CHRONOSTRATIGRAPHY OF A PROGRADED CARBONATE PLATFORM MARGIN: A RECORD OF DYNAMIC SLOPE SEDIMENTATION, WESTERN GREAT BAHAMA BANK

Abstract

Independent analyses to date prograding carbonate platform sediments have enabled development of an integrated chronostratigraphy of the west margin of the Great Bahama Bank GBB The chronostratigraphy permits interpretation of periods of deposition nondeposition and erosion on the platform and margin The integrated chronostratigraphy is based on planktic foraminiferal biostratigraphy calcareous nannofossil biostratigraphy strontiumstratigraphy and magnetostratigraphy Critical to interpreting each type of stratigraphy is a clearunderstanding of slope dynamics and the depositional system The biostratigraphic datainparticular are affected by the extreme dilutionof key microfossils byplatformsedimentsduring margin progradation Because of this factor the highest abundance of microfossils is restricted to thin units of pelagic sediment deposited during temporary intervals when platform sediment supply was greatly reduced However these selective microfossil concentrations are also more likely to yield premature last appearance datums and delayedfirstdatums thanthe dilutedintervals becausethey represent short periods of time Despite these problems the microfossils provide critical age control The detailed chronostratigraphy allowsinterpretation of awell platform evolution Core Unda themore landward location penetrated the oldest sediments earliest late Miocene The more seaward coreClino although deeper bottomed in younger sediments latest Miocene Three major progradational episodes were delineated using seismic stratigraphy lithostratigraphy and information on depositional age Progradation occurred during the late Miocene late early Pliocene and latest Pliocene In the Pliocene shelf and ramp setting margin progradation began during a highstand but also occurred inaforced situation during a fall insea level Rapid reef progradation occurred near theend of the Pliocene and early Pleistocene whenthe platform had aggraded to a nearly horizontal surface The transformation from aramp platform topography to aflat platform culminated in themiddle Pleistocene Age constraints acrossthe west margin of GBB indicate that the seismic reflectors that constitute sequence boundaries are synchronous within our age resolution As expected in a slope setting the sequence boundaries represent condensed time periods of both erosion and nondeposition Downslope plankton onlapping units are correlated to upslope marine hardgrounds and are thought to represent periods of falling sealevel Subsequenthigh dating of additional basinaland shallow borings will provide a rich archive of paleoceanographic changes and will serve as a direct link between thedeep and shallow accumulations