ABSTRACT: Origin of Fine-Grained Holocene Shallow Marine Carbonate Sediment on the Florida-Bahama Platform

Randolph Steinen, Paul Tennet

Fine-grained sediments present in Florida Bay, the inner reef tract of Florida, and the Great Bahama Bank are formed by three processes. The sediments differ primarily in the amount of 4-µm-long aragonite needles that comprise the sediment, but also in the amount of aragonite. Florida Bay muds are composed primarily of equant 1 µm grains; less than 20% are needles. The fine-grained fraction from the inner reef tract of Florida contains more aragonite needles than Florida Bay muds. Some samples contain nearly 90% needles, whereas samples, from adjacent localities contain only 50%. Sediment from both areas is deposited and mixed by bioturbation on the deltas at tidal passes through the Florida Keys. The amount of needle sediment diminishes rapidly away from the ti al passes in Florida Bay. The fine-grained fraction on the Great Bahama Bank is dominated by 4-µm-long needles of aragonite but may contain fragments and whole tests of foraminifers. In general, the more abundant the aragonite-needle content is in any of the sediment, the greater the aragonite content of that sediment.

The three main processes responsible for mud production on the Florida-Bahama Platform are (1) disintegration of skeletal encrustations, mainly red algae and spirorbid worm tubules, on Thalassia grass blades; (2) postmortem disaggregation of aragonite needles from green algae; and (3) direct precipitation of aragonite and Mg-calcite from supersaturated marine waters. Where sufficient nutrients exist in the water column, such as in Florida Bay, mud production is dominated by the epibionts on the Thalassia grass community. Less nutrients in the water column of the inner reef tract results in fewer encrustations. Green algae thrive in these well-circulated waters and produce much of the fine-grained sediment that has accumulated there. The biota on interior portions of the Great Bahama B nk are diminished by lack of nutrients in the water. Here, where few biotic constituents are available to remove calcium carbonate from the water, direct precipitation accounts for about 70% of the sediment; green algae produce most of the remaining sediment. Although most muds are mixtures of the products of these three processes, mud production in most areas is dominated by just one. This results in several distinctive varieties of fine-grained sediment.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990