Abstract: Oncoids to Reefs: Rolling Stones Come to Rest

David Kopaska-Merkel

Bioherms and biostromes (reefs) in the Upper Jurassic Smackover Formation in Alabama are dominated by microbial stromatolites. Oncoids ("algal balls") over- and underlie reefs and are incorporated within reefs. 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. Cup-shaped oncoids and severely flattened oncoids appear to form when more rounded oncoids are grounded for extended periods, and are interpreted to record the incipient stages of reef formation. If sedimentation rate is low, then grounded oncoids grow laterally, forming severely flattened oncoids. If sedimentation rate is higher, then cup- or cone-shaped oncoids may form. The flanks of these oncoids are supported by sediment deposited while the oncoids grow (including other oncoids), and this sediment prevents unrestricted lateral microbial growth; many modern stromatolites exhibit similar stromatolite-sediment relationships. Grounded oncoids that avoid burial continue to grow and eventually become small microbial reefs. This takes place by expansion of single oncoids, by coalescence of oncoids, by trapping of sediment particles between oncoids, and by microbial overgrowth of multiple oncoids and interstitial material. The reefs described here are 2 to 3 cm thick and centimeters to (possibly) tens of centimeters wide. The demise of small reefs appears to involve burial by oncoidal pellet packstone/grainst ne, with or without erosion of the top of the reef. These features and relationships are illustrated with examples from the Manila embayment.

AAPG Search and Discovery Article #90983©1994 GCAGS and Gulf Coast SEPM 44th Annual Meeting, Austin, Texas, October 6-7, 1994