--> ABSTRACT: Prograding Holocene Oolitic Shoreface of the Leeward Margin of Crooked-Acklins Platform, Southern Bahamas, by Rankey, Gene; Jordan, Jessica ; Reeder, Stacy Lynn; #90142 (2012)

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Prograding Holocene Oolitic Shoreface of the Leeward Margin of Crooked-Acklins Platform, Southern Bahamas

Rankey, Gene *1; Jordan, Jessica 1; Reeder, Stacy Lynn 2
(1) KICC, Department of Geology, University of Kansas, Lawrence, KS.
(2) Schlumberger-Doll Research, Cambridge, MA.

Although tidally dominated Holocene ooid shoal complexes are well documented, shoreface oolitic sand accumulations are less well understood. To provide a conceptual model for processes and sedimentological characteristics of progradational oolitic shorefaces, this study characterizes the western, leeward margin of the Crooked-Acklins Platform. Here, islands made of a series of offlapping low beach ridges include complex plan-view geometries that document up to 1 km of westward progradation and southward longshore transport. The top surface of the ridges is a cemented surface, and outcrops of beachrock to dune deposits locally occur at the foreshore. The foreshore includes moderately well-sorted fine-to-medium oolitic sand, with some peloids and skeletal fragments. The gently sloping (<0.5°) bottom from the shore to depths ~4 m includes ripples (wave and current) to flat, burrowed deposits. Sediments in this sandy nearshore region include moderately sorted medium-to-coarse sand consisting of ooids (up to 65% of the sediment), peloids, composite grains, and skeletal fragments. A core from this upper shoreface includes a lower coarsening-upward succession overlain by >130 cm of coarse-fine laminated, oolitic-peloid-skeletal-intraclastic grainstone. In general terms, grain size decreases towards the shelf margin; the outer shelf includes moderately-to-poorly sorted fine-to-medium sand. The outer shelf (>4 m deep) includes less evidence for regular agitation. Instead, burrows and isolated coral heads and small patch reefs with < 1.5 m relief occur. Ooids and oolitically coated grains 150-250 µm in diameter occur across the shelf and can form in excess of 50% of the sediment, even in depths > 10 m. No shelf-margin reef is present; the gradient gradually increases (>0.5°) westward, towards the dropoff. Tides measured on this leeward margin have spring tidal amplitude of ~100 cm and neap tidal amplitude of ~ 50 cm. Wave data illustrate the influence of passage of a front that generated waves with significant wave height of nearly 30 cm, more than twice as large as those otherwise present. Likewise, passage of category-3 Hurricane Irene directly over the area had little impact. At the longest time scale, currents are weak (<10 cm/sec) and southward directed, creating the marked longshore transport. These data provide an actualistic analog for ancient oolitic shoreface successions, and contrast with patterns of tide-dominated shoals.

 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California