Origin Of Calcareous Sediments in the Holocene Pigeon Creek Tidal Lagoon and Tidal Delta, San Salvador Island, Bahamas

Abstract

A sedimentological survey of the southwest arm of the Holocene Pigeon Creek tidal lagoon on San Salvador Island, Bahamas was conducted to determine the origin and distribution of calcareous sediments. Transect surveys, surface samples, and sediment cores were collected and examined to determine the abundance and distribution of sediments and sediment producers in the lagoon and associated tidal delta. The study area has three regions: an extensive tidal to shallow, subtidal flat; a tidal channel near the entrance to the lagoon; and a lobate, ebb-dominated tidal delta. Tidal flat sediments were extensively bioturbated and heavily rooted by Thalassia testudinum. Tidal flat sediments ranged from coarse, calcareous sand to calcareous muds, and were rich in gastropod shells, foraminifera, calcareous algae, and pelletal calcareous mud. The tidal flat sediments are assigned to a pellet-lime wackestone to muddy lime packstone lithofacies. Calcareous green algae are the primary producers of calcareous sediments in the lagoon with species of Halimeda and Penicillus being the dominant genera. Surface samples taken from the coarse channel lag of the tidal channel consisted primarily of whole and fragmented bivalves and gastropods. Most of the bivalves were oriented convex upward. Surface samples taken from ripples, mega-ripples, and sand waves of the tidal delta are dominated by well-sorted, well-rounded, and polished sand-size grains (1-2 mm in diameter). This sediment was dominated by bioclasts (bivalves, gastropods, calcareous algae, foraminifera) and minor ooids. Sediments of the tidal channel and tidal delta are assigned to a lime packstone to grainstone lithofacies. The coarsest fraction (> 2 mm) accumulated on the lee slopes and troughs of large, tidal and wave produced ripples. Ripples for both ebb- and flood tides are strongly asymmetrical, but symmetrical ripples are common, indicating that significant migration of the tidal delta does not occur during normal tidal events. A low energy, shallowing upward cycle is well illustrated in the lagoon, and the presence of a depaurperate biofacies and heavily bioturbated and rooted lithofacies typify a carbonate lagoon depositional environment.