Abstract

The Middle Ordovician limestones of the Bigby, Cannon, and "Dove" units in the Central Basin of Tennessee are facies of one another and were all formed in a shallow shelf environment. The Bigby grainstones were deposited in well agitated waters and contain abraded fragments of numerous fossil allochems. This abrasion possibly furnished fine carbonate material to quieter water areas where the Cannon and "Dove" were being deposited. The Cannon wackestone-packstone contains primarily an algal-mollusk-arthropod fauna, many fragments of which show evidence of biological and mechanical breakdown. This breakdown of fragments within the Cannon depositional environment also supplied fine carbonate grains to the matrix. The "Dove" is a carbonate mudstone containing numerous sparry calcite birdseyes. Much of the fine carbonate material probably formed within the "Dove" depositional environment and few grains were transported from outside. The "Dove" represents algal mat deposition and in some localities has loferite fabric. The numerous birdseyes suggest that it was subjected to burrowing. The algal mat deposition represented by the "Dove" occurred in slightly elevated, exposed areas of a mud flat where the contained algal organic matter may have been oxidized. The organic material present in the Cannon is interpreted to be primarily of algal origin, but because the environment was not subjected to exposure it was not oxidized. The organic matter in the Cannon aided the recrystallization of the original fine carbonate material--thus forming microspar. The absence of organic material due to oxidation resulted in the "Dove" texture remaining micritic and fairly uniform.

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