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The AAPG/Datapages Combined Publications Database

GCAGS Transactions

Abstract


Gulf Coast Association of Geological Societies Transactions
Vol. 44 (1994), Pages 347-353

Oncoids to Reefs: Rolling Stones Come to Rest in the Smackover Formation

David C. Kopaska-Merkel

ABSTRACT

Bioherms and biostromes (reefs) in the Upper Jurassic Smackover Formation in Alabama are dominated by microbial stromatolites. Oncoids ("algal balls") overlie and underlie reefs and are incorporated within them. Oncoids and reefs probably represent microbial responses to differing energy regimes. Subspherical oncoids form under conditions of regular and frequent overturn, whereas flattened oncoids have been interpreted to form under conditions of infrequent overturn. Upwardly expanding oncoids appear to form when subspherical oncoids are grounded (cease rolling) for extended periods and are interpreted to record the incipient stages of reef formation. Grounded oncoids that avoid burial eventually become small microbial reefs or microreefs by expansion of single oncoids, by coalescence of oncoids, by trapping of sediment particles among oncoids, and by microbial overgrowth of multiple oncoids and interstitial material. Small microreefs may coalesce to form larger ones, which consequently exhibit a complex internal fabric. The microreefs described here are up to 3.5 in (9 cm) thick and centimeters to (possibly) tens of centimeters wide; elsewhere in the Alabama Smackover microbial boundstone bodies are much larger. The demise of individual microreefs appears to involve burial by oncoidal pellet packstone/grainstone, with or without erosion of the top of the microreef. These features and relationships are illustrated with examples from Uriah field of southwestern Alabama.


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