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Getting Our Feet Wet: Ancient Alabama Reefs. (Abstracts).

GETTING OUR FEET WET: ANCIENT ALABAMA REEFS. David C. Kopaska-Merkel, Geological Survey of Alabama.

Many of Alabama's ancient rocks and sediments contain fossil reefs or mounds, ecologically similar to oyster reefs modern Mobile Bay. Three examples of these ancient buildups are described.

Upper Ordovician mounds in the Alabama Appalachians are dominated by bryozoans and algae, with bachiopods arid sponges important in some mounds. The bioherms are constructed of bafflestones arid boundstones and tine internal sediment.

The Mississippian Bangor Limestone, deposited on a broad platform that stretched across north Alabama, contains carbonate buildups that grew on topographic highs. A mound in Lawrence County consists chiefly of packstone and grainstone dominated by eclnirnoderrni ossicles amid fragments of fenestrate bryozoarns. In situ colonies of the rugose coral Caninia flaccida compose about 8 percent of the round by volume. Strong currents within the mound are indicated by preferred orientation of corals and by coarse, commonly cross-stratified grainstone inn channels between neighboring coral colonies. Corals are most abundant on the windward side of the mound.

Carbonate mounds flourished in the Upper Jurassic Smackover Formation on the 65-kilometer-hong Saint Stephens ridge. [Ire ridge crest, up to 15 kilometers wide, supported distinct communities of mound builders, which constructed different kinds of mounds. On the southeastern ridge flank, a biodetrital mound is dominated by locally derived debris. The mound incorporated microherms up to I meter thick, which account for 16 percent of the mound. On the northern part of the ridge crest a similar microherm-bearing mouund 8.5 meters thick is directly overlain by a microbial mound 9 meters thick. The microbial mound consists of stromatolite and of thrombolite with three different microstructures: (1) diffuse clots (grumose structure), (2) well-defined clots, or (3) homogeneous microspar. Fenestrae contain the remains of a low-diversity cryptic microbial community.
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Publication:Journal of the Alabama Academy of Science
Date:Apr 1, 2002
Words:297
Previous Article:Geologic History of Alabama's River Systems. (Abstracts).
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