FORMATION AND EVOLUTION OF STEEP CARBONATE PLATFORM MARGINS–LESSONS LEARNED FROM THE MODERN

The depositional and diagenetic processes that form the steep Holocene slopes (40-60°) of Great Bahama Bank (GBB) provide a model for the origin and evolution of ancient steep platform margins such as the Permian Capitan Fm. of the southwestern U.S., and the Devonian of the Canning Basin, Australia. Recent study of the high relief margins in the Bahamas provide us with insight into the development of these and other steeply dipping slope deposits fronting ancient platform margins. The morphology of the GBB margin consists of a gradually sloping platform top that extends to approximately 60 meters water depth, followed downslope by a near vertical wall that abruptly terminates in a 40-60 degree slope at a depth varying from 120-150 meters subsea. These steeply dipping slope deposits extend to depths in excess of 330 meters and consist of coalesced lenticular deposits of coarse grained sediment deposited by a combination of rock fall and grain flow processes. Radiocarbon dating shows that the most recent slope was deposited during the early transgressive phase of the latest rise in sea level from fringing reefs that grew along the margin and tracked sea level rise. The stability of these steep slope deposits is the result of grain size, sorting, and grain angularity coupled with pervasive syndepositional marine cementation.

While recent slope deposits along Great Bahama Bank are likely deposited on a steep margin inherited from the Pleistocene, depositional and diagenetic processes allowed for both the maintenance of the steep profile as well as the probable enhancement of slope declivities through aggradational growth. Submarine cementation is pervasive in these deposits and took place at rates that virtually froze the deposits in place over a short period of time (i.e. months to years). This “geologically instantaneous” cementation preserved many of the slope deposits at the non-stable angle of yield, and also contributes to the deposition of megabreccias as the slope continues to oversteepen and ultimately fail. Understanding the role of syndepositional cementation in maintaining steep slope deposits in the modern, provides a model to better evaluate the evolution and resulting stratal geometries of ancient slope deposits.