CONCLUSIONS
The drilling, coring, and
logging of Leg 166 sites along the Bahamas Transect added important board data
to the seismic images of the margin architecture and the two core borings on the
platform top. The scientific results show the strength of the transect approach
for addressing sea-level objectives, especially when the distal site can provide
a link to the stable isotope proxy of sea-level changes. This approach has
resulted in the following major results regarding sea-level changes in the
Neogene:
- The chronostratigraphic significance of seismic reflection is
documented with unprecedented precision. This precision is achieved by using
a check-shot survey at each site for an exact core-seismic correlation.
- The facies succession in cores corroborate that the observed
seismic sequences are the result of long-term sea-level changes.
- Carbonate platforms shed turbidites during both lowstands and
highstands of sea level, whereas transgressions produce a lull in shedding.
For most of the Neogene, the ratio between highstand vs. lowstand turbidites
is smaller than in the Quaternary. The increase of lowstand turbidites
compared to the Quaternary (Schlager and Chermak, 1979; Droxler and Schlager,
1985) is related to the change in platform morphology from a steepened ramp
to a steep-sided platform.
- High-frequency sea-level changes are recorded in the sediments
along the entire transect. On the platform top, these changes are expressed
in shallow-water packages capped by exposure horizons. In the basin, they
are seen as marl/limestone alternations.
- Orbital precession is the dominant beat for high-frequency
sea-level changes on the Great Bahama Bank throughout the Neogene. This
finding is at odds with the stable isotope proxy that recognizes orbital
obliquity as the dominant force.
- Bundling of the precessional cycles into cycles of orbital
eccentricity emerges as a mechanism to package the high-frequency cycles
into the lower order, seismically imaged sequence.
- The precisely dated sequence boundaries will be an important
data set to assess global synchrony of long-term sea-level changes in the
Neogene. Additional transects from other ocean basins, however, are needed
to complete this task.