Comparison of Two Planktonic:Benthonic (P:B) Curves from the Yazoo Clay (Eocene; Priabonian) of Mississippi

The foraminifera of the Yazoo Clay have been well studied, especially sections from southwestern Alabama and eastern Mississippi. Both benthonic and planktonic foraminifera are abundant and well preserved in the Yazoo enabling correlation to the global geologic time scale and an understanding of late Eocene paleoecology in the region. Significant facies differences exist in the Yazoo Clay from eastern Mississippi to western Mississippi The Yazoo increases in thickness from 51 meters in the Chickasawhay River Valley of eastern Mississippi to more than 138 meters in western Mississippi. Additionally, the western Mississippi Yazoo sections consist of a rather uniform clay facies while sections in eastern Mississippi and western Alabama are differentiated into well defined, lithologically distinct members. Correlation has been established between the two regions using planktonic foraminifera and calcareous nannofossils but many questions remain, especially with regard to sequence stratigraphic interpretations.

A planktonic:benthonic (P:B) ratio curve was constructed for the late Eocene Pachuta Marl Member and the Shubuta Clay Member of the Yazoo Clay in the Mobil #1 Young core, Wayne County Mississippi. This curve was compared to the correlative late Eocene section in the Yazoo Clay of the Mossy Grove core, Hinds County, Mississippi. Both curves show a gradual increase in the P:B values in the lower part of the studied section followed by an interval of high, widely fluctuating values. Numerical P:B values are much higher in this interval in the Mossy Grove core than in the Young core although the overall pattern is similar. The upper part of the studied interval in both cores has lower P:B values. Intervals of fluctuating P:B values in the upper Yazoo of the Mossy Grove core appear to be cyclic in nature and may be related to the development of late Eocene glaciation.