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.
The Mid-Oceanic Ridge is a broad fractured arch whose axis follows the median line of the ocean. It generally covers the center third of the ocean. The ridge provinces are divided into crest provinces and flank provinces. The crest provinces include (1) the Rift Valley, a long axial cleft 15-30 miles wide and 500-1500 fathoms deep; (2) the Rift Mountains which form the sides of the Rift Valley; and (3) the High Fractured Plateau, a rugged plateau which borders the Rift Mountains. The flank provinces consist, on each side, of roughly three steps separated by large scarps. A seismic belt accurately follows the Rift Valley. The topography of the Mid-Oceanic Ridge seems best explained by extensive normal faulting. The mid-oceanic Rift Valley connects with and is probably of the same origin as the African rift valleys.