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

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.