, 1955), this concept of the true character of the edge of the 

continental shelf is clearly brought out. Here the usually prominent 

100- and 200-meter isobaths are subordinated to a line demarcating
34 CONTINENTAL SHELVES OF THE AMERICAS

the zone where the gentle slope of the continental shelf gives way to 

the steeper continental slope. This line has the mean depth prominently marked on it at 4-kilometer intervals. In Algerian waters the 

depth at the edge of the continental shelf varies from 27 to 93 

fathoms (50 to 170 m), with a mean value of 52 fathoms (96 m).

SEDIMENTS

In the past, in addition to taking soundings for hydrographic 

charts, it was customary to sample the bottom. Seamen have recognized for centuries that there exist great differences in the character 

and texture of the sediments on the bottom, even within relatively 

short distances, and the identification and description of these sediment types was an important part of the science of hydrography. 

Originally both surveyors and navigators took soundings with the 

armed lead, which has a concavity at its lower end that is filled with 

tallow or other sticky substance to retain a sample of the bottom. 

With that technique some information about the bottom, though 

often misleading in rocky areas, was readily obtainable. In recent 

years, however, the use of echo-sounding devices has almost completely replaced the leadline in hydrographic surveying, and likewise to a great extent in navigation. The result is, that, although the 

speed with which the surveyor can obtain and record soundings has 

been greatly multiplied, the facility of obtaining and describing 

bottom samples has not been appreciably increased.

Even the most recent navigational charts fall far short of modern 

soil maps in depicting the complex variations that exist over relatively 

short distances. A scale of at least 1:250,000 is required before justice 

can be done to the sediment patterns. At this scale, the ordinary hydrographic chart is insufficient as a source, and special surveys are necessary. The lack of precision that results without such special surveys 

is illustrated by a comparison of bottom-sediment charts compiled by 

the German Navy and the United States Navy during World War II 

but kept secret until after the war. Although largely drawn from 

common sources, many of the resulting charts for the same areas had 

only the vaguest general resemblance.

An example of one of the few areas of the continental shelves in the 

Western Hemisphere for which bottom sediments are known in adequate detail is the zone immediately outside San Diego Bay described 

by Emery and others (1952). In an area about 20 kilometers wide and 

50 kilometers long (1,000 sq km), they obtained 1,656 samples and 184 

bottom photographs, which enabled them to map the sediments on the 

scale of 1:160,000.
REVIEW OF PRESENT KNOWLEDGE 35

Although the echograms obtained during surveying with an acoustic 

sounding device can be interpreted to yield useful information on the 

nature of the bottom (Pratje and Schiller, 1952; Hough, 1952), such 

analysis can only support, not replace, the information obtained by 

actual sampling. Pratje and Schiller prepared a chart of the sediments of the North Sea on a scale of 1:2,200,000, using a sample density 

of about 1 per 100 square kilometers (as compared with the average 

figure of 165 per 100 sq km available off San Diego) and deducing 

intermediate features from the echograms. Allowing for the change 

in scale, this use of echograms permitted mapping with about one-tenth 

the density of sediment samples available to Emery and others (1952).

STRUCTURE

Knowledge of the sediments that cover the submerged land adjacent 

to the continents is poor compared with the corresponding knowledge 

of soils ashore, and knowledge of what lies beneath the surface layers 

is considerably poorer. The visible outcrops that exist on land and 

serve as clues to the geologic structure are masked in the sea both by 

the overlying water and by the fact that in general the land is a region 

of erosion, whereas the sea is a region of deposition. The frequent 

cuts and borings that are made on land as a concomitant of numerous 

commercial and industrial enterprises, and that yield much information on the nature of land formations, are still almost entirely lacking 

on the sea bottom. Only in recent years, in the few places where the 

occurrence of petroleum has been suspected, such as off the coasts of 

southern California, Louisiana, and Texas, and at Cape Hatteras, has 

exploratory drilling been carried out in coastal waters.

GRAVITY

Since gravity observations in deep water are not yet (1956) feasible 

from surface vessels, the possibility of making such measurements is 

limited to countries possessing seagoing submarines. Kecently, for 

use in shallow water, instruments have been developed that can be 

lowered to the bottom or taken to the bottom by divers. Observations 

with such instruments, however, have been mainly in regions where 

active prospecting for petroleum has been carried out.

The existing oceanic gravity coverage has been summarized as to 

extent by Worzel, Shurbet, and Ewing (1955) and by Woollard (1955), 

and as to extent and quality by Karo (1955). The areas most seriously 

deficient in coverage in the Western Hemisphere appear to be Antarctica, the east coast of South America between Trinidad and the 

mouth of the Amazon Kiver, and the entire Arctic from Point Barrow,
36

Alaska, eastward to Labrador. More precise and more closely spaced 

measurements are still needed for most of the rest of the hemisphere. 

Interpretation of gravity measurements on a worldwide scale yields 

useful information on the shape of the earth. For smaller areas, the 

measurements provide information on the crustal structure. As already mentioned, such gravity measurements as have been made in 

shallow water demonstrate that the continental shelves are structurally 

part of the continents, whereas measurements over deep oceanic areas 

confirm the conclusion drawn from other geophysical evidence that the 

earth's crust under the ocean basins differs fundamentally from the 

continental land masses.

GEOMAGNETISM

The earth's magnetic field varies over periods of time, so that, unlike 

most of the other types of investigation that have been mentioned, 

geomagnetic observations must be repeated from decade to decade. 

Until recently it was impossible to conduct such observations at sea 

except from specially built nonmagnetic vessels, and none has existed 

in American waters since the loss of the Carnegie in 1929. The extent 

of current knowledge of the geomagnetic field has been shown by Karo 

(1955). His chart appraising world magnetic data indicates that for a 

large part of North America and most of South America the error of 

the present values for magnetic declination or variation averages 2°. 

Recently developed airborne magnetometers have made it possible to 

conduct magnetic surveys from aircraft. Much of the United States 

and small sectors of the Gulf of Mexico have already been surveyed 

in this way. Instruments are also available which can be towed behind 

a ship, out of the range of its magnetic influence.

Magnetic observations are of practical value in connection with the 

magnetic compass and for detecting local variations in geologic 

structure connected with mineral deposits. They also can yield information on general structure of the earth's crust.

SEISMIC INVESTIGATIONS

The seismic waves generated by earthquakes or by shocks induced 

by explosives have provided geophysicists with meager but basic data 

on the structure and thickness of the crust of the earth. Marked 

differences in the traveltime of these waves through oceanic and continental sections of the earth, as recorded by seismographs, agree with 

and support the evidence derived from other data of a difference in 

composition between oceanic and continental rocks. Recent investigations by Gutenberg (1955), Bath (1954), and Press and Ewing 

(1955) of a particular type of seismic wave (channel wave), which is 

confined to continental rocks, give promise of making it possible to
REVIEW OF PRESENT KNOWLEDGE 37

delimit the margins of the continents when a sufficient number 

of seismologic stations is developed along the seaboard.

At present, in the Western Hemisphere, a few cross sections of the 

outer edge of the continent of North America have been obtained by 

recording reflected or refracted waves produced by manmade explosions at sea. These sections are confined primarily to the western 

North Atlantic Ocean (Bentley and Worzel, 1956), the Gulf of Mexico 

(Ewing and others, 1955), and the coast off California (Tatel and 

Tuve, 1955).

In the expanded search for oil and other minerals of economic 

importance, seismic methods have been the most important of the 

various geophysical techniques used in investigating the continental 

shelf. In small areas of the Gulf of Mexico and off the coast of California intensive seismic investigations have been conducted in recent 

years in order to establish the presence or absence of structural features 

which might localize accumulations of such minerals as oil, gas, and 

sulfur. This exploration has been limited generally to shelf waters 

less than 20 fathoms (40 m) in depth leaving a broad area of the 

shelf unexplored.

IMPROVING THE KNOWLEDGE OF CONTINENTAL,

SHELVES

In the days of Malaspina and Cook, hydrographic surveying was 

recognized as part of the profession of the naval officer. Later, increased application of technology to naval problems tended to produce 

specialization into various branches of the naval sciences, with the 

result that today in such important maritime nations as Canada and 

Norway the hydrographic service operates as a purely civilian establishment. In the United States, also, the surveys of home waters are 

carried out chiefly by the Coast and Geodetic Survey, a civilian 

organization with its own commissioned service.

Today more than ever before, however, the principles and techniques 

used in conducting hydrographic and geophysical surveys are linked 

to the requirements of contemporary naval warfare. The recording 

echo sounder, for example, which has vastly speeded up the work of 

obtaining details of the ocean bottom, is no different in principle from 

the sonar (sound navigation and ranging) with which a destroyer 

determines the range and bearing of a submerged submarine. The 

accurate electronic navigation systems required for controlling a hydrographic survey are precisely those needed for laying or clearing a 

minefield. A naval vessel is an obvious first choice for transporting 

and handling the tons of explosives required in seismic exploration, 

and the hydrophones used to record the resulting sound waves are 

almost identical with the equipment by which a submarine detects
38 CONTINENTAL SHELVES OF THE AMERICAS

approaching vessels. The airborne magnetometer, which has found 

wide application in surveys of the continental shelves, was first developed as a means of finding submerged submarines. The techniques 

of aerial photogrammetry employed in mapping a coast are identical 

whether the product is to be a nautical chart for the use of fishermen 

or a bombardment chart for a fleet.

This list of parallel items could be extended, but it should be sufficient to emphasize the value to a modern naval power of maintaining 

a surveying force in the waters in which it is interested. There is no 

better index of the technical efficiency and state of readiness of a 

navy than the quality and quantity of its hydrographic and geophysical investigations.

The geophysical information than can be collected simultaneously 

from a properly equipped survey vessel while under way includes, in 

addition to continuous fathograms that yield water depth and information on bottom sediments; the temperature structure of the upper 

900 feet (275 m) of water, by means of bathythermographs; bottom 

samples, obtained with "underway" samplers; and observations of 

total magnetic intensity, by means of towed magnetometers. Since 

all the data are referred to the same system of position location, their 

value is greatly enhanced.

Position location has been found to be one of the key items in modern 

geophysical surveying. Because of the expense of operating survey 

vessels and the necessity of having accurate geographic positioning at 

all times, the ordinary navigational procedures are inadequate. Veatch 

and Smith (1939, p. 6) give examples of soundings made before 1930, 

the positions of which were determined by means of the most accurate 

astronomical and dead-reckoning methods, but which proved later to 

be in error at least 6 or 7 statute miles (10 or 11 km). This discrepancy is 3 or 4 times as great as the error customarily admitted to by 

the ordinary navigator. New electronic methods of positioning, however, have been developed which are accurate to distances comparable 

to the length of the ship. The EPI (electronic position indicator) 

system, for example, developed by the U. S. Coast and Geodetic Survey 

(Burmister, 1954), shows an error of less than 200 feet (60 m) in 

over 80 percent of observations for distances up to 300 miles (500 km); 

shoran (short range navigation) is even more accurate over shorter distances (Koussel, 1955); and several other methods are available 

(International Hydrographic Bureau, 1956).

In contrast to the types of observation listed above that can be 

taken simultaneously from a moving vessel, certain other investigations must be conducted from a motionless vessel. These include
REVIEW OF PRESENT KNOWLEDGE 39

sampling of the water for accurate determination of its chemical and 

physical properties at various levels, measuring ocean currents, taking 

core samples of the ocean bottom for elucidating geologic history and 

structure, making shallow-water gravity observations, and conducting 

seismic and acoustic studies. Deep-water gravity observations require 

the use of a submarine. The data obtained by these means supplement 

the results derived from surveys made under way.

Although navies can play a major part in investigations of the 

continental shelf, the results to be obtained are by no means of interest 

to them alone. Most countries with important fishing interests employ 

vessels in investigations of the ocean and its bottom as well as of the 

oceanic flora and fauna. Fisheries research vessels search for bottom 

features that may turn out to be new and undeveloped fishing banks, 

they carry out extensive investigations of the bottom in studying the 

organisms that live on and over them, and they examine the circulation of the waters and their chemical and physical properties. In many 

countries there also exist oceanographic institutions with active programs of investigation of the sea floor, not for any immediate practical 

benefit to be obtained therefrom, but primarily to increase man's 

knowledge of the planet on which he lives.

ACKNOWLEDGMENT

The assistance of Mrs. Suzanne Bershad and Gordon H. MacDougall 

of the Division of Oceanography of the U. S. Navy Hydrographic 

Office is gratefully acknowledged.

SELECTED REFERENCES

Bdth, M., 1954, The elastic waves Lg and Rg along Eurosiatic paths: Arkiv 

for Geofysik, v. 2, p. 295-342.

Bentley, C. P., and Worzel, J. L., 1956, Geophysical investigations in the emerged 

and submerged Atlantic coastal plain Part 10, Continental slope and continental rise south of the Grand Banks: Geol. Soc. America Bull. v. 67, p. 

1-18.

Bigelow, H. W., Jr., 1949, Sea-level changes along the coasts of the United 

States in recent years, by H. A. Manner A discussion: Am. Geophys. 

Union Trans., v. 30, p. 918.

Bullard, E. C., 1954, A comparison of oceans and continents: Royal Soc. Proc., 

A, v. 222, p. 403-407.

Burmister, C. A., 1954, Electronic position indicator: Internat. Hydrog. Rev., v. 

31, p. 113-132.

Emery, K. O., Butcher, W. S., Gould, H. R., and Shepard, F. P., 1952, Submarine 

geology off San Diego, California: Jour. Geology, v. 60, p. 511-548.

Ewing, Maurice, and Press, Frank, 1955, Geophysical contrasts between continents and ocean basins, in Poldervaart, Arie (Ed.), Crust of the earth: 

Geol. Soc. America Special Paper 62, p. 1-6.
40 CONTINENTAL SHELVES OF THE AMERICAS

Ewing, Maurice, Worzel, J. L., Ericson, D. B., and Heezen, B. C., 1955, Geophysical and geological investigations in the Gulf of Mexico part 1: Geophysics, 

v. 20, p. 1-18.

Gealy, B. L., 1955, Topography of the continental slope in northwest Gulf of 

Mexico: Geol. Soc. America Bull., v. 66, p. 203-227.

Gilluly, James, 1955, Geologic contrasts between continents and ocean basins, in 

Poldervaart, Arie (Ed.), Crust of the earth: Geol. Soc. America Special 

Paper 62, p. 7-18.

Gutenberg, Beno, 1955, Channel waves in the earth's crust: Geophysics, v. 20, 

p. 283-294.

Hough, J. L., 1952, Fathogram indications of bottom materials in Lake Michigan : Jour. Sed. Petrology, v. 22, p. 162-172.

International Hydrographic Bureau, 1953, Repertory of technical resolutions; 

depths, contours and tints: Monaco, 4th ed., sec. 22, p. 49-50.

 1956, Radio aids to maritime navigation and hydrography: Special Pub.

39, 447, p. [Monaco] 

Karo, H. A., 1955 World Mapping: Washington, Industrial College of the Armed

Forces, 101 p. 

Kuenen, Ph. H., 1950, Marine geology: New York, John Wiley and Sons, Inc.,

568 p.

 , 1955, Sea level and crustal warping, in Poldervaart, Arie (ed.), Crust of 

the earth: Geol. Soc. America Special Paper 62, p. 193-203.

Lees, G. M., 1954, The geological evidence on the nature of the ocean floors: 

Royal Soc. Proc., A., v. 222, p. 400-407.

Mariner, H. A., 1949, Sea level changes along the coasts of the United States in 

recent years: Am. Geophys. Union Trans., v. 30, p. 201-204.

Murray, H. W., 1947, Topography of the Gulf of Maine, field season of 1940: 

Geol. Soc. America Bull., v. 58, no. 2, p. 153-196.

Pratje, Otto and Schuler, Fredrich, 1952, Bodenkartierung des Seegebietes Hoofden (siidliche Nordsee) mit Hilfe von Grundproben und Echogrammen, 

[Plotting of a chart of the bottom covering in the Southern Bight of the 

North Sea (Hoofden) based on bottom samples and echograms]: Deutsche 

Hydrographische Zeitschr., v. 5, p. 189-196.

Press, Frank, and Ewing, Maurice, 1955, Earthquake surface waves and crustal 

structure, in Poldervaart, Arie (ed.), Crust of the earth: Geol. Soc. America 

Special Paper 62, p. 51-60.

Price, W. A., 1954, Dynamic environments reconnaissance mapping, geologic 

and geomorphic, of continental shelf of Gulf of Mexico: Gulf Coast Assoc. 

Geol. Soc. Trans., v. 4, p. 75-107.

Rosfelder, Andre1 , 1955, Carte provisoire au l/500,000e de la marge continentale 

alg^rienne, note de presentation [Provisional chart of the Algerian continental shelf at the scale of 1/500,000, with explanatory note]: Publications du service de la carte ge"ologique de I'Alggrie, Bull. 5, Travaux des 

Collaborateurs, p. 57-106.

Roussel, G. A., 1955, Electronic surveying in offshore areas: Internat. HydrogRev., v. 32, no. 1, p. 97-109.

Shepard, F. P., 1950, 1940 E. W. Scripps cruise to the Gulf of California Part 

3, Submarine topography of the Gulf of California: Geol. Soc. America 

Memo. 43, p. 1-29.

Shepard, F. P., and Emery, K. O., 1941, Submarine topography off the California 

coast canyons and tectonic interpretation: Geol. Soc. America Special 

Paper 31, p. 1-171.
REVIEW OF PRESENT KNOWLEDGE 41

Stetson, H. C., 1955, Patterns of deposition at the continental margin: Deep-Sea

Research Papers in Marine Biology and Oceanography, v. 3 (supp.), p.

298-308. 

Tatel, H. E., and Tuve, M. A., 1955, Seismic exploration of a continental crust,

in Poldervaart, Arie (Ed.), Crust of the earth: Geol. Soc. America Special

Paper 62, p. 35-50. 

Trask, P. D., 1956, The Atlantis marine geological expedition to Peru and Chile:

Nature, v. 177, p. 454-455. 

Veatch, A. C., and Smith, P. A., 1939, Atlantic submarine valleys of the United

States and the Congo submarine valley: Geol. Soc. America Special Paper

7, 101 p.

Wiseman, J. D. H., and Ovey, C. D., 1953, Definitions of features on the deepsea floor: Deep-Sea Research, v. 1, p. 11-16. 

Woollard, G. P., 1955, Report of the special committee on the geophysical and

geological study of the continents, 1952-54: Am. Geophys. Union Trans., v.

36, p. 695-708. 

Worzel, J. L., Shurbet, G. L., and Ewing, Maurice, 1955, Gravity measurements

at sea, 1952-1953: Am. Geophys. Union Trans., v. 36, p. 326-334.

451437 58 4

POTENTIAL MINERAL RESOURCES OF THE CONTINENTAL 

SHELVES OF THE WESTERN HEMISPHERE

By JAMES F. PEPPER, U. S. Geological Survey

CONTENTS

PagQ

Abstract________________________________________________________ 44

Introduction.-_-__--_ ____ _________________________ _______ 44

Definition of the continental shelf.___________________ _______ 44

Economic minerals of the shelf area____________________ ___ 45

Gulf of Mexico western shelf area_____________________-__ _ ___ 46

Oil and gas____--__-__--_____________________________ __ 46

Sulfur__ --____-__--_____ -_- 49

Gulf of Mexico eastern shelf area_____________________ __ __ 50

Oil and gas_____________________________________ 50

Phosphate________________________._________________ 51

Eastern North America from southern Florida to Newfoundland. __ __ 51

Oil and gas______________________________________-__ 51

Coal_________.__________________________ 52

Iron ore- ___________________________________________________ 52

Monazite and ilmenite_______________________________________ 52

Caribbean shelf area and the insular shelves of the Antilles and the Bahamas oil and gas._____________________________ ___ 53

Eastern South America from British Guiana to Cape Horn____________ 57

Oil and gas___--_-_-_____________________________ 57

Monazite_____________________________________________ 58

Pacific coast of South America and Central America ___________________ 58

Oil and gas___________________________________________ 58

Coal__._ __________________________________ 59

Western North America from Mexico to Alaska _______________________ 60

Oil and gas_____________________________________________ 60

Coal____________________________________ 62

Platinum and chromite_________________________________-_______ 62

Alaska.______________________________________ 63

Oil and gas. _________________________________ 63

Gold---______-_____________________________________________ 63

Conclusions.-.-..---______________________________________ 63

Selected references_____________________________________________ 64

43
44

Page

FIGURE 6. The^ Western Hemisphere showing location of continental shelf 

and known mineral deposits exclusive of petroleum near 

shore________.________ __. - 47

7. Known salt domes and oil seeps in the Gulf of Mexico 48

8. Oil and gas fields near shore in Tierra del Fuego, Chile 59

9. Oil fields near shore in^Peru^and Ecuador_________ __ 60

10. Oil fields near shore in the Los Angeles basin, California 61

ABSTRACT

The continental shelves are the marine terraces that extend from the shoreline to the sharp change in the slope of the sea bottom that occurs at water 

depths of generally somewhat less than 600 feet (183 m). Beyond this outer 

edge of the shelves the depth of water increases rapidly toward the ocean basins.

Geologically speaking, the continental shelves are integral parts of the continents, and their geology and mineral resources are simply an extension of those 

of the adjacent land. Exploration for mineral resources in the shelf areas is 

far more difficult and expensive than on land. One relatively small area, a 

part of the shelf of the Gulf of Mexico off the coast of the United States, has 

been prospected intensively, and economically recoverable deposits of oil and 

gas have been discovered there. The undersea parts of iron ore deposits in 

Newfoundland and coal beds in Nova Scotia and Chile are being mined from 

portals on land.

Continental-shelf areas where prospecting might be successful border land 

areas that contain mineral fuels or other minerals. For example, oil and gas 

may be found in shelf areas in parts of the general Caribbean Sea area, Tierra 

del Fuego and scattered other parts of South America, Lower and southern 

Calfornia, and Alaska; sulfur in part of the Gulf of Mexico; and phosphate on 

the western shelf of Florida. So-called heavy minerals are found in present-day 

or ancient beach deposits in widespread areas: monazite, ilmenite, rutile, and 

zircon in Florida and in Brazil; gold, platinum, and chromite in Oregon; and 

gold in Alaska.

The ever-expanding economy of the world may in the future come to depend 

in substantial part on the mineral resources of the continental shelves.

INTRODUCTION

DEFINITION OF THE CONTINENTAL SHELF

The continental shelf is that part of the ocean floor lying between 

the seacoast and a seaward margin where the slope steepens abruptly 

and plunges rapidly into the abyssal depths of the ocean. The depth 

of water where this change of slope takes place may vary, but for 

convenience of usage most oceanographers assume a depth of 600 feet 

(183 m). The continental shelf is wide where the sea floor inclines 

gently under the sea from a flat coastal plain, but is narrow at places 

where the flanks of mountains plunge into the sea.
POTENTIAL MINERAL RESOURCES 45

The continental shelf is simply the continuation of the continent 

under the sea. It is composed of the same kind of rocks as those 

of the coast of the continent, and the strata under the sea floor usually 

are continuations of rock formations on the coast. Much of the shelf 

surface is a nearly smooth or gently rolling plain dissected at places 

by channel-like depressions which deepen near the outer edge of the 

shelf. At some places, the shelf surface is rough, with hills and 

valleys. At a few places shallow basinlike depressions have been 

formed by solution of underlying limestone, as on the continental 

shelf off western Florida.

At many places rocks of the continental shelf are covered by loose 

sediments derived from the land. These sediments may consist of 

sand, silt, and mud transported to the ocean by large and small river 

systems and deposited over wide areas in geologically recent time, 

as, for example, the sediments of the Mississippi River which are 

spread along the shelf in the western part of the Gulf of Mexico. 

Elsewhere, as along the northern Atlantic shelf, the sediments may 

consist in large part of glacial clay and gravel deposited during the 

ice age. In many areas, however, the rocks of the continental shelf 

are barren of loose sediments, either because such sediments were not 

deposited or because they were swept away by ocean currents.

ECONOMIC MINERALS OF THE SHELF AREA

The mineral resources of the shelf are similar to those of the contiguous land. Where oil and gas fields are present along a seacoast 

other fields can be expected in similar strata and in similar structures offshore. Off the coast of Louisiana to a water depth of 100 

feet (about 30 m) the frequency of occurrence of major geologic 

structures that produce or can be expected to produce oil is almost 

exactly the same as that on the adjoining land (Atwater, 1956, p. 

2626).

Research groups of geologists from university and State organizations and groups of geologists from oil companies have carried out 

large and expanding programs of geologic and oceanographic studies 

on several North American shelf areas with a view toward exploration, especially for oil and gas. Although much has been accomplished toward preliminary mapping of relatively small areas of the 

sedimentary rocks and their structures, a more detailed knowledge 

of the geology of the shelf can be obtained only after a long period 

of exploration, which will involve much effort and money.

Bedded deposits of minerals, such as coal and iron ore, where 

present along the shore, may dip under the sea and continue in the 

same rocks for many miles offshore. Iron ore in Newfoundland,
46 CONTINENTAL SHELVES OP THE AMERICAS

present under the sea floor in millions of tons, is currently being 

mined about 2 miles (3.2 km) from land.

Minerals eroded from the land may be concentrated in placer deposits in beaches and bars along the coasts. Gold, for example, is 

found at Nome, Alaska, and monazite, ilmenite, rutile, and zircon 

are present in some of the ancient beaches in Florida and in presentday beaches in Brazil (fig. 6). Similar placer deposits can be expected farther offshore, beneath the ocean.

Materials such as seashells, sand, and gravel, which are used in 

cement work and roadbuilding, are taken in great quantities from 

deposits along the seashore, or dredged from shallow bayous or bays. 

Inasmuch as these resources are in plentiful supply on most beaches 

and in the shallow bays along the coast, dredging them from the continental shelf is usually not practical at the present time.

The shelf areas of the Western Hemisphere and the insular shelves 

of the West Indies are described in the following pages. The occurrence of oil and other minerals or the prospect of finding them is 

discussed for each area. The information is a synthesis of published 

data from many sources. For more detailed information on any 

subject or area mentioned, the appropriate report in the appended 

bibliography may be consulted.

GULP OF MEXICO WESTERN SHELF AREA

OIL AND GAS

Perhaps no area off the coast of any country has received such an 

intensive examination as the continental shelf off Texas and Louisiana. 

The Gulf of Mexico is shallow for many miles offshore, and the sedimentary rocks of the gulf floor are similar to those which are highly 

productive in the coastal area of Texas and Louisiana. Structures 

similar to those on the adjacent coastal plains from which oil or gas 

are produced have been found on the shelf by intensive geophysical 

exploration (fig. Y). These structures extend along this coastal belt 

eastward through Mississippi into southwestern Alabama, and are 

either known or presumed to be closely related to salt domes. These 

domes, more properly described as salt plugs, are cylindrical masses of 

rock salt, of small horizontal cross section but great vertical dimension, that have been forced upward through great thicknesses of 

gulf-coast sediments from a salt layer far below. The salt in this 

layer is believed to have been a bedded sedimentary deposit that probably formed originally as a single mass in one large basin, rather than 

as disconnected masses in separate basins. This widespread basin 

apparently extends southward to a termination in the northern part of 

the Isthmus of Tehuantepec, Mexico. The area now occupied by the
POTENTIAL MINERAL RESOURCES 47

Monazite, ilmenite, rutile, and zircon sand 

Phosphate

FIODRB 6. The Western Hemisphere showing location of continental shelf and known 

mineral deposits exclusive of petroleum near shore. Continental shelves are shown in 

black.
48 CONTINENTAL SHELVES OF THE AMEBICAS

FIGURE 7. Salt domes and oil seeps In the Gulf of Mexico. Continental shelves are shown

by diagonal pattern.

Gulf of Mexico was probably a continuous salt-depositing basin enclosed on the north, south, and west, and connected with the ocean on 

the east by a relatively narrow, shallow strait.

In the gulf coastal area, exploration spread rapidly following the 

passage by the United States Congress of the Submerged Lands Act 

in 1953. In that year, 73 wells were completed in the Louisiana tidelands area. The successful completion of a number of these wells as 

producers of oil and gas encouraged interested oil companies to extend known production and to test other prospective areas on the continental shelf. As of November 1, 1955, about 840 wells had been 

drilled off the shore of Louisiana, and 65 off Texas. More than 70 percent of the wells drilled in 1955 in these areas found oil or gas in commercial quantities. Offshore fields which are connected to pipelines
POTENTIAL MINERAL RESOURCES 49

produced about 90,000 barrels daily in 1955, but many fields are shutin off the Louisiana coast awaiting the installation of pipelines, and, 

for the same reason, only a few fields in the Texas tidelands area are 

producing.

Gradually, as exploration continues, prospective drilling sites will 

be located in deeper water farther from the coast. In January 1956 a 

well drilled 50 miles (80 km) offshore in 101 feet (30.8 m) of water 

held the record for depth of water and distance from shore. Platforms are now being designed for possible use in water 250 feet (76 m) 

deep. Engineers believe that platforms capable of standing in 600 

feet (183 m) of water will be possible in a few years.

In the area between the oil fields on the continental shelf of east 

Texas and Veracruz, Mexico, about 750 miles (1,200 km) to the 

southward, the sedimentary rocks of the continental shelf may be as 

much as 20,000 feet (6,100 m) thick. Much of the geology of these 

rocks is imperfectly known, but the probability of their containing 

reservoirs of oil or gas is considered fair.

At some places the continental shelf of Mexico is narrow, as along 

the coast north and southeast of Veracruz; few suitable sites for 

locating offshore drilling rigs are available.

The continental shelf of the Gulf of Mexico broadens northward 

and eastward from the Isthmus of Tehuantepec to the Yucatan Peninsula. Oil and some gas have been obtained from the narrow coastalplain regions of northeast Mexico, the Tampico Embayment, and the 

Isthmus of Tehuantepec. The oil fields in the isthmus region occur 

on structural domes formed over deep salt plugs, similar to the oil 

fields that are found over salt plugs off the coast of Texas and Louisiana. The salt plugs are thought to have been extruded from the 

same salt beds that underlie the Gulf of Mexico.

Although no oil is produced from the Yucatan Peninsula at the 

present time, many geologists believe that it is a geologically favorable 

area for oil. The broad shallow shelf that lies along the northwest 

coast of the peninsula is believed to be underlain by a thick section 

of sedimentary rocks. These rocks may contain pools of oil trapped 

on structures in domes over salt plugs or in other types of structural 

or sedimentary traps.

SULFUR

Sulfur ranks second to oil and gas as a potential mineral of the shelf 

area in the western part of the Gulf of Mexico. The sulfur is found 

in the cap rock of some salt domes. Much sulfur has been produced 

from the cap rock of the Lake Washington dome, Plaquemines Parish, 

La., on the Mississippi delta. The surface plant at Lake Wlashington 

is constructed on piles and concrete mats covered with hydraulic fill,
50

and the sulfur is barged from the site through channels dredged 

through the surrounding salt marshes. In 1954, announcement was 

made of the discovery of sulfur about 6 miles (9.6 km) off the south 

coast of Louisiana near Grand Isle. This is apparently the first discovery of sulfur ever made offshore. Test wells are being drilled 

to estimate the extent of the deposit. Of the 230 domes discovered 

by early 1956 in Texas, Louisiana, and Mississippi, 18 contained sulfur 

in sufficient quantity and purity to warrant recovery. Additional 

deposits of sulfur will undoubtedly be discovered both onshore and 

offshore. Despite the many mechanical problems involved in the drilling for and the recovery of sulfur from offshore deposits, the development of these shelf deposits is expected in the foreseeable future.

GULF OF MEXICO EASTERN SHELF AREA

OIL AND GAS

The continental shelf between the Mississippi delta and the Florida 

coast is an area of change in subsurface geology from the area to the 

west. As presently evaluated, the rocks of the eastern gulf shelf do 

not appear to be as favorable for the occurrence of oil and gas as the 

rocks to the west.

The two oil fields discovered in southern Florida produce oil from 

beds at approximately depths of 11,400 to 12,000 feet (3,480 to 3,660 m) 

within a rock sequence that consists largely of limestone, dolomite, 

and anhydrite. These rocks are presumed to extend under the continental shelf off the west coast of Florida and are probably present 

under parts of the Gulf of Mexico and the Caribbean Sea. A forecast 

of the future of exploration in northern and southern Florida, its offshore areas, and southern Alabama should be cautiously optimistic.

A well located offshore in Florida Bay about 7 miles (11.2 km) 

south of East Cape, was completed as a dry hole in November 1955 at 

a depth of 12,631 feet (3,850 m). Small oil shows and some salt water 

in this well supplied encouraging verification of the presence of 

hydrocarbons in porous rocks in the Florida shelf area. Structural 

and stratigraphic features that could hold great amounts of oil and 

gas are reported throughout the coastal areas of western Florida and 

southern Alabama, but the depth to which exploratory wells would 

need to be drilled is a deterrent at the present time. If new pools of 

oil are found along the shore between Alabama and southern Florida 

it will be reasonable to assume that local areas in which the stratigraphic and structural relations are favorable for oil and gas accumulation may exist within this large shelf area. Systematic regional 

and geophysical surveys might provide evidence of structural features 

suitable for oil accumulation.
POTENTIAL MINERAL RESOURCES 51 

PHOSPHATE

The land pebble-phosphate beds of north-central Florida have become one of its most exploited mineral resources. Many tons are 

processed and shipped each year as mineral fertilizer. Because 

phosphate is a valuable resource, other areas in Florida and on the 

gulf shelf have been prospected for possible deposits. The Florida 

phosphates contain a small amount of uranium but they are not 

processed for this element at the present time.

In the eastern Gulf of Mexico, geologic investigations have shown 

that the phosphate-bearing rocks were confined to the inner 20 miles 

(32 km) of the continental shelf. The phosphate content was not 

high, however; only a few areas on the shelf contained as much as 

13.5 percent phosphorous pentoxide (P2O5 ). The pebble phosphate 

now being mined in the interior of Florida has a phosphorous pentoxide content of between 30 and 35 percent.

EASTERN NORTH, AMERICA FROM SOUTHERN FLORIDA 

TO NEWFOUNDLAND

OIL AND GAS

The shelf along the Atlantic coast northward from the Florida 

Keys is practically unknown geologically as compared to the continental shelf of the Gulf of Mexico. Much of the shelf area borders 

coasts where the sedimentary rocks are thought to be relatively thin 

and are not generally considered to be promising for the production 

of oil or gas at the present time.

Long stretches of the coastal plain and continental shelf between 

Florida and New Jersey have not been explored by test wells; the 

few scattered wells drilled to date in this coastal area have failed to 

find deposits of petroleum or natural gas. Geophysical investigations, however, have indicated the presence of sediments 12,000 feet 

(3,660 m) thick at places only 60 miles (96 km) seaward from the 

present coastline, but the mineral potential cannot be guessed at without further knowledge of the rock type and associated structures.

From New Jersey to Maine, also, the continental shelf is not considered geologically favorable for accumulations of oil and gas. The 

sedimentary sequence of rocks in many places is thin, and some of the 

older rocks are metamorphosed.

Farther north small amounts of oil and gas have been produced 

from wells drilled inland from the coastal areas of Newfoundland, 

Nova Scotia, and New Brunswick. No oil wells have been drilled 

along the shores of this region, so far as is known to the writer, and 

the possibility of finding oil in commercial quantities in any of the
52 CONTINENTAL SHELVES OF THE AMERICAS

shelf areas between New Jersey and Newfoundland is not considered 

to be favorable.

COAL

Cape Breton Island, Nova Scotia, contains two general areas which 

have produced coal from beneath the sea. These are the Sydney coal 

field in the eastern part and the four small coal fields of Port Hood, 

Mabou, Inverness, and St. Kose-Chimney Corner in the western 

part.

The Sydney coal field yields about 80 percent of the total output 

of coal in Nova Scotia. The coal is found in 12 beds, ranging in 

thickness from 3 to 7 feet (0.9 to 2.1 m), and is mostly high volatile 

bituminous in rank. On the basis of developed reserves, about 80 

years of mining operations remain, although on the basis of other 

estimated reserves perhaps as much as 180 years of mining can be 

expected. The total estimated reserves for the Sydney field are more 

than 2 billion tons (MacKay, 1947, p. 69).

The coal fields of western Cape Breton Island are located in four 

small structural basins along the coast. The coal in these basins is 

of lower rank and grade than that of the Sydney coal field. According to the Canadian Geological Survey the coal seams on the west 

coast are restricted in immediate economic importance by their quality, steep dip, and almost entirely submarine position. Mining in 

these fields will be confined largely to supplying the domestic market 

until such time as the Sydney fields are depleted.

IKON ORE

The Wabana iron ore of Newfoundland is the one metal deposit 

mined at the present time from an area in the inner edge of the 

American continental shelf under the Atlantic Ocean. The iron ore 

is an oolitic hematite occurring in sedimentary rocks that crop out 

on Bell Island in Conception Bay and dip 8° to 10° NNE. under the 

bay. The iron reserves of this area under the shelf are truly enormous, but only the thicker beds are mined under the bay at the present time. It has been stated that 3y2 billion tons of economic ore 

are available within a 5-mile (8-km) radius of Bell Island; other 

billions of tons are present but are not readily available. One ore 

deposit where the hematite is 30 feet (9.15 m) thick is estimated to 

contain about 90 million tons to the square mile (35 million tons per 

sqkm).

MONAZITE AND ILMENITE

The sea has not always remained at its present level along the 

coasts. Throughout the Western Hemisphere, terraces now lying 

above sea level show that the sea once stood at much higher levels.
POTENTIAL MINERAL RESOURCES 53

These higher terraces and their attendant beaches and bars, similar 

to those of today, are composed of sand and gravel that were eroded 

from the land and transported by streams to the sea. Certain minerals, because they were heavier than others comprising this sand 

and gravel, were concentrated along the shore at places where currents were no longer strong enough to move them. These "heavy" 

minerals became concentrated in ancient and recent beaches in sufficient quantity to be of economic importance to industry. Among 

the minerals mined from these deposits are monazite, ilmenite, rutile, 

and zircon in Florida, monazite in Brazil, gold in Alaska, and platinum in Oregon. Monazite contains the rare-earth elements thorium 

and cerium, ilmenite and rutile contain titanium (an element which 

is finding greatly increased usage), and zircon contains the rare element zirconium.

Ancient beach deposits in northeastern Florida which lie about 50 

feet (15 m) above sea level are being mined for their heavy minerals. 

These minerals constitute only 4 percent of the beach sand, and of 

this 4 percent, monazite is present in amounts of less than 0.5 percent, 

whereas ilmenite constitutes about 40 percent.

Not all the beach deposits that lie at or above sea level have been 

mapped geologically. The location and extent of those that lie beneath the sea can barely be guessed at in the present state of knowledge; for example, a gentle terracelike break in the seaward slope 

of the sea floor off the coast of New Jersey extends for about 190 

miles (300 km) along the east coast of the United States at a depth 

ranging from about 250 to 328 feet (76 to 100 m) and may represent 

an ancient beach. Similar beachlike features have been reported 

from Alaska.

Submarine deposits of heavy minerals are most likely to be concentrated offshore in areas where such minerals are present along the 

seashore. Offshore deposits may be concentrated in beaches built 

during a lower stage of the sea, or they may be concentrated in 

pockets on the sea floor by a winnowing action of stream currents 

flowing from the land or by strong ocean currents flowing along the 

coast. Prospecting for these minerals will be difficult, and recovering them after their discovery will be expensive. Their discovery 

and exploitation must await some economic necessity of the future.

CARIBBEAN SHELF AREA AND THE INSULAR SHELVES 

OF THE ANTILLES AND THE BAHAMAS OIL AND 

GAS

The Greater Antilles, bordering the Caribbean Sea on the north, 

include the islands of Cuba, Jamaica, Hispaniola, and Puerto Kico. 

This island chain is in part a gigantic structural arch that exposes
54

a thick series of sedimentary rocks representing a long span of geologic history. The islands and their associated shelf areas have potential reserves of mineral fuels and other minerals.

Oil has been produced from metamorphosed rock in the Bacuranao 

field in northwest Cuba since 1915. Subsequent exploration was slow 

because a large number of test wells were dry, but after the discovery 

of the Jatibonico field in central Cuba in May 1'954 the southern shelf 

became of interest; the Cuban government has since granted oil concessions for many hundred square miles of the shelf area off the south 

shore. In January 1955 a second oil field, the Cruz Verde, was opened 

one-half mile (0.8 km) to the south of the Bacuranao field, but it was 

not until a new oil well was completed north of the Bacuranao field 

in December 1955 that interest was shown in the northern shelf. 

Companies holding concessions will reportedly spend an estimated 

$35 million in exploration work and in drilling by 1960. Long-range 

plans include offshore drilling and exploration on the Cuban shelf, 

although the extent of this exploration may depend on the success 

of drilling in the nearby coastal plain.

The discovery of new fields in Cuba will undoubtedly intensify the 

search for oil in other islands of the Antilles and, where practicable, 

on the shelf areas along their coasts. Although to date there has been 

no production of oil and gas on the island of Jamaica, the area is of 

interest because the first test well, completed in June 1955, contained 

shows of heavy oil and some gas. The well was completed at a depth 

of 6,300 feet (1,921 m) in green sandy shale, the rock section penetrated being predominantly limestone with numerous beds of sand 

and shale. Later an oil company purchased the concession to all the 

nonmountainous area on both the north and south coasts. The concession includes the right to prospect for petroleum, bauxite, bentonite, 

and other minerals not only on this land but also in the area of the 

Jamaican shelf and the Cayman Islands to the west of Jamaica. The 

oil company reportedly planned to spend at least $2 million in 

exploration by June 1956.

The regional geology of the island of Hispaniola is somewhat 

similar to that of Cuba and Puerto Rico. The sedimentary rocks 

that border the coastal plains consist mainly of limestone and clastic 

rocks that are highly compressed, folded, and faulted. To date the 

search for oil on Hispaniola has been only partly successful. In 

Haiti, 4 wells had been drilled by 1948; the deepest of these was 

abandoned at 8,769 feet (2,673 m). Shows of oil and gas were reported in some of the wells, but all were completed as dry holes, and 

the concessions were abandoned. Shows of gas were found in a well 

drilled in 1955 in the northern part of the He de la Gonave, off Haiti.
POTENTIAL MINERAL RESOURCES 55

In the Dominican Republic, 13 test wells were drilled between 1939 

and 1947. About 13,000 barrels of heavy oil was produced from the 

first of these. Future exploration for oil on the island of Hispaniola 

may be more successful, but the shelf area around the island is very 

narrow and few wells could be located offshore.

No direct evidence of oil or gas accumulation has been found on 

Puerto Kico, but recent investigations made by the U. S. Geological 

Survey have been followed by geophysical investigations in the north 

and south coastal areas. Kesults of the mapping show that along the 

north coast gently inclined sedimentary rocks of Tertiary age underlie a strip as much as 10 miles (16 km) wide, and that more sharply 

folded rocks of this age occur in a narrower strip along the central 

and western parts of the south coast. Furthermore, oil has been found 

in similar rocks in the Dominican Kepublic, and oil seeps are present in 

somewhat older but similar rocks in Cuba. The sea floor slopes relatively steeply along the north side of the island. A strip only a few 

thousand feet (about a km) wide lies at depths of less than 90 feet 

(27.5 m) along this coast, and the edge of the continental shelf, at a 

depth of approximately 600 feet (183 m), is at most only 2 miles (3.2 

km) offshore. Along the south coast an area having a maximum width 

of almost 10 miles (16 km) lies under depths of less than 90 feet 

(27.5m).

The Virgin Islands to the east have geologic features similar to those 

of Puerto Kico, but there is only a narrow belt of sedimentary rocks; 

these rocks are believed to have only slight possibilities for oil.

The Bahama Islands and the Bahama Banks lie east and south of 

the Florida Peninsula and were once probably a southern continuation 

of it. No reasonable assumption can be made as to whether or not the 

Bahamas contain deposits of oil or gas until sufficient test wells have 

been drilled to determine the presence of sedimentary rocks sufficiently 

porous to contain pools of oil or gas. The only subsurface data currently available are from a well completed in the northern part of 

Andros Island (the largest island in the Bahama group) in 1947. 

The well, drilled to a depth of 14,587 feet (4,446 m), was reported to 

contain no shows of oil or gas. The deepest formations reached were 

similar stratigraphically to those of southern Florida. The Bahamas 

were completely surveyed by airborne magnetometer during a joint 

project financed by the several companies that hold exploration licenses on the group. The results of this survey are said to indicate 

that structures are present which may be favorable for the accumulation of oil.

The Lesser Antilles bordering the Caribbean Sea to the east include 

many small islands, most of which have a volcanic-rock core but some
56 CONTINENTAL SHELVES OF THE AMERICAS

of which are capped by marine limestones. They hold little promise 

for production of petroleum except for Barbados, the easternmost 

island of the British West Indies. Structurally Barbados seems to 

have affinities with the Bahamas, for it is part of the outer zone of 

low banks and islands to which the Bahamas belong. Its sedimentary 

rocks, however, seem in part related to those of Trinidad, to the southwest. The core of the island of Barbados is a thick series of sedimentary rock consisting of coarse grit, brownish sand, and dark sandy 

clay which are in places saturated with petroleum and contain lenticular beds of asphalt. No volcanic rocks are present in this sequence, 

and the early geologic history of the island, both stratigraphically and 

structurally, differs, therefore, from that of the volcanic islands of the 

group to the west. Several test wells have been drilled on the island 

of Barbados. Some contained shows of oil or gas but only one, a gas 

well, had commercial production.

The shores of the island of Barbados slope steeply into the ocean, 

and any exploration under the sea floor will likely be by directional 

drilling from the shore.

The northwest part of the coast of Venezuela consists of sedimentary rocks. These and similiar rocks of the contiguous shelf areas 

may prove to be productive of oil or gas. Parts of the northern and 

eastern Venezuelan coasts and islands off the coast contain areas of 

metamorphic rocks; at a few places basic igneous rocks are also present. The belt of metamorphic rocks continues eastward through the 

northern part of the island of Trinidad and the island of Tobago. 

Southern Trinidad contains unmetamorphosed sedimentary rocks, 

however, from which oil has been produced. Some of the oil fields 

extend down to the shore, and in expectation of possible future development, nearly a million acres of the shallow shelf area is under 

lease. In 1952 a directional well which produced oil was completed at 

La Brea on the Gulf of Paria, and since that time several successful 

oil wells have been drilled from platforms in the Gulf of Paria.

Along the west perimeter of the Caribbean Sea active prospecting 

for petroleum is taking place in a number of Central American countries. In British Honduras seismic and geologic explorations have 

been carried on, and data obtained from two nonproductive drill tests 

made in the northern interior are being studied as a guide to possible 

further exploration. Concession blocks include coastal and offshore 

areas. In Guatemala the coastal area and the Departamento del Peten 

in the interior are thought to be favorable for prospecting for oil, and 

geologic exploration is being carried on in these areas. Some concessions have been granted for offshore exploration.
POTENTIAL MINERAL RESOURCES 57

An important upwarp of the Antillean region passes westward 

through southern Hispaniola and Jamaica and across the Caribbean 

Sea into Honduras and Nicaragua. Where this structural feature 

enters Central America the upwarped rocks lie near the surface of the 

Caribbean, thus forming a broad platform on the continental shelf. 

Large concessions have been taken along the coasts of Honduras and 

Nicaragua, both onshore and offshore, and sites for offshore drilling 

have been selected. Other areas of islandlike platforms which lie 

along the arch between Nicaragua and Jamaica under less that 150 

feet (46 m) of water may also be prospected.

Several test wells drilled inland from the Caribbean coast of Costa 

Rica have found oil but did not make commercial production because 

of excessive amounts of salt water. Additional tests are planned in 

several areas, some of which are near the Caribbean coast.

Exploration in Panama has been renewed since oil was found in 

Costa Rican test wells. Earlier drilling programs in Panama dating 

back as far as 1928 did not produce oil in commercial quantity. Recent concessions cover the entire area, and extensive preliminary 

geological surveys and some drilling operations have been undertaken.

EASTERN SOUTH AMERICA FROM BRITISH GUIANA TO

CAPE HORN

OIL AND GAS

The continental shelf from British Guiana to easternmost Brazil, 

where the coast swings southward, is relatively broad. No oil or gas 

has been produced to date, however, along the coast or on the shelf in 

this area. One company has a concession in the northeastern coastal 

region of British Guiana that comprises 3,275 square miles (8,482 

sq km) and includes offshore lands out to the 3-mile (4.8-km) territorial limit. In the Marajo basin of Brazil near the mouth of the 

Amazon River exploration of the past several years has included 

geophysical surveys and the drilling of several test wells. One well, 

drilled through more than 13,000 feet (3,960 m) of sediments, had at 

least one show of oil. The basin undoubtedly extends into the continental shelf. Exploration has recently been active in the inland embayment and coastal areas of Sao Luis in Estado do Maranhao.

The shelf is narrow from easternmost Brazil southward to the town 

of Canavieiras, in the southern part of Estado de Bahia, but broadens 

markedly from there to southern Argentina. The broad shelf passes 

around the Falkland Islands and continues to Tierra del Fuego.

Two basin areas along the coast in easternmost Brazil contain thick 

accumulations of sedimentary rocks. These are the Sergipe-Alagoas 

basin to the north, in which test wells yielded shows of oil and gas,

451437 58 5
58 CONTINENTAL SHELVES OF THE AMERICAS

and the Bahia basin to the south, which is now producing oil. Both 

basins apparently project into the narrow continental shelf, and 

seismic mapping is planned for the shelf areas.

In Argentina the Comodoro Kivadavia oil field, discovered in 1907, 

lies on the Golfo San Jorge. Exploratory drilling has been difficult 

in this field because faulting has produced numerous structural complications, the formations are irregularly bedded, and the sand bodies 

pinch out. Even though the Golfo San Jorge is within the continental 

shelf, the water over much of it appears to be too deep to encourage 

prospecting for oil and gas at the present time.

The basin that lies in the southern part of Chile within the limits 

of Tierra del Fuego has long been known to contain seepages of gas 

and oil. Test wells drilled in the late 1920's near Punta Arenas failed 

to discover commercial quantities of oil. A test well drilled in 1945 

northeast of Punta Arenas on the Isla Grande of Tierra del Fuego, 

6 miles (9.6 km) south of the north end of the Straits of Magellan, 

was completed as an oil well producing from beds at depths of 7,419 

to 7,438 feet (2,261 to 2,267 m). Producing wells completed later in 

this vicinity and in the Argentine sector of Tierra del Fuego are not 

on the seacoast, but these oil discoveries do extend the possible areas 

of exploration in South America and may encourage exploration of 

nearby shelf areas where the water is shallow (fig. 8).

MONAZITE

The beaches of Brazil have been an important source of monazite, 

especially along the coasts of Estado do Espirito Santo and Estado 

de Bahia. In former years, the monazite sand was skimmed from the 

surface of the beaches after each high tide and was then sacked and 

shipped without further separation of monazite from other minerals 

in the sand. Later, as this source was depleted, the higher beaches 

were prospected, and placer deposits along watercourses leading to 

the sea were mined. Submarine deposits of heavy minerals may be 

concentrated off the shores of Brazil.

PACIFIC COAST OF SOUTH AMERICA AND 

CENTRAL AMERICA

OIL AND GAS

On the west coasts of South America and of Central America, shelf 

areas beneath less than 50 feet (15 m) of water are narrow. Some 

interest has been shown in offshore Peru and Ecuador by companies 

which have leased large areas on the shelf. Geophysical surveys offshore from Peru indicate that suitable structures may be present. 

Some successful wells have been developed there by directional drill-
POTENTIAL MINERAL RESOURCES 59

EXPLANATION N

Oil field

Gas field 

4 0 8 Miles

40 8 Kilometers

FIGURE 8. Oil and gas fields near shore In Tierra del Fuego, Chile.

ing from the shore. If depth of water permits, drilling on the shelf 

may be undertaken along the coastal areas of Peru and Ecuador. 

Areas where petroleum has been produced along the coast and tidelands in Peru and Ecuador are shown in figure 9.

COAX.

Coal was mined from under the sea on the Golfo de Arauco in the 

Provincia de Arauco and Provincia de Concepcion in central Chile 

soon after 1852. The mine openings were located on land near the 

beach, and slope entries were driven southwestward under the Pacific 

Ocean. The original workings of one mine were flooded by the sea 

and have long since been abandoned. Coal is being produced in other 

mines from below the sea in the gulf area. The coal, which occurs in 

3 beds to depths of about 1,300 feet (400 m) below sea level, is highvolatile bituminous in rank, bright in appearance, and fragile in 

structure. A system of complex faults in the area has greatly in-
60 CONTINENTAL SHELVES OF THE AMERICAS

FIGURE 9. Oil fields near shore In Peru and Ecuador.

fluenced the location of mines on land and has raised many difficulties 

in mining beneath the sea.

Coal production is reported along the fiordlike channels of southern 

Chile in the Provincia de Magallanes, about 90 miles (144 m) north 

of Punta Arenas. Information is not specific as to whether any of 

the coal beds extend under these narrow inlets or to the continental 

shelf.

WESTERN NORTH AMERICA FROM MEXICO TO ALASKA

OIL AND GAS

Geologists for Pemex, the Mexican government oil monopoly, have 

been exploring the peninsula of Lower California since 1942 and have 

located two sedimentary basins in the central and southern part of 

the peninsula which they believe may contain oil or gas. These basins, 

known as the Sebastian Vizcaino and the Purisima-Iray, have been 

mapped in detail during the past 5 years. Test wells have been 

drilled in both basins, but those drilled in the Purisiina-Iray basin 

were the only one with shows of oil and gas. The area has been 

insufficiently tested, however, and a continued program of well drilling is planned by Pemex.
POTENTIAL MINERAL RESOURCES 61

Bathy metric charts of the area show that a harrow shelf is present 

along the shoreline of these basins. If oil deposits are found along 

the shore of this shelf, their extension by drilling offshore might be 

justified.

The topography of the seafloor along the Pacific coast of the United 

States is in marked contrast to the topography of the submerged 

areas off Texas and Louisiana. The submarine topography along the 

California coast is very irregular and is similar in many ways to the 

topography of the adjoining mountain belts on land. The strip of 

ocean floor between the 100-fathom (600-ft, 183-m) line and the 

shore along the California coast is very narrow, generally less than 

10 miles (16 km), and at places is less than 1 mile (1.6 km) wide.

Oil fields along the California coast are localized in deep sedimentary basins containing sharply folded rocks. Great quantities of oil 

have been found in the relatively small Los Angeles, Ventura, and 

Santa Maria basins, but elsewhere in the California coastal region 

only a few small pools have been discovered (fig. 10).

FIGURE 10. Oil fields near shore in the Los Angeles basin, California. 

451437 68 6
62 CONTINENTAL SHELVES OF THE AMERICAS

The productive basins are at an angle to the general trend of the coast 

and extend westward under the sea. These submerged areas offer 

excellent prospects for the discovery of additional reserves of 

petroleum.

Divers have mapped the extension of known anticlines under sea off 

the coast of California. In many of the areas mapped, high-pressure 

jets were used to wash away sand deposits that obscured the strata; 

however, this was practicable only if the deposits were less than 25 feet 

(7.6 m) thick. Exploration with the reflection seismograph has met 

with considerable success in California, and its use offshore will probably be of great value as exploration is undertaken for new possibly 

oilbearing structures that are entirely beneath the water. Shipmounted drilling equipment capable of recovering cores from rocks 

lying below 1,500 feet (457.2 m) of water has recently been perfected.

The coastal belts of Washington, Oregon, and central and northern 

California have been prospected at geologically favorable points, but 

wells drilled in these areas have not found large accumulations of oil 

or gas. Available information does not suggest that offshore exploration along this part of the Pacific coast and northward to Alaska 

would offer promise of greater success than has been achieved on land. 

Production of some oil from two wells that were drilled below high 

tide level along the coast of Washington indicates that the general area 

is not unsuitable for further prospecting.

COAL

Coal-bearing rocks of Cretaceous age are present along the eastern 

side of Vancouver Island, British Columbia, in the Nanaimo district. 

They crop out along the coast for a distance of 40 miles (64 km) and 

dip generally seaward. Two coal beds had been mined for a distance 

of 2 miles (3.2 km) under the harbor at Nanaimo by 1926, and at that 

time about 20 percent of the coal produced on Vancouver Island was 

by subsea mining.

'.. PLATINUM AND CHBOMITE

Deposits of "black sand," which consist mostly of magnetite and 

small amounts of gold, platinum, chromite, and traces of monazite, are 

present in the beaches of Oregon and in streams that flow into, the 

Pacific Ocean. Platinum, which usually contains iridium, and chromite, containing the element chromium, have not been found in sufficient quantities to justify dredging for them alone. A few ounces of 

platinum is recovered each year, usually as a byproduct after 

separation from placer gold.
POTENTIAL MINERAL RESOURCES 63 

ALASKA

OIL AND GAS

The continental shelf off the south, west, and north coasts of Alaska 

includes a Pacific Ocean area of approximately 140,000 square miles 

(360,000 sq km), a Bering Sea area of approximately 320,000 square 

miles (830,000 sq km), and an Arctic Ocean area which is less definitely delineated. As parts of each shelf area are contiguous to known 

petroliferous areas and the geologic conditions in the shelves offshore 

from these areas are presumably similar to the contiguous land areas, 

the shelf areas may also have potentialities as sources of petroleum. 

Evaluation of the shelf area must necessarily be based mainly on 

comparison with the adj acent land area.

Considerable exploration has been carried on recently in coastal 

areas around the Gulf of Alaska and the Alaskan peninsula. Offshore 

exploration in some parts of this shelf might be difficult, for although 

some places are relatively sheltered, much of the Pacific Ocean shelf 

area of Alaska is exposed to the notoriously violent storms of the 

northern Pacific.

Direct information on the nature of the rocks underlying the shelf 

in the Bering Sea area is provided only by a few islands. Any attempt 

to recover petroleum from most of this shelf area or from shelf areas 

of the Arctic Ocean would be subject to special hazards and difficulties 

arising from the continual or periodic presence of ice.

GOLD

Placer gold, discovered at Nome, Alaska, in 1899, is found at 3 levels 

above water and 1 below. The highest level, an ancient beach now 

70 feet (21.3 m) above the sea, was the richest. The lowest level, 

known as the submarine beach, lies 19 feet (5.8 m) below the present 

level of the sea, and is thought to be the oldest.

CONCLUSIONS

The continental shelf contains a vast store of minerals, some of 

which are sufficiently concentrated to form deposits of great potential 

value. For example, oil is present over salt domes in the Gulf of 

Mexico in known reserves of many millions of barrels. Sulfur, which 

is,in ever-increasing demand by industry, is present also in some shelf 

salt domes and will soon be produced through offshore installations. 

Other minerals, such as iron ore in Newfoundland, and coal in Nova 

Scotia and in Chile, underlie the continental shelf in deposits whose 

reserves are known to be in millions of tons. These materials are 

becoming increasingly important because as the population of the
64 CONTINENTAL SHELVES OF THE AMERICAS

world increases a greater mineral supply will be required to maintain 

a high standard of living. Thus all countries will be forced to intensify 

the search for new deposits within their borders, and as some of these 

deposits become depleted, the mineral potential of the continental shelf 

will become increasingly important as a means of maintaining reserves 

adequate for the ever-expanding economy.

SELECTED REFERENCES

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symposium on the Gulf Coast Cenozoic: Am. Assoc. Petroleum Geologists,

1070 p. 

Bucher, W. H., 1947, Problems of earth deformation illustrated by the Caribbean

Sea basin: N. Y. Acad. Sci. Trans., sec. 2, v. 9, no. 3, p. 98-116. 

Bush, James, 1951, Rock from Straits of Florida: Am. Assoc. Petroleum Geologists Bull., v. 35, no. 1, p. 102-107. 

Cogen, W. M., 1940, Heavy mineral zones of Louisiana and Texas Gulf Coast

sediments: Am. Assoc. Petroleum Geologists Bull., v. 24, no. 12, p. 2069-2101. 

Dale, N. C., 1915, The Cambriaa manganese deposits of Conception and Trinity

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456. 

Dietz, R. S., 1955, Manganese deposits on the northeast Pacific floor: Calif. Jour.

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Emery, K. O., 1952, Continental shelf sediments of southern California: Geol.

Soc. America Bull., v. 63, no. 11, p. 1105-1108. 

Emery, K. 0., and Dietz, R. S., 1950, Submarine phosphorite deposits of California

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Fenneman, N. M., 1938, Physiography of eastern United States: New York,

McGraw-Hill Book Co., Inc., 714 p. 

Foss, M. E., 1932, Faults in Comodoro Rivadavia oil fields, Argetina: Am. Assoc.

Petroleum Geologists Bull., v. 16, no. 6, p. 556-576. 

Gray, F. W., 1927, Mining coal under the sea in Nova Scotia, with notes on

comparable undersea coal-mining operations elsewhere: Second (Triennial)

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Hayes, A. O., 1915, Wabana iron ore of Newfoundland: Canada Dept. Mines Mem.

78, no. 66, Geol. Ser., 163 p. 

Kuenen, Ph. H., 1950, Marine Geology: New York, John Wiley and Sons, Inc.,

568 p. 

Lees, G. M., 1951, Nature of continental shelves: Am. Assoc. Petroleum Geologists

Bull., v. 35, no. 1, p. 108-110. 

MacKay, B. R., 1947, Coal reserves of Canada: Ottawa, Edmond Cloutier,

C. M. G., Ba., L. Ph., King's Printer and Controller of Stationery, 113 p.

[Reprint of chap. 1 and app. A of Rpt. of the Royal Comm. on Coal, 1946.] 

MacNeil, F. S., 1950, Pleistocene shorelines in Florida and Georgia: U. S. Geol.

Survey Prof. Paper 221-F, p. 95-107. 

Mason, J, F., 1952, Petroleum developments in South America and Caribbean

area in 1951: Am. Assoc. Petroleum Geologists Bull., v. 36, no. 7, p. 1330-1358. 

Meyerhoff, H. A., 1954, Antillean tectonics: N. Y. Acad. Sci. Trans., ser. 2, v. 16,

no 3, p. 149-155.
POTENTIAL MINERAL RESOURCES 65

Mouton, M. W., 1952, The continental shelf: The Hague, Martinus Nijhoff, 367 p. 

Oil Weekly, 1946-50, World oil atlas issue[s]: Houston, Tex., The Gulf Publishing Co. 

Oil and Gas Journal, 1954, 1955, Worldwide oil report issue[s]: Tulsa, Okla.,

The Petroleum Publishing Co. 

Palmer, R. H., 1948, Outline of the geology of Cuba: Jour. Geology, v. 53, no. 1,

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Patton, J. L., 1954, Southeastern states hold promise for tomorrow: Oil and Gas

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Pratt, W. E., 1947, Petroleum on continental shelves: Ajn. Assoc. Petroleum

Geologists Bull., v. 31, no. 4, p. 657-692. 

Price, W. A., 1951, Building of Gulf of Mexico, secondary events in a regionally

concordant basin: Gulf Coast Assoc. Geol. Soc., First annual meeting, New

Orleans, Nov. 1951, p. 7-39.

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p. 39. 

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Schuchert, Charles, 1935, Historical geology of the Antillean-Caribbean region of

the lands bordering the Gulf of Mexico and the Caribbean Sea: New York,

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Map 85.

PROBLEMS OF PETROLEUM DEVELOPMENT ON THE 

CONTINENTAL SHELF OF THE GULF OF MEXICO

By EDWIN M. THOMASSON, U. S. Geological Survey

CONTENTS

Page

Abstract__ -____----------__-________-__ _--__-_--_-----_------ 67

Introduction...---.-----------_._._____-__.-_------_-----. 68

Historical background_ :________________._-___-._---_--_---_--- 69

Exploration.._ _____--_-_---____-___-____-______--__-__-----__-_--- 74

Reconnaissance surveying...______._._________._______-__--.___--- 74

Detailed surveying_______________________________________________ 75

Leasing procedures._-_--_-____________----_______--_-___--__----- 76

Development problems..___________________________________________ 77

Operation and production problems..--____________-___ _----___-_-_ 82

Transportation._--____-_-___-____-.___-_____-_._.- --_--___---_-- 83

Economics of continental-shelf oil development_.____ ___--___-_-_-- 85

Magnitude of offshore petroleum industry in the United States..._-__--_-_ 87

The future of continental-shelf mineral operations________-__--_----- 89

References cited...____-___-________-____-_______-__-_-_-_--------- 90

ILLUSTRATIONS

Page 

PLATE 3. Self-contained drilling platform.-.-._________.__.__-.__. Facing 78

4. Small drilling platform and mobile drilling tender _-_--.__. Facing 79

TABLES

Page

TABLE 1. Wells completed on the continental shelf of the Gulf of Mexico.. 87 

2. Estimated production of liquid hydrocarbons from the continental shelf of the Gulf of Mexico------..-----------.------- 88

ABSTRACT

The continental shelf is an extension of the continental land mass which at 

present is covered by the ocean. The geologic characteristics of the shelf are 

similar to those of the adjoining land area, and mineral accumulation on the 

land extends into the continental shelf.

67
68 CONTINENTAL SHELVES OF THE AMERICAS

In economic value, oil and gas are the most important minerals produced 

from the continental shelf. Production of oil and gas from the shelf areas 

of the Western Hemisphere is presently confined to the Gulf of Mexico and to 

the Pacific Ocean off the California coast. The development off California has 

been limited to extension of fields discovered on shore, and almost all wells 

are drilled from shore sites. In the Gulf of Mexico oil and gas development 

has proceeded at a rapid rate, and wells are now located as much as 50 miles 

(80 km) from shore in water almost 120 feet (36.57 m, 20 fathoms) deep.

The problems involved in developing oil and gas on the continental shelf of 

the Gulf of Mexico are both technologic and economic. In addition to normal 

oil-field problems, the offshore operator must consider hydrographic, oceanographic, and meteorologic factors. The design of drilling structures is closely 

related to these factors.

The cost of continental-shelf petroleum operations ranges from 2 to 10 times 

that of equivalent operations on land. These high costs cause most of the 

problems in offshore operations, as the products recovered from the shelf must 

compete with products recovered from oil and gas fields on land. Much research 

is being done in the petroleum industry to devise methods of reducing these costs.

INTRODUCTION

The continental shelf has been defined as that part of the continental 

land mass which is shallowly submerged by the ocean. The continental 

shelf begins at the line of mean low tide wherever land areas meet the 

open ocean, or at the mouths of rivers, bays, and other inland waters 

wherever they meet the open ocean. It extends seaward to the point 

where the slope of the continental mass changes quite abruptly and 

descends more steeply into the oceanic depths. The continental shelf 

is an integral part of the continent, distinguished only by being covered 

with a shallow mantle of water.

Throughout the Western Hemisphere the continental shelves vary 

greatly in their extent. The width of the continental shelf off the 

United States varies from less than 10 miles (16 km) along the coast of 

the Pacific Ocean to approximately 250 miles (402 km) along the 

northern part of the Atlantic Ocean. The average width of the shelf 

off the Atlantic coast of the United States is approximately 73 nautical 

miles (135 km), the average width in the Gulf of Mexico is approximately 59 nautical miles (109 km), and the average width off the 

Pacific coast of the United States is about 18 nautical miles (33 km) 

(U. S. Dept. Interior, 1953, p. 191). The continental shelves off the 

coast of Central and South America show similar variations in extent. 

The total area of the continental shelf adjoining the United States is 

approximately 290,000 square miles (751,100 sq km).

As the continental shelf is an extension of the continental land 

mass which is now covered by the ocean, it may reasonably be inferred that the geologic characteristics of the shelf will be quite 

similar to those of the adjoining land area. In general, where suf-
PROBLEMS OF PETROLEUM DEVELOPMENT 69

ficient data are available, this inference has proved true. For example, the structure of the sedimentary rocks and the occurrence of 

numerous salt domes in the shelf in the Gulf of Mexico are essentially the same as those of the contiguous coastal-plain area.

If the continental shelf is geologically similar to the adjoining 

land area, then it can reasonably be inferred that mineral accumulations on the land area will continue into the shelf. Again, where 

suitable data exist, this inference has largely proved true. Several 

oil fields in the United States along the Pacific coast and the Gulf 

of Mexico extend from the land area into the continental shelf. 

Other fields, lying wholly in the shelf, are geologically similar to 

fields on land.

HISTORICAL BACKGROUND

Although the continental shelves have been known for centuries, 

mineral development on the shelves is a very recent accomplishment. 

Oil was first produced from the continental shelf some time between 

1894 and 1899 in the Summerland field, Santa Barbara County, 

Calif. The field was discovered on land near the shore of the Pacific Ocean in 1894, and by 1899 several wells had been drilled from 

wooden wharves extending several hundred feet into the ocean.

Later production of oil from the continental shelf started in the 

Rincon oil field, Ventura County, Calif., in 1927; Capitan and Elwood fields, Santa Barbara County, in 1929; and Huntington Beach 

field, Orange County, in 1933. As in the Summerland field, these 

were all discovered on land and were extended a short distance onto 

the continental shelf. Drilling operations were conducted from shore 

locations through directional (slant) wells, or from piers or wharves 

extending from the land for short distances into the sea.

Oil operations on the continental shelf of the Gulf of Mexico began in 1936 in the High Island field, Galveston County, Tex. In 

1938, oil production was started in the Creole field, Cameron Parish, 

La., and in 1941-42 production was established in the Sabine Pass 

field, Jefferson County, Tex.; the Caplan field, Galveston County, 

Tex.; and the Rabbit Island field, Iberia Parish, La. In all these 

fields, the continental-shelf activity was an extension of discoveries 

made on shore, and drilling operations were conducted from shore 

by means of directional drilling, from piers or wharves built from 

the shore, or by submersible barges in protected waters, such as had 

been previously used in the marshes of southern Louisiana. The 

entry of the United States into World War II generally ended active exploration and development of the petroleum resources of the 

continental shelf for the period of the war.
70 CONTINENTAL SHELVES OF THE AMERICAS

The greatest impetus to exploration and development of the petroleum reserves in the continental shelf was the discovery of oil 

about 12 miles (19 km) off the coast of Louisiana in the open, unprotected waters of the Gulf of Mexico, in November 1947. It proved 

conclusively that oil accumulations exist under the continental shelf 

at a considerable distance from the shore, and it also demonstrated 

that technological methods had been devised which were suitable for 

the exploration, development, and production of oil and gas from 

the continental shelf.

A major problem in the orderly development of the mineral resources of the continental shelves of the United States has been the 

controversy between the United States and the coastal States, particularly Louisiana, Texas, and California, concerning jurisdiction 

over the shelves. Although it is beyond the scope of this paper to 

present a detailed analysis and account of this controversy, some 

concept of the dispute is necessary for an understanding of petroleum development on the continental shelf.

In October 1945, the Attorney General of the United States invoked the original jurisdiction of the Supreme Court of the United 

States by instituting a suit against the State of California. As of 

that time the State of California had issued oil and gas leases for 

approximately 10,700 acres of the continental shelf, from which approximately 950,000 barrels of oil per month was being produced. 

The following passages concerning the actions before and decisions 

of the Supreme Court are quoted from a report of the U. S. Department of Interior (1953, p. 192-195).

The complaint against California alleged that the United States "is the owner 

in fee simple of, or possessed of paramount rights in and powers over, the lands, 

minerals and other things of value underlying the Pacific Ocean, lying seaward 

of the ordinary low-water mark on the coast of California and outside of the 

inland waters of the State, extending seaward three nautical miles [5.56 km];" 

that California had unlawfully issued oil and gas leases on lands underlying such 

ocean area; and that the State's lessees had entered upon such lands and taken 

oil and gas from them. The Government sought a decree declaring the rights 

of the United States in the area as against California, and enjoining California, 

together with all persons claiming under the State, from continuing to trespass 

on the area in violation of the rights of the United States.

* * * * * * *

The Supreme Court rendered its decision in the case of United States v. California on June 23,1947 (332 U. S. 19). The Court found * * * that the Thirteen 

Original States did not, upon the attainment of independence, acquire any rights 

of ownership in the lands underlying the 3-mile [5.56-km] marginal belt of open 

sea contiguous to their shores, because at that time there was no settled international custom of understanding among nations relative to the control by a 

littoral nation of a marginal belt of ocean waters along its coast; and * * * that 

the acquisition of the 3-mile belt was subsequently accomplished by the National
PROBLEMS OF PETROLEUM DEVELOPMENT 71

Government, and "protection and control of it has been and is a function of 

national external sovereignty." Accordingly, the Court held * * * "that California is not the owner of the 3-mile marginal belt along its coast, and that the 

Federal Government rather than the State has paramount rights in and power 

over that belt, an incident to which is full dominion over the resources of the 

soil under that water area, including oil" * * *.

* ******

As of the date (June 23,1947) when the Supreme Court rendered its decision 

in the case of United States v. California, the State of Louisiana had issued oil 

and gas leases covering a total of approximately 1,419,700 acres of land in the 

continental shelf extending seaward from the coast line of that State. Actual 

development work under such leases had not been extensive, however, and oil 

production from the Continental Shelf off the Louisiana coast was averaging only 

about 30,000 barrels per month at that time.

The State of Louisiana continued, after the announcement by the Supreme 

Court of its decision in the California case, to issue oil and gas leases on submerged lands of the continental shelf off the Louisiana coast. By December 21, 

1948, the total acreage of the continental shelf covered by Louisiana-issued oil 

and gas leases amounted to approximately 2,043,990 acres. Many of the continental shelf areas covered by these State leases were situated seaward of the 

3-mile [5.56-km] limit and, hence, were within the portion of the continental 

shelf as to which the rights of the United States had been expressly proclaimed 

under the principles of international law by Proclamation No. 2667 of September 28,1945 (10 F. R. 12303). (A State statute enacted in June 1938 (Louisiana 

Acts, 1938, p. 169) had purported to extend the seaward boundary of Louisiana 

to a line in the Gulf of Mexico 27 nautical miles [50 km] from the shore.)

On December 21, 1948, the Attorney General instituted before the Supreme 

Court a suit against the State of Louisiana. The complaint in this case was 

similar to the complaint which had been filed against the State of California in 

1945, except that the complaint in the case against Louisiana stated that the 

area in dispute extended seaward for a distance of 27 marine miles [50 km] 

(rather than three nautical miles) from the Louisiana coast.

The Supreme Court decided the Louisiana case on June 5, 1950 (339 U. S. 699). 

The Court followed its previous decision in the California case, stating * * * 

that "If, as we held in California's case, the 3-mile belt is in the domain of the 

Nation rather than that of the separate States, it follows a fortiori that the ocean 

beyond that limit also is."

******* 

On the same day when the Attorney General filed the submerged lands suit 

against the State of Louisiana, he also filed with the Supreme Court a suit 

against the State of Texas. The complaint against Texas indicated that the 

area in dispute was the whole of the continental shelf extending seaward from 

the Texas coast line.

The Supreme Court announced its decision in the case of United States v. 

Texas on June 5,1950 (339 U. S. 707). It was necessary for the Court to devote 

special consideration to an argument vigorously urged by the State of Texas 

respecting the portion of the continental shelf situated within a distance of 

three marine leagues [16.7 km] (9 nautical miles or 10% land miles) from 

the Texas coast line. The State of Texas was not created by the United 

States out of Federal territory (as were the States of California and Louisiana). 

Instead, Texas had been an independent nation, and it was admitted into the
72 CONTINENTAL SHELVES OF THE AMERICAS

Union as a State through the process of annexation. During the period of 

Texas' independence, the Congress of the Republic of Texas, by the act of 

December 19, 1836 (1 Laws of Republic of Texas, p. 133), had fixed the seaward boundary of the Republic of Texas in the Gulf of Mexico at a distance of 

"three leagues from land." Texas contended before the Supreme Court that 

the Republic of Texas had maintained open, adverse, and exclusive possession 

of, and had exercised jurisdiction and control over, the submerged lands underlying that part of the Gulf of Mexico within the 3-league limit; and that, under 

the annexation legislation pursuant to which Texas entered the Union as a 

State, it was provided that Texas should retain "all the vacant and unappropriated lands lying within its limits" (5 Stat. 798), which, according to 

Texas, included the lands of the continental shelf within the 3-league limit.

With regard to the point mentioned in the preceding paragraph, the Supreme 

Court held * * * in a 4 to 3 decision that when Texas ceased to be an independent nation and entered the Union, she became, under the terms of the 

annexation legislation (9 Stat. 108), a sister State on an "equal footing" with 

the other States, none of which had acquired or been granted rights in the 

submerged lands of the Continental Shelf; and that as an incident of the transfer of national external sovereignty from the Republic of Texas to the United 

States, any claim that Texas may have had to the lands of the Continental 

Shelf inside the 3-league limit was relinquished to the United States, since 

property rights in this area are so subordinated to the rights of sovereignty 

as to follow sovereignty.

The Supreme Court in the Texas case, also upheld the rights of the United 

States in the Continental Shelf lands situated seaward of the 3-league limit. 

However, the Court apparently regarded this as being so plain that it did not 

require any particular discussion.

The Court's opinion in the Texas case made it clear that, as regards the submerged lands of the Continental Shelf off the coast of the United States, the 

Federal Government has both imperium (governmental powers of regulation 

and control) and dominium (ownership or proprietary rights).

Following the decisions and decrees of the Supreme Court in the 

Louisiana and Texas cases, practically all oil and gas operations on 

the continental shelf came to a halt except for production from wells 

already drilled and the drilling of a few wells approved by the 

Secretary of the Interior for conservation purposes. However in 

1953 legislation was enacted which clarified the legal status of the 

leases in the continental shelf. The first of these laws, known as 

the "Submerged Lands Act" (Public Law 31, 83d Cong., 1st Sess., 

67 Stat. 29), was approved May 22, 1953. This act provides:

SEC. 3 (a). It is hereby determined and declared to be in the public interest 

that (1) title to and ownership of the lands beneath navigable waters within the 

boundaries of the respective States, and the natural resources within such 

lands and waters, and (2) the right and power to manage, administer, lease, 

develop, and use the said lands and natural resources all in accordance with 

applicable State law be, and they are hereby, subject to the provisions hereof, 

recognized, confirmed, established, and vested in and assigned to the respective States or the persons who were on June 5, 1950, entitled thereto under 

the law of the respective States in which the land is located, and the respective grantees, lessees, or successors in interest thereof.
PROBLEMS OF PETROLEUM DEVELOPMENT 73

Seaward boundaries of the coastal States are defined in the act as 

follows:

SEO. 4. The seaward boundary of each original coastal State is hereby approved and confirmed as a line three geographical miles [5.56 km] distant 

from its coast line or, in the case of the Great Lakes, to the international 

boundary. Any State admitted subsequent to the formation of the Union which 

has not already done so may extend its seaward boundaries to a line three 

geographical miles distant from its coast line, or to the international boundaries 

of the United States in the Great Lakes or any other body of water traversed 

by such boundaries. Any claim heretofore or hereafter asserted either by 

constitutional provision, statute, or otherwise, indicating the intent of a State 

so to extend its boundaries is hereby approved and confirmed, without prejudice 

to its claim, if any it has, that its boundaries extend beyond that line. Nothing 

in this section is to be construed as questioning or in any manner prejudicing 

the existence of any State's seaward boundary beyond three geographical 

miles if it was so provided by its constitution or laws prior to or at the time 

such State became a member of the Union, or if it has been heretofore 

approved by Congress.

The act also retains certain powers in the United States:

SEC. 6 (a). The United States retains all its navigational servitude and 

rights in and powers of regulation and control of said lands and navigable 

waters for the constitutional purposes of commerce, navigation, national defense, 

and international affairs, all of which shall be paramount to, but shall not be 

deemed to include, proprietary rights of ownership, or the rights of management, administration, leasing, use, and development of the lands and natural 

resources which are specifically recognized, confirmed, established, and vested 

in and assigned to the respective States and others by section 3 of this Act.

The net effect of the Submerged Lands Act was to. grant the coastal 

States jurisdiction over the submerged lands of the continental shelf 

and ownership of all minerals therein within the so-called "territorial 

sea."

Following the enactment of the Submerged Lands Act, the Congress provided for the administration of the continental shelf beyond 

(that is, seaward of) the "territorial sea" by the enactment of the 

Outer Continental Shelf Lands Act (Public Law 212, 83d Congress, 

1st Session, 67 Stat. 462). This act, in its first major section, establishes the basic policy of the United States as follows:

SEC. 3 (a). It is hereby declared to be the policy of the United States that 

the subsoil and seabed of the outer Continental Shelf appertain to the United 

States and are subject to its jurisdiction, control, and power of disposition as 

provided in this Act.

(b) This Act shall be construed in such manner that the character as high 

seas of the waters above the outer Continental Shelf and the right to navigation and fishing therein shall not be affected.

The term "outer Continental Shelf" is defined as "* * * all submerged lands lying seaward and outside of the area of lands beneath 

navigable waters as defined in section 2 of the Submerged Lands Act
74

(Public Law 31, Eighty-third Congress, first session), and of which 

the subsoil and seabed appertain to the United States and are subject 

to its jurisdiction and control." (Sec. 2 (a).) The act also provides 

the framework for the issuance of mineral leases in the "outer Continental Shelf" and for the supervision and regulation of operations 

conducted pursuant to such leases.

These two acts did much to resolve the basic conflict concerning 

jurisdiction over the continental shelves contiguous to the United 

States and the ownership of the minerals therein and provided a substantial legal foundation on which private industry can develop 

and produce these mineral resources.

EXPLORATION

In the continental shelf adjoining the United States, exploration for 

oil and gas includes three major phases: prospecting by geologists and 

geophysicists, leasing of potential producing areas, and drilling to 

prove the existence of oil and gas. Prospecting is usually divided into 

reconnaissance surveying and detailed surveying.

RECONNAISSANCE SURVEYING

Reconnaissance surveys are usually conducted to locate those areas 

in which geologic structures favorable for the accumulation of petroleum may occur. If the structures are sufficiently promising, a 

more detailed survey is conducted. Reconnaissance surveys are 

generally either a measure of the magnetic characteristics or of the 

specific gravity of the earth's crust in the area of interest. Instruments 

known as magnetometers and gravity meters are used in making such 

surveys.

In the continental shelf of the Gulf of Mexico, most oil and gas 

accumulations now known are associated with geologic structures 

called salt domes. Throughout the gulf coast region of the United 

States, including the shelf, geologic evidence indicates that the sedimentary formations are underlain by a deep layer of salt. In some 

places, during various periods of geologic time, this deep-lying salt 

bulged, pushing the overlying sediments (including any potentially 

mineral-bearing strata) into arches or domes. The resulting structures are known as deep-seated domes. The arching that produced 

them was usually accompanied by faulting, which is a breaking of the 

strata into large blocks.

In other places, the deep-lying salt pushed up as irregular cylindrical bodies through overlying strata to form cores of salt surrounded 

by sedimentary deposits. The name "piercement dome" has been 

given to this type of geologic structure. In such structures salt may be
PROBLEMS OF PETROLEUM DEVELOPMENT 75

found at any depth, from the surface of the land to 10,000 feet below 

the surface. Oil and gas are sometimes found in the strata overlying 

the salt mass, but the more usual occurrence is in irregular areas at 

varying depths around the flanks of the salt core.

The sedimentary rocks of the gulf coast region, including the continental shelf, consist mostly of sand and clay. Although they have 

relatively low magnetism, salt has practically none. Thus, by use of 

the magnetometer, areas of abnormally low magnetism can be located, 

with sound justification for the belief that such areas represent a salt 

dome.

It was not until World War II that the magnetometer was successfully adapted for work in areas covered by water. By about 1945, the 

magnometer had been so perfected that it could be towed through the 

water behind a boat, and continuous readings could be taken along a 

fixed course. An even faster method of making a magnetometer 

survey consists of towing the instrument through the air behind an 

airplane flying at a fixed altitude, thus covering large areas very 

rapidly. These methods are ideally suited for reconnaissance surveys 

in the continental shelf as they are both rapid and comparatively 

inexpensive.

The gravity-meter survey measures the difference in the force of 

gravity over the earth's surface. As the specific gravity of salt is 

considerably less than that of the normal sedimentary rocks, it is 

comparatively easy to locate salt-dome structures by this method.

In the continental shelf, the first gravity-meter surveys were made 

by stationing the instrument on a tripod in very shallow water. Later, 

the instrument was placed in a diving bell, with an operator, and 

lowered into deeper water. Instruments have now been developed 

which can be lowered from a boat to the sea bottom very rapidly, and 

the readings made by remote control from the boat. This has greatly 

simplified the making of gravity surveys although this method is still 

somewhat slower than with the magnetometer.

DETAILED SURVEYING

The most reliable method of finding the exact location of individual 

geologic structures is by use of the seismograph, which measures the 

time interval required for shock waves to travel from the earth's 

surface to a reflecting stratum and return. Shock waves are generated 

by exploding small charges of dynamite or powder, and the returning 

waves are picked up by small instruments known as geophones.

Seismic operations on the open sea have created many problems. As 

the exact location of each shot point is vital to the success of the 

operation, accurate surveying is critical. Near land, conventional
76 CONTINENTAL SHELVES OF THE AMERICAS

surveying, such as visual methods with transit or alidade, can be used, 

but for use beyond the sight of land other methods had to be devised. 

Modifications of radar-base navigational aid systems are now in widespread use. In one such system, boats as far as Y5 miles (121 km) at 

sea can be located rapidly and precisely by triangulation from shore 

stations through use of electronic signals.

The protection of aquatic life is another major problem during 

seismic operations. It has been found that damage to aquatic life, 

particularly oysters and shrimp, can be minimized, if not avoided 

altogether, by limiting the size of the explosion and by suspending 

the explosive charge about midway between the surface of the water 

and the ocean bottom. This shooting procedure does not affect the 

accuracy of the survey and has become the usual practice in continental-shelf seismic prospecting.

As a measure of protection for marine life, inspectors are required 

to accompany all geophysical surveys. These inspectors are employed 

by the Coastal States and are empowered to enforce all State regulations concerning the size of the explosive charge, method of detonation, and like matters. The United States Government, through the 

Secretary of the Interior, has entered into cooperative agreements with 

the various States, whereby the regulations of the States are adopted as 

Federal regulations applicable to that part of the continental shelf 

under Federal jurisdiction adjoining the State. Under these agreements, the inspectors also accompany seismic crews operating in the 

Federal portion of the continental shelf.

Seismograph exploration of the continental shelf is expensive. The 

cost of maintaining a seismic crew operating on the continental shelf 

is from $60,000 to $100,000 a month as compared to $15,000 to $25,000 

for a crew operating on land. Because there is usually no necessity to 

drill shot holes, however, a crew can map considerably faster on the 

continental shelf, thus reducing the cost per unit area to below 

comparable land costs.

As in all other continental-shelf operations, weather is a major 

factor. The crews are not allowed to leave port during periods of 

storms or rough weather, but seismic boats must be large enough to 

withstand relatively high seas safely in case of unexpected storms.

From the passage of the Submerged Lands Act and the Outer Continental Shelf Lands Act in 1953 until the present (1956) few problems have been encountered by the oil industry in obtaining leases 

on the continental shelf of the Gulf of Mexico. Leases have been 

issued competitively, on the basis of the highest cash bonus bid offered,
PROBLEMS OF PETROLEUM DEVELOPMENT 77

by the agency having jurisdiction over the lands pursuant to the abovecited acts. Sealed bids, rather than auction, are the usual rule. Generally, industry nominates certain tracts as suggested areas to be 

offered for leasing. Following such nominations the agency selects 

all or part of these nominations, and after suitable advertising, accepts and opens the sealed bids at a specified time and place, and 

awards the leases to the successful bidder.

In the Gulf of Mexico, a substantial part of the shelf has been 

plotted on a coordinate system based on the Lambert projection. For 

leasing purposes, maps have been prepared showing the individual 

tracts with coordinates and descriptions thereof.

The provisions of leases follow a general pattern, regardless of the 

leasing jurisdiction, and are usually for a primary term of 5 years "and 

so long thereafter as oil and gas (or other minerals) can be produced." 

The leases usually provide for an annual rental payment prior to discovery and production of the leased minerals and a royalty (generally 

y$ or I/Q of the value of the gross production in the case of oil and gas) 

after production. Other provisions of the leases vary considerably, 

depending on the agency having jurisdiction over the shelf.

DEVELOPMENT PROBLEMS

Once an operator has decided, after conducting preliminary prospecting surveys and obtaining a lease, to test.drill an apparently 

favorable geologic structure, he is confronted with a new series of 

problems. First among these is choosing and obtaining suitable 

equipment on which to base the drilling rig.

In general, two basic types of foundations for drilling rigs are 

used on the continental shelf. The first, which is seldom used, is a 

rock- or earth-filled artificial island sufficiently large to support all 

necessary equipment. One such island is being successfully used in 

relatively shallow water off the coast of California. The construction 

of such an island in water of considerable depth involves excessive 

costs and under current economic conditions is not considered economically feasible, although it is theoretically possible. To date, no artificial island has been used in the Gulf of Mexico.

In more general use are platforms supported on pilings sunk deep 

into the ocean bottom and extending a sufficient distance above the 

ocean surface to escape wave action. These platforms are of four 

general types: self-contained platforms; small drilling platforms 

with mobile drilling tenders; drilling barges; and mobile platforms.

The self-contained platform (pi. 3) is large enough to support all 

drilling equipment and supplies, crew quarters, mess and kitchen 

facilities, and even recreation facilities for off-duty crew members.

451437 58 7
78 CONTINENTAL SHELVES OF THE AMERICAS

Self-contained platforms have a serious disadvantage in that they 

are difficult to salvage and almost impossible to move intact from one 

location to another. They are also expensive to construct, costing 

from a minimum of about $750,000 to more than $1.5 million. However, they have the advantage of providing a permanent base from 

which several wells can be drilled and, if production is obtained, can 

easily be modified to support necessary production facilities such as 

separators and storage tanks.

In order to avoid the high cost of the self-contained platform, the 

oil industry has developed a smaller platform, serviced by a tender 

vessel moored alongside (pi. 4). Only a minimum of equipment, such 

as the derrick and drawworks, is located on the platform itself. Motive power (usually diesel), liquid and dry storage, drill pipe, casing, 

crew quarters, and mess, kitchen, and recreation facilities are located 

on the tender. Originally surplus LSTs (landing ship tank) were 

purchased from the United States Government after World War II 

and converted to use as tenders. More recently, as the supply of LSTs 

became exhausted, the industry began building special vessels for 

tenders.

In order for a tender to serve as a base for drilling operations, it 

must not only be sufficiently large for storage and quarters, but also 

must be sufficiently seaworthy to withstand considerable rough 

weather. As a result, the average tender is as much as 300 feet (121 

m) or more in length. As an economic and space-saving measure 

many tenders contain no motive power, and depend upon tugs to move 

them from one location to another.

The platform-and-tender combination has several advantages over 

the self-contained platform but also has several disadvantages. The 

chief advantages are low cost and high versatility the fixed-platform cost is considerably reduced and the tender can be moved easily 

from one drilling site to another. Also, in the event of serious storms, 

the tender can be moved to a protected location, thus lessening the 

risk of serious damage to valuable equipment.

The chief disadvantage of the platform-and-tender combination is 

that the tender moves with the waves. This is a particularly bad 

feature when operating at substantial distances from shore because 

the ground swell of the ocean often makes it impossible to transfer men 

and materials to and from the tender, resulting in costly rig shutdown.

The initial cost of a tender is high, from $1.5 to $2.5 million, but the 

cost is partly offset by using the tender in the drilling of a number 

of wells.

It seems likely that the platform-and-tender combination will remain a favorite of the oil industry for operations in water less than
SELF-CONTAINED DRILLING PLAT

This platform is located in 95 feet of water 20 miles from land in the Gulf of Mexico. T

190 feet below the floor of the gulf. As many as 10 wells can be drilled by moving the 

by courtesy of the Continental Oil Co.)
GEOLOGICAL SI KVI.Y 

fp^B*

SMALL DRILLING PLAIKJKM AND MOUILL D

This platform is located in the Gulf of Mexico. (Photograph b
PROBLEMS OF PETROLEUM DEVELOPMENT 79

100 feet (30 m) deep. It should be of particular advantage in drilling 

exploratory wells by avoiding excessive cost until oil or gas accumulation is definitely proved.

Although the design of both types of platforms varies somewhat, 

the general construction is similar. A template, which is a framework of hollow, vertical, tubular caissons cross braced both horizontally and vertically and of sufficient length to reach from the ocean 

bottom to above the water level, is constructed on shore. The template (more than one may be used, depending on the size of the platform) is moved to the drilling location by barge and lowered until 

it rests on the ocean bottom. Piling is then driven through the upright caissons and fastened at the top of the template. The piling 

is cut off and leveled at a predetermined height above the water, and 

a prefabricated deck is lowered and fastened into place atop the piling. 

The platform is completed by installing necessary equipment on the 

deck.

The third and fourth types of platforms are the drilling barge and 

the mobile platform. The concept of a drilling barge is not new, as 

such equipment has been used for a number of years in inland waterways and swamp and marsh land of the gulf coast. All necessary 

equipment is mounted on a barge and then, by flooding the hull with 

water, the barge is sunk until it rests firmly on the bottom. After 

drilling operations are completed, the water is pumped from the hull 

and the barge is floated to another drilling site.

Barges were successfully used in drilling in 10 to 20 feet (3 to 6 m) 

of water in marshes and swamps prior to World War II. However, 

the adaption of barges for use in water 50 feet (15 m) or more in depth 

is fraught with difficulties. The latest development consists of a 

barge that can be flooded, but it is mounted with caissons rather than 

drilling equipment. A deck is erected on the caissons well above the 

hull level, and the drilling equipment is mounted on this deck.

Another variation of the drilling barge consists of mounting air 

or hydraulic jacks on the caissons and deck, thus making it possible 

to raise or lower the deck after the barge is in position. Several such 

mobile platform-barges are now in use in the Gulf of Mexico in water 

as much as 100 feet (30 m) deep.

The primary advantages of the mobile platform-barge are stability 

and mobility. The barge hull, resting on the ocean floor, provides 

an extremely stable base for drilling operations, and upon completion 

of the well the entire unit can be floated and moved rapidly to a new 

location.

The chief disadvantage of this type of unit is the extremely high 

cost of both construction and maintenance. Initial construction costs
80 CONTINENTAL SHELVES OF THE AMERICAS

of the completed units have been as much as $4 million, and operating 

costs range from $8,000 to $10,000 per day, compared to operating 

costs of about $3,500 per day for self-contained platforms and about 

$5,000 per day for the platform-and-tender combinations.

In spite of the high cost, it is expected that the mobile platformbarge unit will find great use in future operations, particularly in 

drilling exploratory wells.

The fourth type of platform, the mobile platform, has only limited 

use. It consists of a watertight, seaworthy hull through which several 

steel caissons are fitted. All drilling equipment is mounted on the deck 

of the hull. The platform is floated into position, and the caissons are 

lowered and are forced into the ocean bottom by air jacks. When sufficient resistance is reached, the hull literally climbs up the caissons, 

raising itself well above the water. Upon completion of the well, the 

air jacks are reversed, the hull is lowered to the water, and the caissons 

are lifted out of the bottom. The unit is then moved to the next location. In actual operation the hull can be lowered, the caissons raised, 

the unit moved, the caissons lowered, the hull raised, and drilling commenced in 24 hours or less. The unit has a limited weight-bearing 

capacity, however, and can be used only for relatively shallow wells. 

It was used successfully to drill a series of shallow core-test wells 

(averaging about 2,500 feet (762 m) in depth) to evaluate a sulfur 

deposit in the continental shelf off the coast of Louisiana.

Kegardless of the type of platform used on the continental shelf, 

several design factors must be considered before the platform can be 

built. These can be considered in three groups: oceanographic considerations, drilling depth, and windload.

The oceanographic factors considered in the design of offshore 

structures include storm tides, maximum wave height, length and 

period of maximum waves, horizontal water velocities, and wave 

forms. Usually it is necessary to call on the services of a trained 

oceanographer who can predict within reasonable limits, these factors.

Eelated to the oceanographic factors, but requiring somewhat different consideration, are those concerning bottom conditions. Such 

conditions vary greatly. Along the western part of the coast of 

Louisiana, for example, the bottom sediments are relatively firm and 

will support considerable weight without the necessity of driving 

piling to excessive depths. On the other hand, in the East Bay area 

near the Mississippi River delta, it was necessary to drive piling about 

200 feet (61 m) before any kind of solid bottom was reached, and 

375 feet (114 m) to gain sufficient bearing strength to support a particular drilling platform. The piles for this platform, located in 72 

feet (22 m) of water, were 465 feet (142 m) in length.
_1

PROBLEMS OF PETROLEUM DEVELOPMENT 81

In planning and designing offshore structures it is often necessary 

to drive test piles at the proposed location to determine bottom characteristics, bearing strength, and similar factors.

Factors relating to the load imposed on the offshore structure by the 

drilling operations are relatively easy to calculate, but a basic decision 

must be made concerning whether to design only for the projected 

depth of the well or to design for the maximum capabilities of the 

drilling rig and derrick. The smaller the total design load, the less 

expensive the overall cost of the structure. In general, platforms intended for exploratory drilling are designed to support drilling operations at or near the maximum drilling capability of the rig, whereas 

platforms to be used in drilling development wells are usually designed 

for the projected well depths.

Windload factors are comparatively simple considerations. Usually 

offshore structures are designed to withstand hurricane-force winds 

of about 125 miles .an hour. Such meterologic factors as storm expectancy, direction, frequency, and violence also must be considered 

in the design of offshore structures.

Space considerations are of the utmost importance. It is obvious 

that the cost of offshore structures increases with size. The cost increases are not directly proportional to size increases, and at some point 

there is an optimum ratio of size to cost. It is necessary, therefore, 

to design the structure to conform, as nearly as possible, to the optimum size and still incorporate all necessary features.

One of the early self-contained platforms used in the Gulf of Mexico 

is located about 7 miles (11 km) off Grand Isle. It is a double-deck 

platform, with the decks located 34 feet (10 m) and 45 feet (14 m) 

above mean water level and measuring 206 X110 feet (63 X 34 m). The 

structure, which provides living quarters for 54 men, is designed 

to withstand 120 mile per hour (193 km per hr) winds and 34-foot 

(10-m) waves. It is of template construction, employing 25 templates through which are driven 100 steel pilings. The platform 

weighs about 10 million pounds and costs about $1.2 million.

Corrosion, while primarily an operating problem, nevertheless influences the design of offshore structures to some extent. In all offshore operations a constant battle must be waged against the corrosive 

effect of salt water, and continual scraping and painting are the rule. 

To lessen the corrosive effect of the sea on the steel drilling structure, 

anticorrosion devices, such as cathode protection, monel-metal sheathing on piling (usually at the contact between the water and the air), 

concrete or cement sheathing, and special paints developed to protect 

against corrosion are often incorporated in the design.
82 CONTINENTAL SHELVES OF THE AMERICAS

Once the drilling platform or other structure is erected, the actual 

drilling operations vary little from drilling operations on land. Rotary drilling equipment, drill pipe, casing, and other tools are identical 

with those used for wells of comparable depth on land. However, oil 

and gas operations in the continental shelf create new and unique 

problems, some of which are discussed in the following section. .

The extremely high cost of platform construction has led to the 

perfection of directional (controlled deviation from vertical) drilling 

techniques in the shelf. While directional drilling has been used for 

many years in land operations, it was used very sparingly and usually 

for specialized purposes. One example is in the Oklahoma City field, 

where wells were directionally drilled in order to end almost directly 

beneath the State Capitol Building. In the Gulf of Mexico, however, 

directional drilling has become routine, and single platforms often 

support 4,6, or even 10 wells.

Such widespread use of directional-drilling techniques has led to 

the development of much specialized equipment. Most noteworthy 

is the derrick having an unusually broad base and a unique guidesheave arrangement in the crown-block area. This permits the crown 

block and the rotary table to be moved slightly to 6 or more prefixed 

locations; thus 6 or more wells can be drilled without moving the 

derrick. This has been so successful that several more are on order, 

and other modifications of the design are currently being developed.

Another technique widely used is dual completions completing a 

single well in two separate formations and producing from both simultaneously. Dual completions are not new but are used much more 

extensively in continental-shelf operations than in land operations.

One of the major concerns to operators in the continental shelf is 

the ever-present danger of oil-or gas-well blowouts. A few disastrous 

blowouts of gas wells have occurred in the Gulf of Mexico, with almost 

total loss of the platforms. That no blowout of an oil well has 

occurred is due largely to the extreme caution of the operators and to 

the rigid specifications for the equipment used. Gas-well blowouts, 

although causing heavy financial loss, do not pollute or contaminate 

the water or damage aquatic life as would an oil-well blowout. The 

oil industry is well aware of the hazards of blowouts, as are the Federal 

and State supervisory agencies, and as as a result, automatic storm 

chokes and other specialized equipment are installed on all wells.

The protection of aquatic life and respect for the rights of fishermen 

also pose serious operational problems for the oil industry. Protection of aquatic life is especially serious in conducting seismic surveys. 

Every reasonable effort is made to avoid any damage to aquatic life
PROBLEMS OF PETROLEUM DEVELOPMENT 83

around offshore structures during drilling and producing operations. 

Waste materials are thoroughly treated before being dumped in the 

sea in order to prevent any pollution or contamination. That these 

measures are effective is shown by the increase in the number of fish 

near offshore structures. Probably the fish are attracted by the lights 

and by garbage and other materials dropped from the platforms.

Every effort is made to avoid disturbing the ocean bottom or leaving 

discarded material which could cause damage to fishing nets. Upon 

removal of a drilling structure all piling or other protruding material 

is cut below the level of the mud to avoid leaving anything which 

might snag fishing nets or trawls.

All offshore structures are required to be equipped with adequate 

lights and other devices to warn vessels of their existence, and drilling 

platforms can not be erected in shipping fairways. Beyond the 3-mile 

(5.56-km) limit the character of the high seas is preserved as directed 

by the Congress of the United States in the Outer Continental Shelf 

Lands Act.

The safety and welfare of the workmen is of utmost concern to the 

operators. Standby boats are in constant attendance during drilling 

operations to remove any seriously injured personnel. Radio or radiotelephone communication is maintained at all times with the shore base, 

and life preservers and life jackets are standard equipment on all 

platforms.

TRANSPORTATION

Transportation has created relatively few serious problems- Personnel are usually transported by boat or helicopter. Crews are rotated about once a week and regular trips can be planned and scheduled. 

As boats are slow many companies use helicopters for personnel transportation. Landing space is now incorporated on many offshore structures, and helicopters are able to transport full drilling or operating 

crews in a fraction of the time required by boats. Weather conditions, 

however, often limit the use of helicopters.

Personnel transportation is expensive, regardless of the means used. 

Crew boats must be sufficiently large and have sufficient power to operate safely during severe weather. As a result, most of the boats are 

65 to 85 feet (20 to 26 m) long and are expensive to purchase and maintain. Helicopters require a much greater initial investment and involve greater operational and maintenance costs. Therefore, many oil 

companies lease boats or helicopters from service companies. Contract 

service is usually on a monthly basis at a fixed fee, regardless of the 

actual service time of the vessel.

Equipment transportation, particularly of heavy massive equipment, 

is by seagoing barge and tugboat. Heavy-duty cranes mounted on
84 CONTINENTAL SHELVES OP THE AMERICAS

barges are used to transfer equipment from towed barges to the platform and, during construction of platforms, to lower templates and to 

lift prefabricated deck sections. Weather limits the movement of 

equipment barges, and weather forecasting plays an important part 

in scheduling their movement.

A substantial fleet of vessels is necessary to supply a well being 

drilled in the continental shelf. A minimum requirement would 

probably be three crew boats, one stationed at the platform, one at 

the shore base, and the third in a standby capacity; one or more 

equipment barges with tugs or a self-propelled vessel capable of transporting heavy supplies; and possibly one or more helicopters, either 

in routine service or on standby for emergency.

The transportation of oil and gas to markets on shore involves 

many economic and engineering problems. As on shore, pipelines 

are the most economical and efficient method of moving large quantities of oil and gas. In fact, natural gas cannot yet be transported 

in substantial quantities by any other method. However, the construction of a pipeline from an individual field cannot be justified 

economically unless that field has sufficient reserves to amortize the 

cost of the line within a reasonable period of time and sufficient 

productive capacity to supply the line with enough oil or gas to maintain efficient operations. In many fields in the Gulf of Mexico these 

two requirements have not yet been fulfilled. Much of the drilling to 

date has been exploratory, and as a result many fields are only partly 

developed. Even though the results of this exploratory drilling have 

indicated that the proved reserves of the fields are substantial, the 

daily productive capacity of the fields has remained low.

A pipeline project in the continental shelf is tremendously expensive. On the basis of the few lines constructed to date, it is 

estimated that underwater pipelines in the Gulf of Mexico cost from 

$80,000 to $100,000 per mile.

In the Eugene Island and the Grand Isle areas off the coast of 

Louisiana, underwater pipelines are in operation. The construction 

of these pipelines was an engineering task of major proportions. 

After the route of the line was selected, necessary permission obtained 

from government agencies, and men and materials assembled at the 

shore base, the pipe was prepared for its underwater service. This 

preparation included strengthening the pipe by welding additional 

steel plate around it, coating the pipe with suitable anticorrosion 

asphaltic compounds, wrapping it with fiberglass or other material, 

and finally coating the entire pipe with a 1 to 3 inch layer of concrete. 

The prepared pipe was then placed on a barge and, as sections were 

welded and the welded joints coated and wrapped, gently lowered to
PROBLEMS OF PETROLEUM DEVELOPMENT 85

the ocean bottom. Hydraulic jets were used to cut ditches in the sea 

bottom. Ocean currents and wave action supplied the backfill necessary to cover the pipe. Elaborate testing procedures were used at 

each step of the process to insure a strong leakproof line.

Consideration is being given by the oil industry to the construction 

of a major oil pipeline roughly parallel to the coast of Louisiana to 

service several fields in the Gulf of Mexico, but construction is not 

expected for some time.

Oil is transported by barge in most of the fields. Wells usually 

produce into storage barges moored nearby or into tanks on the 

platforms, which contain all the usual production facilities including 

tanks, separators, and heat-treaters. Transport barges load at these 

facilities when assured of a full load.

Barge transportation of oil is unsatisfactory because of excessive 

cost and undependability. Whereas pipelines can transport a barrel 

of oil to shore at costs ranging from 5 to 10 cents, barging costs have 

been as high as $1.00 a barrel and average from 15 to 25 cents (Petroleum Week, 1955 a-c). Furthermore, adverse weather often makes it 

impossible to undertake barge movement, necessitating a shutdown in 

production. A shutdown not only is costly to the operator but can, 

under some conditions, cause severe damage to the well.

It seems certain that as development progresses in the continental 

shelf, more pipelines will be constructed and barge transportation of 

liquid hydrocarbons will be only a temporary measure.

ECONOMICS OF CONTINENTAL-SHELF OIL 

DEVELOPMENT

The heart of all problems faced by the operator of offshore 

oil operations is the high cost. Every operation, from supplying 

drinking water for the drilling crew to constructing storage tanks, 

costs from 2 to 10 times the amount expended for equivalent services 

on dry land.

Oil and gas operations on the shelf have created a substantial industry in the adjacent coastal areas. While it is difficult to obtain 

exact figures, it is estimated that the oil industry has invested more 

than $1 billion in the shelf operations of the Gulf of Mexico since 

1946. Much of this has been for capital expenditures including lease 

acquisition, marine equipment, drilling rigs, and platforms. Other 

expenditures include the cost of seismic and other surveys, establishment of shore bases, weather forecasting facilities, and radio and 

navigation-aid stations, and drilling and production costs.

In return the oil industry has received less than $250 million for 

oil and gas produced. However, tremendous new reserves have been
86 CONTINENTAL SHELVES OF THE AMERICAS

discovered, and production reached approximately 80,000 barrels of 

oil daily by January 1, 1956. Shelf oil and gas operations should in 

the future return a profit on this tremenodous investment.

There are wide variations in the conditions under which oil and gas 

wells are drilled in the continental shelf. It has been said that there 

is no such thing as an "average" well in the Gulf of Mexico. However, the cost and effort that might go into the drilling of a hypothetical well located on the shelf can be examined briefly. The 

hypothetical well will be located about 9 miles (14 km) from the nearest 

land in 40 to 50 feet (12 to 15 m) of water and about 50 miles (80 

km) from the nearest deep-water harbor. Twelve thousand feet 

(3658 m) may be assumed as the projected depth of the well, and a 

combination platform and tender will be used throughout the 

operation.

About 200 men will be engaged in drilling the well during its various stages. This will include about 36 members of the seismicexploration crew; 24 men engaged in the construction of the platform 

and drilling rig; 30 members of the drilling crew; 10 men supplying 

special services such as well cementing and electrical and mud logging ; 64 marine workers including crew members on the construction 

vessel, the drilling tender, and the crew boats; and 36 supervisory 

administrative, and unclassified personnel.

The platform will require a minimum of 200 tons of steel beams, 

100 tons of steel plates and bracing, more than 350 tons of steel piling, 

and 30,000 board-feet of heavy lumber for decking. After the drilling 

platform is constructed, 7 vessels will be required, including a 3,500- 

ton drilling tender, 3 crew boats, 2 special-service combination lifeboats and workboats, and 1 water cargo barge.

The well will require 80,000 barrels of fresh water, 4,000 sacks of 

oil-well cement, 10,000 sacks of drilling mud and mud chemicals, 

and about 250 tons of steel casing and tubing. Three 1,000-kilowatt 

generators, driven'by diesel engines, will supply motive power for 

the mud pumps, draw works, and rotary table. The diesel engines 

will consume about 120,000 gallons of fuel during the drilling operations. Large quantities of food will be consumed during the 4 

months or more of drilling operations. Miscellaneous supplies, such 

as handtools, paint, cable, rope, bolts, and nuts, must be added to 

the supply list. Operational costs will average about $5,000 per 

day throughout the drilling of the well.

A shore base will be required, at which will be located a dock, 

warehouse, office, communication facilities, and quarters for base 

personnel.
PROBLEMS OF PETROLEUM DEVELOPMENT 87

The above figures, while hypothetical, present some idea of the magnitude of effort required to drill and complete a well on the continental shelf of the Gulf of Mexico.

Most operators believe that offshore costs can be reduced to provide more incentive for the exploration and development of the continental shelf. New platform designs, new operating techniques, 

added experience, and favorable economic conditions will tend to 

reduce operational expenses and provide added incentive. However, 

reserves and production rates must be maintained at high levels to 

insure sufficient operating capital. In the opinion of most operators 

stripper wells have litle or no place on the continental shelf.

MAGNITUDE OF THE OFFSHORE PETROLEUM INDUSTRY IN THE UNITED STATES

Although the previous section gave some indication of the magnitude of the offshore petroleum industry in the United States in terms of 

the approximate total investment in continental-shelf facilities, dollar amounts are difficult to translate into actual wells drilled, reserves proved, or production obtained.

Table 1 shows the growth of drilling activity in the continental 

shelf of the Gulf of Mexico, by years, since 1948. The figures have 

been compiled from various sources and may not conform exactly to 

other published data, though the magnitude of the figures shown is 

believed to be accurate. One difficulty in compiling such data is determining just which wells are located on the continental shelf rather 

than in inland waters such as bays and estuaries.

TABLE 1. Wells completed on the continental shelf of the Gulf of Mexico

Year

1948. ............................... .

1949... ........................... ...

1950... ......._:... ...I. .--.- .:.

1951....- ................ .............

1952... .......................... i...

1953.-.---------- - --- --------

1954... 

1955--- - - 

1956.... .............................

Total

Total

20

73

68

8

67

79

234

360

548

1,457

Oil 

wells

7

29

36

4

47

59

150

220

324

876

Gas 

wells

6

11

9

4

4

5

25

38

60

162

Dry 

wells

7

33

23

0

16

15

59

102

164

419
88 CONTINENTAL SHELVES OF THE AMERICAS

The table shows very vividly the continued expansion of the continental-shelf oil industry since its beginning. The low drilling rates 

during the years 1951 and 1952 are explained by the injunctions issued 

by the Supreme Court of the United States in the cases United States 

v. Louisiana and United States v. Texas. Following these injunctions, 

the Secretary of the Interior permitted no exploratory wells to be 

drilled but did permit the drilling of wells necessary to protect proven 

reservoirs from waste or unequal withdrawals. This was done in the 

interest of conservation.

Production from the continental shelf of the Gulf of Mexico has 

grown steadily. Table 2 shows the approximate annual and cumulative production of liquid hydrocarbons. These data are subject to the 

same criticisms as table 1, but are also believed accurate so far as magnitude is concerned.

TABLE 2. Estimated production of liquid hydrocarbons from the continental shelf of

the Gulf of Mexico

Year

19471.. ___________________________________

1948__ 

1949. _-_--_-._. __ ___ __

1950. __ _____ 

1951.. ___ ___ ________

1952. _____________________________________

1953... ___________________________________

1954. _......._..___________ ______________

1955. _...-..._..______.. __________________

1956. _____________________________________

Production (barrels)

Annual

3,500 

133, 000 

1, 100, 000 

4, 360, 000 

6, 560, 000 

6, 900, 000 

10, 040, 000 

15, 850, 000 

25, 900, 000 

40, 100, 000

Cumulative

20, 000 

153, 000 

1, 253, 000 

5, 613, 000 

12, 173, 000 

19, 073, 000 

29, 113, 000 

44, 963, 000 

70, 893, 000 

110, 993, 000

> There was some production from the continental shelf prior to 1947.

On January 1,1956, about 70 wells were being drilled in the continental shelf of the Gulf of Mexico. An even greater increase can be 

expected as new drilling platforms, more tenders, and more barges are 

completed and placed in operation.

The proved reserves of the continental shelf of the Gulf of Mexico 

have been estimated (Cram, 1956) to be approximately 1.5 billion barrels of liquid hydrocarbons.
89

THE FUTURE OF CONTINENTAL-SHELF MINERAL 

OPERATIONS

It seems safe to assume that exploration, drilling, and production of 

hydrocarbons in the continental shelf will go forward at an everincreasing rate and in ever-increasing water depths. Development of 

methods and equipment to withstand the greater forces generated by 

the deeper ocean waters will be necessary for each new depth of shelf 

area explored. The ultimate water depth in which oil and gas operations can be successfully conducted is problematical. A few years 

ago, 100 feet (30 m) was viewed as the limiting depth. Now successful drilling operations have been conducted in water depths of 100 

feet, (30 m) and equipment in the design or construction stage can 

support drilling operations in even greater water depths, perhaps as 

much as 200 feet (60 m). Thus, so far as the continental shelf is 

concerned, with water depths of approximately 600 feet (183 m), exploration, drilling, and production appear to be limited only by the 

imagination and ingenuity of the oil operators and the equipment 

designers.

Many predictions, including those of the National Petroleum Council, agree that the average daily production from the shelf areas of the 

Gulf of Mexico will reach 250,000 barrels within the next 5 years. 

This increase in production will be accompanied by new methods and 

lower cost.

Already in the experimental stage is electronic remote-control 

equipment which will allow an operator, located at a shore base, to 

control production, to start any well flowing, to switch the well flow 

from one tank to another, and to gage tanks merely by pushing a button on a control panel. The system works by very high frequency 

radio, and experiments indicate that its effective range may extend to 

100 miles (161 km). Such a system would eliminate the necessity of 

maintaining production crews on offshore platforms and would minimize shutdowns due to adverse weather conditions.

Ships from which all drilling operations can be conducted, without 

the necessity of erecting a platform, also represent a future possibility. 

One such vessel, capable of drilling very shallow holes for stratigraphic studies, is currently being tested off the coast of California. 

Future development of this type of equipment might make it possible 

to drill exploration and development wells to depths in excess of 10,000 

feet (3,048 m) without having to erect a platform, thus eliminating 

one of the major cost factors of continental-shelf development.
90

It can reasonably be expected that as production increases more and 

more pipelines will be constructed to handle the oil and gas produced. 

Future pipelines will undoubtedly incorporate automatic control 

equipment which can regulate all operations of the line from control 

points at land bases.

The storage of liquid hydrocarbons in cavities dissolved in shallow 

salt domes represents another possibility. Such storage space has 

been used successfully on land, and if deemed feasible in the continental shelf, might allow ocean-going tankers to load at the site, thus 

reducing transportation and handling costs and speeding the shipment of products to market.

Better and more accurate methods of electronic surveying will probably be developed, as will more accurate navigation aids. Seismicexploration methods capable of locating favorable geologic structures 

more rapidly and more accurately are also a possibility.

These are but some of the promises technology holds for the future. 

But technology cannot replace the vision, judgment, and courage of 

the men who explore for and develop the mineral resources underlying 

the oceans.

REFERENCES CITED

Cram, I. H., 1956, Offshore development proving successful: World Oil, V; 142,

no. 7, p. 78. 

Petroleum Week, 1955a, On the continental shelf a whole new business is in

the making: v. 1, no. 4, p. 14-18.

 1955b, A whole raft of new problems: v. 1, no. 5, p. 14-18.

 1955c, Oil puts new tools to work: v. 1, no. 6, p. 15-20.

United States Department of the Interior, 1953, Years of progress, 1945-1952: 

Washington, U. S. Government Printing Office, 195 p.
INDEX

Page

Alabama______-__.____.____ 46,60 

Alaska........................................ 30,63

Amazon River. 17 

Antllloan Island arc, description......____ 7-8

Argentina....__ : . . 68

Artificial islands for oil-well drilling........... 77

Bahama Banks.. ~ :..... 17

Bahama Islands.. 65 

\ Banks, description... ... . ...___ 3 

Barbados.___ . .. ....___ 8,66

Basins, description... . . _... 3 

Bibliography.... . . . .- 26,39,64,90

Blake Plateau................................ 23

Bottom sampling, techniques_..______ 34,38 

Brazil....__._.......................... 46,67-58

British Columbia, Vancouver Island..____ 62 

British Guiana.... . 57

British Honduras. 66 

Brownson Deep.. 3

California..............!--.. 23,32,33,36,37,61-62,69

Cape Cod, Mass. . 16 

Cape Hatteras, N. O... . 23,36 

Charts, navigation 31 

' Chile.............. ............... 14,33,68,69-60

Chromite____ . - 62 

Coal........................ ......... 62,69-60,62

Continental borderlands, description. . 23,33

Continental . margins, active, bordered by

basins and ridges. 32

active, bordered by aeep-sea trenches.. 32,33

stable...__ 32

Continental shelves, definition.......... 11,29,44,68

glaciated..___^___............ 12,14,15-16

in areas of coral growth . 17 

off large rivers.. 16-17 

off stable lowland coasts... .... 12-14

off young mountain coasts... 14 

origin..__ ... 18-20 

dispute of ownership, original- 70-72 

Outer Continental Shelf Lands Act 73-74 

Submerged Lands Act............ __ 72-73

percentage of earth's surface ... 11 

sedimentation.... 12,18,19-20,45

shelf edge..._................__.__ 11,29

slope..___ . 12 

structural features___________ 12,35,69 

water depth..._..._ . ~ ~ 12,29,34 

width......... 12,14,16,68

Su also geographic nama.

Continental slopes, definition . _ 11,20,29 

height above ocean floors.... .. . 21

off faulted coasts.. ......... 22

Page 

Continental slopes, definition Continued

off large deltas______________ 21 

off stable lowland coasts.________ 21 

off unfaulted coasts_ . _ __ 22 

origin.-..... ...................... 24-26

percentage of earth's crust______ _ 11 

sedimentation...... ...._._ 21,23-24

slope...._ ........................... 21

submarine canyons...... .. 12,21,22-23,32

topography..... .................... 20,22-23

Continents, altitude. ......... 4,28

Costa Rica....__....... 57

Cuba......................................... 53,54

Dominican Republic...... ....._.__.. 8,55

Drilling barges.. ....................... 79

E. W.Scrlpps survey........ . .... 32

Ecuador.. ... ........ 69

Florida........................... 32,33,46,50,51,63

Geomagnetic Investigations....... 36,38,39,75

Georges Bank_____... _.______ 33 

Gold...... . . . . 46,62

Gravity measurements. ... 6-7,35-36,76 

Guatemala . 56 

Gulf of Mexico. ~_.......... 19,31,36,37,46-SO, 51

future of oil production 89-90 

historical background of Industry..___ 69-74 

leasing procedures 76-77 

number of wells___._________ 87-88 

production of liquid hydrocarbons. . 88 

provided petroleum reserves _ .... 88

Haiti..__ ............................. 64

Hispanlola................................... 63,64

Honduras .... 87 

Hudson River....___ . . .... 32

Ilmenlte________________.__ 46,63 

Iron ore. ____.__ . . ... 52 

Island acre, description 7-8 

Isostasy. . . 7,29

Jamaica_____________. ___ 8,53,54,67

Louisiana.____..._........._.. 35,46,48-50

See also Gulf of Mexico. 

Lower California . 61

Maine_________._._..._...._ 51

Maine, Gulf of............................... 16,33

Mexico. . .......... 49

01
92 INDEX

Page 

Mid-Atlantic Ridge.......................... 3

Mississippi River Dalta...................... 17. 49

Motille platform-barges. ____________ 79-80 

Mohorcn ifl6 discontinuity.__________ 0 

Monazlto..__... _____....___. 46,53.58

New Brunswick.__.__._.__.___. 51 

New Jersey... . . ... 61 

Newfoundland......... ..... .. 16,45-46,51,52

Nicaragua.._.... _ . _..__.... 67

Nova Scotia___________._____ 16,51,52

Ocean basins, depth..____________ 3,28,29 

percentage of earth's surface.. _.____ 11 

topography.. 3

Oil and gas, occurrence.... 46-49,

60,51-52.53-57,58-59,60-62, 63 

See also geographic name*. 

offshore-drilling structures, construction 

and daslgn...._..._......._..... 81-82

types................. ............. 77-80

pipelines..... . . 84-85

Oregon. 62

Panama 31,57 

Peru....___ -____.............. 14,68-50

Phlliplne Deep.. .. ................ 3

Phosphate...___..__._ __.___. 61 

Plat form-and-tender drilling rigs.______ 78-79 

Platform barges . 80 

Platinum... ...... 62

Position location, techniques..._____._ 38,76 

Puerto Rico................. ............ 8,53.55

Puerto Rico Trench______________ 7-8

Ridges, description. 

Rio de la Plata..... 

Rises, description...

Page

River-deposited terraces__________.. 17 

Rock types, distribution.___________ 4-5,28 

Rutlle. ....._._....._................ 46,53

Sable Island Bank..______________ 16 

St. Lawrence, Gulf of.____________ 16 

Salt domes..._______________ 46,49-60.74 

Sea level, present._____________ 11,28,29 

Sea-level changes, records___________ 30-31 

relative to land______________ 10 

vertical movement of land areas..____ 10 

Seamounts, description.___________ 3 

Solsmlc Investigations.. ______ 6-6,36-3V, 75-76 

Self-contained drilling platforms..______ 77-78 

Soundings, techniques .____.'.__.__ 34-35,37 

Submarine canyons. See Continental slopes. 

Sulfur........................................ 49-60

Taiuplco Embayment...___ ______ 49 

Tehuanicpec, Isthinusof....... .... . 46,49

Temperature measurements.. ... . 38 

Texas..................__........ 35,46,48-49,50

See alto Oulf ol Mexico. 

Trenches, description..... . . 3

Trinidad..................... . _ 56

Troughs, description ____ ______ 3 

Turbidity currents....._.... ......... 22-24

Venezuela . .. _.... 56 

Virgin Islands .............. . .. 8,55

Washington . . .. . .. 62

Water sampling, techniques. _________ 39

Yucatan Peninsula.. ....... . ..... 49

Zircon. .... ............... ........ 46,53

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