BURIED BEDROCK VALLEYS OF MICHIGAN

In the glaciated Midwest, much can be learned about the geological history of an area by mapping the bedrock surface topography and the buried bedrock valleys. In Michigan, a complex and very irregular bedrock surface underlies the thickest glacial drift on land in North America. In general, the bedrock surface has well defined buried bedrock valley networks with tributaries and main bedrock valleys which can run for tens of miles. These often appear to be in inferred pre-glacial bedrock drainage basins with bedrock surface divides. Other buried bedrock valleys cross bedrock surface divides and cut through broad bedrock highlands and cuestas. Some bedrock valleys are in a nearly parallel pattern over large areas. In other places, the bedrock valleys are short, relatively straight disconnected valleys. Sometimes these appear to be imprinted over an already existing pre-glacial bedrock valley network. There are also broad regional bedrock lowlands and local fjord-like bedrock troughs. Some bedrock valleys and scarps directly overlie deeper structural features. Often bedrock valleys are found preferentially in less resistant bedrock formations. In addition some areas are essentially devoid of bedrock valleys.

There is considerable debate as to the origin of bedrock valleys, but no single mechanism can account for these widely varying bedrock valley types. The extensive buried bedrock valley network suggests that much of the bedrock surface is a slightly glacially modified pre-glacial bedrock surface, the result of long history of pre-glacial uplift and erosion (post Pennsylvanian and post Jurassic). In other places the bedrock surface has also been sculpted by numerous paleo-river channels cut into the bedrock during the numerous glacial ice advances, interglacial periods and the postglacial period over the last 2.5 million years. While in other areas, the bedrock surface has been significantly eroded by direct glacial erosion removing the bedrock valleys.

Knowledge of the location of bedrock valleys may assist in exploration for potential glacial aquifers in bedrock valleys, and in seismic data processing for oil and gas exploration. These bedrock valleys, especially the deeper ones, may contain older glacial, interglacial or pre-Tertiary sediments and paleosols which may be of value for geological age dating.