Pleistocene Reefal and Oolitic Core Sequences from West Caicos, Caicos Platform

Abstract Shallow core boring transects on West Caicos record the late Pleistocene evolution of the leeward margin of Caicos Platform. Throughout the Pleistocene and Holocene, this leeward margin progressively evolved from an open margin to one blocked by emergent topography, resulting in a dramatic shift in depositional facies. A leeward-margin barrier reef formed behind the protection of emergent oolitic grainstone ridges that persisted through the last two Pleistocene and the Holocene interglacial sea-level high stands. Soil crusts separate depositional sequences and define emergent periods. The youngest Pleistocene interglacial produced a classic shallowing upwards barrier reef that built up to a reef flat about 4 meters above present sea level. Pleistocene and Holocene reef sequences are each capped by oolitic grainstones that were spread across the reefs by longshore currents that also promoted beachspit accretion from ooid sources to the north and south. Ooid smothering of reef sequences occurred during the later part of each sea-level high stand. Older Pleistocene units are increasingly recrystallized and cemented, but the youngest retains most of the depositional mineralogy and has little marine or freshwater cement. Where not protected by preexisting emergent topography, the Pleistocene leeward margin sequence comprises repeated shallowing-upwards, burrowed to layered oolitic grainstone sequences. These grainstones retain much of their depositional mineralogy and are only weakly cemented. Well-cemented soil crusts formed during sea-level low stands, separate these oolitic sequences, and form permeability barriers that are responsible for ponding brines derived from overlying Holocene salinas. These brines promoted dolomitization of Holocene storm-derived grainstones below the salinas.