Title

Formation and evolution of Quaternary carbonate foreslopes, Tongue of the Ocean, Bahamas

Date of Award

1991

Availability

Article

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Marine Geology and Geophysics

First Committee Member

Robert N. Ginsburg, Committee Chair

Abstract

Steep marginal slopes around the Tongue of the Ocean record deposition during the early rise of sea level following the last lowstand some 18,000-21,000 ybp. Coarse-grained skeletal sands, gravel, and boulders derived from reefs growing along the marginal escarpment were deposited on slopes of 35-45$\sp\circ$ and rapidly cemented in place. Deposition by rockfall and grainflow resulted in a series of elongate lenses oriented parallel to the slope. Repeated deposition and cementation produced slope deposits that are both laterally discontinuous and internally heterogeneous. Radiocarbon dating of skeletal components and cements indicate that active deposition on the slopes ceased approximately 10,000 ybp as sea level rose above the escarpment and began to flood the top of Great Bahama Bank. Fine-grained, non-skeletal sands and muds derived from the platform are presently bypassing these slopes, resulting in a "modern" example of a drowning unconformity, and are deposited downslope as a wedge of sediment with slope declivities of 25-30$\sp\circ$.Although the steep slopes are no longer sites of active deposition, they are still influencing the ultimate record of slope sedimentation. Arcuate, concave-up cracks in the cemented slope indicate zones of weakness and possibly represent areas susceptible to slope failure. Post-depositional failure of the slope has been observed at several locations.Growth rates of marine, pore-filling botryoidal aragonite cements from the steep marginal slope deposits have been determined by accelerator mass spectrometer radiocarbon dating of samples from the base and top of individual botryoids. Radiocarbon dating of coexisting skeletal components indicates that cementation was syndepositional. These results provide the first quantitative data on growth rates of marine carbonate cements and suggest that geologically instantaneous cementation may be a critical component in stabilizing steep carbonate slope deposits at or above angles of repose.Stable isotopic analyses ($\delta\sp{13}$C and $\delta\sp $O) of the pore-filling cements, combined with accelerator radiocarbon dates, provide the first age constrained, marine non-skeletal isotopic record during the latest deglaciation and provide independent verification of the isotopic changes that occurred in the paleoceans. (Abstract shortened with permission of author.)

Keywords

Geology; Geochemistry

Link to Full Text

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