HIGH-RESOLUTION GEOPHYSICAL LOG STRATIGRAPHY OF THE PALEOCENE PORTERS CREEK AND NAHEOLA FORMATIONS FROM ARCHIVAL CORE DRILLED IN NORTHERN MISSISSIPPI, USA

The transition from the Paleocene Porters Creek Formation to the Naheola Formation in the U.S. Gulf Coast records the rapid progradation of a terrestrial delta plain complex over a mud-dominated shelf. Unfortunately, the thick, massive gray clay of the Porters Creek Formation contains limited observable physical properties upon which to differentiate facies, preventing high-resolution interpretations of stratigraphic patterns. The purpose of this research is to use high-resolution geophysical core logging to elucidate the nature of the regressive interval represented by the Porters Creek-Naheola transition.

We focus on two archival cores drilled by the Mississippi Mineral Resources Institute in Benton and Tippah Counties, Mississippi, USA that contain the entire Porters Creek Formation and the Naheola Formation to its unconformable upper contact with the Eocene Meridian Sand. Logging took place at the Continental Scientific Drilling facility at the University of Minnesota and included gamma density, natural gamma, p-wave velocity, impedance, resistivity, magnetic susceptibility, and spectrophotometry, all measured at 0.5-cm intervals. X-ray fluorescence was also performed at 0.5-cm resolution for select intervals.

The Porters Creek Formation ranges from 70.4 m to 92.1 m thick and the Naheola Formation ranges from 13.1 to 33.8 m thick in the examined cores. A gradual color change from dark gray mud to white kaolinite-rich mud currently defines the contact between the Porters Creek and Naheola. Initial lab results show differences in water sensitivity of Porters Creek clays that suggest smectite-rich and smectite-poor intervals. Planned work includes distinguishing between depositional and diagenetic trends in geophysical log data so that regular patterns representative of depositional conditions can be used to define log facies; these will then be used to examine stratigraphic patterns.