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Shelf to Basin Transition in Middle Ordovician Carbonates in Alabama Appalachians

D. Joe Benson

In the Alabama valley and ridge, Middle Ordovician carbonates are exposed in two northeast-southwest-trending outcrop belts separated by the Helena fault. Northwest to southeast transits across these outcrop belts illustrate a well-defined shallow to deep water transition. West of the Helena fault, the Middle Ordovician is represented by a transgressive-regressive sequence of peritidal and shallow subtidal carbonates of the Chickamauga Limestone, deposited in tidal-flat, low-energy open-shelf, and high-energy shoal environments. Tidal-flat deposits consist of peloidal and intraclastic wackestones and packstones containing abundant exposure indicators. These grade into light-colored, skeletal wackestones and packstones containing a diverse faunal assemblage rich in algae, ndicating deposition in a shallow, low-energy, open-shelf setting. High energy shoal deposition is represented by a 20-80 ft thick sequence of cross-bedded skeletal grainstone. Included within the grainstone are pods of bryozoan-sponge-algal bafflestone and bindstone that represent small mud-rich bioherms. East of the Helena fault, the Middle Ordovician series consists of deeper water carbonates of the Lenoir and Little Oak Limestones and graptolitic shales of the Athens Formation. The Lenoir and Little Oak are composed of dark-colored, even-bedded, skeletal wackestones which, with the exception of scattered Nuia; lack algae, indicating deposition in the deeper part of the photic zone. These wackestones grade southeastward into very finely laminated, argillaceous mudstones and calcareous shales of the Athens Formation. The Athens is dark colored, organic- ich, lacking in bottom-dwelling fauna, and contains common synsedimentary slump structures suggesting deposition in an anoxic, lower slope environment.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.