Reefs of Smackover Formation, An Upper Jurassic Carbonate Shelf Sequence Rimming U.S. Gulf Coast: ABSTRACT

Abstract

The occurrence of reefs and oolitic sands in the Smackover Formation, an Upper Jurassic (Oxfordian) sequence that has been recognized in the subsurface of the U.S. Gulf Coast, adds significantly to our knowledge of depositional models for Jurassic carbonate shelf deposits. Models for the upper Smackover Formation have been based exclusively on subsurface data gathered from non-skeletal carbonate reservoirs. The widespread occurrence of buildups within the upper Smackover requires a revision of the existing models. These reefal buildups are 3 to 40 m thick, commonly elongate, and several square kilometers in plan. They developed seaward of oolite shoals on paleostructures that created subtle topographic highs on an otherwise ramp-like sea floor. Reef-rubble zones have rese voir-quality porosity throughout the Smackover trend, but reef framework is a target only where diagenesis has been favorable. The biota forming Smackover reefs are similar to those described for sponge-algal buildups and patch reefs from the Upper Jurassic of Europe. Reef framework is constructed by digitate and branching stromatolitic blue-green algae and to a lesser extent by agglutinated worm tubes, Tubiphytes morronensis, and marine cements. The biota varies throughout the Smackover trend. Reefs in southern Arkansas and northern Louisiana are more diverse. In these reefs, encrusters and binders produce constructional microframework cavities on and between corals (primarily Actinastrea), skeletal algae (Parachaetetes sp. and Cayeuxia sp.), lithistid and hexatinellid sponges, bryozoans, and hydrozoans. The cavities are commonly filled with marine cements or geopetal sediments. The reefs are commonly unde lain and overlain by subtidal peloidal lime packstones containing oncolites and scattered fossils and they can develop in close proximity to subtidal quartz sands. End_of_Article - Last_Page 578------------