ABSTRACT

Deep test-well information in southwest Alabama and northwest Florida indicates that early differential subsidence of the basement had a marked influence on the thickness and distribution of accumulating coastal plain sedimentary deposits, especially carbonates, evaporites and thick marine shale units. Geophysical data reveal that these differential movements were either directly or indirectly related to lateral variations in the thickness of crustal layers. These crustal thickness variations may have been either the cause of differential subsidence of the basement or the result of deep-seated forces that induced or affected subsidence. Regional gravity anomalies reflect crustal thickness variations associated with the Wiggins uplift and the Mississippi interior salt basin. In the study area a basement high and stratal thinning correspond to the easternmost extension of the Wiggins uplift and a regional gravity minimum. A basement low and stratal thickening correspond to the eastern limb of the Mississippi interior salt basin and a regional gravity maximum.

The configuration of the basin margin during much of Late Jurassic time was controlled by a hinge line that roughly coincided with a system of dense intracrustal masses. These masses probably locate a zone of structural weakness within the crust, basinward from which initial subsidence and marine deposition took place. The basin margin and extent of early marine deposition was also influenced by the Wiggins uplift and the related Conecuh arch. This influence continued until Cretaceous seas advanced beyond the northern extent of this positive feature.