3-D GEOLOGIC MAP AND DIGITAL DATABASE OF BERRIEN COUNTY, SOUTHWESTERN MICHIGAN

Surficial deposits in Berrien County, Michigan, 12->150m thick and late-Wisconsinan or younger in age, cover completely the underlying glacially eroded Devonian shale bedrock. The new county geologic map, 1:50,000-scale, preserves details mapped at 7.5' quadrangle scale; time-transgressive cut-away map figures display surface and subsurface glacial deposits in a series of 34 shingled map units organized from oldest to youngest. The 3-D database presents these stratigraphic units in a series of overlapping map-unit files, which show the surface/subsurface extent of each unit, coupled with files of surface and subsurface topographic contours for all units. Related files define the coincident extent and altitudes of single or multiple sedimentary facies within each unit. Cross-sections, supported by airborne resistivity data and drill holes, reveal the contrasting materials in upland stratified-moraine and coastal till-ridge moraine deposits.

Allostratigraphic map units in the Kalamazoo/Valparaiso morainic systems in the eastern upland are deltaic morphosequences graded to four local glacial lakes dammed behind older deposits along the continental divide. Some 48 ice-marginal glacial deltas onlap each other in a shingled succession from south to north, reflecting Michigan ice lobe ice-margin retreat positions controlled by underlying bedrock topography. In the 3-D model, these stratigraphic units are subdivided into topset, foreset, and bottomset sedimentary facies, and are related to local lake-bottom deposits. A single till sheet overlaps the inner edge of the Valparaiso deltaic complex signaling a regional readvance of the Michigan ice lobe. In the western half of the county, the Lake Border morainic system contains 2-3 till ridges and interbedded lacustrine deposits. Till units, lithic equivalents of the Saugatuck till, are modeled in 3-D as discrete sheet deposits that rise easterly from the bottom of the basin to culminate in moraine ridges. Interbedded lake deposits, 15-80m thick, fill deep basin troughs. Postglacial deposits include meltwater and postglacial terraces graded to Glenwood and Calumet lake levels of Lake Chicago, littoral deposits of Calumet, Toleston, and Nipissing lake phases, thick lake-estuarine/alluvial fill in the lower St. Joseph River, and deposits of Lake Michigan.