BASIN ANALYSIS OF THE LATE MISSISSIPPIAN (CHESTERIAN) THROUGH EARLY PENNSYLVANIAN (ATOKAN) MICHIGAN BASIN, U.S.A

Over 2,000 linear feet of core material and hundreds of well logs were analyzed to evaluate the stratigraphy and basin evolution of Carboniferous strata in the Michigan basin. Rock units were evaluated on the basis of lithofacies type, contact relationships, and existing regional geologic interpretations. The recovery of three distinct pollen and spore assemblages from core confirms the timing of deposition during the late Mississippian Chesterian and early-middle Pennsylvanian Morrowan and Atokan regional stages within the Michigan basin.

The Marshall Formation consists of fine-to-medium-grained, fossiliferous, micaceous, quartz sandstone, which grades into the overlying Michigan Formation. The Michigan is composed of heterolithic deposits of shale, sandstone, carbonate, and evaporite strata deposited in a marine, coastal barrier-to-backbarrier depositional setting. Palynomorphs recovered from the Michigan indicate deposition during the Chesterian Stage. The overlying Bayport Formation is a mixed carbonate-clastic succession composed of seven distinct depositional lithofacies reflecting shoal-water to peritidal environments overlain at erosional contacts by tidally bedded (estuarine?) quartz sandstone. Based on climate sensitive lithotypes (i.e. carbonates and evaporites) and sparse palynomorph content, the Bayport Interval is interpreted as being late-Chesterian in age.

In the southern Michigan basin the Mississippian-Pennsylvanian boundary is marked by a significant disconformity. In cores directly adjacent to the Lucas-Monroe Fault structure, the Bayport Interval contains a pronounced transition from an intensely weathered limestone interval overlain by a paleosol alternating between nodular red-bed and low-chroma mudstones. To the west of the Lucas structure the Bayport interval is truncated by Pennsylvanian fluvial deposits suggesting structural control on deposition. Paleosols, coal, and channelized fluvio-estuarine sandstone of the Pennsylvanian Saginaw Formation reflects a terrestrial to marginal marine depositional system deposited in a distinctly humid environment during the Morrowan through Atokan regional North American stages.