Subsurface Stratigraphy of Cambrian Rocks in Southern Peninsula of Michigan: Michigan Basin

Randall L. Milstein

Rocks believed to be Late Cambrian exist throughout the subsurface area of Michigan's Southern Peninsula. These Cambrian rocks represent the oldest Paleozoic sedimentary deposits in the Michigan basin. Isopach and paleogeologic maps of these Cambrian rock units have been prepared with the data derived from geophysical well logs, descriptive logs, and well samples from 101 oil and gas test wells, mineral extraction wells, and deep disposal wells. A generalized contour map of the Precambrian surface has also been prepared from these data.

The prepared maps indicate that geologic structures known to be associated with the Precambrian surface are reflected upward into overlying Cambrian rock units. In many places, these features reflect upward through the entire Paleozoic section to form prominent bedrock structures.

Comparisons between mapped Cambrian strata and structure contours of the Precambrian surface imply the presence of a unique relationship. The apparent distribution of Cambrian strata around the edges of the Michigan basin, particularly along the basin's eastern margin where the greater part of the data is available, indicates that successively younger beds overlap to lie directly on the Precambrian surface. This suggests that the regional configuration of the Precambrian surface, during the Cambrian, was similar to the present. In addition, the Mount Simon Sandstone, the oldest Cambrian sedimentary deposit in the Michigan basin, was apparently deposited directly on the Precambrian erosional surface. An isopach map of the Mount Simon indicates Mount Simon sediments were deposited in a early circular basin. This would suggest that the Precambrian surface had attained almost circular configuration prior to the deposition of Cambrian sediments. Both observations indicate that an embryonic Michigan basin, in a form similar to its present nearby circular morphology, existed as early as Middle Cambrian.

AAPG Search and Discovery Article #91023©1989 AAPG Eastern Section, Sept. 10-13, 1989, Bloomington, Indiana.