ABSTRACT

The large-scale distribution of ostracodes in the Upper Cretaceous deposits of Alabama and Mississippi was studied quantitatively to assess their usefulness in paleoecology and biostratigraphy. The data base consists of 10,174 ostracode valves that were extracted from 34 samples from the Eutaw Formation, including the Tombigbee Sand Member, the Mooreville Chalk, the Blufftown Formation, the Demopolis Chalk, the Ripley Formation, including the Cusseta Sand Member, the Prairie Bluff Chalk, and the Providence Sand. These formations range from late Santonian through middle Maastrichtian in age. The facies range from lagoonal oyster reefs through inner neritic sands and middle neritic marls to outer neritic, nearly pure chalk.

About 300 specimens were randomly picked from each of the samples. Only samples of nearly the same age were compared to each other for the paleoecological analysis. Data on encasing sediment and associated micro- and macrofauna were used to determine paleoenvironmental conditions, and the interpretations were compared with those of the published literature.

The results of the study suggest that ostracodes are excellent paleoecological indicators and show great promise in the dating of nearshore deposits that lack the more commonly used planktonic foraminifera. The chronostratigraphic ranges of most of the species and corresponding biostratigraphic zonations have been published previously. Certain species, such as "Cythereis" bicornis Israelsky, 1929 and Ascetoleberis plummeri (Israelsky, 1929), are found along a wide range of paleoenvironments, even inner neritic environments which either lack or contain relatively few numbers of planktonic foraminifera, and are restricted to a fairly narrow chronostratigraphic range. These taxa are particularly useful because their ranges can be calibrated by planktonic foraminifera and coccoliths in offshore deposits, then extended into nearshore areas. Moreover, many of the most useful taxa, for example Ascetoleberis plummeri, are easily recognizable by the nonspecialist.