The Modern Carbonate Ramp Slope of Central West Florida

Abstract

ABSTRACT A holistic investigation of the carbonate ramp slope of central west Florida has been completed via analysis of approximately 1,500 km of air-gun seismic reflection profiles tied to two drillcores and bottom samples recovered at 92 stations. Our primary goal has been to characterize depositional processes and products; to compare and contrast results with rimmed-platform slopes; and to develop a model for application to the rock record. The modern depositional environment of the west Florida ramp slope was initiated 12-15 Ma when depositional style changed from seaward-prograding clinoforms to a pelagic, slope-front-fill system bound along the shelf margin by contour-parallel flow of the Loop Current. A seaward-thickening wedge (0-250 m) of late Miocene to Recent pelagic carbonate ooze has been deposited across antecedent mid-Miocene topography. In general, the sea floor dips 1-2° seaward from water depths of about 200-2,000 m, and slopes are relatively smooth. However, contemporary submarine geomorphology is controlled, in part, by earlier slope failures. The primary control of sedimentation along the west Florida slope is Loop Current flow, which acts as a dynamic oceanographic boundary between outer shelf and upper slope, winnows bottom sediments, catalyzes early submarine cementation, and stimulates pelagic production of calcium carbonate. The axis of Loop Current circulation has also migrated laterally in response to climatically controlled sea-level fluctuations, which has resulted in a relatively wide ( 25 km) band of winnowed sands and hardgrounds along the shelf margin. Climatic oscillations have also resulted in cyclic deposition of pelagically derived aragonite (pteropods) on the ramp slope that is out of phase with bank-derived aragonite cycles of rimmed platform slope . There are three major facies belts that parallel isobaths: 1) a hardground facies of intraclastic grainstones and rhodolith rudstones, as well as algal ridges, along the outer shelf at water depths of approximately 200-400 m; 2) a winnowed-sand facies of planktonic forams, intraclasts, glauconite, and reworked phosphorite grains as well as deep-water coral mounds along the shelf margin at depths of approximately 400-600 m; and 3) a bioturbated pelagic-ooze facies along the ramp slope at depths of approximately 600-2,000 m. These facies patterns, geometries, and grain types, along with a general paucity of sediment-gravity-flow deposits, readily distinguish this carbonate ramp slope from better-known rimmed-platform slopes of the Bahamas.