TESTING A SEISMIC INTERPRETATION OF GREAT BAHAMA BANK WITH A COMPUTER-SIMULATION

Sixty million years of carbonate deposition were simulated to test an interpretation of platform development based on seismic data and limit ed well control from northwestern Great Bahama Bank. Seismic profiles of the northwestern Great Bahama Bank document the lateral growth pote ntial of isolated platforms that were welded together by progradation to form the modem bank. The mechanism we proposed responsible for an e volution from aggradation to progradation was sediment overproduction on the platform, the excess of which was transported offbank and which caused a decrease in accommodation space on the marginal slope. Progr adation occurred in pulses that were interpreted to be the result of t hird-order sea level fluctuations. To evaluate the proposed mechanism, 15 input parameters were used to model the platform evolution. The si mulation program, which uses empirical relationships to model basin fi ll, successfully reproduced the geometries seen on the seismic lines, indicating that the proposed interplay of mechanisms could have built the observed platform architecture. The simulation demonstrated, in pa rticular, that in a setting like the Bahamas, a basin must be substant ially filled before progradation can take place, and that sea level ch anges can drive the pulses of progradation. This implies that laterall y stacked sequences often contain the record of sea level changes, and therefore have potential use in sequence stratigraphy. The simulation can also be used to estimate the quantitative importance of individua l factors controlling aggradation and progradation in the Bahamas. We show how close the balance between aggradation and progradation is, an d how small changes in the rate of subsidence or accumulation can caus e immediate switches from aggradation to progradation. In particular, we show that the rate of subsidence exerts the major control on the ti ming of progradation, more so than basin width. Carbonate production r ates similar to modern rates were required to produce the necessary se diment input for progradation, which suggests that carbonate productio n has been consistently high since the early Tertiary. Repeated exposu re and erosion, however, have decreased the overall accumulation rate. The simulation also suggests that the asymmetric progradation in the Bahamas was only possible where there were extreme differences between windward and leeward conditions, with a maximum sediment input of 10% from the windward side.