ABSTRACT

Rb-Sr ages of glauconite pellets commonly fall short of the presumed age of deposition by 10-20%, according to current estimates of the geologic time scale. In part, the young ages are the result of preferential loss of exchangeable radiogenic Sr. Consequently, in pellets where tightly bound, nonexchangeable Rb and Sr have remained a closed chemical system since deposition, removal of the exchangeable cations with ammonium acetate (NH₄OAc) can raise the apparent age to coincide with the expected depositional age. In other cases the age of pelletal glauconite remains younger than the depositional age after chemical treatment. Analyses of NH₄OAc-treated, disaggregated clay-size fractions invariably yield young but rather consistent apparent ages. These are interpre ed as recording the time of diagenetic recrystallization of the glauconite crystallites which compose the pellets.

Glauconite recrystallization appears to have taken place in the Llano Uplift of central Texas at 428, 350, 328, and 265-280 my ago. These ages do not coincide with times of elevated temperatures, deep burial, or tectonism. Instead, they correlate with times of regional emergence above sea level. Apparently a change in pore-water chemistry due to an influx of meteoric water promoted recrystallization, and this inference is supported by the oxygen isotope composition of the glauconite. Diagenesis of glauconite by this mechanism offers an opportunity to date sea-level changes by isotopic methods.