THE MARINE GEOLOGY OF THE BAHAMA ESCARPMENT

A stratigraphic section (2050 to 3960 m) of the Bahama Escarpment off of Cat Island, measured with DSRV Alvin, consists of lower and Middle Cretaceous peritidal, lagoonal, patchreef and backreef limestones unconformably overlain by an intermittent cover of lithified Maastrichtian and Eocene pelagic limestone above 2540 m. Additional dive traverses and dredging elsewhere indicate that sequences similar in age and lithology to the Cat Island section occur along the entire 800 km of escarpment southeast of Cat Island.; These Bahama sequences have great similarity to "bank-interior" and "backreef" deposits characteristic of the marginal zones (1 to 10 km wide) of other ancient and modern carbonate platforms. Reef and forereef debris which constituted the original Bahama Escarpment is absent due to erosion, and the present Bahama Escarpment is therefore an erosional feature. Analogy with the El Abra platform indicates about five km of erosion at 5000 m depth and about one km of erosion at 1500 m depth. Erosion occurs intermittently (pre-Maastrichtian, Paleocene and Oligocene erosional events) as recorded by the unconformable forereef and pelagic cover.; Erosion of the lower escarpment is effected at present by spallation of joint blocks. Jointing may result from (1) lithostatic pressure at four or five km depth exceeding the strength of the limestone comprising the platform, or (2) decompression of previously deeply buried following exposure on the seafloor. Initial erosion may have resulted from (a) dissolution by cold corrosive deep water, or (b) oversteepening of an initial slope by accretionary processes which caused slumping after sufficient relief was attained. The upper escarpment erodes by slumping of large allochthonous blocks rather than by spallation of joint blocks.; Data from other modern and ancient passive margin continental slopes indicate that these slopes are erosional features. That the erosion is not related to orogenic events must be recognized in order to correctly reconstruct the paleogeography and tectonic and geologic histories of mountain belts.; Subsidence of the Cat Island section has been exponential, decreasing from 60 m/my in Early Cretaceous to 40 m/my in Middle Cretaceous time. Extrapolated values for the Late Cretaceous and Tertiary are 20 m/my.