SEDIMENTOLOGY AND STRATIGRAPHY OF THE PENNSYLVANIAN SAGINAW AQUIFER, CENTRAL LOWER MICHIGAN, USA

The Pennsylvanian Saginaw aquifer is composed of the Saginaw and Grand River formations and is an important source of drinking water in central Lower Michigan. The Saginaw aquifer supplies water for over 400,000 residents of a three county area including Lansing, MI. It is currently subject to over-utilization and surface derived contamination. Lack of exposure, pre-Pleistocene truncation of bedrock units, the ambiguity of wireline log correlations, and complexity of relationships within the Pennsylvanian succession in the Michigan basin confounds further stratigraphic study of these units in support of a semi-regional hydrologic flow model by the US Geological Survey.

Several conventional cores, up to 300 ft in length, were recently collected from an approximately 120 acre site of surface derive, industrial contamination near Mason, MI. These cores and gamma ray log data provide an opportunity to study Pennsylvania strata in detail using modern sedimentologic, biostratigraphic, depositional system and sequence stratigraphic methodology. Local core to wireline log correlation at the site provides a basis for semi-regional correlation using several hundred oil and gas gamma ray logs from the surrounding area. These correlations will be used to test stratigraphic models for regional aquifer geometry, and investigate relationships to correlative Pennsylvanian strata in other Eastern Interior Basins.

Preliminary results suggest that the Atokan age succession present in the subsurface near Mason overlies the base Absaroka unconformity. That unconformity increases in stratigraphic significance from north to south out of the basin. Distinct sedimentary facies previously attributed to either the Grand River or the Saginaw formations are found complexly interrelated in vertical succession and in association with significant development of multiple paleosols. These initial findings are in sharp contrast to simple stratigraphic relationships inferred in outcrop suggesting significant hiatus and unconformity between superjacent, fluvial channel-fill sandstone dominated facies of the Grand River Formation and subjacent, heterolithic, mixed marginal marine/deltaic strata of the Saginaw Formation.