GLAUCONITE AS AN INDICATOR OF SEQUENCE STRATIGRAPHIC PACKAGES AND BOUNDARIES IN A CONDENSED PASSIVE-MARGIN SHELF SEQUENCE (PALEOCENE CLAYTON FORMATION, CENTRAL ALABAMA)

Detailed studies of a passive-margin shelf depositional sequence in the Paleocene Clayton Formation (central Alabama) are being undertaken to test two related hypotheses: (1) abundance and maturity of glauconite vary systematically through a depositional sequence in response to sea-level dynamics and associated changes in sedimentation rate; and (2) abundance and maturity of glauconite vary systematically through individual parasequences in response to shorter-term changes in relative sea level. Results generated thus far support both hypotheses. Abundance, mean grain size, and maturity (based on color, morphology, and chemistry) of glauconite show a general decline from the transgressive systems tract through the highstand systems tract. More important, higher-frequency cycles in these same glauconite parameters help to delineate parasequences and associated flooding surfaces within what otherwise may be perceived as a monotonous sequence of calcareous muddy sands and diagenetic limestones. Notably, decreased glauconitization upward through individual parasequences resulted in inverted textural gradients. Unlike parasequences in foreland basin successions, parasequences in this relatively condensed shelf sequence are consistently manifest as fining-upward packages. Frequency of diagenetic limestone beds increases towards parasequence tops. This relationship, which seems counterintuitive and also may be tied to the glauconitization process, requires further consideration.