ABSTRACT

The Smackover Formation in Southwest Alabama is the product of a third-order sea level rise during the Middle Jurassic. There is, however, significant lateral variation in lithologic sequence reflecting the effects of Smackover paleotopography. Paleozoic ridges and Mesozoic horst blocks defined a number of paleohighs which separated southwest Alabama into a series of subbasins or embayments. The Smackover lithologic sequence differes 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 lithologies are commonly dolomitized and locally anhydritic. Initial lower Smackover deposition was restricted to paleolows while 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 assemblange. These wackestones are interbedded with argillaceous, organic-rich mudstones that reflect deeper, more restricted depositional conditions. Sea level rise had inundated most of the paleohighs by the early Oxfordian. 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.