Carbonate Sequence Stratigraphy: Influence of Paleotopography, Eustasy and Tectonic Subsidence: Upper Jurassic Smackover Formation, Vocation Field, Manila Subbasin (Eastern Coastal Plain), USA

Juan Carlos Llinás: Department of Geological Sciences, The University of Alabama, Tuscaloosa, Alabama

Abstract

The concept of shallowing-upward cycles primarily as a result of changes in eustatic sea level has been widely applied for marine formations in the Mississippi Interior Salt Basin, including the Upper Jurassic Smackover Formation. Nonetheless, in certain scenarios, cyclicity might be affected by different factors. Specifically, in the case of basement paleo-highs and associated sub-basins, it is very likely that differential subsidence resulting from tectonic block movement plays a more significant roll in the depositional sequence development than eustacy.

Vocation Field is located in southwestern Alabama, in the eastern margin of the Manila Sub-basin, along the western flank of the Conecuh ridge. In this area, only the upper Smackover Formation has been deposited over a rough relief generated by block tectonic activity associated with Early Mesozoic rifting events. As a result, the Smackover Formation rests unconformably on basement paleo-highs, and overlies siliciclastic sediments of the Norphlet Formation in a down-dip direction. Seismic interpretation shows that differential tectonic subsidence was still active during Smackover deposition. Based on core descriptions, four sub-environments were interpreted within the Smackover Formation, microbial reef complex, shallow lagoon, shoal complex, and sabkha-tidal flat, with the reef and shoal complexes being the main potential petroleum reservoirs. These sub-environments define an overall shallowing upward cycle developed in an evaporate-carbonate setting. Higher order cyclicity is not as evident here as in the Mississippi Interior Salt Basin due to the combined effects of tectonic activity and the high gradient of the depositional paleo-slope.

Distribution and thickness of the reef facies suggest that they developed as a "keep-up" response during the latest phase of a transgressive event, when the Late Jurassic sea reached its maximum flooding level in this area, resulting in the inundation of these basement paleo-highs. The major driving mechanism for this rapid and significant marine incursion probably is tectonic subsidence associated with faulted basement blocks.