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

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.

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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.

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.