ASYMMETRY IN UPPER CRETACEOUS DEPOSITIONAL SEQUENCES OF THE NORTHEASTERN GULF COASTAL PLAIN, ALABAMA, MISSISSIPPI AND TENNESSEE

Three depositional sequences have been mapped and dated biostratigraphically in mid-Santonian to Maastrichtian sediments of the northeastern Gulf Coastal Plain, Alabama, Mississippi and Tennessee. The paleogeographic setting enables a 3-D view of the eastern flank of the Mississippi Embayment, where nearshore paleoenvironments in northern Mississippi and eastern Alabama grade into offshore paleoenvironments of western Alabama and Mississippi. This setting has enabled biostratigraphic calibration of planktonic foraminifera, calcareous nannofossils and ostracodes using graphic correlation in the offshore areas, which are used to biostratigraphically date marine tongues (transgressive peaks) in the nearshore areas. These studies have revealed considerable asymmetry in the Santonian and Campanian depositional sequence in the nearshore areas, which illustrates how local factors such as sediment supply can influence sequence stratigraphic architecture in an overall regime of sluggish eustatic sea level change.

Two major marine tongues interpreted as transgressive peaks (maximum flooding surfaces) extend into the nearshore regions. In eastern Alabama, the Mooreville Chalk occurs as a marly tongue in the middle of the Blufftown Formation. This tongue occurs near the top of the planktonic foraminiferal Dicarinella asymetrica Taxon Range Zone. Another tongue occurs in the ostracode Ascetoleberis plummeri Interval Zone (IZ), which correlates to the lower portion of the planktonic foraminiferal Globotruncana ventricosa IZ of early Campanian age. In northern Mississippi, a tongue also occurs in the Mooreville Chalk, which extends between the relatively nearshore Tombigbee Sand Member of the Eutaw Formation and the Tupelo Tongue of the Coffee Sand. This northern Mississippi tongue occurs, however, in the Ascetoleberis plummeri IZ, correlable with the younger tongue in eastern Alabama. Thus, the older tongue observed in eastern Alabama is not observed in northern Mississippi. The effects of sediment supply, associated with the ancestral Tombigbee River, are believed to have masked the eustatic signal of this earlier sequence.