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.
Figure 29.—Northwest Atlantic Mid-Ocean Canyon
Surveyed positions and extensions; generalized bathymetry simplified from Smith, Soule, and Mosby (1937)
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
[Figure 31.—Forty-eight cross sections of Northwest Atlantic Mid-Ocean Canyon]
![[Figure 31.—Forty-eight cross sections of Northwest Atlantic Mid-Ocean Canyon]](#444483495174585970_fig31full.jpg.id-4955528251302255175.wrap-0.html.html){kind=link}
Soundings originally recorded on UQN-1B sounder with precision controlled ac power in 1952
[Figure 32.—Long profile of Northwest Atlantic Mid-Ocean Canyon]
![[Figure 32.—Long profile of Northwest Atlantic Mid-Ocean Canyon]](#444483495174585970_fig32full.jpg.id-2087238104504283271.wrap-0.html.html){kind=link}
Numerals I-VIII indicate location of profiles of Figure 30.
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.
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.
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).