ABSTRACT: Characterization of Paleogene Transgressive Systems Tracts in Southern Alabama and Mississippi

MANCINI, EARNEST A., and BERRY H. TEW, Geological Survey of Alabama, Tuscaloosa, AL

The Paleogene strata of the eastern Gulf Coastal Plain can be subdivided into 21 genetic packages (depositional sequences) that reflect cycles of marine transgression and regression with accompanying retrogradation and progradation of the shoreline. These packages are bounded by stratigraphic discontinuities, or sequence boundaries, that are typically developed as erosional or transgressive disconformities. Depositional sequences are defined as type 1 or type 2 cycles based on the lower sequence-bounding surface. Each depositional cycle includes a transgressive component. The characteristics of the sediments accumulating in the transgressive systems tract are distinct and contrast with those of the lowstand, shelf margin, and highstand systems tracts. Typically, the transgressive depo its are less than 15 m in thickness and consist of one or more thin parasequences comprised of bioturbated marine-shelf glauconitic, calcareous, fossiliferous sand and marl. The transgressive parasequences grade upward into a condensed section that is transitional to sparsely fossiliferous, carbonaceous clays and cross-bedded sands of the regressive highstand systems tract. Generally, the surface of maximum sediment starvation (maximum transgression), which marks the change from transgression to regression, is not discernable in these beds. Where recognizable, this surface is marked by shell hash and glauconite and phosphate grains. Detrital glauconite dominates the lower part of the transgressive systems tract, and authigenic glauconite is abundant in the upper part of this systems trac as it grades into the condensed section. The transgressive deposits disconformably overlie the lowstand systems tract (type 1 sequence) or shelf margin systems tract (type 2 sequence) where these sequence components are present. Where lowstand or shelf margin deposits are absent, the transgressive sediments disconformably overlie the highstand deposits of the underlying depositional sequence. The contact of the transgressive deposits with the underlying strata generally is marked by a transgressive lag comprised of reworked

fossils, rounded phosphate pebbles, and other clasts. This transgressive surface is generally sharp but can be burrowed; burrows extending below the surface are filled with sediment from the transgressive beds above. Although the sedimentologic and lithologic characteristics of the transgressive systems tract and its bounding surfaces would not be discernable on seismic reflection data, the continuity and relative horizontality of the sand and marl parasequences of this system tract would be easily recognizable.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)