SEA-LEVEL CONTROLLED ARCHITECTURE OF NEOGENE CARBONATE SEQUENCES ALONG THE BAHAMAS TRANSECT

A hierarchy of sea level changes controls the architecture of prograding carbonate sequences on the western side of Great Bahama Bank. Dating shows that the imaged seismic sequences are the result of prograding pulses generated by third order relative sea level changes. Bundling of precession-controlled cycles into longer-term obliquity and eccentricity cycles emerges as a mechanism to create these lower-order, seismically imaged sequences. The precession-controlled cycles are recognized as subaerial exposure surfaces on the banktop and marl/limestone alternations on the slope and basin.

Cores and seismic data along the Bahamas document the ages and facies distribution in the sequences with an unprecedented precision and shed new light on some controversial aspects of carbonate sequence stratigraphy, in particular leeward reefs and highstand versus lowstand shedding. Sequence boundaries are formed by subaerial exposure on the platform top, deep submarine channel incisions on the slope, and by facies changes on the slope and basin. Decreased sediment supply and increased current activity during sea-level lowstands forms hardgrounds on sequence boundaries and on top of lowstand systems tracts. Increased sediment production during sea level highstands produces thick prograding pulses of fine-grained, platform-derived material overlying thin lowstand and transgressive units. Leeward reefs develop during the each transgressive phase on the uppermost slope but are subsequently covered by offbank sediment transport. Redeposited carbonates accumulate during both sea-level highstands and lowstands. Thus, highstand shedding is recognized in the thickness of the entire sedimentary package rather than in the number of turbidites.

In general, sea level changes cause a change in sediment composition, sedimentation rate and diagenesis from the platform top to the basin. The combination of these factors generates impedance contrasts to create seismic reflections. Age consistency of sequence boundaries along the transect document the chronostratigraphic significance of the seismic reflections.