ABSTRACT The Carboniferous Michigan Basin is the subject of conflicting interpretations resulting from the lack of detailed stratigraphic analysis of relevant rock units. In this study, an ~610 m (2000 ft) section of recently acquired core material was evaluated on the basis of lithofacies and stacking patterns, stratigraphic contacts, and well-established regional geologic relations of Mississippian and Pennsylvanian strata. The Bayport formation is composed of seven distinct primary depositional lithofacies reflecting open-marine and shoal-water to restricted peritidal environments, typically capped by an exposure surface. Carbonate-dominated strata of the Bayport formation are interstratified but ultimately transition up section into siliciclastic-dominated strata (previously called the Parma Sandstone) deposited in tidally influenced, estuarine facies. Late Mississippian Bayport strata are sharply overlain by Pennsylvanian-aged siliciclastic lithofacies of the Saginaw Formation. These facies were deposited in a range of terrestrial and marginal-marine environments, from coarse-grained fluvial sandstones at the base (previously known as the Grand River Formation), to the finer-grained channel sandstones and floodplain mudstones of mixed fluvial and estuarine systems in the middle Saginaw Formation. Carbonaceous shales, mudstones, and thin coal intervals characterize the middle to upper Saginaw Formation. In the southern Michigan Basin, an important unconformity at the Mississippian-Pennsylvanian contact is represented by either an incised valley-fill succession or a prominent paleosol above the Bayport formation at the base of the Absaroka section in the Saginaw Formation. In upthrown areas adjacent to a major wrench fault, the Lucas fault in south-central Michigan, the Bayport formation is transitional upward from an intensely karsted limestone to a red-bed paleosol and then to primarily carbonaceous mudrock of the Saginaw Formation. In downthrown areas adjacent to the fault, the formation contact, and systemic unconformity, is a sandstone-on-sandstone contact. Climate-sensitive strata indicate a significant transition from predominantly arid conditions in the Mississippian Bayport formation to humid climate conditions in the Pennsylvanian Saginaw Formation across the Mississippian-Pennsylvanian systemic boundary. Previously, the Bayport formation was considered Meramecian in age; however, palynologic analyses of samples collected from core within the interval indicate a Chesterian (late Mississippian) age, representing a significant revision of existing Michigan Basin stratigraphy.

ABSTRACT Over 2000 linear feet of recently acquired conventional core and hundreds of well logs were analyzed to reevaluate regional, middle Carboniferous litho- and bio-stratigraphic relationships in the Michigan Basin, USA. The main objective of this workshop is to interpret the evolution of the Michigan Basin relative to other more extensively studied, North American cratonic interior basin successions in the Illinois and Appalachian basins. Stratigraphic relationships are evaluated in the Michigan, Bayport, and Saginaw formations on the basis of sedimentary lithofacies, contact relationships, and facies stacking patterns, in addition to new age determinations from several distinct pollen and spore assemblages. Core and well-log cross sections are presented to establish regional stratal geometry and corroborate stratigraphic relationships established in core studies. Biostratigraphic analysis has established the age range of these middle Carboniferous strata, which range from late Mississippian (Chesterian) through the early–middle Pennsylvanian (Morrowan and Atokan) North American stages, with no indication of significant hiatus relative to existing chronostratigraphic resolution. Significant soil horizons and incised valley-fill deposits found at the Mississippian-Pennsylvanian systemic boundary, however, are interpreted to represent the basal Absaroka sequence boundary. Significant variations in eustatic, climatic, and tectonic signals recorded in Carboniferous strata of the Michigan Basin are found to be in close agreement with regional geological relationships established in recent sequence stratigraphic and basin analysis studies conducted in adjacent cratonic interior basins. Shallow, mostly restricted marine, mixed clastic, carbonate, and evaporite strata of the Mississippian Michigan and Bayport formations are overlain by carbonaceous debris-rich, terrigenous clastics dominated marginal marine and terrestrial strata of the Pennsylvanian Saginaw Formation. These strata record the complex interplay among second and higher order eustatic changes, global climate variations, and the culmination of Appalachian orogenic activity along the eastern margin of North America during the middle Carboniferous. These geological factors resulted in the dramatic transition from carbonate-dominated, stable cratonic interior basin sedimentation during the Siluro-Devonian to siliciclastic-dominated strata in the latest Devonian through Carboniferous in the Michigan Basin.

The Mississippian System has the largest subcrop area of any Phanerozoic system in the Michigan Basin, and attains a maximum thickness of 719 m (2,360 ft) northeast of the basin center. The Mississippian formations include, in ascending stratigraphic order: Antrim Shale, the laterally equivalent Bedford and Ellsworth Shales (all Upper Devonian to Kinderhookian); Berea Sandstone (Kinderhookian); Sunbury Shale (Kinderhookian); Coldwater Shale (Kinderhookian to Osagian); Marshall Sandstone (Osagian); Michigan Formation (Osagian to Meramecian); and Bayport Limestone (Meramecian). There are no Chesterian sediments in the Michigan Basin. The Mississippian sediments accumulated conformably on Devonian strata but are overlain with disconformity by Pennsylvanian and, very locally, Jurassic strata. The Kinderhookian, Osagian, and Meramecian series record a decreasing rate of Michigan Basin subsidence through time. Subsidence ceased temporarily during the Chesterian Epoch, and some Mississippian units were eroded from local anticlines in the central basin area during this interval of nondeposition. As a result, basal Pennsylvanian strata rest directly on Meramecian rocks and locally on older Mississippian formations. The Mississippian sediments are primarily shallow-marine deposits consisting largely of shale with subordinate amounts of sandstone, siltstone, carbonates, and evaporites. Fluvial-deltaic deposits make up a significant portion of the section only in the eastern half of the basin. Terrigenous clastics were derived mainly from a source to the northeast of the basin in the Canadian Shield and, to a lesser extent, from the northwest in the Wisconsin Highlands. Significant quantities of oil and gas have been produced from sandstones in the Berea, Marshall, and Michigan Formations, and from carbonates in the Ellsworth Shale. Sandstones in the Coldwater and Marshall Formations were, at one time, extensively quarried for grindstones and construction flagstone, respectively. The Michigan Formation is the chief source of gypsum in Michigan, and the Bayport supplies some of the state’s limestone.