Barnes, David A.¹
(1) Western Michigan University, Kalamazoo, MI

ABSTRACT: Incised Valley Fill Stratigraphic Model for Upper(?) Pennsylvanian, Bedrock Aquifers in Central Lower Michigan

Coarse-grained, Pennsylvanian strata are important aquifers in several central lower Michigan counties. Better understanding of these units may provide increased water supply in urban areas experiencing development pressures. Stratigraphic studies addressing aquifer characterization, water supply, and contaminant hydrogeology, are very limited. Lower Pennsylvanian shale, coal, sandstone, and limestone (Saginaw Fm) were deposited in deltaic, marginal marine to estuarine depositional environments. Overlying, highly discontinuous, Upper (?) Pennsylvanian, coarse-grained, cross-bedded sandstone, siltstone, and carbonaceous shale (Grand River Fm) comprise meandering river deposits. Pennsylvanian strata may be separated by a significant unconformity. Lateral discontinuity of units and lack of detailed data has confounded stratigraphic modeling of these important bedrock aquifer units.
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 300' of Upper(?) Pennsylvanian strata. Sedimentologic study and core to log correlations indicate that laterally continuous, coarse-grained, sandy meandering river, channel-fill facies aquifers; discontinuous, silty, channel-margin facies, and carbonaceous shale, flood plain facies aquitards are present in classic scour-based, fining upwards, point bar successions transitional to marginal marine, estuarine facies upsection. Small scale faults, soft sediment deformation, and slumping at the base of some cored wells coincides with shallow target geophysical evidence and may indicate penecontemporaneous faulting. Faults active in the Pennsylvanian may have controlled the trend of NE-SW and NW-SE conjugate incised valleys formed during Pennsylvanian, glacio-eustic sea level fluctuations. Pennsylvanian strata in central lower Michigan are modeled as a regressive, high stand systems tract (Saginaw Fm) overlain by a regional sequence boundary and laterally discontinuous, low stand to transgressive systems tract, incised fluvial to estuarine valley-fill deposits of the Grand River Fm. The Grand River may be correlative and analogous to highly productive, Pennsylvanian, valley-fill hydrocarbon reservoirs elsewhere in the mid-Continent region.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.