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Coastal Accretion on Leeward Margins of Carbonate Platforms, Turks and Caicos Islands, British West Indies

Harold R. Wanless, Victor Rossinsky, Jr.

Chance creation of a small patch of emergent topography can initiate an expansive sequence of coastal accretion on leeward margins of carbonate platforms. Without initiating topography, leeward margins remain a featureless marine environment where offbank transport smothers attempts at reefal growth and depletes sand reservoirs.

The Pleistocene-Holocene limestones forming West Caicos Island on the leeward (west) margin of Caicos Bank (British West Indies) are an example. Here, two north-south-trending oolitic sand ridges formed about 3 km in from the platform edge in the early Pleistocene. These ridges blocked cross-platform and offplatform flow for a section of the margin. A leeward-margin coral reef formed and built to sea level along this sector.

Ooid production on, and influx to, the windward (east) side of the ridges was swept longshore to the ridge ends and then westward by cross-bank currents toward the platform margin. These are preserved as fossil beach ridges. Continued ooid production (supply) generated a northward- and southward-prograding series of arcuate beach ridges.

At the leeward platform margin, now protected from cross-bank flow, waves and longshore currents moved oolitic sand spits across the reef buildup, smothering it.

As the island elongated north-south, longshore removal of oolitic sand became ineffective, triggering a final stage of bankward (eastward) beach-ridge progradation. Although this sequence was interrupted by several glacial sea level lowerings, the scenario is not dependent on them.

This accreting coastal complex has severely modified both the leeward platform and offplatform facies. With time it could completely separate this leeward margin from the platform interior.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.