304
C. G. ST. C. Kendall· A. Sen
4
Discussion and Conclusions
The laterally stacked sequences of a seismic section are the product of sea-level
changes whose signal can be identified by making a sequence stratigraphic interpretations (Vail et al. 1988). As Eberli et al. (1994) showed earlier a sedimentary simulation can successfully reproduce the geometries seen on a seismic
line. The seismic data documents the fill of the Straits of Andros and the sediments are expressed as a series of onlapping and downlapping wedges with various angles of slope. The simulation output produces a match with the seismic
interpretation. The simulation helped to quantify individual factors controlling
aggradation and progradation in the Bahamas. It showed that different basin
depths effected the timing and extent of progradation. It also showed that there
was a close balance between aggradation and progradation, and that small
changes in the rate of relative sea-level movement and/or carbonate accumulation rates caused immediate switches from aggradation to progradation of the
margin. Progradation took place after the space on the upper slope had been reduced and the sediment transported offbank could fill the remaining space and
extend the platform margin farther basinward. In particular, progradation was
triggered by sea-level drops that shifted sediment production and accumulation
to the margin slope. Carbonate production rates similar to modern rates were
required to produce the sediment necessary for progradation, which suggests
that carbonate production has been consistently high since the early Tertiary. At
the same time, repeated exposure and erosion have reduced the overall preservation and decreased the overall accumulation rate.
Thus, progradation occurred in pulses which are recorded on the seismic
lines and are confirmed by the simulation as a succession of prograding and sigmoidal sequences, with each sigmoid believed to have been formed as the result
of a single cycle of sea-level fall and rise (Eberli and Ginsburg 1989). Each prograding sequence was up to 500 m thick and probably consisted of an offlapping
complex of reefal carbonates covered by calcareous sand. Eberli and Ginsburg
(1989) thought that during the transgressive stages, marginal reefs were established and then buried during the subsequent highstand, when abundant sediment was produced on the flooded bank and transported to and off the leeside
of the bank. Their interpretation was based on findings from the leeside of the
modern bank where early Holocene reefs are covered by offbank transported
sand (Hine et al.1981). The two 1990 core borings on the western margin of the
Great Bahamas Bank have confirmed this interpretation. This justified the use of
benthic accumulation as well as a background pelagic rain to produce the geometries during simulation. Interestingly the simulation suggested progradation of the bank margin continued at sea-level lows when the platform was exposed.
For a carbonate shelf setting with a low rate of subsidence and a high rate of
sedimentation, a very clear stratigraphic signal is produced by a particular sealevel curve. This requires that the rate of carbonate sedimentation was such that
C. G. ST. C. Kendall· A. Sen
4
Discussion and Conclusions
The laterally stacked sequences of a seismic section are the product of sea-level
changes whose signal can be identified by making a sequence stratigraphic interpretations (Vail et al. 1988). As Eberli et al. (1994) showed earlier a sedimentary simulation can successfully reproduce the geometries seen on a seismic
line. The seismic data documents the fill of the Straits of Andros and the sediments are expressed as a series of onlapping and downlapping wedges with various angles of slope. The simulation output produces a match with the seismic
interpretation. The simulation helped to quantify individual factors controlling
aggradation and progradation in the Bahamas. It showed that different basin
depths effected the timing and extent of progradation. It also showed that there
was a close balance between aggradation and progradation, and that small
changes in the rate of relative sea-level movement and/or carbonate accumulation rates caused immediate switches from aggradation to progradation of the
margin. Progradation took place after the space on the upper slope had been reduced and the sediment transported offbank could fill the remaining space and
extend the platform margin farther basinward. In particular, progradation was
triggered by sea-level drops that shifted sediment production and accumulation
to the margin slope. Carbonate production rates similar to modern rates were
required to produce the sediment necessary for progradation, which suggests
that carbonate production has been consistently high since the early Tertiary. At
the same time, repeated exposure and erosion have reduced the overall preservation and decreased the overall accumulation rate.
Thus, progradation occurred in pulses which are recorded on the seismic
lines and are confirmed by the simulation as a succession of prograding and sigmoidal sequences, with each sigmoid believed to have been formed as the result
of a single cycle of sea-level fall and rise (Eberli and Ginsburg 1989). Each prograding sequence was up to 500 m thick and probably consisted of an offlapping
complex of reefal carbonates covered by calcareous sand. Eberli and Ginsburg
(1989) thought that during the transgressive stages, marginal reefs were established and then buried during the subsequent highstand, when abundant sediment was produced on the flooded bank and transported to and off the leeside
of the bank. Their interpretation was based on findings from the leeside of the
modern bank where early Holocene reefs are covered by offbank transported
sand (Hine et al.1981). The two 1990 core borings on the western margin of the
Great Bahamas Bank have confirmed this interpretation. This justified the use of
benthic accumulation as well as a background pelagic rain to produce the geometries during simulation. Interestingly the simulation suggested progradation of the bank margin continued at sea-level lows when the platform was exposed.
For a carbonate shelf setting with a low rate of subsidence and a high rate of
sedimentation, a very clear stratigraphic signal is produced by a particular sealevel curve. This requires that the rate of carbonate sedimentation was such that