Abstract
The Southeastern Bahamas consist of a number of small banks and atolls, stretching from Crooked Island, at the southeastern tip of Great Bahama Bank to Navidad Bank, off the northeastern end of Hispaniola. As part of an investigation of the pre-Holocene deposits of the Bahamas, shallow cores from four of the Southeastern Banks were collected and analyzed.Eleven different rock types make up the cored intervals from Great Inagua, Mayaguana, Crooked Island and Hogsty Reef. Rock types follow each other vertically in a definite order and the resulting depositional sequence repeats itself over the cored intervals. The vertical distribution of rock types resembles a rhythmic or hemicyclic pattern (ABC/ABC), although individual cycles seldom include the complete sequence.Analysis of the depositional cycles shows that upward growth of the banks occurs mainly during relative sea level rises, at the peak of organic productivity over the whole platform. The advent of non-skeletal sediments, such as we see it today on most banks, announces a slackening in the upbuilding of a bank and coincides with the end of a relative sea level rise. Cycles are separated by zones of discontinuity, which reflect low stands of sea level.Cyclicity would thus result from a succession of glacio-eustatic sea level fluctuations, adding thicknesses of carbonate deposits on top of subsiding banks.The structural history of the Southeastern Bahamas seems rather complex. A measure of the thickness of the Lucayan Formation provides estimates of subsidence rates, which show that each bank constitutes a structural entity behaving independently from the others.In addition to the diagenetic features present on other banks, the cores from the Southeastern Bahamas reveal unique and complementary particularities that allow models of carbonate diagenesis to be developed. In contrast with other Bahamian banks, diagenesis barely affected the Pleistocene limestone of Hogsty Reef. Original mineralogies of particles and cements have been preserved to this day thanks to a low influence of freshwater in Hogsty Reef. The cores of Mayaguana show the other extreme, with the shallowest massive subsurface dolomite in the Bahamas. At 10 m below the surface, dolomite occurs in a variety of habits and textures and may have been formed by a combination of fresh and marine waters, either mixed, or affecting the rock in successive stages.