ABSTRACT

The Lower Tuscaloosa occurs at depths of between 9,010 feet and 9,650 feet in the Dupont De Nemours #1 Lester Earnest, section 4,T8S,R13W, Harrison County, Mississippi. The diagenetic history of the section has been determined by means of thin section, SEM and XRD studies. The diagenetic sequence can be resolved into the following stages: 1/ mechanical compaction; 2/ pyrite and nodular siderite development, also very early; 3/ relatively early dissolution of feldspars and lithic fragments; 4/ authigenic chlorite formation; 5/ "wheat seed" and rhombic siderite formation, probably earlier than 6/ syntaxial quartz cementation; 7/ formation of authigenetic vermiform kaolinite; 8/ precipitation of poikilotopic ferroan calcite and ankerite cement; 9/ carbonate cement dissolution, accompanied by further silicate dissolution and minor amounts of additional mechanical compaction.

The ferroan calcite and ankerite cements are both characterized by sweeping extinction, more pronounced in the ankerite. The semi-quantitatively analyzed calcites contain approximately six percent total iron plus manganese. "Minus Cement Porosity" values (sum of volume of open pore spaces plus volume of cement) suggest very early cementation at shallow burial depths, but the cement composition indicates a much deeper site of origin. Close examination of cement-grain contacts suggests that significant grain surface etching and grain replacement occurred. The general applicability of Minus Cement Porosity (MCP) curves for determination of depth of cementation as used by some authors is therefore thrown into doubt, unless replacement carbonate can be clearly separated from true (pore-filling) cements.