QUATERNARY GEOLOGIC FRAMEWORK OF THE GRAND TRAVERSE BAY REGION, MICHIGAN: RELATIONSHIPS TO WATER, LAND, AND ECOLOGICAL RESOURCES

Major landscape elements and surficial geology of the Grand Traverse Bay (GTB) region were formed by subglacial and proglacial processes during the latest advance of the Laurentide ice sheet onto the Lower Peninsula of Michigan in post-Twocreekan time (after ca.11,700 14C yr BP). Inland from GTB, a markedly drumlinized area records divergent ice flow of a sublobe of the Lake Michigan lobe. The drumlin fields are associated with discontinuous, reddish sandy till over stratified sand and gravel. The drumlins and substrate are deeply incised by an anastomosing network of subglacially formed channels. A minimum limit on the glacial advance that formed these features is defined by an elevated and pitted outwash plain-the Mancelona sandur-that slopes southwestward in the direction of proglacial meltwater flow. The sandur was confined between the ice margin of the readvance limit and a higher, older head-of-outwash associated with the outer Port Huron moraine. In deglacial and postglacial time, various geomorphic features and stratigraphy were superimposed (commonly in relation to lake level history), including incised fluvial systems, nearshore scarps, bars, beach ridges, dunes, and extensive, varied coastal and inland wetlands and lakes.

The late Quaternary geology and landscape development is integral to the present natural resources of the area. The highly permeable glaciofluvial sand and gravel dominated uplands are the recharge areas and aquifers for ground-water flow to discharge areas along incised streams, lowland fens and lakes, and nearshore areas of the GTB region of Lake Michigan, including Sleeping Bear Dunes National Lakeshore. Most streams are groundwater-fed and provide habitat for cold-water fish assemblages, including trout species. The relatively high relief of the landscape and gradient of the groundwater system was largely determined by the development of the glacial tunnel valley system that dominates the nearshore region. The calcium bicarbonate dominated water chemistry is probably controlled by the abundance of regionally derived Paleozoic limestone clasts in the glaciofluvial aquifers. Biogenic carbonate precipitation commonly occurs in the surface water resources and wetlands, thus forming additional varied and unique substrates and integral ecosystem components.