FORMATION AND DISTRIBUTION OF OOIDS ALONG A BEACH-TO-OFFSHORE TRANSECT AT PIGEON CAY, CAT ISLAND, BAHAMAS

Ooids are a common component of ancient carbonate rocks, but their origin and distribution in Holocene deposits and modern carbonate sands are still poorly understood. Even though ooids are well documented from North Andros Island and several other regions in the Bahamas, they have only recently been described as a major sediment component on Cat Island. To better understand the origin and distribution of these grains, this study compares the composition and texture of beach sand to six sediment samples collected from the sea floor at approximately 50 meter intervals along an estimated 300 m transect at Pigeon Cay, Cat Island. Underwater surface sediment samples were collected using a rigid plastic container by making one sweep across several wave ripples perpendicular to their crests. Within the sampled material, we found substantial variation in sediment texture and composition along the sediment transect. The beach sand was dominated by fine-grained and very well sorted ooids of spherical to elliptical shape. About 300 m offshore, sand was coarse-grained, poorly sorted, contained more skeletal fragments, and was dominated by angular and irregular grapestone aggregates of ooids and skeletal fragments in micritic matrix. In the middle of the transect, about 100-150 m offshore at approximately 2 m depth, there was a distinct change in sediment composition from poorly sorted aggregates to more rounded and sorted ooids. This suggests that at Pigeon Cay ooids form within a relatively narrow, high-energy, wave-swept shallow offshore environment that appears to be largely devoid of calcareous algae and other sediment stabilizers. From this environment storms and waves transport ooids onshore where they are further sorted and lithified into beach rock and eolianite. Ooids are also transported farther offshore, where in deeper and less energetic water they are deposited together with skeletal fragments and carbonate mud, and lithified into micritic aggregate grains or grapestones. These observations provide important insights into the formation and deposition of ooids in modern carbonate depositional environments of the Bahamas and have implications for understanding and interpreting the geological record of ooid-rich deposits in general.