The record of hierarchical sea-level fluctuations in cores, logs, and seismic data along the Great Bahama Bank transect.

摘要

The basinal and slope sediments on the western side of Great Bahama Bank record the Neogene sea-level fluctuations, which follow a hierarchical stacking pattern dominated by Milankovitch orbital forces. Data from Ocean Drilling Program Leg 166 along a transect of five sites (Sites 1003–1007) in combination with well logging and seismic data were used for this interpretation.; Three orders of sea-level fluctuations are recorded in Neogene sediments by variations in composition and sedimentation rates that change in concert with sea level. High sedimentation rates of neritic carbonates are related to high production on the platform during sea-level highstand. A reciprocal sedimentation with increased pelagic and siliciclastic material transported by currents characterizes sea-level lowstand and subsequent transgression. In large scale (3rd-order) sequences, changes in the composition of turbidites also record sea-level variations. Fourth-order sea-level changes produce coarsening upward cycles, in which the fine-grained pelagic and siliciclastic material grade into coarse-grained neritic carbonates. High-frequency sea-level changes are recorded as marl/limestone alternations in both the “soft”-sediment portion and the lithified older strata. The compositional variations are primarily and not produced by diagenetic processes, which merely enhance the compositional differences.; Spectral analyses show that these high-frequency sea-level changes are strongly related to orbital forces. In Pliocene sections the alternations are mainly caused by climatic changes due to the obliquity (40 kyr), while for most of the Miocene sections precession (23 kyr) dominates the cyclicity. In several portions alternations at a 11.5 kyr rhythm are observed, that are probably caused by the tilt of the Earth axis. This finding indicates climate and sea-level variations of sub-Milankovitch frequencies in the Miocene.; The sedimentary packages are recognized in the geophysical logs and seismic data. However, the hierarchical stacking is not resolved in the seismic data, because acquisition and interferences of the signal obliterate the sedimentary pattern. In particular, seismic sources do not maintain a linear increase of frequency, and tuning effects in wedge-shaped sedimentary bodies result in a low resolution and fragmented seismic image.