DIAGENETIC ORIGIN OF LIMESTONES IN THE LOWER PALEOCENE CLAYTON FORMATION (CENTRAL ALABAMA): IMPLICATIONS FOR SEA-LEVEL DYNAMICS

Highstand deposits in the Pine Barren Member of the Lower Paleocene (Danian) Clayton Formation exposed in central Alabama are characterized by decimeter- to meter-scale alternations of limestones and sandy mudstones. Limestone-mudstone couplets previously have been interpreted to represent parasequences; despite clear evidence of a diagenetic overprint, including microsparitic textures and unusually light oxygen isotopic signatures, limestones were inferred to be primary deposits that reflect episodes of marine flooding and consequent relative clastic sediment starvation. Recent detailed studies of glauconite through these highstand deposits have permitted more confident parasequence delineation and relative sea-level interpretations. Parasequences are defined by asymmetric cycles in glauconite abundance and maturity; both parameters increase abruptly across marine flooding surfaces and then decrease gradually through each parasequence. Notably, limestone beds or nodular horizons occur at or near tops of parasequences, beneath bounding flooding surfaces. These relationships support a mainly diagenetic origin for the limestones. Limestone beds and nodular horizons likely formed preferentially below parasequence boundaries in response to marine flooding, sediment starvation, and consequent prolonged phases of bicarbonate production mediated by anaerobic microbial decay of organic matter in the subsurface. Multiple limestone horizons at parasequence tops may reflect stepwise shifts of critical redox boundaries downward in the sediment column as flooding surfaces evolved (via winnowing, etc.). Observations made for the Clayton Formation have implications for other purportedly diagenetic limestone beds and their relations to sea-level dynamics.