Paleoweathering of Mississippian Monteagle Limestone preceding development of a lower Chesterian transgressive systems tract and sequence boundary, middle Tennessee and northern Alabama

摘要

The contact between the Hartselle Sandstone (Mississippian, lower Hombergian) and the underlying Monteagle Limestone (Mississippian, upper Gasperian) in central Tennessee and northeastern Alabama is a regional disconformity. A paleoweathering surface developed in the uppermost Monteagle is truncated by this disconformity, and only remnant pedogenic features are preserved. These paleosol remnants and associated meteoric diagenetic features are evidence for the subaerial exposure and weathering of the Monteagle Limestone prior to intense erosional scour accompanying deposition of the Hartselle Sandstone. Petrographic evidence for subaerial exposure and meteoric diagenesis includes extensive micritization, reddening, dissolution, and calcitization of allochem grains. Stable carbon and oxygen isotope compositions of whole-rock samples exhibit depth patterns consistent with subaerial exposure, pedogenesis, and meteoric diagenesis. Stable isotope compositions of nonferroan calcite spar occluding moldic pores are considerably depleted relative to Mississippian marine carbonate compositions, further evidence for subaerial exposure and meteoric diagenesis. The paleoweathering surface was modified after burial when reducing conditions developed, resulting in pyritization. Stabilization of high-Mg calcite grains and cements after burial provided a source of magnesium for localized replacement by ferroan dolomite. This disconformity is interpreted to be a third-order Vail/Exxon sequence boundary that marks a dramatic basinward shift in facies belts in the overlying Hartselle Sand-stone, the result of a sea-level fall.