The sandstone petrology of the Cocoa Sand Member of the Eocene Yazoo Formation is not well documented. Acquisition of two cores (#1 Ketler and #1 Young) during the Mobil-Mississippi Project of 1993 has provided the opportunity for a detailed petrologic and depositional analysis.The Cocoa Sand Member is a moderate to well sorted, poorly cemented quartz arenite with an average composition of Q% Fo L4. 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 anhedral to euhedral grains common. They are often encased in tangential clays indicating a possible reworked sedimentary to volcanic source for this sandstone. Heavy mineral analysis supports a volcanic source, but also indicates a metamorphic contribution.The Cocoa Sand shows little compaction with a packing density averaging 43% and the packing proximity averaging 21%. Point and tangential contacts are predominant as point count porosity averages 18.4%. The dominant authigenic phase is Camontmorillonite with lesser amounts of kaolinite, illite, calcite, and heulandite.Core, petrographic, and log analyses indicates that the Cocoa Sand Member of the Yazoo Formation is an isolated, intrabasinal sheet sand that thickens in the down dip direction. Winnowing of the North Twistwood Creek Member of the Yazoo Formation during transgression resulted in the deposition of the Cocoa Sandstone. This is supported by the similarities in composition between the North Twistwood Creek Member and the Cocoa Sand Member and the presence of rip-up clasts found at the North Twistwood Creek-Cocoa Sand Member contact. It is concluded that the Cocoa Sand Member is the initiation of sequence TE3.3 and here called subset TE3.3a of a transgressive system tract. This subset runs from the base of the Cocoa Sand Member to the bottom of the Pachuta Marl. / Department of Geology

The Eocene Cocoa Sand Member of Yazoo Formation is fine grained, moderately to well sorted, poorly cemented, quartz arenite. Surface exposures are poor, but it has been mapped from west Choctaw County, Alabama to eastern Jasper County, Mississippi. In the subsurface, the Cocoa Sand Member is identified by obvious protrusion both in Spontaneous and Resistivity Logs. Northeast to southwest cross-sections (perpendicular to the paleo-shoreline) and northwest to southeast cross-sections (parallel to the paleo-shoreline) were developed, along with isopach maps, to determine the sequence stratigraphic setting and a depositional model of the Cocoa Sand Member. Previous work has interpreted the Cocoa Sand Member as a shelf margin sand deposited as part of a lowstand systems tract or as a transgressive sand. Grain size analysis indicates that the sand coarsens upward and there is evidence in core that the upper contact of the Cocoa sand with the Pachuta Marl is sharp, representing an upper erosion surface. The presence of rip-up clasts at the base of the Cocoa sand member supports the presence of a transgressive surface at the contact with the North Twistwood Creek. Based on the sand thickness distribution as identified in the Cocoa Sand isopach map and cross sections, two sand ridges have been recognized extending nearly parallel to the paleoshoreline across the Mississippi and Alabama. A three stage model is presented suggesting the formation of these ridges during transgression with the source of the sand being from the eroded and reworked underlying North Twistwood Creek Member. / Department of Geological Sciences