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Ostracodes are abundant and diverse in most samples collected from clastic and carbonate facies of the Vicksburg Group (Mint Spring, Marianna, Glendon, Byram, and Bucatunna Formations) and the underlying Red Bluff and Bumpnose Formations. About 92 species have been identified from 125 samples collected from measured sections and cores from Mississippi and western Alabama. Graphic correlation, no-space graph, and principal coordinate analyses of the data have resulted in a zonation that allows more precise correlation of the lithostratigraphic units than heretofore existed in the literature.
The sequence is divided into three successive first-appearance interval zones, defined, oldest to youngest, on Actinocythereis dacyi, Aurila kniffeni, and Actinocythereis rosefieldensis. The top of the A. rosefieldensis Zone is defined by the first appearance of Leguminocythereis quadricostata. In Mississippi, the Red Bluff is placed in the Actinocythereis dacyi Zone; the base of the Aurila kniffeni Zone is approximately coincident with the base of the Mint Spring; and Actinocythereis rosefieldensis first appears in the upper part of the Glendon. The Byram and the Bucatunna, where it is fossiliferous, are included in the Actinocythereis rosefieldensis Zone. In western Alabama, the basal carbonate rocks of the Oligocene, commonly referred to the Red Bluff by other workers, are here con idered to represent the westernmost extent of the Bumpnose; the overlying dark clays, usually referred to the nonmarine to brackish or nearshore marine Forest Hill, are here placed in the Red Bluff. Both the Bumpnose and Red Bluff represent the Actinocythereis dacyi Zone. The ostracode data suggest that the Mint Spring and the lower part of the overlying Marianna of western Alabama are older than the Mint Spring of Mississippi and therefore correlate with the Forest Hill. This correlation is confirmed by analysis of occurrence data for the Pecten perplanus lineage. The upper part of the Marianna in western Mississippi is included in the Aurila kniffeni Zone. No ostracodes were recovered from the Glendon in western Alabama; Actinocythereis rosefieldensis is first found in the Byram.
Published and unpublished planktonic foraminifer data indicate that the Actinocythereis dacyi and Aurila kniffeni Zones are approximately correlative with the Cassigerinella chipolensis-Pseudohastigerina micra Zone (= P18 and P19). The Actinocythereis rosefieldensis Zone approximates the Globigerina ampliapertura Zone (= P20). Published calcareous nannofossil data indicate that the Actinocythereis dacyi Zone closely approximates the Ericsonia subdisticha Zone NP21). The Aurila kniffeni and lower Actinocythereis rosefieldensis Zone correlate with the Sphenolithus predistentus and Helicopontosphaera reticulata Zones (NP22 and NP23, undifferentiated). Ostracode and calcareous nannofossil data from South Carolina indicate that the base of the upper Oligocene Leguminocythereis quadricostat Zone, and, therefore, the top of the Actinocythereis rosefieldensis Zone, is within the calcareous nannofossil Sphenolithus distentus Zone (NP24). The upper part of the Actinocythereis rosefieldensis Zone and the lower part of the Leguminocythereis quadricostata Zone are apparently represented in Mississippi and western Alabama by the unconformity between the Bucatunna and the overlying Chickasawhay Formation. The latter formation is of late NP24 and late Leguminocythereis quadricostata Zone age.
A study of the distribution of ostracode genera thought to be cogent environmental indicators and a species of Trachyleberidea that has trefoil surface ornamentation indicates that the entire sequence below the Byram in western Alabama was deposited in relatively deep waters, that is, probably at outer sublittoral depths. This sequence includes the Red Bluff clays previously thought to represent an eastward tongue of the deltaic Forest Hill Formation. In western Mississippi, deeper water indicators are present in the upper part of the Mint Spring through Glendon interval, but they are not dominant. The lower part of the Mint Spring and Byram are dominated by inner sublittoral forms. In eastern Mississippi, the Red Bluff, Mint Spring, Marianna, and Glendon represent open-marine, probab y middle sublittoral, sedimentation. No ostracodes were obtained from rocks that could unequivocably be said to represent the Forest Hill Formation. The Byram in eastern Mississippi and western Alabama was deposited at inner sublittoral depths, as was the marine part of the Bucatunna. The presence of Jugosocythereis in all the shallow-water facies strongly suggests that tropical to perhaps subtropical conditions prevailed during deposition of these Vicksburgian sediments.
Seven new species are proposed: Ghardaglaia obovata Mumma, Hermanites moorei Hazel, Leguminocythereis quadricostata Mumma and Hazel, L. alata Hazel, L. edwardsae Hazel, Patellacythere comptonae Hazel, and Loxoconcha pseudoinflata Huff and Hazel.
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