21 Geochemistry, Petrology and Tectonics (Posters)

SANDSTONE PETROLOGY OF THE COCOA SAND MEMBER, YAZOO FORMATION (EOCENE), MISSISSIPPI

Paper number: 21-5
Program: Technical Sessions
Time: 1:00 PM – 5:00 PM

Authors

Abstract

The sandstone petrology of the Cocoa Sand Member of the Eocene Yazoo Formation is not well documented. Acquisition of two cores (Ketler and Young) during the Mobil-Mississippi Project of 1993 has provided the opportunity for a detailed petrologic analysis. The Cocoa Sand, deposited as a low-stand shelf margin sand, thickens in the core from 12 feet to 29.5 feet in a downdip direction. It is bounded above by the Pachuta Member and below by the North Twistwood Creek Clay.

The Cocoa Sand is a moderate to well sorted, poorly cemented quartz arenite with an average composition of Q96F0L4. Lithic fragments are dominated by sedimentary rock fragments that appear to be rip-up clasts from the underlying North Twistwood Creek Clay. Quartz grains range from well rounded to angular with embayed and anhedral to euhedral grains common indicating a possible reworked sedimentary to volcanic source for this sandstone. Backscatter images of these grains indicate that they are highly fractured.

The Cocoa Sand shows little compaction with a packing density averaging 43% and the packing proximity averaging 21%. Point contacts are predominant. Intergranular porosity ranges from 23% to 30%. The dominant authigenic phase is Ca-montmorillonite with lesser amounts of kaolinite, illite, calcite, and heulandite.