INTRODUCTION
Descriptions of physical features of the earth's surface are found in
the earliest-known writings. However, the systematic classification of
land forms is relatively recent and followed the development of the
science of physical geology. The natural topographic divisions of the
continents have been classified into physiographic provinces according
to several similar systems (Lobeck, 1939; Fenneman, 1938; Atwood,
1940; and others). These systems take into account form and age of the
relief, as well as the structure of the underlying rocks. Descriptions
are usually given in terms of age, process, and structure, with the
ultimate aim the understanding of the origin and history of topography.
Detailed topographic maps at 1:50,000 or larger are available and are
used in conjunction with direct field observations. More recently
aerial photographs have greatly aided geomorphic studies.
The oceans, in contrast, have been subdivided by oceanographers merely
into basins separated by ridges and swells. This was done on the basis
of widely spaced discrete soundings shown on charts rarely of larger
scale than 1:10 million. The basins were delimited by arbitrarily
chosen and often crudely controlled isobaths. The development and
installation of continuously recording deep-sea echo sounders and their
extensive use in the deep sea provide for the first time detailed
topographic information on the deep-sea floor and thus a new basis for
description and classification.
It is perhaps presumptuous at this time to refer to the topographic
divisions of the sea floor as physiographic provinces when we have
only scant information concerning the structure of each province, the
age, the physical processes, and, in fact, the details of topography.
Therefore, the classification described in the following pages is
presented as a first attempt, with the full knowledge that it will be
modified and expanded by subsequent exploration.
We are only beginning to understand the structural significance of
deep-sea physiographic provinces. We now think that the correlation
of topography and structure will be better under the sea than on land
because of less vigorous erosion at depth in the sea. If this is true,
deep-sea structural patterns may eventually be quite simple to map.
NOMENCLATURE AND CLASSIFICATION OF DEEP-SEA RELIEF
Before the advent of continuously recorded echo-sounding profiles, and
their revelation of the texture of the sea-floor relief, classification
and nomenclature of submarine topography were based on broad closed
isobaths. We can characterize the older system as the bathymetric
system of nomenclature in contrast to that employed in this paper,
which we can call a geomorphic or textural system.
The terms "basin" and "deep" used in the older literature are usually
defined by closed 3000-, 4000-, or 5000-meter contours as represented
on the Carte Générale Bathymétrique des Océans (International
Hydrographic Bureau). For many purposes this terminology is useful,
particularly in describing the habitat of a deep-sea fish or the locale
of a water mass. Consequently some such system should be retained even
though in many areas basin boundaries are difficult to define, and
regardless of the fact that many boundaries cut arbitrarily through
physiographic provinces without regard for local province boundaries.
The Atlantic has been subdivided by Wüst (1940b) (Fig. 4) whose system
is now in general use.
[Illustration: FIGURE 4.--_Major basins of the North Atlantic, after
Wüst (1940)_
Heavy solid lines indicate boundary formed by axis of Mid-Atlantic
Ridge. Light solid lines indicate boundary formed by shelf breaks and
submarine ridges. Dashed lines indicate arbitrary boundaries.]
The nomenclature of deep-sea topography has been considered by
several committees during the past half century. The most recent
recommendations published by Wiseman and Ovey (1953; 1955) are followed
wherever applicable. The older systems of nomenclature, however, are
not rigidly employed since we are dealing with textural provinces based
on profiles obtained with continuously recording echo sounders rather
than bathymetric provinces defined by closed isobaths.
UNITS OF DEPTH AND SLOPE
On the profiles and echograms the vertical scale is in units of
echo-sounding time rather than in units of true depth. In other words,
all depths are calculated under the assumption that the vertical sound
velocity is 800 fathoms per second. Considering that the sound travels
to the bottom and back, the calculation is based on 400 fathoms per
second of lapsed time.
Since the average vertical velocity is, within the area covered, always
slightly less than 800 fathoms per second, the true depth is always
slightly greater than the "echo-time depth" as expressed in "nominal
fathoms". Figure 5 shows the range of corrections which must be
applied in various parts of the area. The spot depths indicated on the
physiographic diagram are in units of true depth corrected according to
Matthews' tables (1939) for regional variations in the average vertical
sounding velocity.
[Illustration: FIGURE 5.--_Sound-velocity corrections for echo
soundings_
Add correction to uncorrected echo sounding to obtain true depth.
Curves I and II are representative of North Atlantic 17°-50°N.
exclusive of the Grand Banks region. Curves III and IV are
representative of the deep-water areas near the Grand Banks. Curves are
based on Matthews (1939) and are for use only where assumed sounding
velocity is 800 fathoms/second. _All soundings mentioned in the text
are uncorrected for sound velocity._]
[Illustration: FIGURE 6.--_Conversion diagram for degrees, per cent
grade, feet per statute mile, and gradient_
(a) Values of gradient from 1:10-1:1500; (b) Values of gradient from
1:100-1:5000]
The inclination of the bottom is given as the tangent of the angle
between the sloping plane and the horizontal expressed as a ratio of
integers. These ratios are referred to as gradients. In Figure 6 slope
values expressed in degrees, per cent grade, feet per statute mile,
and gradient are compared. With a few exceptions all profiles are
represented with a 40:1 vertical exaggeration. To facilitate judging
the magnitude of slopes on these profiles, Figure 7 shows various
gradient ratios at a 40:1 vertical exaggeration. Slope corrections
have not been made to the soundings. Except in special cases such
corrections would make insignificant changes in the 40:1 profile.
All distances are given in nautical or geographical miles (1 nautical
mile = 6080 feet).
[Illustration: FIGURE 7.--_Gradients from 1:5 to 1:8000 shown at a 40:1
vertical exaggeration_
Most profiles reproduced in this paper are at 40:1 vertical
exaggeration. This template is provided to aid the reader in judging
slopes on these exaggerated profiles.]
[Illustration: FIGURE 8.--_Outline of submarine topography_
Line 1, first-order features of the crust; line 2, major topographic
features of the ocean; line 3, categories of provinces and
super-provinces; line 4, provinces; line 5, sub-provinces and other
important features.]
CONTINENT AND OCEAN
The two first-order morphologic divisions of the earth's crust are
continent and ocean. The oceans can be divided into a few major
divisions which are in turn subdivided into categories of physiographic
provinces and then into individual provinces. The area of the present
study is composed of the three major divisions shown in Figure 9:
continental margin, ocean-basin floor, and mid-oceanic ridge. The
discussion and description of the physiographic provinces of the North
Atlantic will follow the schematic outline shown in Figure 8.
[Illustration: FIGURE 9.--_Major morphologic divisions: North Atlantic
Ocean_
The profile is a representative profile from New England to the Sahara
Coast.]
CONTINENTAL MARGIN
DEFINITION AND GENERAL CATEGORIES
The continental margin includes those provinces of the continents and
of the oceans which are associated with the boundary between these two
first-order features of the earth.
_General categories._--In most areas three parallel categories of
provinces can be distinguished in the continental margin (Fig.
10). The relatively flat portions of the submerged continental
platform constitute category I. These provinces are: continental
shelf, epicontinental marginal seas (_e. g._, Gulf of Maine), and
continental-margin plateaus (_e. g._, Blake Plateau). The provinces of
category II include the continental slope, marginal escarpments (_e.
g._, Blake Escarpment), and the landward slopes of trenches. These
provinces mark the edge of the continental block. Category III includes
the continental rise, marginal trench-outer ridge, and marginal
basin-outer ridge complexes.
[Illustration: FIGURE 10.--_Three categories of continental-margin
provinces_
Category I provinces lie on the continental block, Category II
provinces form the side of the continental block, and Category III
provinces are the upturned or depressed margins of the oceanic
depression.]
The most common type of continental margin is made up of continental
shelf (I), continental slope (II), and continental rise (III). (Fig.
10, Sahara and New York). In areas where the continental rise is
well developed it is composed of two parts, the upper and the lower
continental rise (Fig. 15). In some areas the lower continental rise
is replaced by an outer ridge, and the upper continental rise is
replaced by a marginal basin or marginal trench. These two latter
types are illustrated in Figure 10 by profiles marked Blake Plateau
and Puerto Rico respectively. Seamounts and islands occur in all the
continental-margin provinces.
* * * * *
_Category I provinces._--The ocean overflows its basin onto the edge of
the continents. The principal physiographic province of this submerged
portion of the continental platform is the continental shelf which
is present off all the lands of the world. The continental shelf is a
smooth area with very low relief and is nearly everywhere limited to
depths less than 250 fathoms. The continental blocks are also flooded
by epicontinental seas. Some of these have rough bottoms, as the Gulf
of Maine; others are relatively smooth-floored. Marginal plateaus,
where present, lie in depths of 500-1200 fathoms, and many are nearly
as smooth as the continental shelves.
CONTINENTAL SHELF: The continental shelf is a shallow (averaging less
than 100 fathoms), gently sloping (less than 1:1000) surface of low
local relief (less than 10 fathoms) which extends from the shore line
to the shelf break where the seaward gradient sharply increases to
greater than 1:40. Its width ranges from a few miles to more than 200
miles.
Continental shelves border all land areas. Because of their proximity
to shore, their shallow depth, and their importance in navigation the
continental shelves are now the best-known part of the oceans (Veatch
and Smith, 1939; Shepard, 1948).
The transition from the continental shelf to the continental slope is
generally abrupt and is known as the shelf break. This feature ranges
in depth from 20 to more than 100 fathoms and in form from a sharp edge
to a rounded shoulder. The change in the gradient is from less than
1:1000 to greater than 1:40.
EPICONTINENTAL MARGINAL SEAS: The epicontinental marginal seas
are those shallow seas (less than 1500 fathoms) which lie on the
continental block and can be distinguished from the continental shelves
by their greater depth (usually > 100 fathoms) and possibly greater
topographic irregularity. Most of them are enclosed by shallow banks (<
50 fathoms) of the continental shelf and by land. The Gulf of Maine,
the Gulf of St. Lawrence, and the channels of the Bahamas belong to
this class.
MARGINAL PLATEAUS: A marginal plateau is a shelflike feature which lies
at greater depths than the continental shelf and is separated from the
continental shelf by an incipient continental slope. These features
generally lie at depths greater than 100 fathoms and less than 1200
fathoms. They can be distinguished from epicontinental marginal seas
by their lack of a seaward barrier or sill. The surface of a marginal
plateau is generally quite similar to the continental shelf in slope
and in the frequency and magnitude of minor relief features. The Blake
Plateau is the only well-expressed representative of this morphologic
type in the area of the diagram. Well-developed marginal plateaus are
also found off the coast of southern Argentina and east of New Zealand.
* * * * *
_Category II provinces._--The steep slopes which border the continental
block are grouped into category II provinces. Loosely speaking, we are
referring to the continental slope but, because of the complications
imposed by such features as marginal plateaus and marginal trenches, we
distinguish three province types.
CONTINENTAL SLOPE: The continental slope is that relatively steep
(3°-6°) portion of the sea floor which lies at the seaward border
of the continental shelf. It typically drops from depths of 50-100
fathoms to depths of 750-1750 fathoms. The top of the slope is
usually well marked by a relatively sharp shelf break. The base of
the slope, although less definite, is generally abrupt. As a basis of
classification in those few areas where no abrupt change is noted, we
have set the gradient of 1:40 as the lowest gradient of the continental
slope. The setting up of a lower limit for the gradient marks a
departure from the older usage in which the continental slope was
defined as "the slopes leading from the outer edge of the continental
shelves down to the great depths of the ocean" (Shepard, 1948). This
older definition included the continental rise, marginal plateaus,
and marginal escarpments. The continental slopes are a world-wide
phenomenon. The details of their regional distribution in the North
Atlantic are covered in a later section.
MARGINAL ESCARPMENTS: A marginal escarpment is a precipitous escarpment
which forms the seaward slope of a marginal plateau. Such escarpments
begin in depths of 500-1500 fathoms and are 1000 to 2000 fathoms high.
The base of the escarpment is well marked by an abrupt change in slope.
Gradients of marginal escarpments exceed 1:10. The Blake Escarpment is
a marginal escarpment. Similar features are found in the Gulf of Mexico
and off the southeast coast of Argentina.
LANDWARD SLOPES OF TRENCHES: This term was introduced to set apart the
landward slopes of marginal trenches from the usual continental slopes
found off trench-less coasts. These steep slopes (> 1:40) drop from
depths of a few hundred fathoms near a continent to a depth of a few
thousand fathoms in a marginal trench. In the North Atlantic the one
example is north of Puerto Rico. A large part of the circumference of
the Pacific is bounded by such features.
* * * * *
_Category III provinces._--At the base of the continental slope a
gentle gradient continues to the local level of the abyssal floor. This
seaward gradient ranges from 1:100 to 1:700 and extends over a strip
from a few miles to a few hundred miles in width. On many topographic
profiles made at right angles to the slope of the continental margin
three major breaks are visible: the shelf break, the base of the
continental slope, and the point where the near-by level of the
ocean-basin floor intersects the more steeply sloping continental
margin. Since we have limited the continental slope to gradients
greater than 1:40, we split off this lower portion of the continental
margin into a separate province, the continental rise. In the older
"bathymetric" classification of relief the ocean basin--continental
slope boundary was along the 2000- or 2500-fathom contour, an arbitrary
division which cut across the then-unrecognized continental rise. At
the base of the Blake Escarpment lies an enclosed basin, and north
of Puerto Rico the sea floor drops almost directly to the floor of a
marginal trench. These seemingly diverse provinces of continental rise,
marginal escarpments, enclosed marginal basins, marginal trenches, and
outer ridges are placed in category III because of their similarity in
position with respect to the continents and ocean floor and because of
other similarities shown in the deeper structure of the continental
rise.
CONTINENTAL RISE: The continental rise, where present, lies at the base
of the continental slope. Gradients on the continental rise generally
range from 1:100 to 1:700, while the width ranges from a few score to
a few hundred miles. However, gradients as high as 1:50 are observed
in segment 3 of the upper continental rise, and gradients as low as
1:2500 are locally present in segment 2 of the lower continental
rise (Tables 1 and 2). The seaward limit of the continental rise is
generally abrupt, and at this point regional gradients decrease to less
than 1:1000. The depth on the continental rise ranges from 750 to 2800
fathoms. Local relief is moderate to low, and, except for infrequent
seamounts and fairly frequent submarine canyons, the local relief of
the continental rise rarely exceeds 20 fathoms.
The continental rise is well developed over most of the area covered
by the physiographic diagram. The major exceptions are north of the
Iberian Peninsula where the rise is present, but extremely narrow,
and in the southwestern third of the map in the area south of Cape
Hatteras, where it is not present. In this latter area the geographical
position usually filled by the continental rise is occupied by the low,
broad outer ridge and the enclosed marginal basin and marginal trench.
OUTER RIDGE: An outer ridge is a broad ridge generally more than 100
miles wide that rises from less than 100 fathoms to about 1000 fathoms
above the adjacent floor. It lies parallel to the continental margin
and may enclose a basin or trench on the landward side. The local
relief of an outer ridge is generally a little greater than that of the
continental rise but much more subdued than that on the oceanic rises
and mid-oceanic ridges.
MARGINAL BASIN: A marginal basin, where present, lies at the foot of
the continental slope or at the base of a marginal escarpment. It is
slightly shallower than the general level of the ocean basins and is
bounded on the seaward side by an outer ridge. Part of its floor is
generally occupied by an abyssal plain.
MARGINAL TRENCH: A marginal trench is a narrow, steep-sided feature
running closely parallel to the trend of the continental margin; it
is generally at least 1000 fathoms below the general level of the
adjacent ocean floor. It is separated from the ocean floor by a low
outer ridge which rises 100-500 fathoms above the level of the adjacent
ocean floor. The bottom of a trench is generally rugged except near the
deepest spots where it is flat.
The combination of marginal basin and outer ridge replaces the
continental rise east of the Blake Plateau. North of Puerto Rico this
combination is replaced by a marginal trench-outer ridge complex. The
reason for this grouping is discussed in a later section.
Submarine canyons cut across all the continental-margin provinces
except isolated portions of the outer ridge. Submarine canyons range
from less than a mile to more than 10 miles in width and from less
than 10 to nearly 1000 fathoms in depth. Canyons are most abundant on
the continental slope. However, a smaller number persist across the
continental rise. They are also found on the marginal escarpments and
on the landward slopes of trenches. Shepard (1948), Kuenen (1950),
Veatch and Smith (1939), De Andrade (1937), Johnson (1939), and others
have discussed the continental-slope canyons at great length. Canyons
in the continental rise of the North Atlantic were discovered and
mapped by Ericson, Ewing, and Heezen (1951).
REGIONAL DESCRIPTION OF CONTINENTAL MARGIN
This discussion is based on continuously recorded echo-sounding
traverses made by Lamont Geological Observatory expeditions. Profiles
approximately perpendicular to the continental margin are reproduced
in Plates 24 and 25. None is precisely perpendicular, and thus slight
distortions of slopes and widths of the features are unavoidable.
EASTERN NORTH AMERICA: Thirty-four profiles of the continental margin
of eastern North America are presented in Plate 24. The positions of
the profiles are indicated on the index chart in Plate 23. All profiles
show the three categories of continental-margin provinces. Profiles W-1
to W-21 Plate 24 show the more general succession of shelf, slope, and
rise, while profiles W-22 to W-34 show the outer ridge-marginal basin
and outer ridge-marginal trench complexes. Each of the 34 profiles
exhibits a continental shelf although it may range from 20 to 300 miles
in width. On each a shelf break is present at depths of 20-150 fathoms.
Each profile shows a continental slope, the base of which may be from
300 to 1900 fathoms deep.
Northern Grand Banks Sector.--On profiles W-1 to W-6 (Pl. 24), across
the Grand Banks of Newfoundland, the shelf ranges from 120 to 285 miles
in width. Exceptionally strong local relief of 50-100 fathoms is found
on the shelf in profile W-1 northeastward from Newfoundland. The shelf
break, which occurs at 150 fathoms, is abnormally deep--more than twice
the depths found off New England. The continental slope has a typical
gradient of 1:20 but is unusually short as the continental rise is
reached at 725 fathoms. From this depth the continental rise descends
to the 1700-fathom curve at a gradient of 1:140. This gentle slope
is interrupted by a group of exceptionally rugged lower continental
rise hills which rise to 1250 fathoms. Northeast of the hills the
2200-fathom line marks the rather abrupt beginning of the abyssal plain
which slopes seaward at a gradient of 1:1100.
Flemish Cap.--Profile W-2 crosses the Grand Banks, along its widest
east-west axis, and also the semidetached bank called Flemish Cap. The
shelf is much smoother than in profile W-1, except for a small deep of
about 100 fathoms immediately east of Newfoundland. The shelf break at
150 fathoms is followed by a continental slope 150 to 500 fathoms deep
which has a gradient of 1:20. The Flemish Cap is a difficult feature
to classify. It is too large to be a seamount and too shallow to be a
marginal plateau. We must treat it as a part of the continental shelf,
semidetached from the rest by a 650-fathom-deep channel. The eastern
flank of the Flemish Cap slopes off at gradients of 1:100 and 1:60
until at a depth of 650 fathoms the bottom drops precipitously to 1750
fathoms at a gradient of 1:10. Seaward of this point an 85-mile-wide
continental rise has a gradient of 1:65 and 1:250 down to the
Newfoundland Abyssal Plain which is at a depth of 2400 fathoms. Twenty
miles east of the continental rise this profile crosses the Northwest
Atlantic Mid-Ocean Canyon.
On profile W-3 the shelf is quite smooth, and the shelf break is
reached at 60 fathoms. The profile runs slightly oblique to the
continental slope and reveals a series of submarine canyons. The base
of the slope is at 1700 fathoms where the gradient drops to less than
1:200.
[Illustration: FIGURE 11.--_Two east-west profiles of Southeast
Newfoundland Ridge_
Positions of profiles are indexed on Plate 23. Both profiles plotted
from nonprecision soundings (NMC).]
Southeast Newfoundland Ridge.--From the southern tip of the Grand Banks
a broad ridge runs southeasterly toward the Mid-Atlantic Ridge and
forms a natural barrier between the Newfoundland Basin and the North
America Basin [to the south]. Since it is almost impossible to define
a boundary between the continental rise and the ridge, we consider the
Southeast Newfoundland Ridge an extension of the continental rise. The
ridge is 60-100 miles wide, and its crest plunges southeastward from
depths of 1500 fathoms near 50° W. to depths of 2200 fathoms near 45°
W. Profiles N-1 and N-2 (Fig. 11) cross the Southeast Newfoundland
Ridge at about 41.5° N. and 39.5° N. respectively. Profile N-1 is of
poor quality, which probably accounts for the lack of fine-textured
relief. The Mid-Ocean Canyon is again seen at the eastern end of
Profile N-2. Profile W-5 (Pl. 24) crosses the Southeast Newfoundland
Ridge from north to south. The similarity of profiles W-5 and W-23
suggests that the Southeast Newfoundland Ridge is an outer ridge of the
same kind as the one east of the Blake-Bahama region. The northern one
is not so long, and it does not totally enclose a basin. Otherwise, it
is quite similar to the outer ridge east of the Bahamas in relative
position, size, and surface features. The term Southeast Newfoundland
Ridge was proposed by Wüst (1940b; 1943) and the feature has been shown
on bathymetric charts (Tolstoy, 1951) and profiles (Emery, 1950). This
ridge will be discussed again in connection with the Mid-Ocean Canyon
and the ocean-basin floor.
Southern Grand Banks Sector.--Profiles W-4 and W-5 cross the southern
tip of the Grand Banks. The shelf break is at 50 fathoms on both
profiles. On Profile W-4 an apparent gradient of 1:25 extends from
200 to about 1000 fathoms where, after some irregularities probably
associated with submarine canyons, the gradient drops to 1:40. This
lower gradient extends to 1750 fathoms. Profile W-5 is quite similar to
W-4 except that a steep initial slope of 1:5, from about 200 fathoms to
650 fathoms, is followed by a gradient of 1:80 which continues to 1000
fathoms. This same terracelike feature is also seen on W-3, W-4, W-5,
W-6, W-7, and W-8. Below 1000 fathoms a gradient of about 1:50 is found
on profiles W-4, W-5, W-6, W-7, and W-8. Profile W-6 runs south of the
Grand Banks through the epicenter of the 1929 Grand Banks earthquake
and then south through the area passed over by the 1929 Grand Banks
turbidity current (Heezen and Ewing, 1952). The depression marked by
the 1150-fathom sounding on the continental slope in Profile W-6 is
a canyon running south from the Laurentian Channel. The continental
rise is 250 miles wide and has an average gradient of 1:400 over its
deepest third. At a depth of 2750 fathoms the gradient abruptly drops
to 1:2000, and this marks the northern edge of the Sohm Abyssal Plain.
[Illustration: FIGURE 12.--_Laurentian Channel_
Profile replotted from NMC echogram]
[Illustration: FIGURE 13.--_Eastern Channel, Gulf of Maine_
Profile replotted from NMC echogram]
Laurentian Channel.--Between Nova Scotia and Newfoundland a
60-mile-wide, steep-sided, flat-floored channel cuts across the
continental shelf connecting the Gulf of St. Lawrence and the open
ocean. The nearly flat, smooth floor of this channel lies at about
230 fathoms depth. Figure 12 shows a cross-section of the Laurentian
Channel near its seaward end. The origin and physiography of the
channel has been treated by Shepard (1931; 1948); its structure has
been reported by Press and Beckmann (1954). The Laurentian Channel
continues as a steep-sided, box-shaped feature for more than 500 miles
into the Gulf of St. Lawrence.
Scotian Shelf Sector.--The term Scotian Shelf was introduced by
Canadian oceanographers and refers to the continental shelf southeast
of Nova Scotia from the Laurentian Channel to the Gulf of Maine. This
region is illustrated by Profiles W-7, W-8, W-9, W-10, and W-11 which
run at slightly different directions, all starting in the vicinity
of Halifax, Nova Scotia. Along the entire Scotian Shelf a series of
120-fathom depressions are located 10 to 80 miles off shore. A nearly
continuous bank 20-60 fathoms deep and 10-25 miles wide lies along the
seaward edge of the Scotian Shelf. From northeast to southwest this
feature is divided by low saddles into Banquereau Bank (20-40 fathoms),
Sable Island Bank (0-20 fathoms), Emerald Bank (40-60 fathoms), Lahave
Bank (50-60 fathoms), and Browns Bank (20-60 fathoms). These shelf-edge
banks culminate in the low, sandy Sable Island which stretches for
about 25 miles along the outer edge of the shelf. In profile W-7 the
break from the nearly flat shelf to a gradient of 1:50 occurs at 50
fathoms; a second break occurs at 80 fathoms. A gradient of 1:10 is
reached at the 150-fathom curve. Profile W-11 is somewhat similar to
W-7 in the form of the shelf break. Profiles W-8, W-9, and W-10 show
shelf breaks at 50, 60, and 70 fathoms respectively.
The gradient of the continental slope off the Scotian Shelf ranges
from 1:10 to 1:25 along the profiles. In profiles W-7, W-8, W-10,
and W-11 the 1:25 gradient abruptly decreases to 1:70 at about 700
fathoms; in W-9 the 1:25 gradient continues to almost 2000 fathoms. It
is difficult to decide whether to include the 1:40 to 1:60 segments
with the continental slope or with the continental rise. However, since
we have picked the gradient of 1:40 as the minimum gradient for true
continental slopes, these segments fall within the continental rise.
The continental rise thus defined averages 160 miles in width off
the Scotian Shelf. Gradients are generally greater here than in the
continental rise farther south. The "Gully", a large submarine canyon
shown on navigational charts, lies about 25 miles east of Sable Island.
The submarine canyons of the Scotian Shelf have not been accurately
mapped, but the existence of many canyons in this area has been shown
by several fathograms obtained in this vicinity. Figure 14 illustrates
one sounding profile nearly parallel to the shelf near the "Gully".
Several small canyons about 100 fathoms deep occur between 100 and 700
fathoms. Several larger canyons 300-500 fathoms deep and 7-10 miles
wide are crossed on the lower continental slope and upper continental
rise.
[Illustration: FIGURE 14.--_Submarine canyons off the Scotian Shelf_
Profile replotted from NMC echogram runs nearly parallel to trend of
continental slope near Sable Island. On navigational charts largest
canyon is known as the "Gully".]
Gulf of Maine Entrance.--Southwest of the Scotian Shelf there is a
narrow gap in the continental shelf similar to the Laurentian Channel.
This feature, called either the Northeast Trough (Shepard, 1948) or
Eastern Channel of the Gulf of Maine, is 15 miles wide and about 150
fathoms deep; it provides a deep-water entrance to the Gulf of Maine
(Fig. 13). The Gulf of Maine is enclosed by Georges Bank off the New
England shelf, Cape Cod, and southern Nova Scotia. This entrance has
recently been described by Torphy and Zeigler (1957).
Gulf of Maine Interior.--Much of the interior of the Gulf of Maine
has been surveyed in exceptional detail by the Coast and Geodetic
Survey. The reader is referred to Murray's paper (1947) for a thorough
description of the floor of the Gulf of Maine. In general the floor is
extremely irregular with several 20- to 40-fathom "hills" per mile. The
floor is covered by sediment which transmits sound so readily that the
area is noted for exceptionally pronounced sub-bottom reflections from
the rock layers beneath the sediment.
Northeastern United States Sector.--From the northeast tip of Georges
Bank to Cape Hatteras the continental margin is remarkably uniform in
morphologic detail. Profiles W-12 to W-19 differ very little from the
type profile off northeastern United States (Fig. 15). The continental
shelf and slope in this area are better surveyed than in any other
area in the Atlantic. The surveys of the Coast and Geodetic Survey
were contoured and described by Veatch and Smith (1939). The sediment
studies of Stetson (1936; 1938; 1949) and the seismic studies of Ewing
and others (1937 _et seq._) make this geologically the best-known
shelf and slope in the world. Many large and well-mapped canyons cut
the continental slope from Georges Bank to Cape Hatteras. The large
submarine canyons off Georges Bank have attracted great interest
because of their remoteness from rivers and associated discharges of
river sediments.
The continental shelf is 50 to 100 miles wide in this sector. Toward
Cape Hatteras the coastal plain widens as the shelf narrows. The
combined features are called the "emerged and submerged coastal plain."
The gradient of the continental slope ranges from 1:8 to 1:15 and the
base of the slope with one exception is at 1150 ± 100 fathoms. The
shelf break is at about 50 fathoms on all profiles. On profiles W-12,
W-13, W-14, W-15, W-18, and Figure 1 of Plate 4 there is a second break
at 75-100 fathoms.
The break between the continental slope and the upper continental rise
is abrupt at some places and occupies a distance of 5 to 10 miles in
other places (Fig. 16). In each case the gradient of the next lower 30-
to 50-mile segment is 1:100.
All profiles from Georges Bank to Cape Hatteras, a span of more than
500 miles, show both an upper and a lower continental rise (Profiles
W-13, W-19, and Fig. 15). The uniformity in the continental slope
gradient carries over into the continental rise. Both the upper
continental rise and the lower continental rise are divided into three
segments. The width, gradient, and depths of each of the slope segments
are remarkably similar. Representative values based on profiles W-13 to
W-19 and Figure 15 are shown in Table 1.
[Illustration: FIGURE 15.--_Continental margin provinces: Type profile
off northeastern United States._
Profile plotted from PDR records. This profile is representative of the
sector from Georges Bank to Cape Hatteras.]
TABLE 1.--_Characteristics of the continental rise,
northeastern United States sector: representative values of
gradient, depth, and width of slope segments_
Values measured from profiles W-13 to W-19
==========================================================
Depth
Segment Upper Lower Gradient Width
edge edge
==========================================================
Upper continental rise
1 1150 ± 150 1450 ± 200 1:100 ± 20 30 ± 5
2 1450 ± 200 1650 ± 150 1:275 ± 25 40 ± 15
3 1650 ± 150 2150 ± 200 1:90 ± 30 30 ± 15
Lower continental rise
1 2150 ± 200 2350 ± 100 1:250 ± 50 40 ± 10
2 2350 ± 100 2350 ± 100 1:1400 ± 1000 50 ± 20
3 2350 ± 100 2725 ± 100 1:150 ± 30 60 ± 20
----------------------------------------------------------
The upper continental rise and the lower continental rise are
essentially terrace or shelflike features. Each has a relatively
steep (1:50-1:200) outer face (segment 3) and a relatively gentle
(1:250-1:2000) shelflike surface (segment 2). In each case a slope
of intermediate gradient (1:80-1:300) (segment 1) connects the upper
shelflike surface with the next higher face. In the case of the upper
continental rise the next higher face is the continental slope. Other
smaller-scale terracelike features may eventually be correlated along
the strike when more data are available. The local relief exceeds 20
fathoms in the deeper parts of segment 3 of the lower continental rise.
A range of hills extends for a few hundred miles along the base of
the continental rise as indicated on the physiographic province chart
(Pl. 20). These hills, known as the lower continental rise hills,
are 30-100 fathoms high and each is 1 to 3 miles wide. An echogram
(Pl. 3, fig. 4) shows three continental rise hills. The only other
part of the continental rise where relief of more than 20 fathoms is
generally encountered is in segment 1 of the upper continental rise.
The irregularity in this case is probably related to the extensions of
numerous continental-slope canyons onto the continental rise. Relief
of 5-10 fathoms is almost universal in segments 1 and 2 of the upper
continental rise. The echogram reproduced in Figure 1 of Plate 3 shows
typical minor-relief features of the upper continental rise. An oblique
crossing of a submarine canyon on the upper continental rise is shown
in Figure 2 of Plate 3. The smooth topography typical of most of the
remainder of the continental rise is well illustrated by the echogram
shown in Figure 3 of Plate 3.
The Hudson Submarine Canyon cuts across the continental rise in this
sector. A chart contoured by Ivan Tolstoy and the authors from surveys
made in 1949 is shown in Plate 2 (Ericson, Ewing, and Heezen, 1951). A
series of 30 cross profiles is shown in Figure 17.
[Illustration: FIGURE 16.--_Tracings of PDR records of continental and
insular slopes_
(a). Insular slope of Madeira southwest of Funchal.
(b). Continental slope of Europe.
(c). Continental slope off northeastern United States.]
The Hudson Canyon, which is more than 500 fathoms deep and 5 miles
wide in the continental slope (Upper Gorge), narrows to less than 2
miles and shallows to 50 fathoms at the base of the continental slope.
As it cuts across segment 2 of the upper continental rise the canyon
gradually deepens. When it cuts into the upper part of segment 3 the
canyon deepens to 300 + fathoms, widens to 3 + miles, and forms the
Lower Gorge. The canyon gradually narrows and shallows as it cuts
across the lower continental rise. It ends near Caryn Peak where
sediment cores indicate an extensive delta or submarine alluvial cone.
The upper continental rise and the lower continental rise can be
tentatively traced northeastward through the Scotian Shelf and Grand
Banks sectors. The irregular bench at 2250-2450 fathoms on W-6 and
the bench at 2300 fathoms on W-8 and W-11 can probably be referred to
segment 2 of the lower continental rise.
[Illustration]
Near Cape Hatteras the entire character of the continental margin
changes. Benches which were barely discernible farther north widen
to form a series of broad steps which resemble a giant staircase
descending to the depths of the Atlantic. These benches appear to merge
with the benches of the Blake Plateau and Escarpment farther south.
However, insufficient profiles exist to permit a firm correlation.
[Illustration: FIGURE 17.--_Cross sections of Hudson Submarine Canyon_
Replotted from nonprecision soundings (NMC) made 1949-1950]
Blake Plateau Sector.--This sector is divided into two parts, the
northern part from Cape Hatteras to 29° N. (essentially a transition
zone) and the southern or main Blake Plateau between 29° N. and the
northern edge of the Bahamas at 26° N.
The shelf break lies parallel to the coast, about 60 miles offshore,
from just south of Cape Hatteras to Cape Canaveral. The continental
slope extends (at a gradient of 1:40) only to depths of 300-400 fathoms
where the lower gradients (_ca._ 1:1000) of the Blake Plateau are found.
The main or southern Blake Plateau is 170 miles wide (east-west) and
extends from the latitude of Grand Bahama Island to 30° N. From this
point to Cape Hatteras the Blake Plateau narrows and disappears. The
Blake Escarpment forms a precipitous drop to abyssal depths along the
eastern edge of the plateau. The top of the Blake Escarpment lies at
about 550 fathoms, and its base at about 2600 fathoms. The Escarpment
is typically formed by two or three distinct slope segments.
An echogram obtained along a track running southeast from Charleston,
South Carolina, is reproduced in Figure 1 of Plate 7. The continental
shelf extends from the shore at an extremely low gradient to the
25-fathom isobath where a small definite notch marks an increase in
gradient to 1:1000. This gradient continues to the 50-fathom isobath
where it changes to 1:40. At the 90-fathom curve the gradient increases
to 1:120 and continues to 160 fathoms where it finally increases to
1:40. This continental slope drops from 160 fathoms to 280 fathoms
where the gradient flattens, and the surface changes from smooth to
rough, with hills 10 to 20 fathoms high and half a mile to 1-½ miles
wide. These hills, which extend for 4-6 miles along the profile,
directly underlie the Gulf Stream.
For 50 miles seaward of these hills the ocean floor is irregular
between 230 and 300 fathoms. At 90 miles from shore five
eastward-facing scarps 10-20 fathoms high form a striking contrast to
the generally smooth, gently rolling topography. At 300 fathoms the
gradient increases to 1:200, and the sea floor drops for the next 24
miles to 400 fathoms where a few small hills are associated with a drop
in the gradient to 1:1000. Southeast of this point the bottom is smooth
until at a depth of 430 fathoms a steep scarp drops abruptly 30 fathoms
to form a mile-wide depression 20-30 fathoms deep. The southeast side
of this feature rises to 445 fathoms, and southeastward of a few
5-fathom scarps the surface of the Blake Plateau becomes smooth.
[Illustration: PL. 3
FIGURE 1. UPPER CONTINENTAL RISE
FIGURE 2. OBLIQUE CROSSING OF SUBMARINE CANYON
FIGURE 3. SMOOTH BOTTOM OF LOWER CONTINENTAL RISE
FIGURE 4. LOWER CONTINENTAL HILLS
REPRESENTATIVE PDR RECORDS FROM CONTINENTAL RISE OF NORTHEASTERN UNITED
STATES]
[Illustration: PL. 4
FIGURE 1. SHELF BREAK AND CONTINENTAL SLOPE OFF NEW YORK
FIGURE 2. SMALL HILLS ON CONTINENTAL SLOPE OFF DAYTONA BEACH, FLORIDA
FIGURE 3. SMALL HILLS IN THE STRAITS OF FLORIDA
FIGURE 4. SMALL ABRUPT DEPRESSIONS ON THE INNER PART OF THE BLAKE
PLATEAU
PDR RECORDS FROM CONTINENTAL SLOPE AND BLAKE PLATEAU
The positions of all echo-sounding records are shown in Plate 30. Depth
in fathoms.]
[Illustration: PL. 5
FIGURE 1. WEST OF THE CREST OF THE OUTER RIDGE
FIGURE 2. WESTERN SIDE OF OUTER RIDGE NEAR ABYSSAL PLAIN
FIGURE 3. BLAKE-BAHAMA ABYSSAL PLAIN
PDR RECORDS FROM OUTER RIDGE EAST OF THE BLAKE PLATEAU AND FROM THE
BLAKE-BAHAMA ABYSSAL PLAIN
Depth in fathoms: The outgoing "pings" as well as "scattering layers"
are recorded in the 0-400 fathom scale range; the bottom topography may
lie within the range of any multiple of 400 fathoms. The depth scales
indicated on the plates refer to the profiles of bottom topography. For
example--Figure 1 of Plate 5 shows a 300 fathom deep "scattering layer"
over which the bottom profile has been superimposed.]
[Illustration:
Heezen _et al._, PL. 6
PDR RECORD OF OUTER RIDGE SHOWING SUB-BOTTOM HORIZON
One 3 millisecond ping was transmitted and received once each second.
Depth in fathoms.]
The same general succession of topographic features is shown in a
echogram (Pl. 7, fig. 2) taken along a southeast-northwest line east
of Daytona Beach, Florida. The small definite notch at 26 fathoms is
present, but a significant difference between the two echograms is seen
between 90 and 300 fathoms. On the Charleston profile a steep 1:40
gradient slope marks this depth range, while, on the Daytona Beach
profile, the gradient is relatively gentle (1:180); small but probably
significant benches are found at 225, 270, 290, 375, and 385 fathoms.
Both profiles have the same characteristic rugged 5- to 15-fathom hills
at 400 fathoms at a point underlying the Gulf Stream. On the Charleston
profile a broad, gently fractured arch separates the continental
slope from the smooth outer part of the Blake Plateau. The small,
sharp-crested hills noted on the Daytona Beach and Charleston profiles
are also found at the north end of the Straits of Florida (Pl. 4, fig.
2).
Blake Escarpment.--Profiles W-23, W-24, and W-25 and Figure 18
illustrate the form of the Blake Escarpment. The outer edge of the
Blake Plateau abruptly breaks off at about 600 fathoms. Here gradients
increase to 1:30. This segment continues with a few minor breaks to a
depth of 1200 to 1500 fathoms where a narrow bench or at least a major
break in slope occurs. Below this bench the escarpment drops so steeply
that only a few side echoes are recorded. The gradient here exceeds 1:2
in several profiles. At 2400 fathoms there is in places another narrow
bench, but on other profiles the abyssal plain of the floor of the
Blake-Bahama Basin lies directly at the foot of the steepest segment. A
peculiar fact is that along many east-west cross sections the deepest
point in the basin lies directly at the foot of the escarpment. A
similar deepening adjacent to the Campeche and West Florida escarpments
in the Gulf of Mexico has been reported (Ewing, Ericson, Heezen, 1958).
Antilles Outer Ridge.--South of Cape Hatteras a ridge ranging in width
from 60 to 200 miles lies about 100 miles east of the Blake Escarpment
and the Bahama Banks. The ridge has two parallel crests 100 miles apart
which both plunge to the south. At 30°N. the crests average about 1600
fathoms in depth, but at 25°N. they are 2750 fathoms, a drop of nearly
1000 fathoms in 300 miles. The smooth rolling topography of the ridge
between Cape Hatteras and 24°N. resembles the continental rise off New
York or, in some areas, the somewhat stronger relief of the Bermuda
Plateau of the central Bermuda Rise (Pl. 5, figs. 1, 2).
South of 24°N. and in the vicinity of Hispaniola the ridge is poorly
known and difficult to study because of its low relief and the large
errors in most nonprecision soundings taken in such a great depth
of water. North of Puerto Rico the outer ridge appears as a clearly
defined feature between the Puerto Rico Trench on the south and the
Nares Abyssal Plain on the north. Again there are two parallel crests
60 miles apart marked by low relief of 20 to 100 fathoms at a depth of
2750 fathoms.
The Antilles Outer Ridge, continuing to the east, merges with the Lower
Step of the Mid-Atlantic Ridge. But more probably it skirts the Lesser
Antilles to join the continental rise off South America. The crest
zone is covered by _Globigerina_ ooze in the north and red clay in the
south; it is isolated from the silty clays of the continental slope
except on its northwest flank. Sub-bottom echoes appear on fathograms
taken across the outer ridge. Data suggest that a prominent 8- to
10-fathom sub-bottom interface extends over the outer ridge between San
Salvador and 30°N. (Pl. 6).
Bahamas Sector.--The Bahamas sector can be divided into two parts: (1)
the broad (200 miles wide) northern area dominated by broad, shallow
banks broken by relatively narrow, deep (_ca._ 1000 fathoms) tongues
or channels; (2) the narrow southeastern part where the banks decrease
in area and the tongues deepen (to 2200 fathoms) and widen. This
southeastern part tapers to the east in the direction of the Puerto
Rico Trench. The southeast tip of this area is formed by Navidad Bank,
whose eastern slopes drop to the floor of the Puerto Rico Trench.
[Illustration: FIGURE 18.--_Tracing of PDR record of Blake Escarpment_
No soundings were recorded between 1600 and 2400 fathoms, a common
difficulty on this precipitous escarpment. Note how Blake-Bahama
Abyssal Plain slopes toward the base of the escarpment.]
[Illustration]
The Bahama Banks appear to consist of a slab superimposed on the same
surface which forms the Blake Plateau. The Blake Escarpment merges
with the lower part of the eastern slope of the Bahamas. The slopes of
the Bahamas are precipitous; gradients are of the order of 1:4 to 1:8.
Vertical cliffs, which lie just below the 50-fathom curve, have been
reported by lead soundings (Armstrong, 1953). The Tongue of the Ocean
and the Northeast and Northwest Providence channels form a network of
submarine canyons (Hess, 1933). The floor of this canyon system has a
continuous down-slope gradient to the floor of the Blake-Bahama Basin.
Sediment cores from the floor of the Blake-Bahama Basin (2525 fathoms)
(Ericson, Ewing, and Heezen, 1952) contained thick (1-3 m) beds of
graded calcareous sand. The steep slopes of the Bahamas are generally
rocky, and cores here reveal a variety of Tertiary and Cretaceous
sediments. Exuma Sound also is linked by submarine canyons to the
Blake-Bahama Basin. The graded calcareous sands of the Blake-Bahama
Basin were probably carried through this submarine canyon system by
turbidity currents. The topographic benches of Exuma Sound have been
described by Lee (1951).
The southeastern Bahamas from Great Inagua to Navidad Bank consist of
more numerous isolated banks and greater expanses of ocean floor in
the depth range of 1700-2400 fathoms. The basins behind the southern
Bahamas lie below the sill depth between the line of banks. Thus an
abyssal plain lies entrapped in the Hispaniola-Caicos Channel and the
southeastern portion of the Old Bahama Channel. Profile W-29 (Pl. 24)
shows much irregular relief between 1000 and 1500 fathoms.
Puerto Rico Trench Sector.--With the disappearance of the Bahama Banks
at the eastern edge of Navidad Bank, the continental margin assumes
its third mode of expression: the marginal trench-outer ridge complex.
The outer ridge, which nearly disappeared in the southeastern Bahama
sector, again becomes a prominent feature. The last traces of the
marginal plateau merge with the continental slope of Hispaniola and
Puerto Rico. The Puerto Rico Trench develops rapidly east of Navidad
Bank; it lies between the outer ridge and the continental slope or
landward trench slope of the Greater Antilles. The relief of the outer
ridge is somewhat greater than that observed on the outer ridge farther
northwest near the Blake Plateau. The floor of the Puerto Rico Trench
is divided into two parts by a longitudinal ridge. The deepest parts
of both are floored by nearly level trench plains. The deeper one on
the outer or northern side maintains a nearly constant depth at 4358
fathoms (4585 fathoms corrected) for 150 miles (Ewing and Heezen,
1955). The southern or inner trench is more variable in depth, and the
trench plain lies intermittently along its length. Its depth ranges
from 3600 to 4300 fathoms (uncorrected). The walls of the Puerto Rico
Trench are formed by a series of extremely steep segments which show
a remarkable persistence along the trench. Profiles W-32 and W-33
illustrate the typical trench profile north of Puerto Rico. Breaks in
slope are observed at 700 fathoms and at 1500 fathoms; at 2000 fathoms
the gradient steepens to > 1:6. In this region all soundings are side
echoes. The outer or seaward wall of the trench is also characterized
by a succession of laterally persistent slope segments. A bench at 3800
fathoms at the top of a scarp which drops to the bottom of the trench
is characteristic of several profiles.
Anegada Passage.--The Virgin Islands Bank extends 30 miles east of
Puerto Rico along the south side of the Puerto Rico Trench. Between
the Virgin Islands Bank and St. Croix a deep passage cuts through from
the Atlantic to the Caribbean. This deep passage is 130 miles long and
runs along an e-ne-w-sw line. Its walls are extremely steep (9°-43°)
(Frassetto and Northrop, 1957). The structure of this feature has been
studied by Shurbet and Worzel (1957) and by J. Ewing et al. (1957).
SOUTHWESTERN EUROPE AND NORTHWEST AFRICA: The continental margin of
Europe and Africa is illustrated in Plate 25 by only 23 profiles, and
therefore our description of this area cannot be as detailed as that
for North America. For purposes of description we have broken the area
into four sectors of contrasting type: (1) the Anglo-French sector: (2)
the Iberian sector; (3) the Gibraltar sector; and (4) the North African
sector. In all but the three profiles in the Gibraltar sector there is
a well-defined continental shelf. The continental slope is everywhere
present but ranges widely in height and gradient. The continental rise
is extremely well developed off Africa but virtually absent in the Bay
of Biscay. Abyssal plains are shown on almost every profile, but their
depth ranges from 2550 to 3075 fathoms, and their width from 50 to 250
miles.
Anglo French Sector.--Profiles E-1, E-2, and E-3 are representative of
the Anglo-French sector which extends from 45° N. to 60° N. Only the
southern part of this sector is shown on the physiographic diagram. The
continental slope is broken by a prominent bench or marginal plateau at
1000-1200 fathoms. The northwest corner of the physiographic diagram
south to 42° N. is included in Hill's (1956) contour chart. According
to this chart the prominent 1200-fathom bench extends for more than 900
miles along the continental margin from 45° to 60° North Latitude.
The continental slope from the shelf break to the prominent bench
exhibits smaller benches and changes in slope, many of which probably
will be correlatable when more profiles are obtained in this region.
The general gradient of this portion of the slope ranges from 1:10
to 1:30. On the bench individual slope segments range from 1:40 to
1:80. Below the bench the sea floor drops from 1500 to 2100 fathoms at
gradients of 1:15 to 1:30.
In profiles E-1 and E-3 a narrow continental rise about 70 miles wide
with gradients of 1:250 to 1:800 lies at the foot of the continental
slope. This narrow continental rise (Pl. 8, fig. 1) gives away to a
60-mile-wide abyssal plain at about 2500 fathoms depth (Pl. 8, fig.
3). Abyssal-plain gradients are about 1:2000 in this region. Toward
the southeast corner of the Bay of Biscay the 1000- to 1200-fathom
bench disappears (Fig. 16b); the continental rise and continental shelf
narrow to 30 miles. We have only two profiles off the north coast of
Iberia, but Hill's (1956) chart suggests that the slope is relatively
steep with some prominent benches, and that the continental shelf and
continental rise narrow to 10 to 15 miles in width. The prominent Cape
Breton Submarine Canyon lies at the southeast corner of the Bay of
Biscay (Bourcart, 1949).
Iberian Sector.--This sector was described by De Andrade (1937) on
the basis of a large number of discrete soundings by the Portuguese
Hydrographic Service. Relatively few echo-sounding profiles are
available for the area, and little more can be added to De Andrade's
description. The shelf in most places is less than 20 miles in width.
The few echo-sounding profiles available indicate several prominent
benches on the continental slope. Exceptionally large submarine
canyons occur off Cape St. Vincent, Setúbal, Lisbon (2), and Nazaré.
Preliminary investigations by the Lamont Geological Observatory
indicate that Tertiary sediments outcrop on the walls of Lisbon and
Setúbal canyons (Sutton _et al._, 1957). Tertiary sediments have also
been obtained by dredging and cable grappling along the continental
slope of northwest Iberia (Wiseman and Ovey, 1950).
Gibraltar Sector.--The continental margin in this sector is unique. The
straits cut through at about 200 fathoms so that profiles through the
straits show no continental shelf. A typical abrupt continental slope
is also absent since only locally do slope segments have gradients
exceeding 1:45. A series of prominent benches is seen in the sector
from profile E-7 to E-10.
The dominant bench levels in this sector are 300, 600, 850, 1300, 1700,
and 2100 fathoms. Insufficient profiles and dredgings are available
in this area to permit the correlation and dating of these benches.
Photographs in the area show sandy and rocky bottom, and thus dredging
in this area might yield rich rewards in ancient sediments. The benches
are so prominent that a detailed study of the topography should be
equally rewarding. Of particular interest is the manner in which the
broad benches in the Gibraltar area merge with the smaller benches
and breaks in slope of the steep continental slopes of Portugal and
Algeria. The topography in this sector most closely resembles that
of the northern Blake Plateau (profiles W-21, W-22) and the northern
part of the Anglo-French sector. In all these areas many benches are
developed, and steep slopes are only locally developed.
Northwest African Sector.--The continental margin from northwest
Morocco to Dakar is remarkably uniform and rather closely resembles
the northeastern United States sector. The continental shelf and slope
are well developed (Fig. 19). The shelf is 15 to 70 miles in width
and thus is somewhat narrower than either the North American shelf or
the Anglo-French shelf. The shelf break ranges from 50 to 80 fathoms.
The continental-slope gradients range from 1:15 to 1:40 and are thus
somewhat less steep than in the American sectors. Prominent benches
are common at 300, 600, 850, 1200, and 1600 fathoms. The continental
rise is well developed and is compound. The main contrast between the
North African and American sectors is the greater width of the African
continental rise. Off northeastern United States a line of isolated
volcanic peaks cuts across the continental rise and abyssal plain. In
the North African continental margin volcanic peaks are larger, more
numerous, and lie in coalescing lines or along ridges. The Cape Verde
and Canary groups lie in the continental rise near the outer edge of
the upper continental rise. All provinces except the continental shelf
widen from Gibraltar southward toward Cape Verde.
On profile E-11 off Casablanca the distance from the shelf break to
the lower continental rise is only 50 miles as compared with a similar
measurement of 500 miles at Cape Verde. Off Casablanca the continental
slope extends to 1400 fathoms where the gradient drops to less than
1:40 from 1:10-1:20 on the continental slope. The upper continental
rise which widens to more than 100 miles farther south is only poorly
developed off Morocco. No other deep-sea echo-sounding profiles
are available for the Moroccan continental margin. Surveys of the
continental slope made by the French Hydrographic Service during the
past few years will, when published in full, undoubtedly provide much
valuable information on the topographic benches in this important area
(Grousson, 1957).
[Illustration:
HEEZEN _et al._, PL. 8
FIGURE 1. CONTINENTAL RISE WEST OF ST. NAZAIRE, FRANCE
FIGURE 2. BISCAY ABYSSAL PLAIN. NOTE SMALL MID-OCEAN CANYON
FIGURE 3. BISCAY ABYSSAL PLAIN
PDR RECORDS EUROPEAN CONTINENTAL RISE AND BISCAY ABYSSAL PLAIN
Depth in fathoms.]
[Illustration:
Heezen _et al._, PL. 9
FIGURE 1. SMALL-SCALE ROUGHNESS, UPPER CONTINENTAL RISE
FIGURE 2. ROLLING TOPOGRAPHY, UPPER CONTINENTAL RISE
FIGURE 3. CAPE VERDE ABYSSAL PLAIN
FIGURE 4. ABYSSAL HILLS]
[Illustration:
Heezen _et al._, PL. 10
PDR RECORD OF ABYSSAL HILLS, SOUTHEAST OF BERMUDA RISE
Note sub-bottom echos from beneath intermontane basin floor. Depth in
fathoms.]
[Illustration:
Heezen _et al._, PL. 11
Area of each photograph is about 6 by 8 feet.
PLATE 11.--_OCEAN-BOTTOM PHOTOGRAPHS ON THE CONTINENTAL MARGIN AND
OCEAN-BASIN FLOOR_
FIGURE 1. (Station T1-3, photo 27) Depth 260 fathoms, location 47°
42´N., 07° 34´W., just below shelf break west of St. Nazaire, France.
Note small ripples which appear to be superimposed on larger ripples.
FIGURE 2. (Station T1-3, photo 7) Depth 285 fathoms, location several
hundred feet from photograph in Figure 1. Note holothurians and
solitary coral attached to rocks. Ripple marks are less prominent than
in Figure 1.
FIGURE 3. (Stations T1-16, photo 28) Depth 2600 fathoms, location 46°
50´N., 11° 25´W., northern part of Biscay Abyssal Plain. Note tracks of
bottom crawlers, and the prominent conical mounds, each with a central
hole.
FIGURE 4. (Station T1-18, photo 29) Depth 2650 fathoms, location
43° 56´N., 11° 12´W., southern part of Biscay Abyssal Plain. Note
meandering ridge made by subsurface burrower. Note also starfish, upper
left, and quantity of fecal pellets and holes in the bottom.
FIGURE 5. (Station T1-20, photo 53) Depth 2850 fathoms, location 42°
18´N., 14° 47´W., northeastern part of Iberia Abyssal Plain. Note holes
with converging tracks and large mound in upper right.
FIGURE 6. (Station T1-58, photo 14) Depth 3072 fathoms, location 29°
17´N., 57° 23´W., Abyssal Hills southeast of Bermuda Rise. The round
objects are manganese nodules. Note shark's tooth in lower right. Note
also small holes indicating bottom dwellers, and meandering raised
ridge of sub-bottom burrower. Of particular interest are the small
moats surrounding many of the nodules; they are probably scour marks
caused by bottom currents. These currents must be very gentle since
none of the nodules seems to show evidence of recent rolling.
Positions of stations shown on Plate 30.]
Profile E-12 passes from the African coast between Fuerteventura and
Gran Canary toward the northeast. Here the continental slope extends
only to 1000 fathoms. The Canary Islands rise abruptly from the
continental rise. Except for gradients of the order of 1:15 on the
steep slopes of these volcanic islands the gradients of the continental
rise are 1:300-1:1000.
Profiles E-13 and E-14 end on the east near Gran Canary and thus do not
show most of the upper continental rise. They do show the remarkably
wide and nearly level lower continental rise which reaches a width of
more than 500 miles.
Profiles E-15 and E-16 lie off Spanish Sahara. In both profiles the
gradient is 1:10 to 1:20 between the 50-fathom shelf break and a bench
at 300-500 fathoms. In both profiles the gradient drops below 1:40 at
about 1200 fathoms. Both profiles show numerous prominent benches on
the continental slope. The upper continental rise with gradients of
1:350-1:1200 extends to the western limit of the profiles.
In profiles E-17 and E-18 the continental slope becomes gentler,
and only in the upper 500 fathoms of E-18 does the gradient exceed
1:25. The upper continental rise is about 60 miles wide with depths
predominantly about 1600 fathoms, and the lower continental rise lies
at about 2100 fathoms and is very smooth.
[Illustration: FIGURE 19.--_Continental-margin provinces: Type
profiles off Northwest Africa_]
Profiles E-19, E-20, and E-21 cross the continental margin off Dakar
and the Cape Verde Plateau which rises from the lower continental
rise. The Cape Verde Plateau consists largely of the coalescing bases
of the volcanic Cape Verde Islands. The lower continental rise and
the abyssal plain reach their maximum width at this latitude. The
width of the ocean, the width of the Mid-Atlantic Ridge, the width
of the abyssal hills, and the depth at the axis of maximum depth all
reach their maximum values for the North Atlantic at this point. The
characteristics of the continental rise in this sector are listed in
Table 2. The reliability of these figures is much poorer than those
given for northeastern United States, owing to the smaller number of
profiles in this sector.
TABLE 2.--_General characteristics of the continental rise
northwest Africa Sector_
Values measured from Profiles E-11 to E-21
====================================================================
Depth
Segment Upper edge Lower edge Gradient Width
====================================================================
Upper continental rise
1 1200 ± 200 1500 ± 200 1:90 ± 30 30 ± 10?
2 1500 ± 200 1600 ± 200 1:200 ± 100 30 ± 15?
3 1600 ± 200 1800 ± 100 1:100 ± 50 25 ± 15?
Lower continental rise
1 1800 ± 100 2000 ± 100 1:400 ± 200 75 ± 50
2 2000 ± 100 2000 ± 100 1:1500 ± 500 150 ± 50
3 2000 ± 100 2700 ± 100 1:500 ± 200 200 ± 50
Abyssal plain 2700 ± 100 3000 ± 75 1:1250 ± 250 200 ± 50
--------------------------------------------------------------------
A famous submarine canyon, the Fosse de Cayar, lies just north of Cape
Verde. Other submarine canyons are certainly present in the sector
since any profile parallel to the strike of the topography reveals
large irregularities probably related to canyons. Echograms (Pls.
9, 13) taken in the continental rise in this sector show distinct
contrasts in the topographic detail of the sea floor. The rugged
topography of the abyssal hills (Pl. 9, fig. 4) contrasts sharply with
the nearly flat, extremely smooth abyssal plain. (Pl. 9, fig. 3).
The continental rise is nowhere so smooth nor so flat as the abyssal
plain. The continental rise ranges from 10- to 20-fathom rolling hills
5-10 miles in width to 2- to 5-fathom hills a few hundred feet across
(Fig. 19). At the seaward edge of the abyssal plain the echo sounder
penetrates the bottom to reveal interfaces 5-20 fathoms below (Pl.
13, Fig. 4). Sub-bottom penetration of 5-15 fathoms is occasionally
encountered on the continental rise on local topographic highs. Lower
continental-rise hills of the type observed off eastern United States
have not been observed off Africa.
BENCHES AND TERRACES OF THE CONTINENTAL MARGIN
The topography of the continental margin provinces is divided into a
series of benches or terraces. The largest are the continental shelf
and slope (continental terrace) and the upper and lower continental
rise. Superimposed on each of these major features is a series of
smaller benches and terraces which range from features a few miles wide
to simple breaks in the gradient of the continental slope.
Many of these features can be traced for hundreds of miles (Heezen _et
al._, in press); some are intermittent, others change in depth with
distance along the shelf; still others are only locally developed. We
can propose at least four possible origins for terraces or benches:
(1) ancient shore features; (2) structural (or rock) benching; (3)
block faulting; and (4) landslide or slump scars.
The submerged terraces within a few hundred feet below present sea
level can probably best be explained as ancient beaches formed during
the lower sea levels of the Pleistocene. The fact that the same levels
are found along coasts of diverse geology and tectonic development
supports the eustatic origin of terraces between sea level and 70-100
fathoms. The gradients of the continental shelf are so low and the
benches are so persistent that block faulting and slump scars are
excluded as general explanations. The benches of the continental
slope extend to depths of 1500 fathoms and vary in depth from point
to point along the continental slope. These cannot be Pleistocene
eustatic levels unless we consider that they were formed prior to
recent large crustal deformations. Again the persistence of the benches
for many miles argues against a fault-scarp or slump-scar hypothesis.
Thus, while the benches of the continental shelf are probably ancient
beaches, particularly those traced at the same depth for thousands of
miles, the benches of the continental slope are probably rock benches,
while some may represent step faulting.
SUBMERGED BEACHES ON THE CONTINENTAL SHELF: In Table 3 the depths of
terraces or persistent levels of the continental shelf are listed for
selected points in the North Atlantic. There is a remarkable uniformity
in these data; the same levels are found near Newfoundland, in Florida,
and on oceanic islands far from the glaciated areas.
On the basis of data obtained in the North Atlantic it is not possible
to date the different terraces, but probably most were formed in
the period between 12,000 and 5,000 years B.P. when the sea rose in
consequence of the melting of the Wisconsin glaciers. Coring and
dredging on these submerged ancient beaches could probably produce
material datable by the radiocarbon method.
TABLE 3.--_Depth (in fathoms) of prominent continental-shelf
terraces_
Each column based on only one nonprecision echogram
=============================================================================
|St. | |Charles-| | |Bar |St. | |
Placentia|John's,|Norfolk,|ton, |Bimini,|Miami,|bados,|Vincent,|Dakar|Dakar
Bay, Nfd.|Nfd. |Va. |S. C. |B.W.I. |Fla. |B.W.I.|C.V.I. |North|South
=============================================================================
10 12 10 8 10 8 10
15 15 15
20 18 20 20 18 20 20
25 24
30 30 28 30 28 28
35 38 35 35 32
40 38 38 38
42 45 42 45 42 45
50 50
55 55 58 56 54 55 55
68 65 68 65 60
72 76 72
80 80 80 80 85 81 80 78
-----------------------------------------------------------------------------
CONTINENTAL MARGIN BENCHES: On each profile across the continental
margin is a series of benches and changes in gradient which range from
the shelf break to slight changes in gradient on the continental slope.
If a field geologist enters a new area of sedimentary rocks where road
cuts do not exist he invariably goes to the stream valleys, and here
he gets his first and best view of the geologic section. The stream's
gradient is adjusted to the resistance of the rocks over which it
cuts, and the form of the valley-side slopes reveals the nature of the
underlying rocks even if they are grassed over.
This obvious field method had never been fully applied to the
continental margin. Stetson (1936) dredged in the canyons of Georges
Bank, and his hauls included Cretaceous sandstones and Tertiary marls
and green sands. He concluded that the canyons had been cut deep into
the continental margin to expose the underlying Cretaceous rocks,
but he considered the continental slope the product of depositional
processes.
However Upham (1894) had suggested that the continental slope
formed a continuous outcrop of Tertiary and Cretaceous sediments
from Newfoundland to Florida, a suggestion the writers consider
quite probable. That is to say, an analogy can be made between the
continental slope and one face of the Grand Canyon or to an erosional
escarpment bounding a high mesa or plateau like the Book Cliffs of Utah
and Colorado.
Only a few areas of the world are sufficiently well sounded to
provide data for a study of structural benches. One cannot expect
to see identical structural benches in each profile even across a
slope composed of a laterally uniform series of horizontal beds of
contrasting lithologies. The exact mode of erosion, the local system
of jointing, and chance variations in a number of other variables make
it necessary to have a large number of closely spaced, accurately
located profiles. We are fortunate that the Coast and Geodetic Survey
has surveyed virtually the entire continental slope from Georges Bank
to Norfolk, Virginia. Almost all these sounding lines are run at
right angles to the strike of the topography and are thus suitable
for analysis of structural benches. In this same area the dredgings
of Stetson (1936) on Georges Bank and the Esso Hatteras Light test
provide us with information on the stratigraphy of the sediments which
form the continental shelf and slope. The seismic work of Ewing and
collaborators (1937 _et seq._) provides us with further information on
the dips and on the depths of a number of sedimentary rock series of
contrasting lithology.
Fishermen began finding fossiliferous rocks on Georges Bank well over a
century ago. They were not particularly pleased to obtain rocks instead
of fish and generally threw the accursed rocks back into the sea. Some
curious fishermen brought a few of the rocks to shore, however, and in
time some of these were received by the museums (Upham, 1894; Dahl,
1925). These rocks contain Tertiary and Cretaceous fossils. The depths
and positions of recovery of the rocks were generally unknown to the
museums, and no clear idea could be gained of the exact occurrence of
this material. Stetson (1936; 1949) conducted a series of scientific
dredging operations in the Georges Bank area. His aim was to recover
more of these older rocks from known depth ranges and positions.
He concluded that the older rocks outcrop only in the submarine
canyons. He found no consistent depth ranges for the series of Miocene
and Upper Cretaceous rocks obtained.
[Illustration: FIGURE 20.--_Georges Bank canyons_
Chart shows position of sounding lines and dredge hauls used to
construct projected profile and inferred geologic section shown in
Figure 21 (a). Sounding lines from Coast and Geodetic Survey Chart
1313.]
In the Georges Bank area a series of prominent benches continues along
the continental slope. If these are structural benches we should be
able to trace them up the canyons and thus determine the dip of the
formations. If the benches are the result of step faulting or landslide
scars they would not extend up the canyons. In order to test these
alternatives, a series of profiles has been plotted from the surveys of
the Coast and Geodetic Survey. A line was drawn which paralleled most
of the contours of the continental slope for 20 miles or more (Fig.
20). A second line was drawn at right angles to this first strike line.
All sounding lines in the area were projected to this second dip line
along lines parallel to the strike line, and plotted as a composite
projected profile (Fig. 21). If the first line was essentially the
strike of rock layers then we should be able to determine the dip of
the beds by picking the successive benches as they occur on successive
profiles across the canyon. In Figure 21 the results of the analysis
of Oceanographer and Hydrographer canyons are presented. The dredge
hauls by Stetson (1949) have also been projected on this profile. We
see that a major bench occurs at _A_ which passes below the hauls
in which fossiliferous Navarro (Upper Cretaceous) was obtained and
through the upper limit of the hauls where Matawan (Upper Cretaceous)
was obtained. Thus if this horizon or one closely parallel to it is
the Navarro-Matawan contract, the apparent discrepancies of the depth
ranges of Stetson's dredge hauls are explained. Much more important to
the present study is the dating of a prominent structural bench.
[Illustration: FIGURE 21.--_Two projected profiles of Georges Bank
canyons_
Location of soundings for profile (a) shown in Figure 20. Soundings
projected along strike to construct profile. Soundings for both
profiles taken from Coast and Geodetic Survey Chart 1313.]
[Illustration: FIGURE 22.--_Geologic section at Cape Hatteras, Virginia_
Well logs from Swain (1947) and Spangler (1950). Four sounding profiles
made by R. V. ATLANTIS are projected to profile. Note that resistant
formations form prominent structural benches on continental slope.]
The Esso Hatteras Light No. 1 test encountered crystalline rock at
1640 fathoms depth, beneath Lower Cretaceous strata (Spangler, 1950).
The several holes drilled in the vicinity revealed remarkably constant
dips over a wide area. This is in fact true of the whole coastal plain.
Since the Hatteras well is only 17 miles from the continental slope,
it seems reasonable to project the dips to the continental slope.
We can then observe whether prominent benches on the continental
slope correlate with resistant strata in the well. We find (Fig.
22) that they do. In the area between Cape Hatteras and Nova Scotia
several cores have revealed reworked Eocene, Miocene, and Cretaceous
Foraminifera. In 1947 Northrop and Heezen (1951) obtained a photograph
and a core at 500 fathoms on the continental slope. The core contained
Eocene (Jackson) sediments, and the photograph showed a rock ledge
below the marl sampled. Although sediment cores, particularly those of
reworked material, do not provide as reliable information as dredge
hauls, this outcrop of un-reworked Eocene may also be used in dating
the structural benches of the continental slope.
The structural benches between Cape Hatteras and Cape May are
remarkably uniform and persistent (Fig. 23). On the basis of the
extrapolation shown in Figure 22, the structural benches in this area
have been correlated with the formations encountered in the Hatteras
well. North of Cape May a major angular unconformity separates the late
Tertiary and Cretaceous formations. Eocene has not been found north of
Nantucket. Upper Cretaceous has been found at about 400 fathoms off
Georges Bank (Fig. 21), and Lower Cretaceous has been dredged at 200
fathoms off Banquereau Bank, Nova Scotia.
Between 1945 and 1950 workers on the ATLANTIS made several sounding
profiles east of Georgia, North Carolina, and South Carolina. Each
crossed the precipitous Blake Escarpment. It was quite clear that no
sediment could be accumulating on such a steep escarpment and that beds
of ancient sediments and perhaps crystalline rocks must outcrop on the
escarpment. In 1949 and 1950 a few cores were taken on the escarpment
which encountered Miocene and Eocene sediments in depths of 500-800
fathoms (Ericson, Ewing, and Heezen, 1952). The marked similarity of
all topographic profiles further supported the view that the escarpment
was formed by the outcrop of an orderly sequence of horizontal
sedimentary rock layers. With this specific problem in mind a cruise
was made to the Blake-Bahama area on the research vessel ATLANTIS, in
1951. More than 50 cores were obtained. Sediments of Recent to Upper
Cretaceous age were obtained on the Blake Escarpment and from the
steep walls of the Bahama Channels (Ericson, Ewing, and Heezen, 1952).
Seismic-refraction work by Katz and Ewing (1955) and Nafe _et al._
(unpublished) and reflection work by Ewing and Landisman (unpublished)
have revealed that distinct seismic interfaces can be traced into the
structural benches on the Blake Escarpment. The ancient sediments from
the Blake Escarpment and the log of the Andros well allow the dating of
some of these formation contacts. At present the most prominent bench
at 1200-1500 fathoms appears to mark the base of the Upper Cretaceous.
Dredging and further coring on the Blake Escarpment below 1400 fathoms
is one of the most promising projects of its type despite the great
difficulties involved.
[Illustration: FIGURE 23.--_Correlation of structural benches off
northeast United States_
Soundings by Coast and Geodetic Survey; 35° 30´N.-38° 30´N.]
Lee (1951), who made a topographic study of Exuma Sound, Bahamas,
traced several prominent benches through 51 cross sections of the sound.
[Illustration: FIGURE 24.--_Geologic section: Western Europe based on
refraction measurements_
Data from Day et al. (1956) and Hill (1957). Geologic ages are those
assigned by Day on the basis of velocity; they are not based on
dredging or drilling.]
Seismic-refraction profiles have been made across the continental slope
southwest from the English Channel. These studies were initiated by
Bullard and Gaskell (1941) and have been most recently reported on by
Day _et al._ (1956). The seismic section of Day _et al._ (1956) (Fig.
24) suggests that the prominent bench at 1600 fathoms and the short
but steep scarp just below represent the outcrop of the metamorphic
basement on the continental slope. It is postulated that the prominent
bench at 900 fathoms may represent the base of the Mesozoic, and
the smaller bench at 300 fathoms the base of the Miocene. Tertiary
sediments have been obtained from the walls of canyons in the Bay of
Biscay in depths down to about 1500 fathoms (Bourcart and Marie, 1951).
The age assignments in Figure 24 are taken directly from Day _et al._
(1956) and have not been confirmed by dredging on the continental slope.
The writers conclude that the majority of the topographic benches of
the continental slope and other category II provinces are structural
benches which reflect the outcrop of resistant rock layers. This of
course implies that the continental slope is not a simple depositional
feature but a structural or erosional one. Since the structural benches
are present both in the canyons and on the un-dissected slope, the
occurrence of Tertiary and Cretaceous rocks on the continental slope
cannot be explained by erosion of submarine canyons into an otherwise
depositional terrace in the manner implied by Stetson (1949).
The existence of such persistent benches implies that the entire width
of the category II provinces must be at most only thinly covered by
recent sediments. Since the discovery of the great importance of
turbidity currents and the relatively low slopes necessary for their
occurrence, it has been a great puzzle to the writers how sediments
could be permanently deposited on the present continental slope. The
answer is simply that they are not. In addition to the turbidity
currents which provide a mechanism for the seaward transport of
sediment down the continental margin, deep-ocean currents probably sort
and transport much sediment along a course parallel to the continental
slope. It has recently been demonstrated (Swallow and Worthington,
1957) that velocities of 10-20 cm/sec are attained by ocean currents
which flow parallel to the continental slope. The particular
measurement referred to was made at 1600 fathoms on the continental
slope south of Cape Hatteras where a 17 cm/sec southward-moving current
was observed. The strong current is not a local phenomenon since it was
found in the South Atlantic by Wüst (1935) and is predicted by theories
of ocean circulation (Stommel, 1957). Photographs of ripple marks
on the continental slope (See for instance Fig. 13 in Elmendorf and
Heezen, 1957) had indicated high velocities, but it was not possible
to distinguish between a current and an oscillatory origin. The total
effect of slides, slumps, turbidity currents, and strong ocean-bottom
currents is the removal of most of the unconsolidated Recent sediments
from the continental slope.
The deposition of a series of Mesozoic and Tertiary sediments on the
subsiding margin of the continental block has produced a wedge of
sedimentary rock largely of shallow marine facies. Each successive
strata laid down on the shelf was abruptly terminated at the shelf
break by the processes of erosion which continuously or periodically
clear the unconsolidated sediment from the continental slope.
Deposition on the shelf was interrupted by several marine regressions
which produced unconformities in the stratigraphic sequence. Nafe and
Drake (1957) observed that the increase of seismic velocity with depth
and therefore the increase in compaction with depth is more rapid on
the continental shelf than in the deep sea. This is probably in part
the result of erosion of previously deposited sediments and sedimentary
rocks along the unconformities and in part the result of ground-water
cementation during periods of emergence.
Each unconformity should mark a lithologic change and consequently a
change in the resistance to erosion of the rock series. Many structural
benches may indicate surfaces of unconformity. The most recent
unconformity in the sequence lies between the surface of the emerged
Wisconsin continental shelf and the overlying post-glacial shelf
sediments.
The shelf break is defined as the most prominent break in slope between
the continental shelf and continental slope. The most prominent break
may locally be a Pliocene or Miocene structural bench, but elsewhere
late Pleistocene or Recent strata may form the shelf break. Rates of
subsidence, erosion, and sediment supply vary from place to place along
the continental shelf, and the lack of conformity either in depth or in
age of the shelf break is thus easily explained.
The deeper structure of the continental margin indicates a fundamental
structural discontinuity at the base of the continental slope (category
II provinces). It would seem a small extrapolation to attribute a
fault origin to the continental slope. Although faulting may have
played a large part in the earliest history of the category II
provinces, alternate periods of sedimentation and marine planation on
the continental shelf and long-continued erosion by slumps, turbidity
currents, and deep-sea currents on the continental slope, together
with a general subsidence of the area, could have alone produced the
characteristic form of the continental terrace.
Further work on structural benches co-ordinated with a study of ancient
sediments from dredges and cores should enable us to draw a geologic
map of the continental slope of eastern United States and Europe
(Heezen _et al._, in preparation).
GEOLOGY AND GEOPHYSICS OF CONTINENTAL MARGIN PHYSIOGRAPHIC PROVINCES
_Seismicity of the continental-margin provinces._--Plate 29 shows the
distribution of epicenters in the North Atlantic. Except in the Puerto
Rico Trench and the adjacent Antilles Arc, earthquakes are exceedingly
rare in the continental-margin provinces of the North Atlantic. From
the Bahamas through the Grand Banks the only earthquakes reported since
1910 are two near the Laurentian Channel. In the eastern Atlantic an
earthquake belt crosses the continental margin near Gibraltar but
does not seem to be directly associated with the Atlantic continental
margins. If the continental slope in the area covered by Plate 29 is a
fault scarp, we must infer that the motion has now been arrested.
* * * * *
_Magnetic anomalies and continental-margin provinces._--The first
continuously recorded total-intensity magnetic data for the Atlantic
were obtained in 1946 on a flight from Long Island, New York, to
Bermuda, and return (Keller _et al._, 1954). The first data from a
ship-towed magnetometer were obtained in 1948 (Heezen, Ewing, and
Miller, 1953).
The initial measurements showed (1) several large (> 300γ) anomalies
on the continental shelf, (2) a large (> 500γ) anomaly over the
continental slope, (3) an exceptionally smooth field over the category
III provinces, (4) rough fields with 5- to 15-mile wide 100 to 200γ
anomalies over oceanic ridges and rises, (5) enormous anomalies (>
1000γ) over seamounts and islands, and (6) large anomalies (> 500γ)
over the crest of the Mid-Atlantic Ridge.
The large anomalies on the continental shelf were considered to
indicate large volcanic cones buried by the sediments of the
continental shelf (Miller and Ewing, 1956). The anomaly over the
continental slope was considered to be the magnetic-edge effect
associated with the abrupt edge of the continental block. The rough
fields over the oceanic rises and ridges were attributed to variations
in the susceptibility of the crustal rocks, probably related to
volcanic extrusions and intrusions. The anomaly over the crest of
the Mid-Atlantic Ridge was not recognized as a general pattern until
later when many measurements were made. The major puzzle for which
no explanation was readily obtained was the origin of the smooth
field over the continental rise. The smooth or quiet field has been
observed on each crossing of the category III provinces, and even
such a sharp-sided feature as the Puerto Rico Trench failed to show a
magnetic anomaly (Davidson and Miller, 1956). We can thus state that
all category III provinces are characterized by a smooth magnetic field.
The significance of this point is not yet clear, but it must be of
major importance to the full understanding of the continental margin.
_Crustal structure and continental-margin provinces._--Maurice Ewing
and his associates have made more than 1000 seismic-refraction
measurements to determine the crustal structure of continental margins.
Most of the published results pertain to the eastern continental margin
of the United States (Drake et al., in press), but much material will
soon be published on the continental margin of Sahara, Sierra Leone,
Brazil, Argentina, Guiana, Spain, Morocco, the Gulf of Mexico, and the
Caribbean Sea. In addition extensive work has been accomplished by
Bullard, Gaskell, and Hill in the western approaches to the English
Channel. In some areas seismic, gravity, and magnetic measurements were
made along the same lines and have been subjected to an integrated
analysis (Worzel and Shurbet, 1955c).
In Plate 26 four profiles replotted from published sections are
shown. Each profile is plotted at 40:1 exaggeration, and depths are
indicated in fathoms and kilometers to facilitate comparison with
the profiles and topographic descriptions given elsewhere in this
paper. Some stations which lie up to 100 miles from a profile have
been projected along the strike of the topography. In general each
profile indicates (1) a thick lens of sediment and sedimentary rock
beneath the continental rise; (2) a major discontinuity in crustal
structure at the base of the continental slope; and (3) a wedge
of low-velocity sedimentary rocks ("unconsolidated") overlying a
lens of higher-velocity ("semi-consolidated and consolidated")
sedimentary rocks beneath the continental shelf. The basin, filled
with higher-velocity sedimentary rocks, is formed by an upturned
lip of crystalline basement rocks near the continental slope. Local
variations in thickness and velocity give rise to interesting minor
configurations, but the basic structure is nearly identical in all the
sections. The main contrast is that the category III provinces are
underlain by thick sedimentary rocks in the case of the continental
rise, and successively thinner accumulations of sedimentary rock are
associated with the marginal basins and marginal trenches.
Since the main difference in the continental-margin sections seems
to be a greatly enhanced continental-rise sedimentary section to
the north as compared with the Blake Plateau--Bahama--Puerto Rico
region, it seems reasonable to conclude that the main difference in
the topography is the result of different sedimentation rates or a
different time of origin, rather than of a vastly different structural
origin. The sedimentary rock column beneath the continental rise is
truly geosynclinal in thickness. Whether this great lens of sediment
and sedimentary rock will eventually be deformed into a mountain range
to form a new addition to the continent as advocated by the accretion
school or whether it will lie forever on the sea floor is one of the
great speculations of geology.
* * * * *
_Sediment distribution and physiographic provinces of the continental
margin._ The recent sediments of the category I provinces are largely
sands, silts, silty lutites, and carbonates (Stetson, 1938; Shepard
and Cohee, 1936; Newell, 1955). All these sediments can be assigned to
the orthoquartzite suite of Pettijohn (1957). Judging from the ancient
sediments dredged from the continental slope and obtained by drilling,
the category I provinces have been receiving orthoquartzite sediments
at least since the Cretaceous. In category II provinces Recent
sediments are either lacking or are very thin. Those Recent sediments
which temporarily remain in the province range from foraminiferal
lutites, found in such areas as the Bahamas, to silty lutites
relatively poor in pelagic fossils, found in the higher latitudes. In
general category II provinces are characterized by nondeposition or
erosion.
The sediments of the category III provinces include both pelagic
and clastic terrigenous sediments. The outer ridge is covered by
_Globigerina_ ooze in depths shallower than about 2200 fathoms and by
red clay in deeper areas. The continental rise is generally covered
by silty foraminiferal lutites, but, where submarine canyon deltas
spread out from the mouths of submarine canyons (Ericson _et al._,
1951, 1952), beds of graded sand and silts from a few centimeters
to a few meters in thickness are interbedded with lutites. In the
marginal trenches and marginal basins graded calcareous sands alternate
with low-carbonate lutites. The category III provinces of northwest
Africa are dominated by two groups of volcanic islands and seamounts
which contribute flows and volcanic detritus to the continental-rise
sediments. In general the category III provinces are dominated by
sediments ascribable to the graywacke suite of Pettijohn (1957).
* * * * *
_Past, present, and future of continental-margin physiographic
provinces._--It takes only a little imagination to see a historical
or genetic sequence in the four profiles of Plate 26. The Puerto Rico
Trench--outer-ridge profiles thus may represent a continental margin in
youth, the Blake--Bahama--outer-ridge profile a margin in late youth or
early maturity, and Newfoundland and northeast United States profiles a
margin in late maturity or old age.
The lens of sediments that has filled the marginal trench off
northeastern United States is truly geosynclinal in thickness (Drake
_et al._, in press). The sediments of the shelf lens are similar in
lithology to the orthoquartzite suite of a mio-geosyncline (cf. Kay,
1951), and the thicker continental-rise lens is probably similar in
lithology to the graywacke-volcanic suite of a eugeosyncline. (Kay,
1951). It seems virtually certain that ancient orthogeosynclines were,
before deformation, closely analogous to the continental margins. The
major problem is completing the sequence from filled geosyncline to
folded mountains is the mechanism by which the earth's crust beneath
the geosyncline thickens by 20-30 km. Although we cannot predict the
future of the present continental margin with any assurance until the
mechanism of thickening is understood, it seems probable that before
deformation the older orthogeosynclines were similar in most details to
the modern continental margins.
It seems possible that the present continental margins will in some
future geological period be uplifted into folded mountains.
OCEAN-BASIN FLOOR
GENERAL DESCRIPTION
The second of the three basic topographic divisions of the oceanic
depression is the ocean-basin floor. Included in this division are
those provinces of the oceanic depression that are not included in the
continental margin or the mid-oceanic ridge.
The ocean-basin floor is divided into three categories: (1) abyssal
floor, (2) oceanic rises, and (3) seamounts and seamount groups. The
first category includes two types of provinces, abyssal plains and
abyssal hills, which occupy the deepest portion of the ocean-basin
floor. Included in these provinces are such features as abyssal
gaps and mid-ocean canyons. The second category includes the larger
positive features of the ocean-basin floor, and the large seamounts and
seamount groups fall in the third category. The landward limit of the
ocean-basin floor is the 1:1000 gradient isopleth along the continental
margin. Along the mid-oceanic ridge the boundary is taken as that scarp
or scarp zone where the average level rises appreciably above the axis
of maximum depth of the basin floor. Broad elevations which rise above
the basin floor as isolated rises are termed oceanic rises and are
included in this discussion even though they may be structurally more
closely related to the mid-oceanic ridge or the continental margin.
ABYSSAL PLAINS
_General discussion._--An abyssal plain has been defined (Heezen,
Ewing, and Ericson, 1954) as "an area of the deep-ocean floor in which
the ocean bottom is flat and the slope of the bottom is less than
1:1000".
Abyssal plains have been found in all oceans, characteristically at the
foot of the continental rise. Koczy (1954; 1956) and Gaskell and Ashton
(1954) have described the abyssal plain south of the Bay of Bengal.
Koczy (1956) has described plains in the equatorial Atlantic on either
side of the Mid-Atlantic Ridge. On Expedition Deep Freeze workers
on the USS GLACIER, using a PDR, discovered an abyssal plain in the
Weddell Sea. Workers on the R. V. VEMA in 1957 confirmed the existence
of abyssal plains in the Argentine and in the Cape and Angola basins
of the South Atlantic. Menard (1955) has described abyssal plains off
the California and Alaska coasts. Abyssal plains are really important
and are present off all coasts except those having a long, continuous,
unfilled marginal trench.
The abyssal plains shown on the physiographic diagram have been named
in order to facilitate referencing. The abyssal plains of the western
Atlantic between Newfoundland and the West Indies are best known. The
"abyssal plain south of Newfoundland" (Heezen _et al._, 1954) has been
named the Sohm Abyssal Plain following the usage of Murray (1912) who
referred to this area as "Sohm Deep" in several publications. The
abyssal plain south of the Bermuda Rise referred to as the "abyssal
plain in the Nares Basin" (Heezen, Ewing, and Menzies, 1955; Luskin et
al., 1954) is named on the province chart (Pl. 20) the Nares Abyssal
Plain, also following the usage of Murray (1912), who named this area
the Nares Deep. The abyssal plain southwest of the Bermuda Rise was
included in the Nares Abyssal Plain until the discovery in 1955 that
the two parts were separated by a steep abyssal gap at 24° N. 68° W.
necessitated the splitting off of this plain from the Nares Abyssal
Plain. The name Hatteras Abyssal Plain used on the map was suggested by
the close proximity of the very prominent Cape Hatteras.
An abyssal plain is probably present in the Newfoundland basin, but no
PDR soundings have been obtained in that area. The name Newfoundland
Abyssal Plain is suggested subject to confirmation by a precision
survey.
The outer ridge which runs south from Cape Hatteras paralleling the
coast lines of the Bahamas encloses the Blake-Bahama Basin (Ericson,
Ewing, and Heezen, 1952); a narrow abyssal plain is found along its
western margin. This plain is named the Blake-Bahama Abyssal Plain.
Between the Greater Antilles and the Bahama Banks is a depression
called the Old Bahama Channel in the north; farther south it is known
as the Hispaniola-Caicos Channel. The Hispaniola-Caicos Channel in
particular contains an abyssal plain which, although much smaller
than the large, deep-sea abyssal plains, has all the characteristics
of slope flatness and sediment composition. This is called the
Hispaniola-Caicos Abyssal Plain.
The smallest, yet perhaps the most striking, Atlantic abyssal plains
are those at the bottom of the Puerto Rico Trench at depths of about
4358 fathoms (4585 corrected). They exhibit the proper degree of
flatness, low gradient (Ewing and Heezen, 1955), and the shallow-water
clastic sediments typical of abyssal plains.
South of Cuba in the southwest corner of the diagram lies a major
deep-sea trench. Only the eastern end of this Cayman Trench is shown
in the diagram. On the floor of this steep-walled trench lies a narrow
trench plain or system of semi-connected trench plains which lie at
depths of about 3000 fathoms south of Guantanamo, Cuba, but deepen to
3692 fathoms south of the Cayman Islands. These are known as the Cayman
Trench Plains. This area will be described in more detail in a later
publication devoted to the topography of the Caribbean.
In the eastern Atlantic each abyssal plain has been crossed at least
three times by ships employing a PDR, a coverage which, although
sufficient to establish their existence, is vastly inferior to the
coverage of the western Atlantic abyssal plains. The Biscay Abyssal
Plain occupies a large portion of the Bay of Biscay. It is connected
by an abyssal gap at 43° 30´N., 12° 00´W., to the Iberia Abyssal Plain
which lies west of the northern two-thirds of the Iberian Peninsula. A
small abyssal plain west of the Tagus River takes its name from this
river. The great abyssal plain which skirts Madeira on the east and
south and extends over a vast area to the west is referred to as the
Madeira Abyssal Plain. The Canary Islands and associated banks form
the boundaries of a small oval depression known as the Canary Abyssal
Plain. The vast abyssal plain which skirts the African continent west
of Cape Verde is named the Cape Verde Abyssal Plain.
The eastern margin of the map just reaches the abyssal plain in the
western Mediterranean which Heezen and Ewing (1955) have named the
Balearic Abyssal Plain.
Abyssal plains were not discovered until the use of continuously
recording echo sounders was extended to abyssal depths following
World War II (Tolstoy and Ewing, 1949). While the early nonprecision
echo sounders were sometimes adequate to distinguish the limits
of particular plains, real advances in their study required the
development of precision sounders and the extensive use thereof
(Heezen, Ewing, and Ericson, 1954). Since the very existence of abyssal
plains has been known for less than 10 years and only recently have any
of them been adequately delineated, the problem of their nomenclature
has never before arisen.
In the cases of the Nares and Sohm abyssal plains we have taken the
name Murray has given to the "deeps" in which the plains occur. For
Cape Verde, Iberia, and Newfoundland abyssal plains we have followed
the name Wüst (1940b) proposed for the basin within which the plains
lie. The Biscay, Tagus, Hatteras, Blake-Bahama, Hispaniola-Caicos, and
Balearic have been given the name of a prominent near-by land area, in
accordance with recognized practice. The Horseshoe Abyssal Plain takes
its name from the horseshoe-shaped line of seamounts which surrounds it
except on the eastern side.
* * * * *
_Regional description._--The relief of the abyssal plains is so low
that ordinary small-scale exaggerated profiles (Pls. 22, 27) reveal
nothing of its detailed character. In a few areas exceptionally good
PDR records have been obtained which show relief of a few feet (Pl. 12).
SOHM ABYSSAL PLAIN: The best-studied abyssal plain is the Sohm Abyssal
Plain south of Newfoundland. In addition to the 1953 VEMA PDR survey
(Heezen _et al._, 1954) the area was studied extensively in 1955 and
crossed by the trans-Atlantic cruises of 1954 and 1956. The plain is
T-shaped and generally about 200 miles wide; depths range from 2700 to
3000 fathoms (uncorrected). The depth increases in each arm of the T
toward the stem where the slope changes to south. The greatest depth is
found at the south boundary of the plain near 29° N. In the northern
part of the plain small topographic irregularities are unknown, but
toward the south peaks 50 to 500 fathoms high increase in number until
they finally replace the plain. North of the Bermuda Rise the plain
surrounds a line of huge seamounts. However, even here no small hills
are found. In the strip extending between 30° and 37° N., the east and
west boundaries of the plain are formed by scarps 200 to 800 fathoms
high. The boundary on the north side of the Bermuda Rise is formed by a
distinct shallowing, but no scarp is seen.
There is some justification for separating the northwest arm of the
Sohm Abyssal Plain north of the Bermuda Rise from the rest of the Sohm
Abyssal Plain since a mid-ocean canyon runs from the northwest to the
central sections of the plain. A sill might lie northeast of the
northeast tip of the Bermuda Rise. Here the bottom gradient changes
slightly, and an extremely large magnetic anomaly is associated with
the region. However, the plain narrows only slightly at this point,
and, since we cannot tell how many individual basins were filled
to form an abyssal plain, we will consider that the Sohm Plain now
includes the whole area, keeping in mind that the northwest arm may
have only recently merged with the rest.
South of Newfoundland the continental rise merges with the abyssal
plain with no scarp or line of hills. Here the boundary is taken at
the point where the gradient is gentler than 1:1000. This is not an
entirely arbitrary definition since an appreciable change in gradient
generally occurs near this point. South of the boundary the gradient
gradually decreases to 1:3000 at 37° N., beyond which it decreases even
more gradually and reaches 1:5000 at the southern edge of the plain.
HATTERAS ABYSSAL PLAIN: Northeast of the Bermuda Rise on a line between
Long Island (N. Y.) and Bermuda a small area of abyssal hills lies
between the continental rise on the northwest, the Bermuda Rise on the
southeast, the Sohm Abyssal Plain on the northeast, and the Hatteras
Abyssal Plain on the southwest. Although this area has been traversed
by more tracks than any area of similar size in the Atlantic, the
relationships here are still somewhat obscure. This area of some 6000
square miles is one of irregular relief, but it is not known if it
is of the character of an irregular sill or a complex abyssal gap,
although the latter seems more probable. The northwest margin of the
Hatteras Abyssal Plain is abruptly formed by the lower continental
rise hills. The eastern boundary is generally distinct and in places
is formed by a 50- or 100-fathom scarp, but generally it is not so
impressive as the east and west boundaries of the Sohm Abyssal Plain.
South of about 34° N., the western boundary of the Hatteras Abyssal
Plain is formed by the outer ridge. In some sectors the eastern flank
of the outer ridge is rather gentle, and the abyssal plain gently laps
the ridge without a sharp break. This boundary is generally sharp north
of 27° N. but to the south is less definite. The Hatteras Plain slopes
southeast to about 31° N. where the direction of slope changes to
south. A gradient of about 1:3000 and depths of about 2900 fathoms are
reached at 25° N. Within about 60 miles of Vema Gap the plain is broken
by numerous mid-ocean canyons which converge on the gap (Pl. 14, fig.
3). The Hatteras Abyssal Plain has no known seamounts or sea knolls, in
contrast to the Sohm Abyssal Plain and Nares Abyssal Plain.
The sediments of the Hatteras Abyssal Plain resemble the northern
abyssal plains in having many beds of quartz silts (Pl. 28) and
contrast sharply with the Blake-Bahama Abyssal Plain where calcareous
sands are abundant. On the west the outer ridge forms a continuous
barrier to turbidity-current sediments coming from the near-by Bahamas
so that the only source of turbidity-current sediments is from the Cape
Hatteras region to the north or possibly from the Hudson Canyon via the
suspected abyssal gap mentioned above. The type of sediment is entirely
consistent with the gradient of the plain which slopes southward from
the Hatteras region.
[Illustration:
Heezen _et al._, PL. 14
FIGURE 1. SUB-BOTTOM ECHOES ON RECORD FROM SOUTHERN BERMUDA RISE
FIGURE 2. MID-OCEAN CANYON NO. 2
FIGURE 3. MID-OCEAN CANYON IN HATTERAS ABYSSAL PLAIN, WEST OF VEMA GAP
PDR RECORDS OF MID-OCEAN CANYON NO. 2 AND CANYONS AT VEMA GAP
Depth in fathoms.]
[Illustration:
Heezen _et al._, PL. 15
Area of each photograph is about 6 by 8 feet.
PLATE 15.--OCEAN-BOTTOM PHOTOGRAPHS OF SEAMOUNTS
FIGURE 1. (Station V4-12, photo 16) Depth 700 fathoms, location 35°
12´N., 15° 18´W., on seamount of the Horseshoe Group. Note bioclastic
debris lodged in depressions on the rocky bottom. Photographs shown in
Figures 1, 2, and 3 are no more than a few hundred feet apart.
FIGURE 2. (Station V4-12, photo 10) Depth 700 fathoms, location 35°
12´N., 15° 18´W. Rippled sandy bottom.
FIGURE 3. (Station V4-12, photo 7) Depth 710 fathoms, location same as
Figures 1 and 2. Note current ripple marks. The photograph indicates an
appreciable current from left to right.
FIGURE 4. (Station V4-16, photo 19) Depth 1100 fathoms, location 35°
07´N., 13° 04´W., on the side of Ampere Seamount. Note winnow row of
dark and light gravel-sized detritus which resembles similar material
on the Rift Mountains shown in Figure 1 of Plate 19.
FIGURE 5. (Station V4-18, photo 26) Depth 75 fathoms, location 35°
10´N., 12° 55´W., near the top of Ampere Seamount.
FIGURE 6. (Station V4-14, photo 47) Depth 1100 fathoms, location 35°
12´N., 15° 22´W. on same seamount as Figures 1-3. Bottom appears
to consist of light-colored sediment thinly veiled by dark coarser
sediment. The white polka dots appear to represent piles of the
underlying light sediment brought to the surface by burrowers.
Positions of stations shown on PLATE 30]
BLAKE-BAHAMA ABYSSAL PLAIN: The Blake-Bahama Basin is a long narrow
basin between the great Blake Escarpment and calcareous Bahama Island
banks on the west and the outer ridge on the east. A narrow abyssal
plain 20 to 70 miles wide spreads out from the base of the escarpment
(Pl. 5, fig. 3). The plain is shallowest and widest just east of
the Northeast Providence Channel and deepens and narrows toward its
northern and southern ends. Sediment cores taken 60 miles off the mouth
of Northeast Providence Channel contained graded beds of calcareous
sand a few meters thick (Ericson _et al._, 1952). The material was
obviously derived from the Bahamas, presumably through the action of
turbidity currents running out through the Providence Channels, the
southern entrance of Exuma Sound, and the numerous submarine canyons
which dissect the slopes off the islands. The depth of the plain ranges
from 2600 to 2750 fathoms, and its area is about 7000 square miles.
NARES ABYSSAL PLAIN: South and southeast of the Bermuda Rise lies the
Nares Plain, a 37,000-square-mile abyssal plain that slopes eastward
from Vema Gap toward the Mid-Atlantic Ridge. From 62° to 64° W. its
northern and southern boundaries are abrupt, and only three or four
small abyssal hills have been observed in the plain. East of 64° W.
the gradient of the plain exceeds 1:2500, and the frequency of hills
increases until at 61° W. the plain consists of a series of fingers
which extend into the abyssal hills. The sediment cores obtained in
the Nares Abyssal Plain consist of alternating red clays and quartz
silts (Ericson _et al._, 1952; in press). The Nares Abyssal Plain is
the deepest of the broad abyssal plains of the ocean-basin floor. It
is also the farthest from the land. If the turbidity currents smoothed
the Nares Plain they would all have had to flow through Vema Gap
since the outer ridge-trench complex to the south prevents currents
from this area from reaching the Nares Abyssal Plain. The idea of a
route through Vema Gap is supported by the fact that the plain slopes
eastward from the gap.
HISPANIOLA-CAICOS ABYSSAL PLAIN: The depression between the Bahamas and
the coasts of Cuba and Hispaniola deepens east of Cay Lobos and reaches
its maximum depth of 2220 fathoms northwest of Cape Francis Viejo of
Hispaniola. At Cay Lobos the channel is a steep V-shaped depression
with a maximum depth of 400 fathoms, but to the east this V-shaped
channel opens out to an abyssal plain about 1500 fathoms deep. Between
this point and Great Inagua Island the flat floor slopes east reaching
a depth of 1575 fathoms just southwest of Inagua. This portion is
called the Old Bahama Abyssal Plain. South of Great Inagua is another
V-shaped channel in which the depth drops from 1700 fathoms on the west
to 2200 fathoms on the east. From this point to Cape Francis Viejo the
Hispaniola-Caicos Abyssal Plain is nearly flat. Sediments collected
from this plain confirmed the turbidity-current origin of the smooth
topography. The cores contained plant debris, shallow-water fossils,
and a variety of reworked older material (Ericson, Ewing, and Heezen,
1952; Ewing and Heezen, 1955; Ericson _et al._, 1955).
PUERTO RICO TRENCH (ABYSSAL) PLAINS: A trench plain is an abyssal
plain in the bottom of a deep-sea trench. Since the discovery of the
Puerto Rico Trench (Abyssal) Plains similar features have been reported
for the Middle America Trench (Fisher, 1954), the Kurile-Kamchatka
Trench (Udintsev, 1955), and the Peru-Chile Trench (Zeigler, 1958).
Two trench plains are known from the Puerto Rico Trench. The smaller
one occupies a basin south of a median ridge, and the larger one lies
along the deep axis of the trench. They range in width from 1-2 miles
to about 12 miles. Sediment cores taken in the trench plain contained
beds of graded calcareous sand containing fragments of _Halimeda_ and
shallow benthic Foraminifera (Ericson, Ewing, and Heezen, 1952). The
depth of the larger trench plain ranges only slightly from 4358 fathoms
(4585 fathoms corrected). The trench shallows eastward, and the plains
disappear.
CAYMAN TRENCH (ABYSSAL) PLAINS: The Cayman Trench contains trench
plains very similar to those in the Puerto Rico Trench. The limits of
the Cayman Trench Plains shown on the physiographic diagram may not be
too reliable; it has not been redrafted since the most recent tracks
shown on the track chart were obtained.
BISCAY ABYSSAL PLAIN: The Bay of Biscay is occupied by a large abyssal
plain which begins a few miles north of the northern limit of the map
and ends at the Theta Gap at 43° 30´N. The plain ranges from 2550 to
2650 fathoms in depth and is about 200 miles wide. Cores of graded
sand and silt have been obtained from the floor of the plain. Several
mid-ocean canyons 3 to 5 fathoms deep and a quarter of a mile to 2
miles wide have been observed in the plain (Pl. 8), but data are not
sufficient to determine their exact courses. It is suggested that they
converge on Theta Gap and that some may be extensions of submarine
canyons of the continental slope.
IBERIA ABYSSAL PLAIN: South of Theta Gap and west of the northern third
of the Iberian Peninsula lies an oval abyssal plain of about 25,000
square miles. The depth of this plain ranges from 2770 fathoms near
Theta Gap and at the base of the continental rise to 2820 fathoms near
the eastern margin of the abyssal hills province. The plain fans out
from Theta Gap and gradually merges with the normal westward slope from
the continental rise of north Iberia.
TAGUS ABYSSAL PLAIN: West of the canyons off the mouth of the Tagus
lies a small circular plain of 7100 square miles. Its depth ranges from
2650 to 2700 fathoms, and it slopes westward with slightly fan-shaped
contours extending from the mouths of the two large canyons. A recent
sediment sampling campaign revealed numerous graded silt and sand
layers in the plain (Pl. 28).
HORSESHOE ABYSSAL PLAIN: A small abyssal plain of 5200 square miles
lies in the center of a horseshoe-shaped ring of seamounts, just west
of Gibraltar.
MADEIRA AND CAPE VERDE ABYSSAL PLAINS: From Gibraltar to Cape Verde and
extending southward to the Sierra Leone Rise at 10° N. a vast abyssal
plain parallels the coast of Africa. For most of its length the western
boundary of this plain is 750 miles from the coast. The average width
of the plain is 200 miles, and it occasionally reaches 300 miles. An
eastward extension of the abyssal hills approaches within 200 miles of
the Canary Islands, forming a natural constriction which divides the
plain into a northern part, the Madeira Abyssal Plain, and a southern
part, the Cape Verde Abyssal Plain.
The Madeira Abyssal Plain occupies 150,000 square miles. The
Azores-Gibraltar Ridge forms its northern boundary, and the abyssal
hills its western boundary. Depths near the seaward limit of the plain
reach 2970 fathoms. The northern part of the Madeira Plain is broken
by a series of low scarps (Pl. 13, fig. 3). The gradient of the plain
between successive scarps is about 1:1500.
The Cape Verde Abyssal Plain occupies 200,000 square miles of sea floor
(Pl. 9, fig. 3). The transition from abyssal plain to abyssal hills is
gradual. Many hills are scattered in the plain near the boundary. Small
scarps of the kind observed in the Madeira Abyssal Plain have not been
found.
Recent soundings southwest of Madeira indicate that a low topographic
feature which might be called the Madeira Rise continues into the
northeast part of the area shown as Madeira Abyssal Plain. This abyssal
plain is thus somewhat smaller than indicated in the drawings. Two PDR
records from this rise area are illustrated (Pl. 13, figs. 1, 2). Since
the limits of the Madeira Abyssal Plain are based on only four sounding
profiles, future surveys will cause relatively great changes in this
portion of the diagram.
ABYSSAL HILLS
_Definition and distribution._--An abyssal hill is a small hill that
rises from the ocean-basin floor and is from a few fathoms to a few
hundred fathoms in height and from a few hundred feet to a few miles
in width. The term abyssal hills province is applied to those areas of
the ocean-basin floor in which nearly the entire area is occupied by
hills--that is, the province lies at approximately the depth of the
adjacent abyssal plain but lacks a smooth floor. Isolated abyssal hills
and groups of abyssal hills also occur in the abyssal plains.
[Illustration: FIGURE 25.--_Index chart, location of abyssal hills
profiles illustrated in Figures 26 and 27_
Limits of abyssal hills provinces shown by dotted lines.]
[Illustration: FIGURE 26.--_Eleven profiles, Western Atlantic Abyssal
Hills Province_
40:1 vertical exaggeration; positions shown on Figure 25.]
Abyssal hills are found along the seaward margin of most abyssal plains
and probably occur in profusion in basins isolated from adjacent land
areas by ridges, rises, or trenches. In the North Atlantic the abyssal
hills form two strips parallel to the Mid-Atlantic Ridge for virtually
its entire length. The Bermuda Rise is bordered on the southeast by
abyssal hills which join with the strips adjoining the Mid-Atlantic
Ridge (Fig. 25). Southeast of the Bermuda Rise the abyssal plain is
absent, and consequently the Western Atlantic Abyssal-Hills Province
exceeds 500 miles in width.
[Illustration: FIGURE 27.--_Eight profiles, Eastern Atlantic Abyssal
Hills Province_
Vertical exaggeration 40:1; positions shown on Figure 25.]
In the North Atlantic the axes of maximum depth on the eastern and on
the western sides of the Mid-Atlantic Ridge lie in the abyssal-hills
province. This pattern probably continues through the South Atlantic,
Indian Ocean, and South Pacific oceans. Individual abyssal hills
are identical to the smaller hills which rise from the steps of the
Mid-Atlantic Ridge and are probably of the same origin. The abyssal
hills and the Lower Step of the Mid-Atlantic Ridge can be distinguished
topographically only by the contrast in level.
_Regional description._--The abyssal hills within the area of the
physiographic diagram are illustrated by profiles WH-1-11 and EH-1-8
reproduced in Figures 26 and 27 respectively.
WESTERN ATLANTIC: The abyssal-hills province ranges from a few miles
to more than 125 miles in width from the north edge of the map to the
Southeast Newfoundland Ridge. Along profile WH-1 the province consists
of about a dozen hills 3-4 miles wide and 50-100 fathoms high, while
in WH-2 the hills are 4-10 miles in width, and some exceed 200 fathoms
in height. The abyssal-hills province appears to pinch out at the
Southeast Newfoundland Ridge. Southwest of the Southeast Newfoundland
Ridge the abyssal hills form a belt 60-120 miles wide which skirts
along the north edge of Corner Rise. In this area, as illustrated by
WH-3, -4, and -5, the largest abyssal hills nowhere exceed 250 fathoms
in height. The individual hills range from 3 to 12 miles wide; maximum
depths between individual hills frequently exceed the depth of the
adjacent abyssal plain by 50-100 fathoms.
Along the northwest corner of Corner Rise the abyssal-hills province
pinches out. South of Corner Rise the province appears on both the east
and the west sides of the Sohm Abyssal Plain, and isolated abyssal
hills become more numerous within the abyssal plain. In profile WH-6
the abyssal hills within the province reach 300 fathoms in height and
are 3-5 miles wide. A few higher peaks occur such as the two in profile
WH-6, one 500 fathoms high and 20 miles wide and the other 700 fathoms
high and 15 miles wide. In addition eight isolated hills 100 to 175
fathoms high rise from the plain in the same profile. To the south, the
frequency of hills within the plain increases, and the hills province
widens. In profile WH-7 the province increases to 35 miles in width
on the east side of the Sohm Plain and to 100 miles in width on the
west side. The smaller individual hills in the plain are 200 fathoms
high, in contrast to the average of 100 fathoms in WH-6. Twenty-one
individual hills rise from the plain in WH-7, in contrast to 15 in
WH-6. In WH-8 the abyssal plain is represented by a flat-floored valley
only 12 miles wide; the remaining 500-mile width of the abyssal floor
is occupied by abyssal hills. Individual hills are 300 to 600 fathoms
high and 5 to 10 miles wide (Fig. 28; Pl. 10). If we filled all the
depressions on WH-8 with 100 fathoms of sediments the profile would
closely resemble WH-7.
[Illustration: FIGURE 28.--_Tracing of PDR record, abyssal hills,
southeast of Bermuda Rise_] WH-9 and WH-10 are very similar to WH-8
except that individual hills seem to become broader and lower toward
the south. The height of individual hills in WH-10 ranges from 100 to
250 fathoms. South of WH-8 the abyssal-hill province splits into two
strips which border the Nares Abyssal Plain. The western strip lies
below the eastern scarp of the Bermuda Rise Scarp Zone, and the eastern
strip follows the Mid-Atlantic Ridge, sending a narrow strip westward
along the northern edge of the outer ridge north of Puerto Rico. A
number of hills 50 fathoms high and 2-5 miles wide are scattered over
the eastern part of the Nares Abyssal Plain. Abyssal hills are rare
in the abyssal plain west of 65° W. The abyssal-hill province along
the southwestern Bermuda Rise is extremely narrow and intermittent. No
abyssal hills have been recorded for the Hatteras Abyssal Plain, and
only locally is the province developed along the western side of the
Bermuda Rise. Locally abyssal hills are moderately well developed along
the northern margin of the Bermuda Rise.
EASTERN ATLANTIC: The abyssal floor west of the Anglo-French shelf is
extremely narrow; the abyssal plain and abyssal hills combined are only
80 miles wide. The abyssal-hills province is absent, but, as shown on
EH-1 and EH-2 (Fig. 27), large hills rise from the seaward edge of
the Biscay Abyssal Plain. These hills are 5-10 miles wide and 100-400
fathoms high. West of the Iberia Abyssal Plain (EH-3) the abyssal-hills
province is wider and better developed than near the Biscay Abyssal
Plain.
South of Gibraltar the abyssal floor widens southward; northwest of
Cape Verde it is more than 200 miles wide. Profile EH-6 runs obliquely
through the province and indicates an exaggerated width for the
province. Individual hills are 5-10 miles wide and 100-600 fathoms high
(Pl. 9, fig. 4). The abyssal-hills province is widest, and the hills
are the highest where the depth is greatest, a relation similar to that
observed on the opposite side of the Mid-Atlantic Ridge in the sector
southeast of the Bermuda Rise.
* * * * *
_Origin of abyssal-hills topography._--The topography of the abyssal
hills is considered to represent the original surface buried beneath
the abyssal plains and perhaps beneath the continental rise.
This explanation agrees well with the smooth topography and the
distribution of deep-sea sands in the abyssal plain. The origin of
the hills topography is a more difficult problem. We have no rock
samples from these provinces. Individual hills cannot be studied with
present seismic-refraction techniques; they do not seem to have a
characteristic magnetic anomaly; and Worzel has only recently developed
a method of gravity measurement which can be applied to a feature as
small as an abyssal hill. At present we have only topography as a basis
of speculation of their origin. As far as we know, individual abyssal
hills are not discernibly different from the smaller hills of the steps
of the Mid-Atlantic Ridge or much of the topography of the oceanic
rises, and thus there is no reason to assume that the abyssal hills
have a different origin.
We have noted that abyssal hills range from 50 to 600 fathoms in height
and with some exceptions from 4 to 10 miles in width. However, many
of the hills are about 200 fathoms high and 5 miles wide. The hills
are very steep-sided, and frequently the sounder simply records a
series of "highlights" from a succession of peaks (Pl. 10; Pl. 9, fig.
4). Soundings are difficult to obtain in the abyssal-hills provinces
because of the great depth and primarily the preponderance of steep
slopes (Pl. 10). We do not know whether individual hills are conical
or elongate or, in fact, if they run in narrow, sinuous ridges as
suggested by Tolstoy (1951). Accurate, detailed mapping of portions
of the abyssal-hills provinces on scales of approximately 1:20,000
or larger should provide additional limits on theories of origin.
At present we cannot decide among vulcanism, faulting, or folding,
although folding seems the least probable.
OTHER MAJOR FEATURES OF THE ABYSSAL FLOOR
_Abyssal gaps._--If two adjacent but distinct abyssal plains have
no through passage at or below the level of the higher plain, they
are said to be separated by a sill, a ridge, or a rise, depending on
the dimensions of the feature involved. However, several plains are
connected by constricted passages.
An abyssal gap is a constricted passage connecting two abyssal plains
which, in the vicinity of the gap, lie at different levels. The sea
floor slopes down continuously through the gap from the higher to the
lower abyssal plain, at a gradient considerably greater than that of
either adjacent abyssal plain.
Prominent abyssal gaps are known in the western North Atlantic (Vema
Gap) and in the eastern North Atlantic (Theta Gap). An abyssal gap
connects the Colombia and Venezuela abyssal plains of the Caribbean.
* * * * *
_Mid-ocean canyons._--Whereas most submarine canyons are furrows cut
into the continental-margin provinces, a class of canyon is found in
the abyssal plains, generally associated with abyssal gaps.
A mid-ocean canyon is a steep-walled, flat-floored persistent linear
depression, 1-5 miles wide and a few to more than 100 fathoms deep,
which occurs in an abyssal plain.
Each mid-ocean canyon discovered in the North Atlantic leads to an
abyssal gap in a manner which suggests a genetic relationship. All
recognized mid-ocean canyons parallel the adjacent continental margin.
Although all slope downward continuously they cut across the regional
slope of the abyssal plain.
Mid-ocean canyons or features resembling them have been found in the
Indian Ocean Bay of Bengal (Dietz, 1953), in the northwest Atlantic
(Ewing _et al._, 1953), the western Atlantic, and the equatorial
Atlantic (Beckmann, in preparation). Gibson (1958) has reported one
in the Gulf of Alaska. Three in the Atlantic and one in the Gulf of
Alaska have been mapped. Although mid-ocean canyons probably join with
continental-margin submarine canyons, no such connection has been
mapped. Menard (1955) has recorded from the North Pacific several
crossings of features resembling the mid-ocean canyons of the Atlantic,
which he refers to as deep-sea channels.
* * * * *
_Regional description._--Abyssal gaps and mid-ocean canyons are found
in both the western Atlantic and eastern Atlantic.
[Illustration: FIGURE 29.--_Northwest Atlantic Mid-Ocean Canyon_
Surveyed positions and extensions; generalized bathymetry simplified
from Smith, Soule, and Mosby (1937)]
[Illustration: FIGURE 30.--_Eight east-west profiles showing Northwest
Atlantic Mid-Ocean Canyon_
Soundings obtained with NMC sounder run with precision controlled ac
power in 1949]
[Illustration: FIGURE 31.--_Forty-eight cross sections of Northwest
Atlantic Mid-Ocean Canyon_
Soundings originally recorded on UQN-1B sounder with precision
controlled ac power in 1952]
[Illustration]
WESTERN ATLANTIC: The major abyssal plains of the western Atlantic are
connected from north to south by a series of abyssal gaps and mid-ocean
canyons. The Northwest Atlantic Mid-Ocean Canyon (Fig. 29) begins in an
unknown area north of the limit of the physiographic diagram (Ewing _et
al._, 1953). It runs south parallel to the continental margin of the
Grand Banks on the west and to the Mid-Atlantic Ridge on the east until
it reaches the Southeast Newfoundland Ridge, where it cuts through a
narrow abyssal gap. After passing through the gap, the canyon turns
west, broadens, and is finally lost in the Sohm Abyssal Plain. The
mid-ocean canyon is about 3 miles wide and 50 fathoms deep north of the
Southeast Newfoundland Ridge (Fig. 30). The west wall is about 10-15
fathoms higher than the east wall (Fig. 32). The canyon has been mapped
by more than 80 echo-sounding profiles (Fig. 31). The average slope of
the sea floor at right angles to the axis of the canyon in most places
exceeds the gradient of the canyon. The gradient west to east is about
1:1500, while the gradient of the canyon from north to south is 1:2250.
[Illustration: FIGURE 32.--_Long profile of Northwest Atlantic
Mid-Ocean Canyon_
Numerals I-VIII indicate location of profiles of Figure 30.]
The mid-ocean canyon forms gentle meanders (Fig. 29). As the gradient
of the canyon decreases, the canyon changes its cross-section form from
a narrow, deep canyon to a broad, shallow one (Figs. 31 and 32).
The mid-ocean canyon follows an exceptionally narrow and deep abyssal
gap through the Southeast Newfoundland Ridge. At its narrowest point
the gap is no wider than the mid-ocean canyon.
Mid-Ocean Canyon No. 2 is nearly identical to the Northwest Atlantic
Mid-Ocean Canyon in shape and form (Pl. 14, fig. 2). It has been traced
for only 350 miles through the northwest arm of the Sohm Abyssal Plain.
There is a strong suggestion of a low sill running northeast of the
Bermuda Rise toward the "tail" of the Grand Banks. Such a sill would
cut off the northwest arm of the Sohm Abyssal Plain. Mid-Ocean Canyon
No. 2 would in this case be associated with this abyssal gap connecting
the higher northwest arm with the deeper central and southern parts of
the Sohm Plain. A large magnetic anomaly which crosses the plain at
this point is possibly evidence of a buried ridge beneath the abyssal
plain. Mid-Ocean Canyon No. 2 has been traced from the vicinity of
Kelvin Seamount eastward to the supposed sill. As in the case of the
Northwest Atlantic Mid-Ocean Canyon the gradient at right angles to
the canyon axis generally exceeds the gradient of the canyon floor. A
systematic difference in depth between the two walls was not found in
Mid-Ocean Canyon No. 2.
A small area (8000 square miles) of abyssal hills lies between the
northwest arm of the Sohm Abyssal Plain and the northern edge of
the Hatteras Abyssal Plain. Although many sounding lines have been
run through this area it is not yet clear whether this area is best
referred to as an abyssal gap or a sill, although the evidence seems
to favor a gap. The western end of the Sohm Plain seems to reach a
smooth sill a few dozen miles east of Caryn Peak, and the abyssal plain
surrounding Caryn Peak, at the mouth of the Hudson Canyon, seems either
to be isolated or to be connected with the Hatteras Abyssal Plain to
the south through an abyssal gap.
An abyssal gap may possibly connect Sohm Plain to the eastern end of
the Nares Abyssal Plain, but in consideration of the eastward gradients
of the Nares Abyssal Plain it seems probable that no gaps cut all the
way through the wide abyssal-hills province which separates the two
plains. The gap connecting the Hatteras Abyssal Plain and the Nares
Abyssal Plain southwest of the Bermuda Rise has been named Vema Gap
after the Research Vessel VEMA which has been used most extensively in
this area. Vema Gap is about 20 miles wide and 70 miles long; its long
axis is oriented approximately west-east. The Hatteras Plain reaches
a depth of 2900 fathoms a few miles west of the gap. The gradient of
the plain is about 1:3000 at that point. The Nares abyssal plain to
the southeast lies at about 3070 fathoms and slopes eastward with a
gradient of about 1:3500. The floor of the gap, in sharp contrast to
the adjacent plains, slopes eastward at an average gradient of 1:300.
The edge of the Hatteras Abyssal Plain is cut by several mid-ocean
canyons 20 fathoms deep and a mile wide which develop a few miles to
the west and converge on the abyssal gap. Associated with Vema Gap is
a large magnetic anomaly similar to the one observed near the supposed
gap north of the Bermuda Rise, which again could possibly be evidence
of a prominent buried ridge forming the sill of the gap.
The Old Bahama Channel Abyssal Plain and the Hispaniola-Caicos Abyssal
Plain are probably connected by a steep-walled abyssal gap which runs
west to east south of Great Inagua Island.
[Illustration: FIGURE 33.--_Tracing of PDR record across Theta Gap_]
EASTERN ATLANTIC: The only abyssal gap known in the eastern Atlantic
was sounded by M/V THETA and is here named Theta Gap. This gap lies
off the northwest cape of Spain between the Biscay and Iberia abyssal
plains. Its existence is based on only a few profiles, one of which is
illustrated in Figure 33. Small depressions a few fathoms deep and 1-2
miles wide were observed in the Biscay Abyssal Plain and may represent
mid-ocean canyons, which may possibly connect with Theta Gap (Pl. 8,
fig. 2). Insufficient precision-sounding tracks are available in this
area to determine the exact nature of these features.
* * * * *
_Origin of abyssal-floor topography._--The explanation that abyssal
plains represent portions of the abyssal floor buried beneath sediments
transported by turbidity currents, and that the abyssal hills represent
this unburied surface, has been offered by Heezen _et al._ (1951; 1954)
and by Menard (1955). Abyssal gaps are pictured as passages through
which turbidity currents flow from a higher plain to a lower one.
Sediment must have filled in the Hatteras Plain to the present depth of
the western end of Vema Gap before the Nares Abyssal Plain could begin
to form. Ewing _et al._ (1953) suggest that the Northwest Atlantic
Mid-Ocean Canyon was probably formed by turbidity currents which flow
from the vicinity of Greenland to the Sohm Abyssal Plain. Whether the
canyon-forming process was largely erosional or depositional remains to
be seen, but the narrow, deep abyssal gap in the Southeast Newfoundland
Ridge suggests erosion. The parallelism of the Mid-Ocean Canyon with
the Mid-Atlantic Ridge even suggests a tectonic origin. The deep-sea
sands of the canyon floor and its continuous gradient argue as strongly
for a turbidity-current origin. The features of the smooth parts of
the abyssal floor seem clearly the result of deposition by turbidity
currents (Ericson _et al._, 1955; in press).
OCEANIC RISES
_Definition and distribution._--An oceanic rise is a large area
(measured in hundreds of miles), not connected to or included in a
mid-oceanic ridge or connected to a continental rise, which rises a few
hundred fathoms above the surrounding abyssal floor. The topography of
an oceanic rise ranges from gentle to extremely rugged.
Included in this classification is the Bermuda Rise and Corner Rise of
the North Atlantic, the Rio Grande Rise of the South Atlantic, and a
number of similar but unnamed features in the Indian and Pacific oceans.
* * * * *
_Regional description._--Only three oceanic rises are situated in the
North Atlantic. They are: Bermuda Rise, Corner Rise, and Rockall Rise.
Of these only the Bermuda Rise has been crossed by many echo-sounding
traverses.
BERMUDA RISE: In the center of the North America Basin and surrounding
the island of Bermuda is an oval asymmetrical arch about 300 by 600
miles with the long axis oriented northeast. The topography of the
Bermuda Rise is relatively gentle as compared to the Mid-Atlantic Ridge
but considerably more rugged than the continental rise (Pl. 27). The
rise is usually distinctly marked on the west by a 10- to 100-fathom
decrease in depth and a change from the monotonously smooth abyssal
plain on the west to gently rolling hills, 20-50 fathoms high and 2-10
miles wide (Pl. 16). In contrast the eastern edge of the rise is marked
by a series of scarps 300-900 fathoms high (Pl. 17), and the adjacent
rise topography is relatively rugged. West of the scarp zone the
topography is quite smooth to the western margin of the rise (Pl. 16).
An apron 30 miles wide surrounds the 2300-fathom high pedestal which
supports the island of Bermuda (Pl. 20). From this apron Ericson,
Ewing, and Heezen (1952), reported quantities of shallow-water-derived
carbonate clastics.
North of Bermuda the rise is marked by a number of high seamounts which
continue across the adjacent abyssal plain and form the Kelvin Group of
seamounts. Many of these appear to be flat-topped.
The Bermuda Rise is bounded on the east and north by the Sohm Abyssal
Plain, on the west by the Hatteras Plain, and on the south by the Nares
Plain. For a short distance on the northwest and a few hundred miles on
the southeast the rise is bordered by abyssal hills. The western edge
of the Bermuda Rise is formed in places by a scarp, but particularly
in the northwest the Bermuda Plateau seems to dip gently beneath the
Hatteras Abyssal Plain.
The details of the topography of the Bermuda Rise are demonstrated by
the series of radiating profiles in Plate 27.
Bermuda Pedestal.--The islands of Bermuda lie along the southeast rim
of a flat-topped pedestal whose surface lies less than 20 fathoms below
sea level. The sides of the Bermuda Pedestal slope at gradients of 1:5
to 1:30 and fall rapidly to the 2300-fathom contour where the base of
the pedestal is reached. The pedestal is 50 miles by 80 miles at its
base. Cores containing Tertiary sediments and recent reef detritus, and
in one case basaltic rock, have been obtained from the sides of the
Bermuda Pedestal (Pl. 28).
Bermuda Apron.--Encircling the base of the Bermuda Pedestal is a
smooth, gently sloping apron or depositional terrace. The width of the
apron is irregular and ranges from 40 miles on the east to 20 miles
on the west. Gradients decrease to 1:700 away from the pedestal, but
the outer edge of the apron in places has gradients of 1:90. On some
profiles hills up to 125 fathoms in height occur near the outer edge of
the apron and become increasingly numerous until at a distance of 45
miles from the pedestal the apron is not recognizable as a topographic
feature. The limits of the apron will ultimately be defined by a study
of the sediments.
Bermuda Plateau.--The Bermuda Plateau is an oval area of 90,000
square miles, which lies roughly in the center of the Bermuda Rise.
The topography is characterized by low hills and rather extensive
intermontane valleys (Pl. 16). Few individual hills exceed 50 fathoms
in height. The topography of the sub-province is well illustrated in
Plate 16. The depth is 2400 to 2700 fathoms. The plateau is bounded on
the southeast by the Bermuda Scarp Zone. On the west it extends almost
to the edge of the rise. On the north the Crescent Peaks and Muir
Seamount Group have made it difficult to define the edge of the plateau.
Crescent Peaks.--Northwest of Bermuda a crescent-shaped line of conical
peaks borders the Bermuda Apron. Individual peaks are 600 fathoms
high and 4 to 6 miles wide at their base. This range of peaks forms a
distinct sub-province which rises from the Bermuda Plateau.
Bermuda Scarp Zone.--The Bermuda Plateau is broken along its eastern
margin by a series of scarps (Pl. 17). There appear to be two systems
of scarps, one trending about N. 55° W. and the other about N. 35°
E. Individual scarps range from 100 to 700 fathoms in height. The
intersecting set of lines shown on the province chart (Pl. 20)
indicates prominent scarps in this area. Dashed lines represent more
speculative scarps. A further study of this interesting area is
underway, and it is hoped that a more-detailed mapping of these scarps
will be possible. Needless to say, dredging on these scarps should
bring rich rewards in fossil sediments and igneous rocks which are
undoubtedly exposed on these precipitous cliffs. The general character
of the scarp zone is illustrated in Plate 17 by an echogram. There seem
to be three or four major ne-sw scarps. In the east-central Bermuda
Rise the areas between the scarp zones remain at virtually the same
depth as the unfractured Bermuda Rise farther west. However, toward the
southeast the inter-scarp areas drop as a series of steps. The depth
along the base of each scarp is nevertheless deeper than the next lower
step. The smaller topographic features of the southeast part of the
scarp zone are very similar to the abyssal hills to the southeast and
are thus probably of the same origin. The strips between successive
scarps tend to shallow to the southeast and reach minimum depths just
before the next scarp is reached.
Muir Seamount Group.--In 1945 workers on the U.S.S. MUIR discovered a
large seamount 140 miles northeast of the Bermuda Islands. Subsequent
reconnaissance surveys by the Lamont Geological Observatory (Tolstoy,
1951; Tolstoy and Ewing, 1949) revealed that the peak was asymmetrical
in east-west profile (considerably steeper on the east) and that the
seamount was elongate northeast-southwest parallel to the axis of
the Bermuda Rise. Cores taken near its summit contained Eocene to
Pleistocene sediment. Additional seamounts have been discovered in the
area by Worzel and Shurbet (1955) and Northrop and Frosch (1954). The
asymmetrical profile and the elongate shape parallel to the axis of the
Bermuda Rise suggest that Muir Seamount is a tectonic uplift rather
than a volcanic pile, but admittedly undersea vulcanism need not always
produce symmetrical cones. As known, Muir Seamount is 35 miles wide
(northwest-southeast), 60 miles long (northeast-southwest), and rises
1700 fathoms above the Bermuda Rise. The minimum sounding recorded is
846 fathoms. The Muir Seamount and near-by associated peaks apparently
are not directly related to the Kelvin Seamount Group farther north.
The north and northwest margin of the Bermuda Rise, like that on
the west, is in some places a scarp and in other places a gentle
transition. Near the northwest corner of the Bermuda Rise a range of
hills each about 20 fathoms high and 3 miles wide follows the margin of
the rise for many miles. The northeastern margin of the Bermuda Rise is
abrupt in most places, and in some places a single scarp 500 fathoms
high is all that separates the Bermuda Rise from the adjacent abyssal
plain.
Sediments of the Bermuda Rise.--Most of the Bermuda Rise lies below the
depth of 2500 fathoms, and thus the sediment ranges from foraminiferal
clay through red clay with a predominance of red clay. In the vicinity
of the Bermuda Pedestal calcareous detrital sediments have built a
depositional apron around the base of the pedestal in depths of 2300 to
2450 fathoms (Pl. 28). The seamounts of the Bermuda Rise are rocky, as
shown by photographs and bottom samples. Several cores from the Muir
Seamount revealed a variety of Tertiary foraminiferal lutites. With the
exception of the detrital sediments of the Bermuda Apron, the scoured
crests and flanks of the larger seamounts, and the steeper scarps, the
Bermuda Rise is covered preponderantly by pelagic sediments. This is in
sharp contrast to the surrounding abyssal plain and near-by continental
rise, where cores reveal frequent alterations of detrital and pelagic
sediments (Pl. 28).
CORNER RISE: Much less extensive and less well known is Corner Rise
which lies directly south of the Grand Banks on the east side of the
Sohm Abyssal Plain. Corner Rise is so named because its northwest
boundary with the abyssal floor forms a sharp, nearly right-angled
corner (Pl. 20). The main part of Corner Rise is formed by a group of
large, poorly surveyed seamounts. Several of the peaks rise to 1500
fathoms. The area is represented in profile I of Plate 22 (between
mile marks 1000 and 1200). Photographs taken on Corner Seamount at the
northwestern extremity of Corner Rise showed rippled and hard-clay
bottom to 1200 fathoms. The fact that these seamounts apparently form
a prolongation of the Kelvin Seamount Group suggests the need for
detailed investigation of the latter seamounts and the probability that
additional volcanic seamounts may be found along the same trend.
[Illustration: FIGURE 34.--_Natural scale profile, Kelvin Seamount
Group_]
ROCKALL RISE: Southwest of Rockall Bank between 19°W. and 23°W. on
the 50th parallel, rising from the abyssal floor of the northeastern
Atlantic, is an ill-defined area of irregular topography which seems
not to belong to the Lower Step of the Mid-Atlantic Ridge. Little is
known of this area, and its classification as a rise may not survive
more detailed study. The area is illustrated in Plate 25 by profile E-1.
SEAMOUNTS OF THE OCEAN-BASIN FLOOR
A seamount is defined as any isolated elevation which rises more than
500 fathoms above the sea floor. Those seamounts which lie entirely on
oceanic rises have been described as part of the rise topography. Now
we will describe the seamounts of the abyssal floor.
KELVIN SEAMOUNT GROUP: An impressive row of large conical peaks runs
from the vicinity of Georges Bank for 600 miles toward the northeast
tip of the Bermuda Rise. A profile plotted at natural scale which
crosses five of the largest seamounts is reproduced in Figure 34. The
line of seamounts runs across the continental rise, abyssal plain, and
the Bermuda Rise, seemingly little affected on crossing the province
boundaries. The larger seamounts such as Kelvin Seamount are about 2000
fathoms higher than the adjacent plain and are as much as 35 miles in
diameter at the base. Those which rise from the continental rise or the
abyssal plain do so abruptly, suggesting that their bases are partially
buried. The tops of most of these seamounts lie between 550 and 850
fathoms, and at least a few are flat-topped. They are thus very similar
to the flat-topped guyots of the Pacific in size, shape, and the range
of depths of their flat summits (Hamilton, 1956). Photographs taken on
these seamounts showed rock, ripples and live solitary corals.
CARYN SEAMOUNT: A small conical peak, 1000 fathoms high, whose base is
8 miles in diameter, lies in the abyssal plain west of the Bermuda Rise
(36° 45´N.). A magnetic survey of the peak has been published by Miller
and Ewing (1956). They found an exceptionally large anomaly associated
with the peak, which clearly showed its volcanic origin. Cores from the
peak contained manganese nodules, altered volcanic rocks, and Upper
Cretaceous to Recent fossils, which shows that it is at least as old
as Upper Cretaceous. The base of the peak rises from the abyssal plain
which extends out from the Hudson Submarine Delta.