Deposition of Carbonate Mud Beds Within High-Energy Subtidal Sand Dunes, Bahamas

Robert F. Dill, Randolph P. Steinen

Laminated, carbonate mud beds are being deposited in the interisland channels of the Exuma Cays in the Bahamas. They are associated with stromatolites and interbedded with ooid sands that form large migrating subtidal dunes on flood tidal deltas and bars. Currents up to 3 knots sweep in and out of the 4-8 m deep channels 3 hours out of every 6 hours, creating a high-energy bank margin environment not usually considered to be the site of mud-sized particle deposition. Mud deposits reach thicknesses of 1 m and have individual beds 2-5 cm thick. When exposed to flowing seawater, bed surfaces become encrusted with carbonate cement and algal mats. The white interior of mud beds between the crusts appears homogeneous, is soft, and has the consistency of "tooth paste." Loose uncemented ooid and is found above and below the mud beds, showing that both are occupying the same depositional environment. Rip-up clasts of the crusted mud beds, formed by scour of underlying sands, are carried throughout the channels and accumulate as a lag deposit within the troughs of migrating dunes. Some clasts are colonized by algal mats that trap ooid and skeletal sands forming stromatolite structures that can grow up to 2 m high.

The probable sources of the fine-grained carbonate muds are whitings and Bahama bank sediments resuspended by tropical storms to produce dense clouds of fine-grained carbonates that are carried by ebb tidal flow to the bank margin. Microscopic examination of the soft bedded muds, washed gently by a stream of water, reveals they are composed of pelletoid aggregates of flocculated aragonite particles. The pelletoids have grain sizes similar to the ooid sands and exhibit hydraulic characteristics that could account for the muds being deposited in what is usually considered a high-energy sand environment. However, the muds seem to be preferentially deposited in the dune troughs. Modern Bahamian associations of pure aragonite mud beds deposited within ooid sand dunes, rip-up clasts, stroma olites, and strong subtidal currents are new finds that mandate caution when using these features as indicators of shoreline or quiet water in ancient carbonate deposits.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.