A NEW GEOLOGIC MAP FOR BARRY COUNTY, MICHIGAN

Barry County, Michigan was mapped for the first time at the 1:24,000 scale through a combination of USGS STATEMAP, Great Lakes Geologic Mapping Coalition, and EDMAP projects by five different mappers. A composite map was then made at the 1:62,500 scale. Some edits and generalizations were made to develop consistent units across the completed quadrangle maps. The area includes deposits from both the Lake Michigan and Saginaw Lobes of the Laurentide Ice Sheet. Cross-cutting relationships indicate that the Lake Michigan Lobe overrode the Saginaw Lobe terrain in the interlobate area. The most dramatic glacial features on the map are northeast-southwest trending Saginaw Lobe tunnel valleys, which were recognized and interpreted for the first time during this mapping. These valleys, many of which contain eskers, cross a broad high-relief, hummocky ice marginal terrain interpreted as the result of stagnation of the Saginaw Lobe margin. Extensive outwash fans, some of them highly pitted, begin at the termini of the fans. The tunnel valleys nearest the ice margin contain thick sand and gravel fills, making them significant targets for aquifer delineation. By contrast, the tunnel valley reaches more distant from the ice margin contain very thin coarse-grained fills.

Six deep, continuously cored rotosonic borings, along with water well logs, were used to develop a preliminary stratigraphic framework for the Saginaw Lobe deposits. This framework includes 3 or more diamicton units in the north, separated by laminated, silty and clayey glaciolacustrine deposits, transitioning to multiple diamictons separated by sand and gravel units in southern Barry County and adjacent Calhoun County. The bedrock surface, rising in elevation to the south, may have provided the topographic barrier to impound glacial lakes in the northern part of the area. In general, good aquifers are expected to be extensive in the outwash fans, concentrated in distal tunnel valley lowlands compared to the adjacent uplands, and scarce both in the proximal tunnel valley reaches and adjacent uplands in the northern part of the county. This map should be useful in planning for future groundwater withdrawals, aggregate mining, as well as for other purposes.