Records from the abyssal plain immediately adjacent to the abyssal hills (Pl. 13 Fig. 4) and from the flat-floored tongues in the abyssal hills (Pl. 10) reveal some of the deepest and strongest sub-bottom echoes. Good sub-bottom echoes are common in the Bermuda Plateau.
The sub-bottom reflecting layers frequently crop out, and the overlying sediments thicken and thin, revealing apparently noticeable variations in the rate of accumulation of sediments. Outcropping of sub-bottom layers on the steeper slopes indicate slumping, while the deepening of the sub-bottom reflecting horizon in valleys indicates a greater rate of deposition. High-frequency sound is normally strongly attenuated by transmission through sediments. The observation of sub-bottom reflections with high-frequency sound pulses (12 kc) indicates (1) that the surface sediment is uniform and is of low density, and (2) that a fairly sharp density change occurs beneath this surface layer of low-density material. In areas such as the outer ridge from 22° to 29° N. Lat. and the southern Bermuda Rise, it can be safely assumed that the upper layer consists of deep-sea red clay. Density measurements on red clay have indicated values of 1.25 to 1.45. The lack of sub-bottom reflections over the parts of the abyssal plains close to the continents is attributed to the numerous sand and silt layers found in the cores which reflect most of the sound. The occurrence of good reflections beneath the outer edges of the abyssal plains could be explained by either assuming that for a long geologic time no sand-or silt-carrying turbidity current has reached this area, or that red clay is deposited here much faster than elsewhere.
An extremely prominent sub-bottom reflector observed over a vast area of the east tropical Pacific has been identified by coring with a 10-cm thick bed of white, vitreous ash. This suggests that sub-bottom reflections found elsewhere may, in general, represent ash horizons. This, of course, would presuppose ash falls so vast that some record should have been preserved on land. There is no reason to assume that there is but a single cause of deep-sea sub-bottom echoes.
The widespread occurrence of the sub-bottom interface on the deeper isolated rises may be of great importance if it be interpreted as evidence of a sudden change in sedimentation resulting in a change from higher- to lower-density sediment. It is just conceivable, however, that some unstable diagenetic process may cause a sudden increase in compaction at a depth corresponding to the sub-bottom reflection.
The sub-bottom reflections in depths of 2600 fathoms on the southern Bermuda Rise and the outer ridge is about .02 second after the bottom echo, and this indicates a layer about 10 fathoms thick. At a rate of deposition of 1 cm/1000 years this change in sediment type would have occurred 20 million years ago.
In a remotely situated oceanic area the factors controlling whether red clay or Globigerina ooze is laid down are largely related to depth and temperature of the bottom water. These two factors are related to those which control the solubility of the carbonate and thus the type of bottom deposit. Emiliani and Edwards (1953), from a study of oxygen isotopes in benthic Foraminifera in Tertiary deep-sea sediments from the eastern Pacific, concluded that the temperature of Pacific bottom water decreased 8° C. from the Eocene to the present. This should have caused a great increase in the solution of carbonate assuming other factors unchanged. Sub-bottom reflections then may also be interpreted as the result of a change in the temperature or the circulation of bottom water in the deep basin. Extensive, basin-wide sub-bottom reflectors, whether the result of vast beds of ash or widespread changes in pelagic sedimentation, imply events of global importance. The further investigation and identification of these reflectors should produce data of far-reaching application in geology, climatology, and paleo-oceanography.
SUMMARY OF PROVINCE CHARACTERISTICS
The Atlantic Ocean floor consists of three major morphological divisions: (1) continental margin, (2) ocean-basin floor, and (3) Mid-Oceanic Ridge. The continental margin is formed by three categories of provinces which represent (1) the submerged continental platform, (2) the steep edge of the continental block, and (3) the raised or depressed edge of the ocean floor. The topographic detail of the continental margin is predominantly smooth except for the submarine canyons and minor irregularities of the upper continental rise. A close correspondence of topography and distribution of recent sediments is apparent. For example, deep-sea sands are found in the submarine canyons and on the canyon deltas of the lower continental rise. The continental slope appears to be a thinly veneered or bare outcrop of Tertiary and Mesozoic sediments. Individual topographic benches can be traced for many miles along the strike. On the basis of published descriptions and dating of dredged rock, certain prominent benches are identified as the outcrop pattern of various Cretaceous and Tertiary formations. The lower continental rise can be directly traced into the outer ridge at Cape Hatteras. The upper continental-rise and the marginal-trench provinces lie between the abrupt continental slope and the outer ridge. Seismic-refraction measurements in the continental margin indicate the greatest thickness of sedimentary rocks under the upper continental rise. Thus if we consider the initial form as an unfilled depression it would have been remarkably similar to the form of the present marginal trenches.
The ocean-basin floor lies between the continental margin and the Mid-Oceanic Ridge and consists of the deeper abyssal floor and the elevated oceanic rises. On the abyssal floor adjacent to the continental margins are found the flattest surfaces of the earth. These abyssal plains apparently were built by turbidity-current deposits. The unburied abyssal floor is represented by the abyssal hills. The oceanic rises are broad uplifts which rise from the abyssal floor through a series of scarps. Oceanic rises are covered with pelagic sediments except locally near islands and seamounts. The crustal structure of oceanic rises differs significantly from the typical abyssal floor in having lower velocities and generally thicker crustal layers.