ABSTRACT: Microfossil Characteristics of Systems Tracts in the Upper Cretaceous Deposits of Mississippi and Alabama
PUCKETT, T. MARKHAM, and ERNEST A. MANCINI , The University of Alabama, Tuscaloosa, Alabama 35487-0338
Three unconformity-bounded depositional sequences are recognized in the Upper Cretaceous (middle Santonian-Maastrichtian) deposits of Alabama and Mississippi. These sequences are: UZAGC-3.0 (Eutaw Formation, Mooreville Chalk, and lowest Demopolis Chalk); UZAGC-4.0 (Demopolis Chalk, including its Bluffport Marl Member, and most of the Ripley Formation); and UZAGC-5.0 (uppermost Ripley Formation and Prairie Bluff Chalk). These sequences are correlated to the global chronostratigraphic framework using planktonic foraminifera and calcareous nannoplankton data. Evolution of Late Cretaceous planktonic microfossils and nannofossils was relatively slow, resulting in broad zonations. Evolution of ostracode faunas was more rapid and complex, with specific types (clades) being characteristic of specific paleoenvironments. Because of their close association with specific environments, ostracodes are excellent indicators of particular systems tracts.
The transgressive systems tract (TST) in the UZAGC-3.0 sequence has a depauperate benthic ostracode and foraminiferal fauna, but has very high numbers of non-keeled planktonic foraminifera. These TST deposits mark the change from a siliciclastic-dominated shoreline system to a hemipelagic depositional system. The maximum flooding surface/event (MFS) is difficult to pinpoint in these sequences due to the lithologically monotonous section of chalk and marl, but is recognized at the inflection point from increasing to decreasing P/B ratios. Microfossil communities were extremely stable during deposition of late transgressive/MFS/early highstand sediments. Late highstand deposits are characterized by a migration of distinct faunas from the nearshore paleoenvironments of the basin margins seaward into the former hemipelagic areas.
AAPG Search and Discovery Article #90908©2000 GCAGS, Houston, Texas