Abstract: Diagenesis in the Upper Cretaceous Tuscaloosa Formation: New Answers to Nagging Questions

Paula L. Hansley

The Upper Cretaceous Tuscaloosa Formation has long been an enigma because of its anomalously high (>20%) porosity at a depth of 6,100 m. In a petrologic study of cores taken from 2,800 to 6,300 m depths in southwestern Mississippi and southeastern Louisiana, most porosity is interpreted to have been produced by dissolution of lithic fragments, feldspars, and primarily carbonate cements. Paragenesis of authigenic phases in fluvial to nearshore marine quartzarenites and sublitharenites of the Tuscaloosa was complex and included chamosite grain coatings and quartz overgrowths, calcite and siderite cement, dissolution, albite overgrowths, ankerite cement, dissolution, clinochlore grain coatings and idiomorphic quartz, illite/smectite, and kaolinite. The identification of two different hlorites in this study may explain the controversy over the timing of chlorite precipitation in the Tuscaloosa. In shallow cores, the chlorite polytype is chamosite that displays an edge-to-face morphology. Chamosite grain rims appear to have been an early alteration product of detrital Fe-oxyhydroxide and illite/smectite grain coatings. In the deepest cores, the chlorite polytype is clinochlore except where chamosite has been preserved. Clinochore also occurs as grain rims on detrital grains and as overgrowths on chamosite, suggesting that it formed by diagenetic alteration of chamosite. Clinochlore occurs only in pores where ankerite cement has dissolved. Albite and pore-bridging illite/smectite have been noted in only the deepest cores. Pore-filling kaolinite is most abundant in shall w sandstones. Oil staining on albite and kaolinite suggests that oil migration into the Tuscaloosa occurred relatively late in the diagenetic history.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994