Depositional History of Smackover Formation in Southwestern Alabama

Joe D. Benson

The Smackover Formation in southwestern Alabama is the product of an overall Middle Jurassic transgression. However, significant lateral variation in lithologic sequence reflects the effects of Smackover paleotopography. Paleozoic ridges and Mesozoic horst blocks defined a number of paleohighs, which separated southwestern Alabama into a series of subbasins or embayments. The Smackover lithologic sequence differs significantly from basin to paleohigh.

Initial transgression of Smackover seas reworked the upper surface of the underlying Norphlet clastics and resulted in deposition of intertidal to shallow subtidal algally laminated mudstones and peloidal and oncoidal wackestones and packstones. These lower Smackover rocks are commonly dolomitized and locally anhydritic. Initial lower Smackover deposition was restricted to paleolows, and subaerial clastic deposition continued over the still emergent paleohighs. As sea level continued to rise, these lower Smackover deposits graded upward into skeletal and peloidal wackestones that contain a sparse, somewhat restricted, faunal assemblage. These wackestones are interbedded with argillaceous organic-rich mudstones that reflect deeper, more restricted depositional conditions. By the early xfordian, the sea level rise had inundated most of the paleohighs. Ooid and oncoidal grainstone shoals developed across paleohighs and along the updip margin. In the basin centers, skeletal and peloidal wackestone/packstones were being deposited. As the rate of sea level rise decreased, the shoals began to prograde basinward and lagoonal environments developed behind the shoals in some areas. Sea level fluctuations led to the formation of stacked shallowing-upward sequences. Evaporitic sabkhas developed along the updip margin and prograded basinward behind the shoals, eventually terminating carbonate deposition.

AAPG Search and Discovery Article #91036©1988 GCAGS and SEPM Gulf Coast Section Meeting; New Orleans, Louisiana, 19-21 October 1988.