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~ 
The Shelf-Slope TransitionCanyon and Upper Sllope 

Sedimentary Processes on the 

Southern Margin of Georges Bank 

By PAGE C. VAlENTINE 

An examination of sedimentary environments in water depths 

of 150--600 meters and of how they are determined by the 

interaction of available sediment, bottom currents, and seafloor morphology 

U.S. GEOLOGICAL SURVEY BULLETIN 1782 
DEPARTMENT OF THE INTERIOR 

DONALD PAUL HODEL, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

UNITED STATES GOVERNMENT PRINTING OFFICE: 1987 

For sale by the Books and Open-File Reports Section, U.S. Geological Survey, 

Federal Center, Box 25425, Denver, CO 80225 

Library o\f Congress Cataloging In Publication Data 

Valentine, Page C. 

The shelf-slope transition-canyon and upper slope sedimentary processes on 

the southern margin of Georges Bank. 

(U.S. Geological Survey bulletin; 1782) 

Bibliography: p. 

Supt. of Docs. no.: I 19.3:1782 

1. Marine sediments-Georges Bank. 2. Continental shelf-Georges Bank. 3. 

Continental slopes-Georges Bank. 4. Ocean bottom-Georges Bank. 5. 

Oceanography-Georges Bank. I. Title. II. Series. 

QE75.B9 no. 1782 557.3 s 87- 600040 

[GC383] [551.46'083'345] 
CONTENTS 

Abstract 1 

Introduction 2 

Acknowledgments 6 

Sedimentary provinces of the Georges Bank region 7 

Georges Bank Shel:f 7 

Submarine canyom: 7 

Outer shelf-upper :dope 8 

Currents and sediment transport 9 

Currents on Georges Bank 9 

Currents in Lydonia Canyon and environs 10 

Sediment transport 10 

Sediment texture of the shelf-slope transition 11 

Texture of canyon noors 12 

Texture of canyon rims 14 

Distribution of silt and day-the mudline 15 

Distribution of very fine sand ( 4 <P) 15 

Texture of outer shelf-upper slope 16 

Canyon morphology and bottom currents 19 

Hudson Canyon 20 

Georges Bank canyons 20 

Speed of bottom currents 20 

Sediment texture and bed forms 20 

Canyon shape and volume 21 

Canyon energy levels 21 

Synthesis 24 

Bottom currents ~~4 

Sediment texture and transport 25 

Canyon size and energetics 25 

Regional sediment transport 26 

References cited 26 

FIGURES 

1. Map of Georges Bcmk showing regional physiography, study areas, and major 

bottom-current patterns 3 

2. Map of Oceanographer and Heel Tapper Canyons and the two outer 

shelf-upper slope study areas showing sample and dive localities 4 

3. Map of the outer shelf-upper slope study area that indudes the branching head 

of Filebottom Can~ln showing sample localities and band of coarse sediment 

on upper slope 5 

4. Map of Lydonia and Gilbert Canyons showing sample and dive localities 6 

5. Map of Hydrographer Canyon showing sample and dive localities 7 

Contents Ill 
6. Graphs of mean grain size versus water depth and silt and clay content versus 

water depth of samples from can)On floors and the outer shelf-upper slope 

study areas on the southern margin of Georges Bank 13 

7. Map of the head of Oceanographer Canyon showing the distribution of very 

fine sand and silt and clay on the canyon floor and rim 17 

8. Graphs showing sediment texture along three traverses across the shelf-slope 

transition 18 

9. Map showing the distribution of Gulf Stream warm-core rings along the 

western North Atlantic margin, 197 6-83 19 

10. Cross sections of canyon mouths at the 200-m isobath 22 

11. Graphs showing the relationship between canyon morphology and energy level 

for 12 Georges Bank canyons 23 

12. Map of the Oceanographer-Lydonia Canyon area showing the directions of 

sediment transport in the shelf-slope transition 27 

TABLES 

1. Steady current velocities required for initiation of movement and for suspended 

transport of quartz sediment of uniform size 11 

2. Grain-size distribution of samples from 11 stations on 2 traverses in the 

shelf-slope transition study areas near Oceanographer Canyon with and without 

carbonate matter 12 

3. Dimensions of 12 submarine canyons on southern Georges Bank and of 

Hudson Canyon on the U.S. Middle Atlantic Shelf 22 

IV Contents 
The Shelf-Slo-pe Transition-Canyon and Upper Slope 

Sedimentary Processes on the Southern Margin of 

Georges Bank 

By Page C. Valentine 

Abstract 

This study of sedimentary processes on the southern margin of 

Georges Bank was based on observations from research submersibles of 

bottom features, on size analyses of 

sediment samples, and on measurements of current strength and flow 

direction made from submersibles and 

from long-term moored current-meter 

arrays of other investigators. The principal study area was the shelf-slope 

transition to a depth of approximately 

600 m in a region that extends from 

west of Oceanographer Canyon eastward to Lydonia Canyon. 

Sedimentary environments present in the transition from shelf to gullied 

upper slope and from shelf to submarine canyon floors are determined by 

the interaction of available sediment, 

bottom currents, and sea-floor morphology. Georges Bank is isolated from 

the North American continent by the 

Gulf of Maine, and the chief source of 

sediment for the bank margin is Pleistocene glacial deposits on the bank 

crest. The smooth, gently sloping outer 

shelf steepens slightly to become the 

upper slope, which is dissected by gullies having steep walls. Surficial sediment of the outer shelf and upper slope 

generally is slightly rippled and 

becomes smoother and finer grained 

as water depth increases. Superimposed on this terrain, submarine canyons of varying size and wall steepness 

incise the Georges Bank Shelf and contain a variety of sedimentary facies, 

including burrowed and collapsed 

Pleistocene outcrops, sand accumulations surfaced with large and small bed 

forms, almost featureless silty sand, 

and a pavement of ice-rafted gravel. 

Three patterns of strong bottom 

currents recognized in the shelf-slope 

transition region are attributed to tidal 

currents, Gulf Stream warm-core rings, 

and an unknown source. Regional tidal 

currents are very strong on the bank 

crest, but they diminish in strength as 

water depth increases toward the shelf 

edge and are weak on the upper slope. 

However, tidal flow is affected by canyon morphology, and at outer shelfupper slope depths (15Q-600 m) in 

large canyons having steep walls, the 

currents are very energetic. Tidal currents reach speeds of 75-1 00 cm/s in 

Oceanographer Canyon along a canyon floor covered by coarse sediment 

and large bed forms; exposures of burrowed Pleistocene silt are present on 

the lower walls. The silt is disaggregated by extensive bioerosion and by 

mass-wasting processes and is an 

important secondary sediment source 

for large and medium canyons. By contrast, small canyons such as Heel Tapper Canyon are relatively tranquil; they 

are shallow embayments of the upper 

slope, and, like the gullied upper slope, 

their walls and floors are covered by 

silty sand. 

Gulf Stream warm-core rings 

impinge on the Georges Bank margin 

intermittently; anticyclonic currents in a 

ring flow northeastward at about 50 

cm/s as the ring drifts southwestward. 

These strong currents winnow very fine 

sand and silt and clay from the upper 

slope between 200 and 300 m water 

depth. A strong current of unknown 

origin flows dominantly westward at 

about 50 cm/s across the rims of 

Oceanographer and Lydonia Canyons 

and transports shelf sand onto canyon 

walls while exposing deposits of icerafted gravel. 

Within the shelf-slope transition, 

net transport is off the Georges Bank 

Shelf onto the gullied upper slope and 

into the canyons. Sediment movement 

is most rapid in three areas-from the 

shelf westward across the east rims of 

large canyons, in a narrow band northeastward along the upper slope, and 

along the lower walls and floors of large 

canyons, where net transport of sand is 

slow and is directed up or down the 

canyon and where fine sediment is winnowed and transported away in suspension. Sediment movement is less 

rapid on the outer shelf; on most of the 

upper slope, including the gullies; on 

the floors of small and medium canyons; and around some canyon heads. 

Moderately energetic canyons such as 

Lydonia may be accumulating sediment most rapidly; they trap a large 

volume of shelf sand as well as 

bioeroded silt from outcrops on their 

lower walls, and currents along the canyon floors are relatively weak. 

Twelve canyons on the southern 

margin of Georges Bank are ranked by 

energy level on the basis of sediment 

texture, bed forms, and current observations. The relationship between 

energy level and canyon size and 

shape means that current and sediment dynamics in other canyons may 

be predicted if their bathymetry and 

regional tidal flow are known. 

Abstract 1 
INTRODUCTION 

At the outer edge of the continental shel~ the gently 

sloping sea floor steepens and becomes the continental 

slope. The increase in slope at the shelf break is chiefly a 

function of sediment supply, the hydrodynamic environment, and the structural transition from continental to 

oceanic crust. In general, bottom currents decrease in 

energy as water depth increases, to result in less effective 

sediment transport and in a reduction in sediment size 

away from continental sources. The physiographic boundary between the shelf and slope may advance seaward or 

retreat landward in response to changes in the volume of 

sediment supplied and to changes in current patterns and 

water depth. These principles hold in a broad sense, but 

patterns of sedimentation are modified regionally by many 

factors. Geological processes of the shelf-slope transition 

have been extensively researched, and papers on recent 

advances in the field are collected in two volumes, one 

describing the geology of continental slopes (Doyle and 

Pilkey, 1979), and the other describing the shelf break 

(Stanley and Moore, 1983). 

Various sedimentary provinces are present in close 

proximity where the Georges Bank Shelf passes into two 

different physiographic provinces, namely, the heads of 

submarine canyons and the adjacent gullied upper slope. 

This report examines the transition in sedimentary facies 

from the outer shelf at about 150 m depth down to about 

600 m on the southern Georges Bank margin (fig. 1 ); 

describes the sedimentary processes, current patterns, and 

sea-floor morphology that make the canyon and upper 

slope environments similar in some respects but very 

different in others; and provides a synthesis of the sedimentary environment of the shelf-slope transition. fur the 

Georges Bank region, this information may prove useful 

in predicting and tracking the movement of water masses 

and currents and of the plankton and sediment particles, 

including pollutants, transported by them. In a broader 

sense, this study may contribute to an understanding of the 

evolution of continental shelf and slope environments during a high stand of sea level. 

The principal study area was the shelf-slope transition to a depth of approximately 600 m in a region that 

extends from west of Oceanographer Canyon eastward to 

Lydonia Canyon (fig. 1 ). The most extensive observations 

and samplings of the sea floor were made in the northern 

part of Oceanographer Canyon and on the upper slope 

east (Filebottom Canyon area) and west of the canyon 

mouth during many dives of the research submersibles 

Johnson-Sea-Link, Alvin, and Nekton Gamma in 1974 and 

from 1980 to 1984 (figs. 2, 3; details of these dives were 

given by Valentine, Uzmann, and Cooper, 1980, 1984; 

Valentine, 1983, 1985; Valentine, Coope:t; and Uzmann, 

1984; Cooper and others, 1987). 

A series of dives with the Johnson-Sea-Link was 

made to each of two sites on the east and west rims of 

Lydonia Can)Qn (shown by star-shaped symbols in fig. 4; 

Cooper and others, 1987), and in 1984, a single dive was 

made to a former current-meter station on the east rim of 

Lydonia Canyon. In 1984, single dives of the Johnson-SeaLink were made also to the floors of Hydrographer (fig. 

5), Gilbert (fig. 4), and Heel Tapper (fig. 2) Canyons. 

Alvin dives were made along the axis and walls of Lydonia 

Canyon in 1980 (Butman and others, 1983; Twichell, 

1983). Sediment from the shelf-slope transition around the 

head of Oceanographer Canyon and in the area of Filebottom Canyon was collected by grab sampler from the 

surface ship in 1983 and 1984. All observations were 

made and all samples were collected during June, July, 

August, or September. 

All samples used in this study were wet sieved to 

separate gravel, sand, and mud fractions. Size analyses 

were conducted by sieving for gravel and by using a rapid 

sediment analyzer for the sand fraction (Schlee, 1966) and 

a Coulter Counter1 for the silt and clay fraction. Textural 

analyses of eight sediment samples from the floor of 

Lydonia Canyon that were reported in previous studies 

(Butman and others, 1983; Twichell, 1983) are incorporated here. All size data are given in weight percent. 

Statistical parameters for all samples were calculated by 

the method of moments (Krumbein and Pettijohn, 1938). 

fur purposes of comparing the textural differences 

of samples and relating these differences to the influence 

of bottom currents, the weight percentages of sediment 

size classes were recalculated by omitting data on gravel of 

pebble size and larger (grain diameter greater than 4 mm 

or -2cp ). Thus, sediment samples plotted in figures 6B, 7, 

and 8 and discussed in the related text are compared on 

the basis of their constituent size classes that are most 

susceptible to transport by the prevailing currents. Gravel 

of pebble size and larger most likely was carried into this 

region by ice rafting during the Pleistocene, and presentday bottom currents are too weak to transport it farther. 

The samples from the outer shelf-upper slope areas and 

the floors of Oceanographer, Hydrographer, Gilbert, and 

Heel Tapper Canyons contained little or no gravel; only 

three samples contained more than 5 percent gravel of 

pebble size and larger, and the recalculation without gravel 

data affected the other size classes by less than 2 percent. 

Approximately 30 percent of the samples from the rim 

around the head of Oceanographer Canyon contained 

appreciable gravel; for most samples, the recalculation 

affected the finer size classes by less than 10 percent, 

although four samples were affected by 12-28 percent. 

1 Any use of trade names in this report is for descriptive purposes 

only and does not constitute endorsement by the U.S. Geological Survey. 

2 The Shelf-Slope Transition on the Southern Margin of Georges Bank 
:; 

I 

& 

0 

:II 

w 

EXPLANATION 

-.MEAN BOTTOM FLOW 

...... TIDAL BOTTOM FLOW 

...,_ 

~~~~ 

\ ¥" 

Bathymetry in meters 

Figure 1. Map of Georges Bank showing physiography of the region, isolation of the 

bank from continental sediment sources, and major bottom-current patterns. Heavy lines 

indicate study areas, which are mapped in more detail in figures 2-5, 7, and 12. Major, 

predictable bottom currents on the bank are described briefly here. (1) The anticyclonic 

mean residual current flows at speeds of 5 cm/s or less, except on the northern edge where 

its speed reaches approximately 10 cm/s. (2) Anticyclonic Gulf Stream warm-core-ring 

currents flow northeastward at speeds of 50 cm/s or more on the upper slope at depths of 

200-300 mas the rings drift southwestward along the bank margin. (3) Rotary semidiumal 

tidal currents have their strongest flow across the bank-magnitudes of major axes of 

ellipses are as great as 10-20 cm/s on the bank margin and are more than 40 cm/s on the 

bank crest. Surface currents have similar patterns but flow faster than do bottom currents. 

Base map from Uchupi (1965). This figure is based in part on Butman and Beardsley (1987) 

and Butman and others (1987). 
68°20' 

EXPLANATION 

• SAMPLE 

15' 

40°~50' • CURRENT METER 

- DIVE TRAVERSE 

[@) SAND DUNES / 

- NET SEDIMENT TRANSPORT/ 

[) GRAVEL PAVEMENT/ 

0 5 KILOMETERS 

AREA SHOWN IN 

FIGURE 3 

Figure 2. Map of Oceanographer and Heel Tapper Canyons and the two outer shelf-upper slope study 

areas. One shelf-slope study area was sampled by submersible during traverse C; the other is shown in figure 

3. Dive traverses in the head and on the floor of Oceanographer Canyon are not shown (see Valentine, Cooper, 

and Uzmann, 1984; Valentine, Uzrnann, and Cooper, 1984}. Map base from Carpenter and others (1982). 

Information on samples from sites plotted here is given in figures 6--8. 

4 The Shelf-Slope Transition on the Southern Margin of Georges Bank 
18' 

17' 

16' 

68°02' 

EXPLANATION 

0 SHELF Ofll SLOPE SAMPLE 

• FILE80TTOM CANYON FLOOR 

SAMPLE 

(] ALONG·SHELF CURRENTS 

AND COARSE SEDIMENT 

500 METERS 

q 

Figure 3. Map of the outer 

shelf-upper slope study area 

that includes the branching 

head of Filebottom Canyon. 

Information on grain sizes of 

samples along traverses A 

and B is plotted in figure 8, 

and data for all samples are 

plotted in figure 6. Samples 

were collected from surface 

ship and from submersible; 

all samples from below 350m 

were collected by submersible. Sediment coarsens on 

the upper slope between 200 

and 300 m (stippled area) 

because of winnowing by 

along-slope currents flowing 

principally to the northeast. 

Bathymetry is unpublished 

work by the author. 

Introduction 5 
0 

67°55' 

EXPLANATION 

• SAMPLE 

• CURRENT METER 

- DIVE TRAVERSE * MULTIPLE DIVE TRAVERSES 

{ill] SAND DUNES 

- NET SEDIMENT TRANSPORT 

[] HARD SANDY GRAVEL 

5 KILOMETERS 

50' .45'. 

Figure 4. Map of Lydonia and Gilbert Canyons showing 

where current meters were deployed in 1980-82 as part of 

the Lydonia Canyon experiment (Butman, 1987; Butman and 

Conley, 1984; Butman and others, 1983; Moody and others, 

1984). Data from only the eight samples from Lydonia 

Canyon floor and the nine samples from Gilbert Canyon floor 

The study of sediment texture, bed forms, and 

sea-floor morphology is complemented by observations 

made during a recent, long-term investigation by other 

workers of currents and sediment transport in the shelfslope transition on Georges Bank. In particulat; Lydonia 

Canyon and its adjacent shelf and upper slope were 

studied intensively with a network of 20 moored instrument arrays that collected data during periods of several 

months to 2 years, and the physical oceanography of this 

canyon is better documented than that of any Georges 

Bank canyon (Butman, 1987; Butman and Beardsley, 

1987; Butman and Conley, 1984; Butman and others, 

1983, 1987; :Moody and others, 1984). 

are plotted in figure 6. Lydonia samples are part of the data 

presented by Butman and others (1983) and Twichell (1983), 

and samples from the canyon rim and the adjacent shelf 

were used by them to map bottom texture. Map base from 

Carpenter and others (1982). 

Acknowledgments 

This research was undertaken in cooperation with 

scientists of the National Marine Fisheries Service of 

NOAA (U.S. National Oceanographic and Atmospheric 

Administration), and cruises were funded by NOAA's 

Office of Undersea Research. I appreciate the support of 

the crews of the Johnson-Sea-Link submersible and support ship R.V. Edwin Link, Harbor Branch Oceanographic Institution, and the Alvin and support ship R.V. 

Lulu, Woods Hole Oceanographic Institution. In particulat; I thank Elliott A. Finkle (NOAA), Joseph R. Uzmann 

(National Marine Fisheries Service), and Richard A. 

6 The Shelf-Slope Transition on the Southern Margin of Georges Bank 
Cooper (University of Connecticut) for their interest and 

support during the course of this study, Brad Butman 

(USGS, U.S. Geological Survey) for sharing with me his 

insights on the current and sediment dynamics of Georges 

Bank and the preliminary results of the Lydonia Canyon 

experiment, and John S. Schlee (USGS) and Robert 

Oldale (USGS) for their helpful reviews of the manuscript. 

SEDIMENTARY PROVINCES OF THE 

GEORGES BANK REGION 

Georges Bank Shelf 

Georges Bank (fig. 1) is part of the continental shelf 

off New England and is covered by Pleistocene sand and 

gravel that were deposited when glaciers extended over its 

northern edge (Schlee, 1973). On the crest of the bank, 

water depths range from 3 to 40 m. The bank edge to the 

north is at about 60 m, below which the sea floor dips 

steeply into the Gulf of Maine. The southern, or seaward, 

edge of the bank is at about 200 m and is far from the 

bank crest. At present, the bank is isolated from continental sediment sources by the Gulf of Maine basin. Lateral 

movement of sediment onto the bank from the Scotian 

Shelf to the northeast and from the Middle Atlantic Shelf 

to the southwest is impeded by topographic lows in 

Northeast Channel and Great South Channel. The seaward edge of the bank is dissected by submarine canyons 

and gullies. 

The most comprehensive regional survey of sediment distribution showed that gravel is concentrated on the 

northern and eastern parts of the bank and that the 

remainder of the bank is covered by sand that decreases in 

size toward deeper. water both to the north and to the 

south, thus suggesting sediment transport in those directions (Schlee, 1973, pl. 1; Schlee and Pratt, 1970). A more 

recent study based on closer spaced sampling on the shelf 

in the vicinity of Oceanographer and Lydonia Canyons 

also recorded a regional decrease in mean grain size 

toward the shelf edge (Butman and others, 1983). Howevet; the sediment pattern becomes more complex near the 

canyon heads where local concentrations of very fine sand 

are present. The silt and clay content of shelf sediment is 

low, commonly 5 percent or less, and only rarely reaches 

10 percent in some areas near the canyons (Hathaway, 

1971 ). Vigorous currents atop the bank decline in strength 

as water depth increases toward the shelf break (Butman, 

Beardsley, and others, 1982). 

Submarine Canyons 

The southern edge of Georges Bank is indented by 

numerous submarine canyons of varying size that extend 

69°00' 

fiYDROGRAPfiER CANYON 

EXPLANATION 

• SAMPLE 

• CURRENT METER 

(!ill SAND DUNES 

- NET SEDIMENT TRANSPORT 

0 

~ 

2 KILOMETERS 

Figure 5. Map of Hydrographer Canyon. Data 

from samples collected from the canyon floor by 

submersible in 1984 are plotted in figure 6. Current 

meters were deployed in 197 4 and were reported 

on by Keller and Shepard (1978) and Shepard and 

others (1979). Map base from Carpenter and others 

(1982). 

across the slope to the deep sea floor. Although canyon 

rims are northward extensions of the outer shelf edge, 

canyon environments can vary considerably from those of 

the outer shelf and upper slope at equivalent depths. The 

canyons have opposing walls and are oriented normal to 

the outer shelf edge; thus, they and the bank margin 

interact in different ways with the major current systems of 

the region. 

Of the many incisions into the southern edge of 

Georges Bank, only a few extend an appreciable distance 

into the shelf. Oceanographer Canyon is the largest of 

these, and it reaches 22 km north of the shelf-slope 

transition (fig. 2). This canyon is the most thoroughly 

studied of all the Georges Bank canyons. It contains a 

Sedimentary Provinces of the Georges Bank Region 7 
wide range of sedimentary environments, and in many 

ways, it resembles other large and medium canyons 

(Hydrographer, Gilbert, Lydonia), but it differs from the 

smaller ones. The canyon rim carries the shelf break from 

a depth of approximately 200 m on the outer shelf to about 

130 m at the canyon head. The canyon is a smooth-walled, 

broadly.sinuous feature except for its lobate head formed 

by several tributary side valleys. The canyon floor ranges in 

depth from about 190m in its head to about 1,200 mat its 

mouth, where it crosses the outer edge of the shelf. 

The sedimentary facies in the northern part of 

Oceanographer Canyon are much more complex than 

those found on the surrounding shelf; they have been 

described and their origin has been discussed elsewhere 

(Valentine, Uzmann, and Cooper, 1980, 1984; Valentine, 

Cooper, and Uzmann, 1984). The facies are briefly 

described below. The canyon rim is veneered by gravelly 

sand and gravel or by a tightly packed gravel pavement; the 

gravel probably was transported by ice rafting and rKJW is 

immobile. The upper walls are covered by sand transported from the shelf; this sand is commonly rippled and is 

somewhat silty as a resuk of mixing by burrowing organisms with an underlying sandy silt deposit. Ripples extend 

down to a depth of 350m in some places on the upper 

walls, and their crests are oriented normal to the canyon 

rim, indicating transport along canyon walls. Similarly 

oriented ripples are also present in patches on the lower 

walls and are abundant on the canyon floor from the head 

down to at least 630 m, the depth limit of this study in 

Oceanographer Canyon. The lower canyon walls are exposures of Pleistocene semiconsolidated sandy silt that in 

places is covered by a thin layer of sand. The silt drapes the 

canyon walls and is part of a thick wedge of glacial 

outwash that was deposited before and during the last rise 

in sea level and that now underlies the outer shelf and 

upper slope. Silt outcrops commonly are disaggregated by 

bioerosion and by related spalling, sliding, and slumping. 

The canyon floor down to at least 630 m is covered by sand 

containing little silt and clay (generally 5 percent or less); 

the sand has been transported from the shelf and forms 

dunes as high as 3m. Strong bottom currents have been 

observed to flow from the shelf across the eastern canyon 

rim and also up and down the canyon along the walls and 

floor. 

By contrast, the sedimentary environments of small 

canyons are less diverse. Observations in Heel Tapper (fig. 

2), Atlantis, and Veatch Can}Uns indicate that their walls 

and floors north of the shelf edge generally are covered by 

silty sand that is slightly rippled in some places; bioeroded 

Pleistocene silt is exposed locally on the upper walls. 

Outer Shelf-Upper Slope 

Two areas of the outer shelf-upper slope of Georges 

Bank have been sampled for this study (fig. 2). The most 

thoroughly studied area is 8 km east of the mouth of 

Oceanographer Canyon and encompasses the branching 

head of Filebottom Canyon, a gully that does not penetrate 

the shelf edge (fig. 3).1bur dives and grab sampling from 

a surface ship were conducted here from 1982 to 1984. 

During a single dive in 1982 (traverse C, fig. 2) in a similar 

geologic setting 20 km west of Oceanographer Canyon, 

five samples were collected; they are included in the data 

base. 

In the area east of Oceanographer Can}Un, the 

outer shelf from 175 m water depth is relatively flat and 

smooth and dips gently seaward. At the shelf break (220 

m), the inclination of the sea floor increases to about 5°. 

The upper slope extends uninterrupted down to the heads 

of two gullies that commence at 300-350 m and that are 

part of the branching head of Fllebottom Canyon. Between 

the gully heads, the sea floor slopes 10°-15°. The gullies 

are narrow and steep walled, and their relief is as much as 

150m. 

The outer shelf-upper slope sampled in traverses 

A-C is almost completely covered by sand in which the silt 

and clay content increases with depth, ranging from less 

than 5 percent on the outer shelf to about 36 percent at 

640 m, the greatest depth examined in this setting. Rippled 

sand was observed above 190 m on traverse B in August 

1982. The asymmetrical ripples were parallel to the shelf 

edge and had their steep faces oriented downslope, indicating offshelf transport at that time. This pattern is 

probably altered by variations in the strength and direction 

of bottom currents with time. The sea floor on the 

outermost shelf and upper slope below the rippled sand is 

commonly shaped into conical mounds 5-10 em high and 

10-15 em in diameter that presumably are constructed by 

burrowing annelid worms; these features are also characteristically present on the upper canyon walls. In addition, 

patches of glacial gravel are present between about 200 

and 300 m on the uppermost slope. 

The sand sheet that covers the outer shelf and upper 

slope is thin, revealing in small, isolated exposures the 

Pleistocene semiconsolidated sandy silt that is so prevalent 

on the walls of Georges Bank canyons. On the gully walls 

of Filebottom Canyon, slopes of 200 -~, similar to the 

slopes of outcropping silt observed on canyon walls, 

suggest that the gullies are cut into the underlying silt. The 

thin layer of sand on the gully walls appears to lie at its 

angle of repose, for on one occasion, a slight disturbance 

of the sea floor by the submersible triggered a sand flow. 

A sample from the gully floor at a depth of 558 m and just 

below the flow area is composed of 51 percent very fine 

sand and 25 percent sik and clay. The supply of sand to the 

gully walls apparently is adequate to replace sand removed 

by mass-wasting processes, as silt exposures are sparse 

there. Moreover, the lack of exposed silt and the instability 

of the sand layer on the steep walls limit burrowing by 

8 The Shelf-Slope Transition on the Southern Margin of Georges Bank 
crabs, and bioerosion is negligible compared to that 

observed in Oceanographer Canyon. 

The sand that covers the narrow gully floors is 

unrippled. Abandoned tubes of epibenthic worms collect 

in small piles along the axes and serve as a somewhat 

unstable substrate for concentrations of large venus flytrap 

anemones. The presence of anemones and fine-grained 

sediment and the absence of rippled sand suggest weak 

currents, low turbidity, and slow sediment movement on 

the gully floors, in direct contrast to the energetic sedimentary environment on the floor of Oceanographer 

Canyon at equivalent depths. 

CURRENTS AND SEDIMENT TRANSPORT 

Currents on Georges Bank 

Currents can be described as Eulerian flow, which is 

flow past a fixed point measured by a current meter, and as 

Lagrangian flow, which is the path of a water particle and 

is measured by current drifters. The clockwise motion of 

the mean residual current on Georges Bank was first 

identified by Bigelow (1927) and has been substantiated by 

both Eulerian and Lagrangian measurements (Butman, 

Beardsley, and others, 1982). Recently, students of currents on the southern flank of Georges Bank have 

described bottom currents in terms of Eulerian flow, and 

the current speeds cited in the present paper refer to 

Eulerian flow measured at stations in shelf, slope, and 

canyon environments. In predicting the effect of currents 

on sediment transport, we must keep in mind that Eulerian flow speed and direction at one site may differ greatly 

from flow in adjacent areas, especially if the sea floor 

changes markedly in depth or if the bottom topography is 

irregulat: 

Long-term current observations provide information on the origin and interaction of the many kinds of 

currents present on Georges Bank (Brown and Moody, 

1987; Butman, 1987; Butman and Beardsley, 1987; 

Butman, Bothner, and others, 1982; Butman, Beardsley, 

and others, 1982; Butman and others, 1983, 1987; Moody 

and others, 1984 ). The major elements of the current 

regime are (1) the mean residual current; (2) lowfrequency (subtidal) currents that have periods of greater 

than 24 hours to 1 month and that are caused by density 

effects, wind stress, and ocean circulation phenomena 

such as Gulf Stream warm-core rings; (3) semidiurnal 

tidal currents that have periods of 12-24 hours; and (4) 

unpredictable high-frequency currents that have periods of 

12 hours or less and that are caused by surface and 

internal waves. The results of the studies cited above are 

used in the following summary of flow patterns on Georges 

Bank (fig. 1 ). 

The mean residual current is stronger near the 

surface than it is near the sea floor. It is most vigorous in 

a narrow band along the steep northern flank of the bank 

where flow is to the northeast at almost 30 cm/s near the 

surface and at 10 cm/s near the bottom. On the southern 

flank, the mean current is weaker; near-surface flow is 

about 15 cm/s, and bottom flow is 5 cm/s or less. The 

mean flow is anticyclonic around the bank and generally 

follows the isobaths. 

Low-frequency currents are also primarily oriented 

along isobaths, but in contrast to the mean residual flow, 

they are characterized by flow reversals lasting several 

days. These currents vary greatly in strength. fur example, 

along the upper slope, anticyclonic currents associated 

with southwestward-drifting Gulf Stream warm-core rings 

flow northeastward at speeds of 80 cm/s or more near the 

surface and 50 cm/s near the bottom (Butman and Conley, 

1984). 

Semidiurnal tidal currents are the most consistently 

energetic currents on Georges Bank. The tidal current can 

be represented by a vector that extends from the center of 

an ellipse to its perimeter; the tidal vector makes one 

revolution around the ellipse in a tidal period, reaching two 

maxima and two minima along the major and minor axes, 

respectively (Moody and others, 1984). Tidal flow and 

direction usually are averaged for a 29-day period. The 

strongest tidal flow on Georges Bank is oriented 

northwest-southeast across isobaths; near-surface currents along the major axis of the tidal ellipse reach speeds 

of 80-100 cm/s on the shallow bank crest and 30-40 cm/s 

on the northern and southern flanks of the bank. Nearbottom tidal currents reach speeds of more than 40 cm/s 

on the bank crest and 10-20 cm/s on the bank margin 

(Moody and others, 1984). 

High-frequency currents are not discussed here. 

Storm -generated currents are intermittent and vigorous 

on the sea floor of the bank crest (3-40 m), but they do 

not play a significant role in the shelf-slope transition 

(150--600 m) under study here. The occurrence and 

effects of high-frequency internal waves are poorly 

understood. 

The currents described above rework sediment on 

the bank and transport it away from the crest. Observations and sampling from a submersible dive on the northern flank of the bank in 1983 revealed that sand and gravel 

are moving downslope and mixing with silt and clay in the 

Gulf of Maine. On the southern flank of the bank, 

sediment moves south onto the upper slope and into the 

submarine canyons that incise the bank and southwest 

toward the shelf off southern New England (Butman, 

1987; Butman and Moody, 1983; Valentine, 1983; Valentine, Uzmann, and Cooper, 1980, 1984; Valentine, Cooper, 

and Uzmann, 1984). There is also evidence that silt and 

clay, and perhaps very fine sand, are transported southwestward in suspension and are deposited on the shelf off 

Martha's Vineyard (Bothner and others, 1981; Twichell 

and others, 1981). 

Currents and Sediment Transport 9 
Currents In Lydonla canyon and Environs 

In an effort to define patterns of circulaOOn and 

sediment movement in a submarine caD)Un and on the 

adjacent shelf and upper slope, an array of moored 

current meters was deployed in and around Lydonia 

Canyon (fig. 4) for periods ranging from 4 months to 2 

years (Butman, 1987; Butman and_ Beardsley, 1987; 

Butman and Conley, 1984; Butman and others, 1983, 

1987). This canyon is about 35 km east of Oceanographer 

Canyon and the upper slope area under study here. 

Oceanographer Canyon is larger than Lydonia Canyon, 

but they have similar geologic and hydrodynamic settings. 

The instrument array included 20 stations (18 of which are 

shown in fig. 4) where current meters were placed at 

different depths. R>urteen stations were on the canyon 

floor, walls, and rim; three stations were on the shelf near 

the canyon head; and three stations were on the upper 

slope near the can}Un mouth. 

Monthly mean flow on the shelf and upper slope to 

245m near Lydonia Can}Un measured 1 to 5 m above 

bottom (1-5 mab) was weak, generally less than 6 cm/s, 

and was directed offshelf and downslope across isobaths 

(Butman and Conley, 1984). During months when Gulf 

Stream warm-core rings touched bottom on the upper 

slope, monthly mean flow was reversed to the northeast 

and reached 18-24 cm/s. Mean near-bottom flow on the 

canyon floor calculated for periods of 5-6 months had a 

speed of 5-6 cm/s or less (Butman and others, 1983). In 

the canyon head, it was directed downcanyon at 277 m ( 5 

mab) and upcan)Un at stations at 554 m ( 6 mab) and 584 

m (6 mab). 

Low-frequency, or subtidal, near-bottom currents 

on the shelf and upper slope near Lydonia Canyon principally flowed northeast and southwest along isobaths 

(Butman and Conley, 1984). On the shelf at 100-120 m 

near the canyon head, the speed of these currents along 

the major axis of the current ellipse (computed for 

monthly segments) was on the order of 10 cm/s or less. 

Longshelf wind stress in large part was responsible for this 

flow. On the upper slope at 245 m (5 mab ), the magnitude 

of the major axis of the ellipse ranged from about 6 to 12 

cm/s and could be attributed, in part, to flow within Gulf 

Stream warm-core rings. In the canyon, low-frequency 

currents were oriented approximately north and south 

along the canyon axis, and the magnitude of the major axis 

of the current ellipse was less than 5 cm/s. 

The lunar semidiurnal (M2 ) tidal current having a 

period of 12.42 hours is by far the strongest tidal flow in 

the Georges Bank region. According to Moody and others 

(1984), it causes more than 80 percent of the tidal 

variance represented by the ftve major components of the 

astronomical tide. The major axis of the tidal ellipse 

(computed for monthly segments and averaged for periods 

of 2-6 months) is oriented normal to isobaths on the shelf 

and slope (Butman and Conley, 1984). The bank crest 

experiences the most vigorous tidal currents, and their 

speeds decrease as water depth increases. Nevertheless, 

the currents measured at two stations on the shelf near the 

canyon were strong, and the magnitudes of major axes of 

ellipses for near-bottom currents ranged from 15 to 18 

cm/s. Currents were somewhat weaker at a slope station, 

reaching speeds of only about 6 cm/s at 245m (5 mab). 

Tidal currents in Lydonia Canyon were oriented along the 

canyon axis and parallel to isobaths on the walls. On the 

floor of the canyon head, the major axis of the tidal ellipse 

was oriented along the canyon, and its magnitude was 

about 6 cm/s at 277m (5 mab ), similar to speeds observed 

at an equivalent depth on the upper slope. However, 

farther downcanyon, analogous tidal currents on the floor 

at 595 m (5 mab) were significantly stronger and reached 

speeds of about 16 cm/s. 

Sediment Transport 

The discussion of regional current patterns so far 

has been presented in terms of monthly mean currents and 

the magnitudes of major axes of tidal and low-frequency 

current ellipses based on hour-averaged observations computed for monthly segments. In order to determine better 

when and where sediment is moved, we must evaluate 

short -term current speeds near the sea fhor and their 

effect on sediment transport. The steady current speed 

required to initiate movement of uniformly sized quartz 

sediment on a flat bed has been determined empirically 

under ideal conditions (table 1; Butman, 1987, and references therein). Howevet; these values are only an approximation of natural conditions because variations in sediment texture and bottom roughness and biological binding 

of sediment particles influence the shear velocities required 

for particle movement. fur a smooth bottom, the required 

current speed 1 mab to move sediment ranges nonuniformly from 30 to 44 cm/s for grain sizes ranging from 

coarse silt (5 ~) to coarse sand (1 ~ ), whereas for a 

bottom roughness of 3 em, the required current speed is 

lower and ranges from 15 to 28 cm/s. 

The strongest currents on the shelf near Lydonia 

Canyon and in the canyon itself are those generated by the 

semidiurnal tide and possibly by high-frequency internal 

wave motion; higher on the bank, tidal and storm currents 

are most important; and on the upper slope, low-frequency 

currents associated with Gulf Stream warm-core rings 

dominate (Butman, 1987; Butman and Moody, 1983; 

Moody and others, 1984). The maximum hour-averaged 

bottom-current speeds recorded during periods of 1.5 

months from late 1980 to late 1982 at stations on the shelf 

at 99 m (1 mab ), 119m (1 mab ), and 126m (1 mab) near 

Lydonia Canyon ranged from 42 to 62 cm/s. During a 

5-month period in 1980-81, two stations in the canyon axis 

10 The Shelf-Slope Transition on the Southern Margin of Georges Bank 
Table 1. Steady current velocities required for initiation of movement (uJ and for suspended transport (us) of quartz sediment 

of uniform size 

[u0 is the steady current velocity needed for grains to become part of the bed load, and u1 is the velocity needed for grains to be transported in 

the suspended load. Critical shear velocities for threshold of movement (u.0 ) and for suspension (u •• ) are given for quartz particles of each size. 

For the calculations of u0 and u,, logarithmic flow in the bottom layer and a flat sea floor were assumed. Currents were calculated for 1 and 5 m 

above bottom ( mab) for a smooth bottom having a roughness equal to grain diameter (d) and for a rough bottom having a constant roughness 

of 3 em. For example, the equation for initiation of movement is u0,. = (u.Jk:) In (z/z0 ), where u.0 is shear velocity (in millimeters per second); 

k is von Karman's constant (0.4); z is distance above bottom (in millimeters); z0 is a roughness length scale, where z0 = roughness/30 (for smooth 

bottom, z0 = d (mm)/30; for rough bottom, z0 = 30 mm/30). Adapted in part from Butman (1987, table 1; see references therein)] 

Smooth bottom Rough bottom 

d d U•c U•s Bed load 

(4>) (mm) (cm/s) (cm/s) u0 (cm/s) 

1 mab/5 mab 

5 0.031 0.86 0.2 30/33 

4 .063 1.07 .8 35/39 

3 .125 1.25 3.1 39/44 

2 .250 1.38 9.4 40/46 

1 .500 1.60 21.9 44/50 

at 277 and 595 m (both 5 mab) recorded maximum 

hour-averaged current speeds of 49 cm/s; during four 

deployments from December 1980 to July 1982, a station 

on the nearby slope at 245 m (5 mab) recorded maximum 

current speeds ranging from 45 to 69 cm/s (Butman, 

1987; Butman and Conley, 1984; Butman and others, 

1983). 

Although these speeds are substantial, maximum 

hour-averaged data (and the magnitudes of major axes of 

current ellipses) tend to obscure the presence of even 

stronger currents of short duration that are important in 

the transport of coarse sediment. fur instance, observations averaged for 7.5-minute intervals recorded at the 

station at 277 m in the head of Lydonia Canyon during late 

November and December 1980 show a series of speed 

peaks that are strongly correlated with the semidiurnal tide 

having a period of 12.42 hours. During the 33-day period, 

the bottom-current speed exceeded 50 cm/s four times 

when the current flo\ved upcanyon (north); it exceeded 40 

cm/s sixteen times, mostly when the current flowed 

upcanyon; and it exceeded 30 cm/s twenty-eight times 

when the current flowed upcanyon and twenty-two times 

when the current flowed downcanyon. Hour-averaged data 

for the same period do not show speeds greater than 50 

cm/s and show only four peaks exceeding 40 cm/s 

(Butman and others, 1983, figs. 8-25b, 8--25c). Most of 

the energy in these short bursts of speed can be attributed 

to tidal forces that were strengthened at times by highfrequency internal waves; the low-frequency component 

generally accounted for less than 10 cm/s (hour-averaged 

data) of the total current speed and was directed south and 

east (Butman and others, 1983, fig. 8--25b ). The magnitude of the major axis of the tidal ellipse at this station was 

only about 6 cm/s, and the monthly mean current speed 

was about 4 cm/s downcan}On. A short record of 7.5-

minute observations at the station at 595 m in the axis of 

Lydonia Canyon shows a similar series of strong flows that 

Suspended load Bed load Suspended load 

us (cm/s) u0 (cm/s) us (cm/s) 

1 mab/5 mab 1 mab/5 mab 1 mab/5 mab 

7/8 15/18 3/4 

26/29 18/23 14/17 

96/108 22/27 54/66 

275/313 24/29 162/200 

602/690 28/34 378/466 

correlate with semidiurnal tides (Butman and others, 

1983, fig. 8--26c). The strongest near-bottom currents at 

the shelf, upper slope, and canyon stations in the Lydonia 

Canyon region are capable of transporting as bed load 

sediment particles as large as coarse sand ( 1 4>) even if 

current speeds calculated for a smooth bottom are 

required (table 1). 

As part of the Lydonia Canyon experiment, current 

meters also were deployed from January to July 1982 in 

the axis of Oceanographer Canyon 35 km to the west (fig. 

2; Butman and others, 1983). One station was in the head 

at 223 m ( 4 mab ), and the other was farther downcanyon 

at 554 m (6 mab). Mean flow was downcanyon at both 

stations. The strongest currents were tidal and were oriented along the axis; mean and maximum speeds observed 

in 7.5-minute data without regard to direction were 20 

cm/s and 73 cm/s, respectively, in the canyon head and 28 

cm/s and 96 cm/s at the deeper station. These relatively 

long term records support direct observations of strong 

currents from submersibles in Oceanographer Canyon 

and conclusions based on asymmetry of sand dunes that 

net sediment transport is downcanyon at the two stations 

(Valentine, Cooper, and Uzmann, 1984). 

Observations of currents, sediment texture, and bed 

forms during dives in Oceanographer Canyon and other 

canyons and on the adjacent slope suggest that the circulation patterns revealed in the study of Lydonia Canyon 

and its environs are generally applicable to an investigation 

of the sediment dynamics in the shelf-slope region in and 

around Oceanographer Canyon. 

SEDIMENT TEXTURE OF THE SHELF-SLOPE 

TRANSITION 

We have seen that the strongest currents in the large 

Georges Bank can}Qns are related to tidal flow up and 

Sediment Texture of the Shelf-Slope Transition 11 
Table 2. Grain-size distribution of samples from 11 stations on 2 traverses in the shelf-slope transition study areas near 

Oceanographer Canyon with and without carbonate matter 

(Samples were collected by the submersible Alvin in 1982 Traverse B is east of Oceanographer Canyon (fig. 3); traverse Cis west of it (fig. 2)] 

Carbonate 

content 

(weight 

percent) 

Weight percentage of sediment in major size classes 

with (and without) carbonate matter 

Differences in weight percentages in each 

size class caused by removal 

of carbonate content 

Traverse 

B 

B 

B 

B 

B 

B 

c 

c 

c 

c 

c 

Depth 

(m) 

180 

200 

250 

356 

455 

608 

206 

250 

350 

445 

595 

2.32 

3.53 

3.03 

3.05 

4.54 

6.22 

4.39 

3.20 

5.24 

6.86 

6.74 

a 

Gravel and 

sand 

(-14»-4;) 

98.51 99.50 

96.02 97.64 

95.81 97.03 

92.10 93.80 

80.10 82.22 

67.35 69.44 

93.74 95.92 !95.46! 96.62 

88.91 90.95 

77.14 79.04 

64.21 66.30 

b 

Silt 

0.91 r.35 2.24 1.76 

3.03 2.28 

5.51 4.77 

16.48 f12.45 

27.11 22.20 

4.35 !3.54! 2.95 2.46 

8.68 7.13 

18.79 (16.17 

24.76 (25.98 

down the canyons along the floors and walls, parallel to the 

major regional tidal forces that transport water in and out 

of the Gulf of Maine across Georges Bank. In contrast, 

although the seaward-facing outer shelf-upper slope is 

oriented approximately east-west, normal to the major 

regional tidal forces, the strongest currents here flow east 

and west along the slope and are related to Gulf Stream 

warm-core rings. The effect of currents on sediment 

transport through the shelf-slope transition in canyons and 

on the outer shelf-upper slope of the bank margin can be 

assessed indirectly by comparing the texture of surficial 

sediment in the same depth interval in both regions. 

Textural comparisons of sediment from different 

environments could be complicated by the fact that finegrained biogenic carbonate matter becomes an increasingly important component of upper slope sediment as 

water depth increases. furaminifers and calcareous nannofossils in abundance could increase the proportion of the 

finer size classes in the sediment. Samples collected by 

submersible at eleven stations on two traverses on the 

outer shelf-upper slope were split and analyzed with and 

without the carbonate fraction (table 2). West of the mouth 

of Oceanographer Canyon (traverse C, fig. 2), carbonate 

contents ranged from 4.39 percent at 206 m to 6.74 

percent at 595 m; east of the canyon (traverse B, fig. 3), 

contents ranged from 2.32 percent at 180 m to 6.22 

percent at 608 m. As expected, carbonate matter increased 

as depth increased, but the amount of increase was small. 

Size analysis sh<med that the maximum combined decrease 

in the weight percentage of silt- and clay-sized particles in 

samples treated with 20 percent glacial acetic acid did not 

exceed about 2 percent in any sample. Thus, comparisons 

of surficial sediment texture at the same depth in different 

environments of the shelf-slope transition are not significantly affected by carbonate matter. The following observations on sediment texture are mainly from Oceanographer Canyon and the nearby gullied outer shelf-upper 

c 

Clay 

(94»-11;) 

0.57 

1.75 0.11 0.60 

1.16 0.68 

2.37 1.42 

3.40 5.33 

5.55 8.3 

1.91 1.12 !0.98! 0.89 

2.40 1.92 

4.07 4.81 

11.04 7.73 

oa 

+0.99 

+1.62 

+1.22 

+1.70 

+2.12 

+2.09 

+1.72 

+0.70 

+2.04 

+1.90 

+2.09 

ob ob+oc 

-0.56 -0.44 -1.00 

-0.48 -1.15 -1.63 

-0.75 -0.48 -1.23 

-0.74 -0.95 -1.69 

-4.03 +1.93 -2.10 

-4.91 +2.82 -2.09 

-0.81 -0.93 -1.74 

-0.49 -0.23 -0.72 

-1.55 -0.48 -2.03 

-2.62 +0.74 -1.88 

+1.22 -3.31 -2.09 

slope; they are supplemented by observations from the 

floors of Lydonia, Gilbert, Hydrographet; and Heel Tapper Canyons (figs. 2-5). 

Texture of Canyon Floors 

The shelf-slope transition of Georges Bank contains 

a variety of sedimentary environments within and between 

the canyons. Sedimentary processes are very active in large 

canyons, and striking differences in the sedimentary environments are evident when comparing the sediment found 

on the floors of large canyons with that on the outer shelf 

and upper slope in the same depth interval. The texture of 

the canyon-floor sand is described here in terms of the 

median grain size of the sand fraction, following the 

practice of fulk (1980); these descriptive terms are not to 

be confused with the mean grain size of the whole sample, 

which is plotted in figure 6A. The discussion of currents in 

Figure 6. Graphs of (A) mean grain size versus water depth IIIIo.... 

and (h) silt and clay content versus water depth of samples from ~~~~"""" 

canyon floors and the outer shelf-upper slope study areas on the 

southern margin of Georges Bank. Data plotted in both graphs 

are for the same samples. A, Sediment coarsens on the upper 

slope between 200 and 300 m as a result of winnowing by 

along-slope currents. Upper slope sediment from water depths 

greater than about 300 m is finer grained than sediment from 

Oceanographer, Hydrographer, and Gilbert Canyons from the 

same water depths, indicating that currents on the canyon floors 

are stronger than those on the upper slope. Sediment from the 

floors of Heel Tapper Canyon and slope gullies (Filebottom 

Canyon floor) is no coarser than upper slope sediment at 

equivalent depths. 8, The weight percentages of silt and clay 

plotted here were calculated after data on gravel of pebble size 

(4 mm; -2 ;) and larger were deleted. Silt and clay contents of 

Georges Bank Shelf samples to depths of 200 m (not plotted) are 

typ!cally 5 percent or less (fig. 8; Hathaway, 1971). Note 

decrease in silt and clay in upper slope sediment between 200 

and 300 m and increase below 300 m (see fig. 8). Sediment from 

the floors of Heel Tapper Canyon and slope gullies (Filebottom 

Canyon floor) contains a large percentage of silt and clay. 

12 The Shelf-Slope Transition on the Southern Margin of Georges Bank 
5 0.031 

~ EXPLANATION 

en CANYON FLOOR OUTER SHELF -UPPER SLOPE 

• OCEANOGRAPHER • FILEBOTTOM CANYON AREA b. -+- HYDROGRAPHER WEST OF + HEEL TAPPER CANYON /!:. I!:. b. • GILBERT A FILEBOTTOM CANYON FLOOR 

E9 LYDONIA b. • • • ~0 b. HEEL TAPPER E9 0 • E9 

E9+ • C/) 

E9 0 a: • LU 

~ 

LU 

(J) 

0 E9 AA 

::E ~ .....J 

z • .....J :::> ~ 

:I: 

.... 

• 0 z Q.. • LU ~ +. N 

LU oO 0 (J) 

N • 00 -+- • U5 0 • E9 • z 

z • "g -+oo • • • • <t 

• • a: 

<( 0 &0 • E9 (.!) 

a: • • • • z (.!) -to • • •• •• <( 

z 0 • • • .. 

LU 

<( 00 • • • ::E 

w • • • • :E • • • 

• I 

0 1.00 

100 200 300 A 400 500 600 

40 EXPLANATION 

CANYON FLOOR OUTER SHELF-UPPER SLOPE b. 

b. 

+ 

b. • OCEANOGRAPHER 0 FILEBOTTOM CANYON AREA • b. -+- HYDROGRAPHER + WEST OF HEEL TAPPER CANYON • 

4 b. 

> • GILBERT A FILEBOTTOM CANYON FLOOR 0 • oo 

<( E9 LYDONIA _J 30 b. HEEL TAPPER "-0 (.) 

0 • 

z 

<( 

~ 0 • • ...J E9 U5 • ~ 20 E9 • z 

w E9 b. 

(.) od9 

a: 

w • E9 

Q.. 0 b. 

~ + :I: • ~ 10 0 E9 w E9 oOcf)O CD • 

:;: ~ • 

• 

oi o+

0 

• • 

• 0 ~ • ~ . . . ~· • 0 ... ~. 

100 200 300 400 500 600 

WATER DEPTH,IN METERS 

8 

Sediment Texture of the Shelf-Slope Transition 13 
a previous section shOwed that bottom flow in Oceanographer Canyon is more vigorous than that in Lydonia 

Canyon. This obseiVation is supported by differences in 

sediment texture and bed forms on the floors of the t\W 

canyons. Oceanographer Canyon, the largest on western 

Georges Bank, has dunes as high as 3 m composed of 

coarse to medium sand containing little silt and clay down 

to 630 m; the floor of Lydonia Canyon is covered by 

rippled fine to very fine sand containing 15 to 20 percent 

silt and clay except for an inteJVal from about 550 to 600 

m, where small dunes of medium sand are present (fig. 6; 

Butman and others, 1983; Twichell, 1983). These t\W 

canyons lie 35 km apart on the southern flank of the bank 

and are subject to the same regional current regimes. 

Although Lydonia Canyon is the fourth largest canyon in 

the region, it has only one-fifth the wlume of Oceanographer Canyon. The differences in current strengths on the 

canyon floors may be due to differences in canyon size and 

shape. Hydrographer Canyon is the second largest canyon. 

Its sedimentary environments resemble those of Oceanographer Canyon, as strong currents flow up and down its 

axis, and dunes and rippled sand are present on its floor 

(Southard and Stanley, 1976, p. 368; Keller and Shepard, 

1978). 

In 1984, the floors of Hydrographer, Gilbert, and 

Heel Tapper Canyons were visited to obseiVe bed forms 

and collect sediment samples that would provide a broader 

base for estimating the relative strengths of bottom currents in large and small canyons. In the head of Hydrographer Canyon at a depth of 254-263 m, the floor of the 

western arm is covered with rippled sand and dunes as 

high as 0.5 m. They are asymmetrical and have steep lee 

faces oriented upcanyon to the northwest. Sediment collected at three locations in this area is medium to fine sand 

containing less than 6 percent silt and clay (figs. 5, 6). 

During the dive, a current having a velocity of 5-10 cm/s 

flowed upcanyon from the southeast; during a period of 15 

minutes, its velocity increased to at least 75 cm/s ( estimated), and the current flowed at that velocity for more 

than an hour. Sand was obseiVed moving across the 

bottom, and the water became very turbid, making visibility poor. The current theoretically was strong enough to 

transport coarse and medium sand in the bed load and 

fine sand in suspension. The dive was ended because the 

submersible had to exert maximum power to remain 

stationary and could not be maneuvered safely in the 

strong current. Two similar events of strong upcanyon flow 

have been observed on the floor of Oceanographer Canyon-one in the main axis at 292m (Valentine, Cooper, 

and Uzmann, 1984), and the other in the east arm at 

175m. 

Gilbert Canyon is the third largest canyon, and a 

dive along its floor between the depths of 611 and 479 m 

revealed rippled, asymmetrical sand dunes as high as 1.5 

m, whose shapes indicated that net flow and sediment 

transport were downcanyon there. Nine samples collected 

from the dunes consisted of coarse to medium sand at 611 

m becoming medium to fine sand upcanyon; the silt and 

clay content was less than 4 percent (figs. 4, 6B). 

Heel Tapper Canyon is one of the smallest canyons 

in the region. It lies just west of Oceanographer Canyon, 

in the same oceanographic setting, and one would assume 

that they both experience the same regional tides and 

currents. However, a dive along the floor of Heel Tapper 

Canyon from 631 to 422 m revealed a uniform expanse of 

very fine to fine silty sand; silt and clay contents decreased 

from 37 to 13 percent as depth decreased (figs. 2, 6B). 

Ripple marks were rare. The floor of this small canyon 

most resembles the surface of the upper slope near the 

mouth of Oceanographer Canyon and represents a much 

less vigorous environment than that in the large canyon. 

The upper slope east of Oceanographer Canyon is 

cut by gullies that are oriented north-south parallel to the 

trend of the adjacent canyon. The texture of the bottom 

sediment in the relatively shallow gullies should indicate 

whether they, like Oceanographer Canyon, affected currents by channeling the flow. However, samples collected 

from the floors of two gullies that form the head of 

Filebottom Canyon do not differ systematically in the 

amount of silt and clay they contain from samples collected 

at equivalent depths elsewhere on the slope (figs. 3, 6A, 

B). The gully heads extend up to about 350m water depth, 

which is below the level at which along-slope currents 

dominate the bottom circulation. Although the gullies are 

aligned parallel to the major axes of tidal ellipses, tidal 

currents apparently are weak there; for example, the 

magnitudes of tidal ellipses are 6.3 cm/s or lower at 

analogous sites west of Lydonia Canyon (Butman and 

others, 1983, fig. 8-24, table 8-3), and bottom morphology does not appear to play a role in modifying current 

patterns. 

Texture of Canyon Rims 

The orientation of Oceanographer Canyon is normal to the shelf edge and parallel to the regional tidal flow, 

and without more information, there was little reason to 

suspect significant differences in the sedimentary environments of the opposing eastern and western canyon rims. 

Howeve:t; obseiVations from submersibles revealed that the 

eastern rim is covered by an extensive deposit of densely 

packed gravel (gravel pavement), whereas the western rim 

is covered by gravelly sand (fig. 2; Valentine, Uzmann, and 

Coope:t; 1980, 1984; Valentine, Cooper, and Uzmann, 

1984). Gravel in the pavement probably was ice rafted 

during the last glaciation. Dives on the eastern rim in 1978 

and 1982 between lat 40°24' N. and 40°27' N. (fig. 2) 

encountered a strong bottom current flowing westward at 

speeds as great as 50 cm/s. The large area of exposed 

14 The Shelf-Slope Transition on the Southern Margin of Georges Bank 
gravel and the presence of deposits of shelf sand in the 

downslope lee of cobbles and boulders suggest that the 

current is strong enough to maintain the gravel exposure 

on the eastern rim but has no similar effect on the western 

nm. 

An analogous, but finer grained, gravel deposit is 

present on the eastern rim of Lydonia Canyon (fig. 4; 

Twichell, 1983), and a long-term current-meter record 

indicates that bottom flow there is dominantly to the west 

(station LCQ of Butman and others, 1983). A dive to the 

current-meter site (fig. 4) documented the presence of 

packed, coarse sandy gravel and corroborated previous 

interpretations based on grab sampling and side-scan 

sonographs. Moreove~; a bottom current having a speed of 

about 40 cm/s flowed to the west throughout the 90-

minute duration of the dive. ' 

The origin of the westward-flowing current is not 

known. Observations of current speed and direction and 

sediment texture indicate that it is a persistent flow across 

the eastern rims of Oceanographer and Lydonia Canyons. 

The wide extent of the exposed gravel and evidence that 

shelf sand is moving through it into the canyons suggest 

that the eastern canyon rims are an important pathway for 

the transport of sand off the shelf. By contrast, the gravelly 

sand on the western rim of Oceanographer Canyon probably reflects the presence of weaker or less favorably 

oriented currents there and a slower transport rate of shelf 

sand into the canyon. 

Distribution of Silt and Clay-The Mud line 

The transport of silt- and clay-sized particles is-very 

sensitive to current strength. Relatively strong currents of 

some tens of centimeters per second are required to place 

them in suspension, but they remain in the water column 

and can be transported long distances by currents of only 

· several centimeters per second (Reineck and Singh, 1980, 

fig. 2, modified from Sundborg, 1967). The strong tidal 

and storm currents on Georges Bank have winnowed most 

of the silt and clay out of the shelf sediment. Today there 

is little or no silt or clay on the bank crest, less than 5 

percent in sediment on the southern flank of the bank, and 

not more than about 10 percent in sediment from the shelf 

near the heads of Oceanographer and Lydonia Canyons 

(Hathaway, 1971; Schlee, 1973; Butman and others, 1983). 

Most samples from the floors of Oceanographer, Gilbert, 

and Hydrographer Canyons contain less than 5 percent silt 

and clay down to at least 630 m, whereas the fine fraction 

of samples from the outer shelf-upper slope and Heel 

Tapper Canyon steadily increases below 300m and reaches 

35--40 percent at about 640 m (fig. 6B). The marked 

decrease in grain size on the upper slope below 300 m is 

related to a decrease in current ener&Y that is not observed 

in the same depth interval on the floors of the more 

energetic Oceanographer, Gilbert, Hydrographer, and 

Lydonia Canyons. 

The level at which silt and clay do not increase 

significantly with depth was termed the "mudline" ,by 

Stanley and Wear (1978) and Stanley and others (1983), 

who interpreted it to represent an ener&Y boundary below 

which deposition prevails over erosion. Those authors have 

shown that the mudline may be above or below the shelf 

edge, depending on factors that include shelf width, sediment supply, and current strength, and that it rarely 

coincides with the shelf edge. The four types of mudline 

outlined by Stanley and others (1983) are defined by depth 

and position relative to the shelf edge; type I, the deepest, 

is on the upper slope, and type IV is shoreward of the shelf 

edge. The slope near Oceanographer Canyon best represents a type I setting. The mudline appears to be below 

640 m, the deepest site sampled here. The amount of silt 

and clay in the sediment increased steadily to that level and 

did not exceed 36 percent, whereas the amount of very 

fine sand ( 4 cp) exceeded that of silt and clay in almost all 

samples from water depths greater than 350 m. Hathaway 

(1971) reported that samples collected in this area from 

lower on the slope than 640 m water depth contained 

70-95 percent silt and clay. Several factors are responsible 

for the relatively deep mudline in this part of Georges 

Bank. The supply of silt and clay is low because the bank 

is isolated from continental sediment sources and strong 

currents have swept the shelf and uppermost slope sand 

clean of fine-grained sediment. A relatively large amount 

of very fine sand ( 4 cp) is being transported off the shelf in 

this region, and the upper slope in water shallower than 

300 m experiences strong along-slope currents that remove 

very fine sand, silt, and clay. 

Distribution of Very Fine Sand (4 cfJ) 

Although sediment examined in the present study 

from the shelf adjacent to Oceanographer and Lydonia 

Canyons had uniformly low concentrations of silt and clay, 

the amount of very fine sand ( 4 cp) it contained ranged 

from 1 percent to more than 50 percent. Particles in the 

4-cp size class range in diameter from 0.125 mm (3 cp) to 

0.063 mm ( 4 cp ). Particles of silt, clay, and very fine sand all 

require a stronger current to initiate transport than to 

maintain them in suspension (table 1). Once in suspension, they may travel long distances in weak currents. In 

contrast, sand having a diameter of 3 cp or larger requires 

a substantially stronger current to place it in suspension 

than to move it as bed load. Butman and Moody (1983) 

suggested that suspension of 4-cp sand is common on 

Georges Bank. Although 4-cp sand travels in suspension, it 

requires a current three to four times as strong as that 

required by coarse silt (5 cp) to remain in suspension (table 

1). Thus, in a region of strong current activity, the 

Sediment Texture of the Shelf-Slope Transition 15 
distribution and abundance of 4-<P sand may be a sensitive 

indicator of relative current strength. A concentration of 

4-cP sand in sediment that contains little silt and clay may 

identify an area where currents are too strong to allow the 

deposition of finer particles from suspension. Sediment 

from the bank crest, where currents are strong, commonly 

contains. little or no 4-cP sand; samples from deeper water, 

where currents are weaker and circulation patterns less 

uniform, contain larger and more variable amounts of 4-cP 

sand, and discrete areas having high concentrations of 4-<P 

sand are near Oceanographer (fig. 7) and Lydonia Canyons (Butman and others, 1983, fig. 8-10c). 

Samples from the floor in the northern part of 

Oceanographer Canyon and samples from the adjacent 

shelf had markedly different contents of 4-<P sand. Samples 

from the shelf had a wide range of 4-<P contents, possibly 

reflecting variable flow directions, current strength, and 

source areas around the canyon head. Deposition predominated in areas east and west of the CaD)'On head, where 

4-<P sand made up 50--60 percent of the sediment. On the 

other hand, samples from the floors of both Oceanographer and Gilbert Canyons contained less than 3 percent 

very fine sand and supported observations that strong 

persistent bottom currents flow up and down the narrow 

axis to depths of at least 630 m, winnowing 4-</J sand from 

coarser sediment and transporting it out of the canyon. 

The shelf-slope transition on the outer edge of 

Georges Bank is not as varied texturally as it is in and 

around Oceanographer Canyon; the change from outer 

shelf to upper slope depths is gradual, and surficial 

sediment is more uniform in texture. The outer shelfupper slope sediment along traverses A, B, and C (figs. 2, 

3) from about 150 to 300m depth contained less 4-q, sand 

by weight than did shelf sediment in some areas at depths 

of 120-160 m near the canyon head. Therefore, the 

differences in weight percentage of 4-<P sand alone indicate that currents on the outer shelf-upper slope are 

stronger than those in several shallower areas adjacent to 

the CaD)'On head. This difference in current strength would 

not have been detected if only the silt and clay rontents of 

sediment from the two areas had been compared, for they 

are similar (fig. 6B). 

Texture of Outer Shelf-Upper Slope 

In general, very fine sand ( 4-cP ), silt, and clay 

contents increase as water depth increases on the outer 

shelf and upper slope. At depths shallower than about 200 

m, the outer shelf texturally resembles the floors of large 

canyons. On the upper slope from 200 to 300 m, sediment 

texture is variable (and patches of glacial gravel are 

present), but between 350 and 600 m, the surficial sediment is much finer than it is in the same depth interval on 

the floors of large canyons (figs. 6A, B). A closer look at 

the trends of iooividual sediment size classes along two 

parallel traverses (A and B) that extend from the outer 

shelf onto the upper slope and into the gullied head of 

Filebottom Canyon reveals the expected decrease in coarse 

sediment and increase in fine sediment with increasing 

depth (fig. 8). Sediment samples from 475 to 600 m on the 

slope between the two gullies had the same textural 

variations observed on traverses A and B at equivalent 

depths. 

Howevet; the increase in fine sediment with depth 

was reversed between 200 and 300 m, where mean grain 

size coarsened from fine to medium sand. Very fine sand 

( 4 q,) was present in low percentages on the outer shelf, but 

between 170 and 200 m, it increased as water depth 

increased. This trend was interrupted between 200 and 

300 m, where very fine sand contents declined and were 

variable, but it resumed below 300 m, and 4-cP sand 

accounted for 40-50 percent of the sediment at about 600 

m. The silt and clay content varied similarly. 

Fine-sand (3 q,) content increased from a range of 

20 to 30 percent on the outer shelf to 40 to 50 percent 

near 200 m. It then decreased abruptly and fluctuated 

between about 7 and 23 percent in the interval from 200 to 

300 m; below 450 m, it ma~e up 10-20 percent of the 

sediment. 

Medium sand (2 <P) was the most abundant sediment on the outer shel( declining from 50-60 percent 

there to 0-10 percent on the upper slope. However; 

between 200 and 300 m, medium sand fluctuated in 

abundance and constituted 30-50 percent of the sediment. 

Coarse sand (1 q,) was present in low percentages along 

each traverse except in the depth interval from 200 to 300 

m, where it ranged from 10 to 27 percent on traverse A 

and from 30 to 45 percent on traverse B. 

Sediment shoaler than 180 m was moderately sorted 

(standard deviation ranged from 0.82 to 1.03 cP ), but that 

from deeper water contained greater percentages of fine 

sediment and generally was poorly sorted (st. dev. = 1-2 

q, ). The depletion of fine sediment in the depth interval 

from 200 to 300 m was a local reversal in a trend toward 

increasing fines with depth and was not reflected in sorting 

values. The textural patterns described here were present 

also in the shelf-slope transition some 30 km to the west, 

as indicated by fwe samples collected along a traverse from 

200 to 600 m water depth near Heel Tapper Canyon (figs. 

2, 8, traverse C). 

These textural variations within the shelf-slope transition suggest that the size classes most sensitive to 

transport increase in abundance on the outermost shelf 

down to about 200 m in response to a general decrease in 

the strength of bottom currents. Fine sand (3 cP ), which in 

this. environment moves as bed load, is deposited preferentially, followed by very fine sand ( 4 cP ), silt, and clay, 

which travel in suspension (fig. 8; table 1). The marked 

decline in abundance of medium sand (2 cP) and coarse 

16 The Shelf-Slope Transition on the Southern Margin of Georges Bank 
17(2) 

+ 

+ 18(7) 

EXPLANATION 

+ GRAB SAMPLE 

+ 17(6) 

5 WEIGHT PERCENT VERY FINE SAND 

101 

+ 19(6) 

(7l WEIGHT PERCENT SILT AND CLAY 35(7)+ G WEIGHT PERCENT VERY FINE SAND 

0 SEDIMENT CONTAINING MORE THAN 

40o29' 40 PERCENT VERY FINE SAND 

0 1500 METERS 

Bathymetry in meters 19(94, 

Figure 7. Map of the head of Oceanographer Canyon 

showing the distribution of very fine sand {0.063 mm; 4 <P) 

and of silt and clay in samples from the canyon rim and floor. 

The weight percentages plotted here were calculated after 

data on gravef of pebble size (4 mm; -2 <P) and larger were 

deleted. Silt and clay content is generally low in all areas, 

except in several samples collected in the eastern arm below 

an escarpment of semiconsolidated sandy silt. Very fine sand 

sand (1 cp) from shallow areas to 200 m is caused largely 

by the increase in finer sediment there. The trend toward 

increased fine sediment with depth is reversed between 

200 and 300 m, and this reversal can be attributed to the 

erosion of fine sediment by strong currents that flow 

northeastward along the upper slope when anticyclonic 

Gulf Stream warm-core rings are present on the margin 

(figs. 3, 8). These currents, in combination with tidal flow, 

reached maximum hour-averaged speeds of 69 cm/s at 

245m (5 mab) near Lydonia Can}On (Butman, 1987). By 

contrast, the magnitude of the major axis of the M2 tidal 

current ellipse was less than 10 cm/s at this depth near 

Lydonia Can}On (Moody and others, 1984), and storm 

currents do not affect the bottom on the southern flank of 

9' 

+18(5) 

CANYON 

68°081 

+12(2) +21(4) 

14(3) 

57(1) 

•••• 

content is also low except in areas on the east and west rims 

where it is 50 percent or more and, thus, defines areas where 

bottom currents are relatively weak. See text for further 

explanation. Map base from Valentine, Uzmann, and Cooper 

{1984). Bathymetry does not extend north of the canyon 

head to where the northernmost samples were collected. 

Bathymetry differs somewhat from that shown in figure 2, 

which was obtained by different methods. 

Georges Bank at water depths below about 100-150 m 

(Butman and Beardsley, 1987). 

During an 8-year interval from 1976 through 1983, 

89 warm -core rings were identified west of long (fJ0 W. by 

use of satellite imagery (Celone and Chamberlin, 1980; 

Celone and Price, 1983; Fitzgerald and Chamberlin, 1981, 

1983, 1984; Mizenko and Chamberlin, 1979a, b; Price and 

Celone, 1984). Many of the rings did not encounter the 

upper slope of Georges Bank, but 19 rings are judged to 

have affected the sea floor in the area of Hydrographer; 

Oceanographer; and Lydonia Canyons during a cumulative period ofl7 months (fig. 9). In general, these rings 

ranged in diameter from 50 to 150 km and drifted 

southwestward at speeds of 3-8 km/day. Occasionally they 

Sediment Texture of the Shelf-Slope Transition 17 
50 

40 

30 

20 

10 

0 

60 

50 

40 

30 

20 

10 

0 

I- 50 

z 

w 

(.) 40 

a::: 

w 30 

a.. 

I- 20 

::J: 

(.!) 10 

w 

;: 0 

50 

40 

30 

20 

10 

0 

40 

30 

20 

10 

0 

TRAVERSE A 

1¢ 

~ .... 3¢ J\ /'\ lJ[ .-.f\. 

4¢ 

SILT AND CLAY ~~­

/· 

,.L/ ,--~-A ---

" 2 3 4 5 6 

TRAVERSE B 

1¢ 

2¢ 

.••••••••••••.•••••• ~-\.~~ 

4¢ 

SILT AND CLAY • _j_· -1\7-

kic--:L--· - 2 3 4 5 6 

TRAVERSE C 

1¢ 

395 

+--4-¢ -7-~ 

!8\.41::------(-

SILT AND CLA~· 

.1· 

2 3 4 5 6 

WATER DEPTH IN HUNDREDS OF METERS 

Figure 8. Graphs showing textural variations along three traverses across the shelf-slope transition. Traverses 

A and 8 are parallel and extend from the outer shelf onto the upper slope and onto gully floors in the head of 

Filebottom Canyon (fig. 3). Traverse C extends from the outer shelf onto the upper slope west of Heel Tapper 

Canyon (fig. 2). Weight percentages plotted here were calculated after deletion of data on gravel of pebble size 

(4 mm; -2 q,) and larger. Graphs for each size class show that as water depth increases, the weight percentages 

of fine sediment (3 q,, 4 q,, silt and clay) increase and the weight percentages of coarse sediment (2 q,, 1 q,) 

decrease. An exception to this trend is the interval on the upper slope from 200 to 300 m (stippled pattern) where 

fines have been winnowed by along-slope currents (see also fig. 3). 

18 The Shelf-Slope Transition on the Southern Margin of Georges Bank 
I 

GULF STREAMS 

WALL 

Figure 9. Map showing the number of warm-core rings (and their residence time in 

months) in segments along the western North Atlantic margin from 1976 to 1983. During 

the 8-year period, 89 rings spawned by the Gulf Stream drifted west of long 60° W., but 

many of them did not touch the continental margin. Note that 19 rings were active in the 

study area for a total of 17 months, or for about 2 months per year on average. Dotted 

line is typical path of warm-core ring (WCR). For sources of data, see references in text. 

Canyon names: C, Corsair; L, Lydonia; 0, Oceanographer; Hy, Hydrographer; V, 

Veatch; A, Atlantis; B, Block; H, Hudson; Wi, Wilmington; Ba, Baltimore; Wa, Washington; N, Norfolk. 

remained in an area of the margin for several weeks. fur 

example, a long-term current-meter record &om 245m (5 

mab) on the upper slope east of Lydonia Canyon shows 

that two rings produced strong northeastward currents 

that reached hour-averaged speeds of 30-35 cm/s and 

dominated bottom flow for 8 consecutive weeks in late 

1980 and early 1981. When these currents combined with 

high-frequency (tidal) currents, bottom flow reached houraveraged speeds of 40-45 cm/s and even greater shortterm speeds (see station LCI data in Butman and others, 

1983; Butman and Conley, 1984). Presumably, very fine 

sand ( 4 q, ), silt, and clay were winnowed and transported in 

suspension, while fine sand (3 c/J) was moved as bed load, 

along and down the slope and into gullies and can~ns that 

incise the margin here. The removal of fine sediment by 

warm-core-ring currents could also account for the 

patches of lag gravel in the depth interval &om 200 to 

300 m on the upper slope. 

CANYON MORPHOLOGY AND BOTTOM 

CURRENTS 

Previous studies have shown that the most energetic 

currents in submarine canyons off eastern North America 

(1) are characterized by a semidiurnal tidal period, similar to that of the M2 barotropic tide, and (2) are much 

stronger than M2 tidal currents on the continental slope at 

equivalent depths (Keller and Shepard, 1978; Butman, 

1987; Butman and Conley, 1984). The strong semidiurnal 

flows in canyons have been attributed partly to M2 tidal 

flow and partly to the movement of internal tides (internal 

gravity waves of tidal frequency) along a density interfa~e. 

Internal tides have been recognized by the presence of: (1) 

changes in temperature and density profdes that are 

correlated with current flow; (2) time delays of current 

flows at a series of stations along a canyon floor; and (3) 

packets of high-frequency currents having periods of less 

Canyon Morphology and Bottom Currents 11 
than 4 hours (Shepard, 1975; Gordon and Marshall, 

1976; Keller and Shepard, 1978; Shepard and others, 

1979; Hotchkiss and Wunsch, 1982; Butman, 1987). 

Hudson Canyon 

Hotchkiss and Wunsch (1982) studied tidal currents 

in Hudson Canyon. Data were collected during a 15-week 

deployment of current meters and temperature-pressure 

sensors (September 1977 to January 1978). The instrument arrays were placed in the axis between 350 and 780 

m water depth and extended 275-375 m into the water 

column. On the basis of their own study, of observations in 

canyons by other workers, and of theory, Hotchkiss and 

Wunsch concluded that: (1) surface tides generate internal 

tides at the edge of both the shelf and the canyon that are 

trapped when they enter the canyon; (2) the strength of 

internal tides depends on the strength of surface tides; (3) 

internal tides increase in strength as canyon length 

increases (in a canyon of uniform cross section); (4) 

internal tides increase in strength as cross-sectional area 

decreases within a canyon; and ( 5) in general, internal 

tidal currents are strongest in long canyons that are located 

in regions of strong surface tides, that have nearly parallel 

walls, and that have walls and floors sloping at appropriate 

angles. 

Hudson Canyon is by far the largest and longest 

canyon on the U.S. East Coast. The floor is covered 

predominantly by silt and clay, although sand has been 

reported in the canyon head; axial currents have been 

reported to reach speeds of 33 cm/s at 221 m (3 mab ), 30 

cm/s at 795 m (5 mab ), and, in a storm-related event, 60 

cm/s at about 725 m (Keller and others, 1973; Keller and 

Shepard, 1978; Stanley and Freeland, 1978; Hotchkiss 

and Wunsch, 1982). 

Georges Bank Canyons 

Tidal currents also are enhanced in some of the 

Georges Bank can}Ons. The largest of these canyons are 

smaller and shorter than Hudson Can}{)n. fur example, 

landward of the shelf edge, Oceanographer Canyon is only 

about 0.7 as large and 0.8 as long as Hudson Can}On; 

other Georges Bank canyons are much smaller. Nevertheless, Oceanographe~; Hydrographe~; Gilbert, and probably 

Lydonia are more energetic than Hudson Canyon, as 

indicated by bottom currents, sediment texture, and bed 

forms observed on the canyon floors. 

Speed of Bottom Currents 

Direct measurements of bottom flow by current 

meters are notably sparse in the Georges Bank canyons 

and are limited to Gilbert, Hydrographet; Lydonia, and 

Oceanographer Canyons. Short -term observations 25 em 

above the bottom lasting approximately 30 minutes were 

made by Stetson (1937) in Gilbert Canyon at 402 m and 

421 m, where speeds of less than 5 cm/s were recorded, 

and on the floor of Lydonia Canyon at three stations 

between 384 and 475 m, where speeds of approximately 

5 cm/s were recorded. Stetson's observations were too 

brief to characterize adequately the flow regime in these 

canyons. 

In 1974, three stations were occupied for 11.5 days 

on the floor of Hydrographer Canyon at 348m, 512 m, 

and 713 m (fig. 5), and maximum speeds greater than 50 

cm/s (3 mab) were recorded (Keller and Shepard, 1978). 

Recently, current-meter arrays were deployed for 4-5 

months at several stations on the floor of Lydonia Canyon 

(282 to 1,554 m) and at two stations in Oceanographer 

Canyon at 223 m and 554 m (figs. 2, 4; Butman and 

others, 1983; Butman and Conley, 1984). Of the two 

canyons, Oceanographer has more active bottom currents. 

As described in the section, "Sediment Traasport," maximum bottom-current speeds (7.5-minute average) in 

Lydonia were 50-60 cm/s (5 mab), and those in Oceanographer were 96 cm/s (6 mab). The most consistently 

strong currents observed in Lydonia Can}On were semidiurnal tidal flows (Butman and Conley, 1984; Butman 

and others, 1983). They were part of the Georges Bank 

tidal regime in which major axes of tidal ellipses were 

oriented approximately north-south across the shelf edge 

and parallel to both the canyon axes and the dip of the 

continental slope. However, near-bottom mean tidal flow 

on the upper slope near the canyon (about 6 cm/s at 245 

m) was much weaker than that on the canyon floor (about 

16 cm/s at 595 m), suggesting that the strength of tidal 

currents is enhanced in the medium to large canyons such 

as Lydonia and Oceanographer. 

Sediment Texture and Bed Forms 

Indirect indicators of sediment dynamics such as 

bed forms and texture also suggest that the floors of the 

Georges Bank canyons are characterized by varying levels 

of current activity. Oceanographer Canyon may be the 

most energetic canyon, for it has extensive areas of rippled 

sand on its walls and large asymmetrical dunes as high as 

3m on its floor down to 630 m (Valentine, Cooper, and 

Uzmann, 1984; Valentine, Uzmann, and Cooper, 1984). 

Dunes and rippled sand are also present in Hydrographer 

Canyon (Southard and Stanley, 1976; Keller and Shepard, 

1978) and were observed in 1984 during dives to the floors 

of both Hydrographer and Gilbert Can}Ons. Lydonia 

Canyon appears to be less active. The rippled sand on its 

floor is finer grained (Butman and others, 1983; Twichell, 

1983) than that found in Oceanographer, Gilbert, and 

Hydrographer Canyons, although the source is the same 

(figs. 6A., B). The floor of Lydonia Canyon was traversed 

by a submersible in a series of dives from 630 to 220 m, 

20 The Shelf-Slope Transition on the Southern Margin of Georges Bank 
and low dunes were observed on the floor only at 600 m 

(B. Butman, unpub. data, 1980). Limited observations of 

bed forms and sediment texture in Veatch and Atlantis 

Canyons at about 450 m on the can)On floors have 

recorded rippled sand only (J.R. Uzmann, unpub. data, 

1975-77) and suggest that these two small canyons are 

even less energetic than Lydonia. Heel Tapper Canyon is 

somewhat larger than Atlantis but smaller than Veatch. 

The texture of sediment on the floor of Heel Tapper 

Canyon resembles that of sediment on the adjacent upper 

slope and indicates that small canyons and the upper slope 

are in similar, relatively low energy, sedimentary environments (figs. 6A, B). 

Canyon Shape and Volume 

The canyons of western Georges Bank have similar 

physiographic and hydrographic settings. They all incise 

the shelf edge and open seaward, they all receive sediment 

from the adjacent continental shelf, and in general, they 

would be expected to experience the same tidal, storm, 

and oceanic currents. However, we have seen that some of 

the canyons are more energetic than others in terms of 

bottom-current strength and sediment transport. The most 

obvious differences exhibited by the canyons are their 

volume and shape. Hotchkiss and Wunsch (1982) concluded that the strongest internal tides should occur in 

long canyons having nearly parallel walls and having walls 

and floors sloping at appropriate angles. The authors did 

not treat canyon mlume as a characteristic that enhances 

current flow. An empirical correlation between volume and 

energy level exists for the Georges Bank canyons, and 

apparently the largest canyons naturally approach the 

criteria set forth by Hotchkiss and Wunsch for amplifying 

tidal energy. Canyon volume was calculated for the part of 

the canyon between the head, the deepest penetration of 

the shelf, and the mouth, which lies in a vertical plane 

drawn through the nominal shelf break at the 200-m 

isobath (table 3). The volume calculation has been 

restricted to the part of each ·can)On that incises the shelf 

and encompasses water depths characteristic of the shelfslope transition where bottom currents are most active. 

Canyon volumes presented here are crude first approximations as they were calculated without data on variability 

in width and wall steepness along the length of the canyon. 

The 12 Georges Bank canyons listed in table 3 range 

widely in mlume, from Shallop Canyon, the smallest (0.66 

km3), to Oceanographer Canyon, the largest by far (37.72 

km3). Sections drawn across these canyons show that each 

is much wider at its mouth than it is deep (fig. 10); canyon 

width ranges from 5.80 to 12.46 km, and height ranges 

from 0.20 to 1.02 km (table 3). The canyons are broad and 

shallow in cross section, although several of them can be 

described as narrow and deep relative to the others. 

Canyon length and mouth height determine the differences in canyon volume (the correlation is significant at 

r = 0.88 and 0.92, respectively), as the width of the canyon 

mouth varies surprisingly little among the 12 canyons. 

Canyon Energy Levels 

The two largest canyons, Oceanographer and 

Hydrographer, also are the most energetic, as indicated by 

sediment texture, bed forms, and a few current -speed 

observations (fig. 11). The largest canyons are long and 

relatively deep. Lydonia Canyon is the third longest canyon 

(17.87 km) and is 4.72 km longer than Gilbert Canyon 

(13.15 km), but the bed forms on the canyon floors 

indicate that currents in Gilbert are stronger. An explanation may lie in the facts that Lydonia has a much shallower 

mouth than Gilbert has, its volume is only two-thirds as 

large, and its walls are sinuous and highly digitated. 

Hotchkiss and Wunsch (1982) implied that canyons 

having steep walls trap internal tidal energy most effectively. Therefore, it may be possible to evaluate the energy 

level of the canyons by comparing the steepness of their 

walls. Bathymetry shows that canyon walls v~ greatly in 

slope angle and that lower walls are generally steepest (fig. 

10). Observations suggest that the strongest currents flow 

along the canyon floor. A comparison of the slope angle of 

the lower walls that extend 100-400 m above the floor at 

the canyon mouths reveals a correlation between canyon 

energy and wall steepness. The most energetic canyons 

have lower walls that slope 25°-35°, whereas the lower 

walls of the most tranquil canyons slope only 4°-~ (fig. 

11, table 3). 

Oceanographer and Hydrographer Canyons are 

large, long, and relatively narrow and deep; have steep 

lower walls; and display strong bottom currents. Gilbert 

also is an energetic canyon; it is large and deep and has 

steep walls, but it is much shorter than the other two 

canyons. Lydonia is long, but it is shallow and sinuous, has 

rough walls, and is only moderately active. The somewhat 

smaller and shorter canyons, Veatch, Powell, and Welker, 

are poorly known but may be less energetic than Lydonia. 

The remaining canyons, Dogbody, Atlantis, Heel Tapper, 

Shallop, and Alvin, are significantly smaller than the 

others. Their heads are very shallow and wide, their walls 

are gently inclined, and they can be regarded as little more 

than embayments of the upper slope. If observations in 

Heel Tapper Canyon are typical, these small can)Ons are 

the least energetic of all Georges Bank canyons and 

probably experience relatively weak currents similar to 

those reported from the outer shelf and upper slope by 

Butman and Conley (1984). 

The chief reasons for currents in some of the 

Georges Bank canyons being stronger than those in the 

much longer and larger Hudson Canyon off New Jersey 

are presumably the regional difference in tidal energy and 

the relatively low angle of Hudson's lower walls (15° -22° at 

the mouth). (Here we are considering semidiurnal and 

Canyon Morphology and Bottom Currents 21 
Tabla 3. Dimensions of 12 submarine canyons on southern Georges Bank and of Hudson Canyon on the U.S. Middle Atlantic 

Shelf 

[Canyons are listed in order of increasing volume. Mouth width was measured across the canyon along the projection of the shelf edge at the 200-m 

isobath. Mouth height was measured from the canyon floor at the mouth to the 200-m isobath. Canyon length was measured along the axis from 

the rim at the canyoa head to the center of the canyon mouth at the 200-m isobath and closely approximates the axis length along the canyon floor. 

Canyon dimensions are first approximations and were measured without regard to variations in canyon width and wall steepness. Measurements 

for Georges Bank canyons are from bath~etric maps of CaiJ?Cnter and others (1982), and measurements for Hudson Canyon are from a U.S. 

National Oceanic and Atmospheric Admtnistration bathymetnc map (1975)] 

Location Mouth Mouth Canyon Volume= Slope M of lower 

Canyon Oat N., long W.) width= a height= b length= c abcl6 wall at mouth 

(km) (km) (km) (km~ West East 

Shallot> ....... . Atlantis ....... . 

Dogbody ...... . Heel Tapper ... . Alvin ......... . Powell ........ . Welker ....... . Veatch ....... . 

Lydonia ...... . Gilbert ....... . 

Hydrographer .. . Oceanographer .. Hudson ....... . 

40"02,' 69"11, 

40"05,' 70011, 

40"08', 6S050' 

40"17', 68°17' 

40"05,, 70027' 

40031', 6~' 

40"14', 6S033' 

40"01,, 69"39' 

40031', 6~42' 

40023', 6~54' 

40"11,, 69"04, 

40"29', 6S010' 

39"35', 72°25' 

7.93 0.20 248 0.66 4 4 

6.11 .20 5.11 1.04 5 5 

6.28 .30 3.67 1.15 10 5 

8.00 .30 3.65 1.46 5 7 

12.46 .20 4.98 2.07 5 4 

5.80 .32 7.96 2.46 10 15 

8.83 .45 7.93 5.25 12 15 

7.40 .52 9.13 5.86 15 15 

5.90 .45 17.87 7.91 8 15 

7.46 .73 13.15 11.93 15 20 

6.22 .75 20.28 15.77 25 35 

8.94 1.02 24.82 37.72 35 35 

11.40 .85 35.60 57.49 22 15 

diurnal flows in the canyon and not relatively rare nontidal 

events such as turbidity currents and storm-related currents.) The surface tides, from which the internal tides are 

generated, are appreciably stronger on the southern edge 

of Georges Bank and on the bank itself than they are 

along the shelf edge and shelf off New Jersey. The 

magnitude of the major axis of the M2 bottom-current 

ellipse is 15-18 cm/s near the Georges Bank canyons, 

whereas it is about 7 cm/s or less near the Middle Atlantic 

canyons (Moody and others, 1984, pl. 9). Comparisons of 

AlVIN 

[0 

ATLANTIS 

r . ·<J 

DOG BODY 

POWELL 

LYDON/A 

[ •... m~ 

· .. 

OCEANOG~APHE~ 

[ ~------·:::::::::::: 

HUDSON 

SHALLOP 

[ 

HEEL TAPPER 

___________ _...---

WELKER 

~-·· 

VEATCH 

GILBERT [·········~ ----~-

HYDROGRAPHER 

SEA LEVELL 500 

1000 

1500 

.] 

0 1500 METERS 

Figure 10. Cross sections of canyon mouths at the 200-m isobath. Horizontal line above each section is sea level. No vertical 

exaggeration. 

22 The Shelf-Slope Transition on the Southern Margin of Georges Bank 
en ...Jen ...Jw 

<tw 

40 

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oo 

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CANYON 

WALLS AT 

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High- energy 

canyons 

OE,W OE 

Ow 

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uo 

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~LL 

OE Moderate-energy 

canyons 

oz 

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canyons 

LL z 10 05 

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en 

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CANYON ~ VOLUME 

AXIS 

LENGTH 

MOUTH 

WIDTH 

MOUTH 

HEIGH 

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Ow 

Ow 

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OCC HYD C/l LYD VCH WEL PWL All DB HT SHL ALV 

EXPLANATION 

0 

• 

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0 

• 

0 

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0 0 0 

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• SAMPLES GEORGES BANK CANYONS 

Figure 11. Graphs showing the relationship between canyon 

morphology and energy level for 12 Georges Bank canyons. Energy 

levels shown are based on sediment texture, bed forms, and current 

observations. The most dynamic canyons are large and long and 

have relatively deep mouths and steep lower walls (E, east wall; W, 

west waiQ. Observations include visual, film, and video documentation; samples are sediment collections. See discussion In text for 

explanation of ranking of canyons for which no direct observations 

or samples are available. Canyon dimensions and slope of walls 

measured from bathymetric maps by Carpenter and others (1982). 

Canyon names: OCG, Oceanographer; HYO, Hydrographer; GIL, 

Gilbert; L YO, Lydonia; VCH, Veatch; WEL, Welker; PWL, Powell; 

AlL, Atlantis; DB, Dogbody; HT, Heel Tapper; SHL,Shallop; ALV,Aivin. 

Canyon Morphology and Bottom Currents 23 
can")On energy levels based on can")On size and shape 

probably are most valid when limited to can")ODS within the 

same hydrodynamic region. Indeed, a comparative study 

of canyon energetics based on morphology, sediment texture, and bed forms of modern (or ancient) canyons that 

fails to result in a logical ranking may indicate that 

regional differences existed in the energy of modern (or 

ancient) surface tides. 

If the relationships among can}On shape, size, energy 

levels, and surface tides are valid, we should be able to 

determine relative energy levels in a suite of can")Ons within 

the same region in parts of the world that are not as well 

studied as Georges Bank. If bathymetry were adequate, 

canyons could be ranked by energy level on the basis of 

size and shape, including wall steepness. A knowledge of 

the strength and orientation of surface tides and relatively 

few sediment samples from the floors of a few canyons in 

a region conceivably could be used to estimate maximum 

bottom current speeds and to determine where erosion, 

transport, and deposition were most likely. 

SYNTHESIS 

Georges Bank is a recently drowned coastal plain, 

underlain by Cretaceous and Tertiary strata that are 

covered by a relatively thin veneer of Pleistocene sediment 

derived from glacial transport and fluvial outwash. The 

youngest beds include a seaward-thickening wedge of 

Pleistocene semiconsolidated sandy silt that is at least 300 

m thick below the southern shelf edge; it is overlain by 

unconsolidated sand and gravel that thins and becomes 

finer toward the southern margin. 

The bank crest is shallow (3-40m) and sediment is 

coarse; it marks the terminus of Pleistocene glaciers that 

scoured the Gulf of Maine, and strong tidal and storm 

currents have removed much of the finer sand and silt and 

clay. The bank is physically isolated from continental 

sediment sources, and currents are actively transporting 

sediment from the bank crest into deeper water. Although 

sediment is being transported away from the bank in all 

directions, the shelf-slope transition areas that surround 

the bank are dissimilar texturally. The northern and 

eastern margins separate the bank from the Gulf of 

Maine and Northeast Channel and are covered by gravel 

and gravelly medium to coarse sand. These areas lie close 

to the coarse bank crest, are relatively shallow (100-200 

m), and are subjected to vigorous tidal and storm currents 

that transport coarse material as bed load and winnow 

away fines in suspension. By contrast, the surficial sediment on the southern, or seaward, margin of the bank is 

medium to fine sand; it is far from the crest, it is deep (200 

m or greater), and it is affected by currents that are 

generally weaker than those on the northern and eastern 

bank margins. 

Sedimentary processes are very complex within the 

shelf -slope transition on the southern margin of Georges 

Bank. Variations in the sedimentary environment result 

from the interaction of many factors: (1) Bottom-current 

patterns are diverse in this region. Currents vary in 

strength, flow direction, and frequency. The most effective 

currents in sediment transport are energetic, strongly 

oriented, and areally limited. (2) The chief sediment 

source for the bank margin is sand and gravel from the 

crest and middle shel~ but an additional important source 

is the semiconsolidated silt exposed on can")On walls and 

extensively eroded by burrowing organisms and masswasting processes. The presence of ice-rafted gravel 

increases the textural complexity of the sea floor: The 

gravel deposits are present on canyon rims, in some areas 

of the outer shelf and upper slope, and as scattered clasts 

in all environments of the shelf-slope transition. (3) Finally, 

the morphology of the sea floor provides the setting for the 

diverse sedimentary environments that characterize the 

shelf-slope transition. On the southern bank margin, the 

sea floor varies from gently inclined outer shel~ upper 

slope, and canyon floors to more steeply dipping and 

sinuous canyon and gully walls. The shape of the sea floor 

affects the strength and direction of bottom currents and 

the downslope movement of sediment particles. 

Major aspects of sedimentary processes and environments in the shelf-slope transition on the southern 

Georges Bank margin to a depth of about 600 m are 

summarized as follows. 

Bottom Currents 

Mean bottom flow is anticyclonic around Georges 

Bank and has an off-shelf component at the bank margins. 

This flow generally has a speed of 5 cm/s or less on the 

southern flank of the bank and transports silt and clay in 

suspension off the bank and southwestward to the shelf off 

southern New England. 

The semidiurnal M2 tidal flow is the major, persistent bottom current that transports sediment on the bank 

and in the large canyons. The orientation of the major axes 

of tidal ellipses is across Georges Bank and the upper 

slope and parallel to the trend of the can}Ons. The 

magnitudes of the major axes of tidal ellipses are more 

than 40 cm/s on the bank crest, are 15-18 cm/s on the 

shelf near the heads of large canyons, and diminish to 

about 6 cm/s on the upper slope. Within large can}Ons, the 

strongest currents flow up and down the canyon along the 

floors, have a period similar to that of the M2 tide, and 

reach short-term speeds of 50-100 cm./s. 

A strong current of unknown origin flows westward 

across the east rims of Oceanographer and Lydonia 

Canyons at speeds of about 50 cm/s. This current transports shelf sand onto the canyon walls and exposes large 

areas of pavement made of ice-rafted gravel. 

24 The Shelf-Slope Transition on the Southern Margin of Georges Bank 
Gulf Stream warm -core rings move onto the upper 

slope and then drift southwestward toward the Middle 

Atlantic region. During 8 years, 19 rings were present near 

Hydrographer, Oceanographe:r; and Lydonia Canyons for 

a total of 17 months, or about 2 months per year. A single 

ring may remain in an area for a month. Anticyclonic 

currents in the rings flow northeastward along the upper 

slope at speeds as great as 50 cm/s or more and winnow 

very fine sand, silt, and clay from the sea floor between 200 

and 300 m water depth, exposing patches of glacial gravel. 

The fine sediment presumably is transported alongslope 

and downslope and into adjacent canyons and gullies. 

Sediment Texture and Transport 

The texture of sediment on the canyon floors is an 

important indicator of bottom-current strength. All canyons receive sediment from the shelf and, to varying 

degrees, from the erosion of canyon walls. Sediment on the 

floors of large canyons resembles that on the adjacent 

shelf. Sediment in medium canyons has a variable texture 

but generally is finer grained than that in large canyons. 

The floors of small canyons are similar to the upper slope 

in texture. 

Lower canyon walls in medium and large canyons 

are Pleistocene semiconsolidated silt covered locally by 

sand being transported to the canyon floor. These walls 

are major sites of erosion. Bioerosion, chiefly burrowing, 

produces small chunks of semiconsolidated silt. Weakened 

and oversteepened walls erode further by the spalling and 

sliding of large pieces of the wall. The clasts are abraded 

and softened by moving sand on the canyon floor; they 

disintegrate, and in large canyons, individual particles are 

transported away by strong currents. Near the mouth of a 

large, deep canyon (Oceanographer), lower walls are 

subvertical exposures of Tertiary and Upper Cretaceous 

limestone and siltstone that erode by a combination of 

biological activity, fracturing caused by unloading, and 

rock falls. 

The upper slope is veneered by silty sand that has 

been transported from the shelf. The sand is thin near the 

heads of steep gullies, revealing the underlying Pleistocene 

semiconsolidated silt into which the gullies are cut. Gully 

walls are steep (20° -40°) and are covered by silty sand 

lying at its angle of repose. This sediment flows periodically onto the gully floors, where apparently it accumulates, as bottom currents are too weak to transport it 

downslope. Perhaps the sand deposits grow until turbidity 

flows or mass-wasting processes carry it onto the lower 

slope and rise. At present, the sand is pervasive and is 

accumulating too rapidly for organisms to construct burrows into the underlying silt; thus, bioerosion of the ~~lly 

walls is minimal. 

Silt and clay constitute only a small percentage of the 

surficial sediment on the bank crest and on the outer shelf. 

Their distribution is a measure of the vigor of bottom 

currents in those areas. On the slope, silt and clay increase 

in abundance as depth increases; they constitute about 35 

percent of the sediment near 600 m. The mudline is below 

this level as silt and clay increase to 70-95 percent on the 

lower slope. On the floors of large canyons, silt and clay 

constitute a small percentage of the surficial sediment, but 

on the floors of smaller canyons, they are more abundant. 

Very fine sand ( 4 cp) is a good indicator of subtle 

changes in the strength of bottom currents. Like silt and 

clay particles, it travels in suspension, but to remain 

suspended, it requires stronger currents than do silt and 

clay. Very fine sand constitutes a small percentage of shelf 

sediment but increases in abundance as depth increases on 

the outer shelf and upper slope. An exception to this trend 

has been observed on the upper slope, where 4-cp sand and 

silt and clay have been removed by the erosive action of 

Gulf Stream warm-core-ring currents between 200 and 

300 m. Very fine sand is accumulating in several areas 

around the heads of Oceanographer and Lydonia Canyons, where bottom currents apparently weaken locally but 

remain strong enough to transport silt and clay. 

Canyon Size and Energetics 

Large canyons such as Oceanographer, Hydrographer, and Gilbert are much more energetic than small 

canyons, and their sedimentary environments are more 

diverse. Bottom currents are strong, are reversing, and 

presumably are produced by the M2 semidiurnal tide, at 

times enhanced by high-frequency internal waves. Canyonfloor currents have speeds of 50-100 cm/s, and ripples and 

dunes are present to depths of at least 600 m. Somewhat 

weaker currents flow along canyon walls and transport 

sand from the shelf to the canyon floor. Extensive outcrop~ 

of the Pleistocene semiconsolidated silt that drapes the 

lower walls are eroded by organisms and by processes of 

mass wasting. Fine-grained sediment is winnowed from the 

canyon floor deposits and transported out of the canyon. 

Sand moves upcanyon and downcanyon along the floor, 

and net transport varies in different areas of the canyon 

floor. Benthic fauna and habitats are diverse in large 

canyons. 

Small canyons such as Heel Tapper are relatively 

tranquil, as indicated by sediment texture. Bottom currents are weak, and the magnitude of the major axis of the 

M2 tidal ellipse probably is less than 10 cm/s, as it is on the 

adjacent upper slope. Ripples and other bed forms are 

largely absent. Silty sand transported from the shelf veneers 

the canyon walls and floor, which are cut into the underlying Pleistocene silt. Exposures of the silt ,are uncommon, 

and burrows are rare. Sediment becomes finer grained as 

depth increases, and texture resembles that of the adjacent 

gullied upper slope. The small canyons are embayments of 

Synthesis 25 
the upper slope. Benthic fauna and habitats are not as 

diverse as those in large can)Ons; howevet; a faunal depth 

zonation based on water temperature was observed in both 

small and large canyons. 

Medium canyons such as Lydonia are less energetic 

than large canyons. Tidally induced bottom currents are 

weaker; ripples and large bed forms are present to a 

limited extent. Sediment on the canyon floor is finer 

grained than that in large canyons, and much of the silt 

eroded from the walls remains on the canyon floor. Sediment accumulation may be most rapid on the floors of 

medium-sized canyons because they trap considerably 

more shelf sand and eroded silt than do small canyons, and 

less sediment is transported away than in large energetic 

canyons. 

Canyons and the outer shelf-upper slope areas 

experience the same regional tide. However, within the 

same depth interval (200-600 m ), the sediments on the 

floors of large canyons and on the adjacent gullied slope 

are dissimilar because bottom currents are not uniform in 

strength. The flow of water through large canyons is 

enhanced by the canyon shape; a correlation exists between 

canyon size and shape and current strength within the 

canyons. All Georges Bank canyons generally are shallow 

and wide at their mouths. Within the morphological limits 

of the canyons, those that are large, long, narrow, deep, 

and steep walled, such as Oceanographer and Hydrographer, experience strong tidally induced bottom currents. 

Small, wide, shallow canyons, such as Heel Tappet; experience weak currents similar to those on the upper slope. 

We do not yet know whether warm-core-ring currents 

affect the small canyons. Small, narrow gullies on the 

upper slope, like those in the head of Filebottom Canyon, 

do not enhance the strength of bottom currents, as 

indicated by the presence of fine-grained sediment on their 

floors and by the absence of ripples and other bed forms. 

If the strength and orientation of the regional tide 

and the size and shape of canyons determine current 

dynamics within canyons, and if current strength and 

canyon morphology are related in a systematic way, then it 

may be possible to make a preliminary assessment of 

current and sediment dynamics in canyons in poorly 

studied areas of the world by analyzing bathymetry and 

tidal data. 

Regional Sediment Transport 

In a broad sense, the bank crest is a shallow 

topographic high, constructed of coarse, glacially and 

fluvially transported sediment that is wasting away under 

the effect of strong tidal and storm currents. On the 

southern bank margin, net sediment movement is downslope off the shelf into canyons or onto the gullied upper 

slope. Howevet; because of local variations in current 

strength and bottom morphology, sediment transport is 

not uniform throughout the shelf-slope transition (fig.12). 

Transport is most rapid in areas of strong currents such as 

the east rims of Oceanographer and Lydonia Canyons 

where gravel pavement is exposed, along the upper slope 

where Gulf Stream warm-core rings winnow sediment, 

and in large canyons where strong currents move sand 

along the canyon floor and remove fine-grained sediment. 

Net transport is upcanyon in response to strong axial 

currents along some reaches of Oceanographer and 

Hydrographer Canyons. Sediment movement is slower and 

sediment is accumulating on the upper slope, including in 

the gullies, on the floors of medium and small canyons, 

and on the shelf around the heads of some canyons. Thus, 

for example, sand moves rapidly across the exposed gravel 

pavement on the east rim of Oceanographer Canyon; it 

slows and accumulates on the upper wall where alongcanyon currents are weak; and it nioves more rapidly 

across the lower walls owing to an increase both in wall 

steepness and in the speed of tidal currents. On the 

canyon floot; it moves most rapidly as it is transported 

short distances back and forth, up and down the canyon by 

strong currents. Net movement on the floor is slowly up or 

down the canyon, depending on the asymmetry of current 

flow. 

The current systems and related sediment-transport 

pathways described in this study are important aspects of 

the sedimentary environment on the Georges Bank outer 

margin. Understanding these systems and pathways can 

help us to elucidate the sedimentary history and modern 

development of the margin and to predict and track the 

movement of water masses and indigenous plankton and 

larvae, as well as pollutants that may be introduced into 

the environment. 

REFERENCES CITED 

Bigelow, H.B., 1927, Physical oceanography of the Gulf of 

Maine: U.S. Bureau of Fisheries Bulletin, v. 40 (1924), pt. 

2, p. 511-1027. 

Bothner, M.H., Spiker, B.C., Johnson, P.P., Rendigs, R.R., and 

Aruscavage, P J., 1981, Geochemical evidence for modem 

sediment accumulation on the continental shelf off southem New England: Journal of Sedimentary Petrology, v. 51, 

no. 1, p. 281-292. 

Brown, W.S., and Moody, J.A., 1987, Tides, in Backus, R.H., 

ed., Georges Bank: Cambridge, Mass., Massachusetts 

Institute of Technology Press, in press. 

Butman, Bradford, 1987, Physical processes causing surficial 

sediment movement, in Backus, R.H., ed., Georges Bank: 

Cambridge, Mass., Massachusetts Institute of Technology 

Press, in press. 

Butman, Bradford, and Beardsley, R.C., 1987, An introduction 

to the physical oceanography of Georges Bank, in Backus, 

R.H., ed., Georges Bank: Cambridge, Mass., Massachusetts Institute of Technology Press, in press. 

26 The Shelf-Slope Transition on the Southern Margin of Georges Bank 
:a 

.. • 

; 

:::s 

a 

• 

I 

~ 

68°201 10' 

EXPLANATION 

[J ACCUMULATION OF VERY FINE SAND 

D FINES WINNOWED 

[3 GRAVEL 

\ ' 

~ ~ 

~ 

~ 

~ 

~ 

~ ~ 

~ 

68°00' 

Figure12. Map of the Oceanographer Canyon-Lydonia Canyon area. Sediment transport 

directions in the shelf-slope transition area are shown by arrows; arrow weight represents 

relative current strength. The interpretation is based on observations of sediment texture, 

bed forms, and current speed and direction and is extended by analogy in areas where data 

are unavailable. Movement is most rapid on the eastern rims of canyons (where ice-rafted 

gravel is exposed) and on the canyon floors. Net movement is offshelf and downslope, but 

50' 

(;;-> 

~ '-1000 

~ C" 

~ 

~ ~ 

67°401 

~ ~ 

~ 

0 5 KILOMETERS I I I I I I 

Bathymetry in meters 

r:novement varies in direction along the axes of large canyons. Major currents are generally 

tidal except for westward flow across the eastern canyon rims and for along-slope flow 

associated with Gulf Stream warm-core rings. Very fine sand (4 ;) accumulates around 

canyon heads, and 4-4» sand, silt, and clay are eroded from the upper slope by 

warm-core-ring currents. Sand dunes are present on the floors of high-energy canyons. 
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Vermersch, J.A., Schlitz, R., Limeburner, R., Wright, 

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mean circulation on Georges Bank: Journal of Physical 

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Butman, Bradford, Bothner, M.H., Noble, MA., and Twichell, 

D.C., 1982, The Lydonia Canyon experiment-Preliminary 

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Butman, Bradford, and Conley, SJ., 1984, Lydonia Canyon 

experiment-Data report for moored array deployment I, 

October 1980-April 1981: U.S. Geological Survey OpenFile Report 84-201, 223 p. 

Butman, Bradford, Loder, J.W., and Beardsley, R.C., 1987, The 

seasonal mean circulation-Observation and theory, in 

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Butman, Bradford, and Moody, J.A., 1983, Observations of 

bottom currents and sediment movement along the U.S. 

East Coast Continental Shelf during winter, Chapter 7 of 

McGregor, B.A., ed., Environmental geologic studies on 

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U.S. Geological Survey, p. 7-1-7-60. (Volume 3 is available from the U.S. National Technical Information Service, Springfield, VA 22161, as report PB84-187954.) 

Butman, Bradford, Noble, M.A., Moody, J.A., and Bothner, 

M.H., 1983, Lydonia Canyon dynamics experiment-Preliminary results, Chapter 8 of McGregor, B.A., ed., Environmental geologic studies on the United States Mid and 

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v. 3, North Atlantic region: U.S. Geological Survey, p. 

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Carpenter, G.B., Cardinell, A.P., Francois, D.K., Good, L.K., 

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Cooper, R.A., Valentine, P.C., Uzmann, J.R., and Slater, R., 

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Fitzgerald, J.L., and Chamberlin, J.L., 1981, Anticyclonic 

warm-core Gulf Stream eddies off the northeastern United 

States in 1979: Annates Biologiques (ICES), v. 36, p. 

44-51. 

--1983, Anticyclonic warm-core Gulf Stream rings off the 

northeastern United States in 1980: Annales Biologiques 

(ICES), v. 37, p. 41-47. 

--1984, Anticyclonic warm-core Gulf Stream rings off the 

northeastern United States in 1981: Annates Biologiques 

(ICES), v. 38, p. 29-33. 

Folk, R.L., 1980, Petrology of sedimentary rocks (2d ed.): 

Austin, Tex., Hemphill Publishing Company, 182 p. 

Gordon, R.L., and Marshall, N.F., 1976, Submarine canyonsInternal wave traps?: Geophysical Research Letters, v. 3, 

no. 10, p. 622-624. 

Hathaway, J.C., ed., 1971, Data file, Continental Margin Program, Atlantic Coast of the United States, v. 2, Sample 

collection and analytical data: Woods Hole Oceanographic 

Institution Reference No. 71-15, 496 p. 

Hotchkiss, F.S., and Wunsch, Carl, 1982, Internal waves in 

Hudson Canyon with possible geological implications: 

Deep-Sea Research, v. 29, no. 4A, p. 415-442. 

Keller, G.H., Lambert, Douglas, Rowe, Gilbert, and Staresinic, 

Nicholas, 1973, Bottom currents in the Hudson Canyon: 

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