Title

Climatic and oceanographic controls on the Neogene sedimentary framework of the outer west Florida carbonate ramp

Date of Award

1987

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Earth Sciences

Advisor(s)

Hank Mullins

Keywords

eastern Gulf of Mexico.

Subject Categories

Geology

Abstract

Carbonate ramps are gently sloping depositional surfaces where shallow-water coarse-grained facies pass basinward into fine-grained deep-water sediments, with no abrupt change in slope. The objective of this study is to integrate the depositional history of the west Florida outer ramp into a paleoclimatic and paleoceanographic framework for the eastern Gulf of Mexico.

The Neogene stratigraphic evolution of the central west Florida outer ramp has been controlled by climatic oscillations, and punctuated by oceanographic events. The establishment of the present Loop Current--Gulf Stream system in the middle Miocene set up the modern depositional environment on the west Florida ramp. The Loop Current presently forms a dynamic barrier to off-platform transport and promotes biological productivity by marginal upwelling. Late Pleistocene invigoration of the Loop Current broadened the winnowing effect of the current, and produced hardgrounds on the outer shelf.

The modern outer ramp is dominated by pelagic sediments that become finer-grained with increasing water depth. Coarse skeletal sand and hardgrounds of the outer shelf, winnowed sand on the shelf margin, and cyclically-bedded oozes on the slope are each controlled by changes in climate, sea level, and oceanic current patterns. Both mineralogic and sedimentologic cycles occur in slope sediments. Aragonitic sediments with high insoluble residues occur in glacial intervals; sandy calcitic sediments accumulate during interglacials. These cycles are linked to climatic cycles in the Milankovitch band, and result from variations in pelagic biological productivity and terrigenous dilution.

The mineralogic cycles are 180$\sp\circ$ out of phase with Bahamian cycles of the same age and lack their asymmetry. Two types of mineral cyclicity apparently may develop in slope carbonates: one that responds to climatically-induced productivity variations in the pelagic realm (west Florida), and one that is caused by shallow-water platform productivity and off-bank transport associated with interglacial highstands (Bahamas). Both models must be considered in the study of ancient cyclic sequences, where the depositional environment (rimmed platform or ramp) is often obscure. An understanding of the processes on a modern ramp slope, such as west Florida, may prove valuable in paleoclimatic and paleoenvironmental analysis of epicontinental carbonate sequences and ramps in the rock record.

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