FORAMINIFERAL PALEOECOLOGY OF THE MARIANNA LIMESTONE (OLIGOCENE; RUPELIAN) IN THE MOBIL-MISSISSIPPI CORES, WAYNE COUNTY, MISSISSIPPI

Foraminifera from the Marianna Limestone (Oligocene; Rupelian) were studied in two cores from Mobil-Mississippi core project in Wayne County, Mississippi with the goal of testing the sequence stratigraphic model of the Marianna. In both cores, the lower .6 m of the Marianna in the study area consists of fine grained, glauconitic chalk known as the Mint Spring Member. The upper 9.5 m of the Marianna consists of foraminiferal wackestones with abundant Lepidocyclina mantelli and is referred to informally as the “Chimneyrock” facies. The Marianna Limestone is correlated with depositional sequence MMR in the St. Stephens core with the Mint Spring Member representing the transgressive systems tract (TST) and the “Chimneyrock” facies associated with the highstand systems tract (HST).

Both planktonic and benthic foraminifera were obtained from the two cores. The cores were sampled at 10 cm intervals throughout the Marianna. The benthic foraminiferal faunas identified in the samples show little variation throughout the cores and have been assigned to the Discorbis-Quinqueloculina assemblage. This assemblage has been associated with paleodepths of less than 30 m in Eocene and Oligocene rocks elsewhere in the Gulf Coastal Plain. The assemblage obtained from the Marianna in the Mobil-Mississippi cores differs from that obtained from the Marianna in the Little Stave Creek section in that the Little Stave Creek section has more diversity and deeper water assemblages throughout the Marianna. The Planktonic:Benthic (P:B) ratios obtained from the Mobil-Mississippi cores shows an increasing value through the Mint Spring which is consistent with a TST. The P:B ratio in the “Chimneyrock” facies however fluctuates repeatedly from samples with relatively high P:B values to relatively low P:B values. At Little Stave Creek, the P:B ratio in the Marianna is similar but the “Chimneyrock” facies does not show the small scale fluctuations.

The paleoecologic results from the Marianna of the Mobil-Mississippi cores are consistent with a TST and HST within a depositional sequence. Further, the small scale fluctuations in the P:B ratio in the “Chimneyrock” facies are interpreted as representing parasequences within a highstand systems tract associated with a “catch-up” type carbonate platform.