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Microfossil Characteristics of Deposits of Systems Tracts in the Upper Cretaceous Strata of Mississippi and Alabama
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 and their correlative units); UZAGC-4.0 (Demopolis Chalk, including its Bluffport Marl Member, and most of the Ripley Formation and their correlative units); and UZAGC-5.0 (uppermost Ripley Formation and Prairie Bluff Chalk and their correlative units). These sequences are correlated to the global chronostratigraphic framework using planktonic foraminifera and calcareous nannoplankton data. Evolution of Late Cretaceous planktonic foraminifera and nannofossils was relatively slow, resulting in zonations with durations of up to four million years. Evolution of ostracode faunas was more rapid and complex, with specific types 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 deposits characteristically contain a depauperate benthic ostracode and foraminiferal fauna but very high numbers of non-keeled planktonic foraminifera. These deposits mark the change from a siliciclastic-dominated shoreline system to a hemipelagic depositional system. The maximum flooding surface/event 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 planktonic/benthic foraminiferal ratios. Microfossil communities are extremely stable during deposition of late transgressive systems tract and early highstand system tract sediments. Late highstand systems tract deposits are characterized by a migration of distinct faunas from the nearshore paleoenvironments of the basin margins seaward into the former hemipelagic areas.
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