Marine geology of the Bahama Escarpment

Abstract

A 2 km thick stratigraphic section of the Bahama Escarpment off Cat Island in the northwestern Bahamas has been measured and sampled by submersible. The escarpment, from 3940 m upwards, consists of lower and middle Cretaceous peritidal, lagoonal, patch-reef and backreef limestones unconformably overlain by an intermittent cover of Maastrichtian and Eocene pelagic limestone. Data from additional dive traverses and dredging elsewhere on the Bahama Escarpment indicate that shallow-water platform limestones similar in age and lithofacies to the Cat Island sequence occur along the entire 800 km of the escarpment as far as Navidad Bank. Comparison of our section to stratigraphic data from coeval Mesozoic carbonate platforms surrounding the Gulf of Mexico and extending up the east coast of North America as well as from other ancient and modern carbonate platforms reveals that the Bahama sequences have the greatest similarity to deposits characteristic of the marginal zones (1–10 km wide) of these platforms. In particular, the Bahama sequence is most similar to the “bank-interior” and “backreef” portions of the marginal zone and not to the “reef” portion. It is clear from both modern and ancient examples that a purely constructive carbonate platform margin should consist of a reef and associated forereef deposits, and not only of “bank-interior” peritidal and lagoonal strata, as exposed along the Bahama Escarpment. Therefore, the reef and forereef debris which constituted the original seaward edge of the Bahama must have been removed by erosion. The amount of erosion is estimated as 1–5 km and is effected at present by spallation of joint blocks. Lithostatic pressure at 4–5 km depth may exceed the strength of the limestone comprising the platform, resulting in the pervasive jointing observed in the lower portion of the escarpment. Subsidence of the Cat Island Escarpment has been exponential, decreasing from 60 m/m.y. in the Early Cretaceous to 40 m/m.y. in the Middle Cretaceous. Extrapolated values for the Late Cretaceous and Tertiary are 20 m/m.y.