The lower Tuscaloosa occurs at depths between 9,010 and 9,650 ft in the du Pont de Nemours 1 Lester Earnest well, Sec. 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) early pyrite and nodular siderite development; (3) relatively early dissolution of feldspars and lithic fragments; (4) authigenic chlorite formation; (5) "wheat seed" and rhombic siderite formation, probably early; (6) syntaxial quartz cementation; (7) formation of authigenetic vermiform kaolinite, and precipitation of poikilotopic ferroan calcite and ankerite cement; and (8) carbonate cement dissolution, possib y 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 semiquantitatively analyzed calcites contain approximately 6% 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. However, the cement composition indicates a much deeper site of origin, and close examination of cement-grain contacts suggests that much grain-surface etching and grain replacement have occurred. The general applicability of minus cement porosity curves for determining depth of cementation (as used by some authors) is therefore thrown into doubt, unless replacement carbonate can clearly be separated from true (pore-filling) cement .

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