Incised Valley Fill Stratigraphic Model for Coarse-Grained, Upper(?) Pennsylvanian, Bedrock Aquifers in Central Lower Michigan

Barnes, David A., and Niah Venable
Department of Geosciences, Western Michigan University, Kalamazoo, MI

Pennsylvanian, coarse-grained clastics are important aquifers in several central lower Michigan counties. A predictive model for stratal geometry, spatial distribution, and controls on aquifer properties of these units may provide for more reliable water supply in areas of intensive agricultural use and in urban areas experiencing development pressures. Lower Pennsylvanian shale, coal, sandstone, and limestone (Saginaw Fm) were previously interpreted as deltaic, marginal marine to estuarine deposits. Overlying, Upper (?) Pennsylvanian, coarse-grained, strata (Grand River Fm) are thought to have originated as coarse-grained, meandering river, channel-fill facies. These Pennsylvanian strata may be separated by a regional unconformity. Lateral discontinuity on a regional scale and a lack of detailed biostratigraphic, lithostratigraphic and sedimentologic data has confounded modeling of these important bedrock aquifer units.

High quality, conventional core and gamma-ray log data collected from 10 closely spaced (~150 acres) ground water monitoring wells near Mason, MI represent as much as 100m of Pennsylvanian sandstone, siltstone, and carbonaceous shale. Sedimentologic study and core to log correlations indicate that fining upwards, meandering river, channel-fill aquifer facies are laterally continuous within the study site and are as thick as 20-25m. Laterally discontinuous aquitards comprising silty channel-margin facies and carbonaceous shale flood plain facies are present at the top of fining upwards, point bar successions and are transitional to marginal marine, estuarine facies up-section.

Pennsylvanian strata in central lower Michigan are tentatively modeled as a regressive, high stand systems tract (Saginaw Fm) overlain by a regional sequence boundary and regionally discontinuous, low stand to transgressive systems tract, incised fluvial to estuarine valley-fill deposits of the Grand River Fm. Regional wire-line log correlation and study of pollen and spores from carbonaceous shale facies are in progress to test this model.

AAPG Search and Discovery Article #90031©2004 AAPG Eastern Section Meeting, Columbus, Ohio, October 3-5, 2004