The Floors of the Ocean: 1. The North Atlantic / Text to accompany the physiographic diagram of the North Atlantic

by Bruce C. Heezen; W. Maurice Ewing; Marie Tharp

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.