Carbonate Facies and Sedimentation of Patch Reefs off Bermuda

Abstract

Transportation of skeletal sands from patch reefs on the Bermuda platform down channels in the reef face results in a wedge or apron of reef-flank material. These sediments physically and bathymetrically separate the reef mass from surrounding lagoonal environments. Distance of sediment transport off the reef top is small (on the order of tens of feet), and only minor amounts of reef debris are present at upper limits of the lagoon floor near the base of reefs. Normal wave action and mass wasting caused by storms are believed to be the major mechanisms of sediment transport. Sediment compositions observed in reef-top, reef-flank, and lagoonal environments provide the basis for recognition of their corresponding facies. Smaller sub-environments include sediment pockets, sand channels, reef-face channels, reef flanks, interreef lagoons, and open lagoons. Their microfacies are transitional and, in the order listed, display the following trends: from the reef top lagoonward, the abundance of Homotrema, coral, and red algae in the sediment decreases, and the abundance of Halimeda and Foraminifera (excluding Homotrema) increases. Interreef lagoons are distinguished from open lagoons by a lower mollusk content in the sediment. From the reef top lagoonward, mean grain size generally decreases and sorting progresses from poor to very poor. Sand-size particles dominate sediments of patch-reef facies. The absence of silt and clay is characteristic of the reef top. Except for concentrations of coral sticks in reef-face channels, gravel is distributed nearly evenly in patch-reef environments, in part, because of the high sediment content of Halimeda (25-60 percent), which commonly contributes whole, granule-size plates. Thus, distribution of bioclastic sediments from recent Bermudan patch reefs takes place on a small, local scale to form reef-flank sediments and results in recognizable sedimentary facies. These facies are accounted for by a working model based on substrate control, biofacies development, particle breakdown, and sediment transport by wave action.