Use of Sedimentary simulations for dating sequence boundaries and measuring ...
299
Eustatic Sea Level. The Haq et al. (1987) sea-level curve (Fig. 5) was used for
modeling the Bahamas/Straits of Andros dataset. The size of the sea-level excursion directly controlled the high-frequency geometries of the sequences.
Tectonic Movement. The subsidence history of the Bahamian/Straits of Andros
dataset was derived directly from the seismic section and was initially modeled
by the simulation with no sediment fill. The thickness of the sediment measured
on the seismic crossing the platform was used as a first approximation to determine the changes in the rate of subsidence. It can be seen that these rates
changed several times through the 30-Ma period, and at different locations
across the basin, reflecting low frequency changes in accommodation space at
different time intervals as well as different locations. A constant rate of 0.009
m/ka was used for 30 to 10 Ma and faster rates has to be used thereafter in order
to ensure that there would be enough space available to accommodate the carbonates as they were generated (Table 1c).
Carbonate Deposition. Benthic carbonate production rates mimic the response
to photosynthesis of carbonate-producing organisms. Most of the sediment was
produced by organisms that were dependent upon light, so production rates decrease rapidly with increasing water depth (Schlager 1981). The simulation
modeled accumulation, not production. This accumulation was modeled as
from a combination of benthic and pelagic sources. In the simulation, the resulting geometries were very sensitive to small changes in the accumulation rate
with depth and time. Note that the accumulation rate fell rapidly in water deeper
than 50 m. (Table 1d)
Pelagic deposition is a critical source of carbonate sediment and comes directly from the water column. In the simulation this pelagic rain is defined as accumulation which varied as a function of time (Table Ie). This rate was used to
modulate the progradational fill of the basin by the benthic carbonate, enabling
the filling the downslope basin so the progradation could occur. It is believed
that in nature this pelagic material included the mud-sized aragonite needles
that were transported offbank on the Great Bahamas Bank, probably during
high stands of sea level on the banktop (Wilber et al. 1990) and in offbank positions during lowstands.
Carbonate Parameters. As in nature, the simulation limits the growth of carbonate buildup crests to sea level. Excess carbonate accumulation, which would
cause the buildups to rise above sea level, was transported off the buildup and
deposited as talus and turbidite. The simulation algorithm assumes that all the
carbonate talus of the margin came from the "reef" crest. The carbonate parameters were such that the angle of repose was 20°, and the distance of transport of
the apron was 1 km for the apron of sediments and 10 km for turbidites (Table
If). These values were set at the beginning of each simulation run. The percentage of the talus that was transported downslope off the carbonate platform into
the basin was an input parameter of 20% (Table 1). By determining the respec-
299
Eustatic Sea Level. The Haq et al. (1987) sea-level curve (Fig. 5) was used for
modeling the Bahamas/Straits of Andros dataset. The size of the sea-level excursion directly controlled the high-frequency geometries of the sequences.
Tectonic Movement. The subsidence history of the Bahamian/Straits of Andros
dataset was derived directly from the seismic section and was initially modeled
by the simulation with no sediment fill. The thickness of the sediment measured
on the seismic crossing the platform was used as a first approximation to determine the changes in the rate of subsidence. It can be seen that these rates
changed several times through the 30-Ma period, and at different locations
across the basin, reflecting low frequency changes in accommodation space at
different time intervals as well as different locations. A constant rate of 0.009
m/ka was used for 30 to 10 Ma and faster rates has to be used thereafter in order
to ensure that there would be enough space available to accommodate the carbonates as they were generated (Table 1c).
Carbonate Deposition. Benthic carbonate production rates mimic the response
to photosynthesis of carbonate-producing organisms. Most of the sediment was
produced by organisms that were dependent upon light, so production rates decrease rapidly with increasing water depth (Schlager 1981). The simulation
modeled accumulation, not production. This accumulation was modeled as
from a combination of benthic and pelagic sources. In the simulation, the resulting geometries were very sensitive to small changes in the accumulation rate
with depth and time. Note that the accumulation rate fell rapidly in water deeper
than 50 m. (Table 1d)
Pelagic deposition is a critical source of carbonate sediment and comes directly from the water column. In the simulation this pelagic rain is defined as accumulation which varied as a function of time (Table Ie). This rate was used to
modulate the progradational fill of the basin by the benthic carbonate, enabling
the filling the downslope basin so the progradation could occur. It is believed
that in nature this pelagic material included the mud-sized aragonite needles
that were transported offbank on the Great Bahamas Bank, probably during
high stands of sea level on the banktop (Wilber et al. 1990) and in offbank positions during lowstands.
Carbonate Parameters. As in nature, the simulation limits the growth of carbonate buildup crests to sea level. Excess carbonate accumulation, which would
cause the buildups to rise above sea level, was transported off the buildup and
deposited as talus and turbidite. The simulation algorithm assumes that all the
carbonate talus of the margin came from the "reef" crest. The carbonate parameters were such that the angle of repose was 20°, and the distance of transport of
the apron was 1 km for the apron of sediments and 10 km for turbidites (Table
If). These values were set at the beginning of each simulation run. The percentage of the talus that was transported downslope off the carbonate platform into
the basin was an input parameter of 20% (Table 1). By determining the respec-