INTERPRETATION OF GLACIAL DEPOSITS AT DRAINAGE-BASIN SCALE FOR USE IN GROUND-WATER MODELING

In a cooperative effort for ground-water modeling of the St. Joseph River basin (Michigan and Indiana), the USGS Geologic Discipline worked together with the USGS Water Resources Discipline to establish a glacial geologic framework for the hydrostratigraphic units in the basin. It is recognized that a hydrogeologic model can more closely approximate reality if it is based on a geologic map that accurately identifies contrasting geologic units.

The area of the St. Joseph River watershed and adjacent Lake Michigan coast was entirely constructed by complex glacial processes originating from three lobes of the Laurentide continental ice sheet. These lobes were first described by Leverett and Taylor (1915) and include from west to east: the Lake Michigan lobe, the Saginaw lobe, and the Huron-Erie lobe. Each lobe has its own history of timing of advance and retreat and left different depositional assemblages. The cross-cutting relationships of various deposits associated with each lobe indicate that retreat of the lobes from the drainage basin was asynchronous. The last depositional event in the basin was a meltwater deposit from the Huron-Erie lobe that cuts across the entire basin.

From regional-scale mapping of the St. Joseph River we have characterized six significant geomorphic terrains of internally consistent assemblages of surficial deposits (1-Lake Michigan lobe terrain, 2-Lake Michigan lobe/Saginaw lobe terrain, 3-Northern Indiana stacked terrain, 4-Saginaw lobe sculpted terrain, 5-Huron/Erie lobe complex terrain, and 6-St. Joseph River outwash terrain). Each of these terrains has unique combination of physiography and substrate with predictable parameters for hydrogeologic modeling.