Stromatolites, Ooid Dunes, Hardgrounds, and Crusted Mud Beds, All Products of Marine Cementation and Microbial Mats in Subtidal Oceanic Mixing Zone on Eastern Margin of Great Bahama Bank

Robert F. Dill, Christopher St. C. G. Kendall, Randolph P. Steinen

The interisland channels along the eastern margin of the Great Bahamas Bank contain lithified structures that owe their origin to recent marine cementation. This cementation appears to be commonly associated with a complex microbial community of plants and microorganisms living within a bank-margin oceanographic mixing zone. In this region, reversing tidal and wind-driven currents flow up to 3 knots (150 cm/sec) three hours out of each six-hour tidal period. Here, marine-cement crusted, carbonate mud beds are found interbedded within migrating ooid sand bars and dunes and are associated with growing, lithified stromatolites up to 2 m in height. These laminated mud beds are found with thicknesses of up to 1 m in subtidal depths of 4 to 8 m (12 to 25 ft). The muds appear to be homogeneous, but closer examination by SEM and under a microscope reveals they are composed of pelletoid aggregates of needle-shaped aragonite crystals with diameters of up to 50 µ. The size of these soft pellets is similar to the smaller grains of ooid sands that are abundant in the area. This size similarity could explain why both the mud beds are found in similar high-energy hydraulic regimes as the ooid sands, but does not suggest how or why the aggregates of pure aragonite needles form.

A high production of ooid sand within this bank margin environment permits the formation of natural levees along the margins of tidal channels. The back sides of these levees are being lithified by marine cements to form hardgrounds. Skeletal and ooid sand dunes stabilized by Thallasia in channel bottoms also are becoming lithified. Grapestones form at the distributaries of flood tidal deltas of ooid sand. All of these features have a common attribute: they are continually in contact with the turbulent mixing-zone waters.

During tropical storms and hurricanes, fine-grained carbonate muds, originally formed from precipitation and biogenic sources, are set into suspension over the shallow bank, forming dense clouds of muddy warm saline bank water. Ebb tidal currents move these suspended mud clouds to the bank margins where they are mixed with oceanic water. Below this zone where sediment-laden, slightly hypersaline bank waters and the cooler open ocean waters mix, one finds not only the crusted, pelletoid-laminated mud beds interbedded with ooid sands but also stromatolites and marine-cemented hardgrounds. Recognizing similar features in ancient carbonate deposits could be useful in determining if they too were formed in a subtidal mixing-zone environment of a broad carbonate platform or an open-water co tinental shelf ramp backed by a broad, shallow bank.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.