INTERPRETATION AND DISCUSSION

The slope sections of the western flank of the Great Bahama Bank are characterized by a low abundance of turbidites. Determining the number of turbidites by counting them in cores and in FMS images taken of low recovery sections revealed that only up to 12% of the total sediment mass was deposited by turbidites at Sites 1003 and 1007. Even if only half of all the mass gravity flows can be detected with this method, turbidite abundance would still be considered low.

Turbidite deposits occur predominantly on the lower slope (Site 1003) and at the toe of the slope (Site 1007), whereas most turbidites bypass the upper slope (Site 1005). The following results were found by dividing these turbidite packages into sea-level highstand and lowstand deposits based on seismic and sedimentary sequence analyses:

1. At Sites 1003 and 1007, more sediments were shed during sea-level highstands than during lowstands. By comparing the turbidite deposits at the two sites, we found that most highstand turbidites occurred on the lower slope, but most lowstand turbidites occurred at the toe of the slope. This pattern indicates that not only the frequency but also the location of deposition varies with changing sea level. In addition, the pattern is likely the result of the morphology of the platform. At a low-angle ramp, the carbonate production shifts upslope with the rising sea level and downslope with the lowering sea level, allowing for carbonate production and export at both times but with different locations of deposition.
2. The average thickness of highstand turbidites is 1.5 times higher than the average thickness of lowstand turbidites, which supports the findings of Mullins (1983) and Schlager et al. (1994). They found that for isolated platforms, higher sedimentation rates occur during sea-level highstands and produce thicker packages of turbidite deposits.
3. The sedimentary composition is indicative of sea level. Turbidite deposits in general are characterized by shallow-water components, whereas the background sediment contains planktonic foraminifers and more fine-grained, muddy particles. Generally, the composition of turbidite and background sediments shows only slight changes from high to low sea level because the environmental change is limited on the ramp-like Miocene-Pliocene profile of the Great Bahama Bank. In fact, there are no significant differences in composition between highstand turbidites and lowstand turbidites in the Miocene. In some cases, the signal may even be reversed. Shallow-water components indicative of sea-level lowstands at Site 1007 can be found in background sediments deposited during sea-level highstands at Site 1003 (Table 8, Table 9). These compositional differences within the two sites indicate a shift of the carbonate production zone toward the basin during lowstands and toward the platform during highstands. This result indicates that the approach of using grain composition to determine highstand vs. lowstand turbidites in ancient turbidite systems will be best applicable along steep-sided platforms (Reijmer et al., 1991; Harris 1994; Vecsei and Sanders, 1997).
4. The distribution of frequency and the location of turbidite deposition follow a complex pattern that is controlled by the interplay of sea-level changes and platform morphology. At the lower slope (Site 1003), more turbidites are recorded in the Burgidalian and Langhian than at the toe of the slope (1007). In the Seravillian, however, both sites record a high occurrence of turbidites; but from the Tortonian to the Zanclian, more turbidites were deposited at Site 1007 (Table 6, Table 7). This change in the deposition of turbidites indicates a downslope shift of the depocenter in the Tortonian, which may be related to the long-term sea-level lowstand in the late Miocene.

In several sequences, turbidites are deposited either during sea-level highstand or lowstand along the entire transect. Highstand shedding of turbidites occurs in Sequences h (upper Tortonian), l (middle Seravillian), and o (lower Langhian) at both Sites 1003 and 1007; while lowstand turbidites at both the lower slope and the toe of the slope are found in Sequences m and n (lower Seravillian and upper Langhian). This distribution does not seem to be correlated to relative sea-level changes. Sea level was very high during Sequences m and n. Therefore, these sequences should be dominated by highstand shedding of carbonates, but they are not. Similarly, the highstand shedding in Sequences c, h, l, and o cannot be explained by a relative high sea level, because sea level is relatively lower than during Sequences m and n. The data suggest a complicated distribution of frequency and location of deposition of turbidites along ramp-like carbonate platforms. About one fourth of the sequences show a change from highstand turbidites at the lower slope (Site 1003) to lowstand turbidites at the toe of the slope (Site 1007). Slight changes in slope angles and rates of sea-level changes seem to be responsible for these diverse patterns.

Our results seem to be inconsistent with the findings in Pleistocene sections of the Bahamas slopes where a clear highstand shedding of turbidites is documented (Droxler and Schlager, 1985; Reijmer et al., 1988). The difference can be explained by the change in platform morphology. During the Miocene and early Pliocene, the Great Bahama Bank developed from a ramp-like platform into the modern steep-sided platform. On the ramp-like platform, the carbonate production zone moved up and down the ramp with fluctuating sea level, resulting in a slight change in sediment production. Nevertheless, the thickness variations in highstand vs. lowstand turbidites confirm the assumption that the slope-to-basin sedimentation is higher during high sea level. As is the case with the production zone, the turbidite depocenters shift up and down the ramp with sea level; at sea-level highstand, the depocenter is further upslope than during sea-level lowstand. Furthermore, in such a setting, compositional differences are minor compared to steep-sided platforms where lowstand turbidites are dominated by eroded lithoclasts and abraded grains (Reijmer et al., 1988).