Eustatic Controls on Diagenesis in Platform to Slope Carbonates, Neogene to Quaternary, Subsurface Great Bahama Bank

Leslie A. Melim, Gregor P. Eberli, Peter K. Swart

Although the role of eustatic sea level changes on carbonate sedimentology has been well described, its effect on diagenesis can be more difficult to identify, particularly in deeper water sediments. This study has identified both near surface and shallow burial diagenetic patterns that can be attributed to eustatic sea level fluctuations in two cores drilled on the western margin of Great Bahama Bank.

Near surface diagenesis occurs during sea level highstands in the form of marine cementation in shallow-water grainstones. During sea level lowstands, subaerial exposure leads to meteoric diagenesis and development of caliche horizons in the platform top facies. In the deeper water facies, the absence of sedimentation during lowstands and during early transgression allows the formation of marine hardgrounds characterized by cementation, dolomitization and, in places, phosphate precipitation.

In addition to the near surface diagenetic effects, eustatic sea level fluctuations control variations of fluid flux into the platform (e.g. meteoric versus marine fluids) and thus directly effect shallow burial diagenesis. In the platform facies, the primary effect has been the development of extensive vuggy to cavernous porosity due to the development of meteoric lenses during sea level lowstands. Meteoric fluids did not extend into the deeper water facies, even during the latest Pleistocene lowstand. Rather, these sediments are characterized by extensive alteration of aragonite to calcite and/or dolomite in marine pore fluids. The large amplitude, high frequency, eustatic sea level fluctuations of the Plio-Pleistocene have contributed to this diagenesis by ensuring a dynamic fluid low system continually adjusting to new base levels and input of different fluids. In addition, the migration of these fluids was strongly influenced by the presence of the early diagenetic hardgrounds that are relatively impermeable and acted as flow barriers.

Eustatic sea level, therefore, plays a more important role in the diagenesis of carbonate sediments than previously thought. Not only do shallow-water carbonates show characteristically distinct diagenesis during sea level highstands and lowstands but the diagenesis of slope carbonates can also be tied to sea level changes. In addition, shallow burial diagenesis also shows a strong influence from eustatic sea level in both shallow and deep-water facies.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995