CARBONIFEROUS STRATIGRAPHY OF MICHIGAN REVISITED: IMPLICATIONS OF REVISED BEDROCK GEOLOGY MAPS OF CENTRAL JACKSON COUNTY, MICHIGAN, USA

Bedrock geology maps generated from water-well lithologic logs (6,000) and drill cores reveal new details of the stratigraphy of Carboniferous rocks in the Jackson, Michigan Coal Province (JCP, new term). The relation of voids created by mid-1800’s underground mining, to ground instability along I-94 in Jackson County has renewed interest in the distribution of coal in the JCP. Geologic maps are needed to address engineering, ground-water flow, contaminant transport, and water-supply issues.

Units mapped include Mississippian Marshall Sandstone (regional aquifer), Michigan Formation (mixed siliclastic-evaporite), Bayport Limestone (carbonate-siliclastics), Pennsylvanian Saginaw Group (coal/black shale), and Grand River Formation (sandstone).

Interpretations of stratigraphic relations include 1) erosion of the Bayport dissected this marine unit and produced 40m of topographic relief on the Mississippian surface, 2) coal, black shale, and fine clastics formed a blanket fill atop the Bayport, with thickest coal (1m) in braided stream channels near the contact of Marshall/Bayport, 3) Late Pennsylvanian erosion dissected the Saginaw coal sequence, with channels incised to the top of Marshall, 4) late sandstone forms a blanket atop the dissected terrain, 5) pre-Pleistocene erosion produced a third series of channels that further dissect the Carboniferous sequence, and 6) the current Grand River occupies the primary coal-bearing paleochannel.

Noteworthy observations are 1) a basal sandstone, the Parma as shown in earlier maps is conspicuously absent, indicating sandstone outcrops in the JCP are Grand River equivalent, and 2) despite multiple continental glaciations, the Grand River and several tributaries reoccupied former water ways, demonstrating the phenomenon of palimpsest superposition, an obscure process that retained geographic memory spanning more than 300 million years of earth’s history.