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