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

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.