Glauconite as an Indicator of Sequence Stratigraphic Packages in a Lower Paleocene Passive-Margin Shelf Succession, Central Alabama
Abstract
The Lower Paleocene Clayton Formation in central Alabama comprises a complete third-order depositional sequence that accumulated mainly on a passive-margin marine shelf. Glauconite occurs throughout the sequence, providing the opportunity to systematically evaluate changes in glauconite abundance and character that resulted from sea-level-mediated fluctuations in sedimentation rates expressed at both the systems-tract and parasequence scale. To this end, detailed studies of glauconite were carried out using a combination of reflected and transmitted light microscopy, microprobe analyses, and x-ray diffraction studies. Total glauconite abundance increases upward from lowstand systems tract (LST) incised valley-fill sands through the transgressive systems tract (TST) and condensed section (CS) and then generally decreases through the highstand systems tract (HST). Parasequences in the CS/HST are defined by asymmetrical cycles characterized by abrupt increases and gradual decreases in glauconite abundance. Although detrital glauconite is common in the LST and TST, most glauconite grains are authigenic. The relative abundances of various authigenic glauconite morphotypes vary with total glauconite content. Mature morphotypes (e.g., mammillated and lobate grains), as well as glauconitized skeletal grains and glauconite-coated detrital grains, are prevalent in the condensed section and lower parts of parasequences, while less mature varieties (e.g., vermicular grains) dominate parasequence tops. Decreases in glauconite maturity upward through parasequences also are indicated by lighter grain colors, decreasing K content, and increasing importance of glauconite smectite relative to glauconite mica. Observations from this study indicate that glauconite can be an effective tool for delineating sequence stratigraphic packages and bounding surfaces, particularly in relatively sediment-starved, passive-margin shelf successions. Notably, in quiet-water shelf sequences, sea-level-controlled changes in glauconitization result in fining-upward parasequences.