Paleontologic Characteristics of Systems Tracts in Upper Cretaceous Depositional Sequences in the Eastern Gulf Coastal Plain

Ernest A. Mancini, T. Markham Puckett, and Berry H. Tew

The Upper Cretaceous deposits of the eastern Gulf Coastal Plain principally consist of marine carbonates and carbonate-rich siliciclastics. Three depositional sequences can be recognized in these strata. The paleontologic characteristics of these sequences are useful in discerning their component systems tracts and in delineating the physical surfaces that define them. The lower bounding unconformities and initial flooding surfaces are marked by an accumulation of vertebrate fossil teeth and bone (shark, ray, bony fish, mosasaur, plesiosaur, turtle, and crocodile). The maximum flooding event can be demarcated by an accumulation of shark teeth, highly bored and encrusted species of oysters, and molluscan steinkern. Usually, however, this event is not marked by a physical s rface but rather by a shift in the planktonic to benthonic foraminiferal ratio. The basal deposits of the transgressive systems tract characteristically contain a concentration of planktonic foraminifera similar to the condensed section deposits. The remainder of the marly and chalky sediments of the transgressive systems tract have a lower percentage of planktonic foraminifera but significantly higher counts than the silty and sandy deposits of the highstand systems tract. Typically, an oyster reef develops in the transgressive systems tract immediately above the initial flooding surface. Biostratigraphically important species of ammonites, belemnites and oysters occur in the transgressive deposits but are usually absent in the highstand systems tract. The deposits of the highstand syst ms tract generally contain higher concentrations of terrestrial plant material than the transgressive systems tracts.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California