Well data suggest that the Precambrian basement of the Michigan basin is highly faulted with vertical tectonics exerting a dominant control over basin structures. A single deep test on the mid-Michigan gravity high penetrated at least 5,000 ft (1,500 m) of Precambrian(?) red beds, suggesting the presence of a rift or graben.

An embryonic form of the Michigan basin was present in Cambrian time. The basin, in approximately its present dimensions, was created during the Middle Ordovician. Two phases of maximum subsidence occurred during the Paleozoic: 4,000 ft (1,200 m) of Silurian sediment, topped by several major unconformities, is followed by 3,800 ft (1,140 m) of Devonian strata. Variations in thickness and sedimentary facies in the Silurian

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and Devonian support the concept of two intrabasin platforms: one in southwestern Michigan (Allegan) and the other in southeastern Michigan (St. Clair). A generalized depocenter lies directly northwest of Saginaw Bay. There were important deviations from this depocenter, particularly during the Ordovician and Middle Devonian.

The major northwest-trending folds in the central part of the Michigan basin were generated during Late Mississippian or Early Pennsylvanian time, and are probably the result of the regeneration of dominantly vertical movements along basement faults.

Major petroleum production in Michigan is from (1) Ordovician dolomitized fault zones; (2) Silurian pinnacle reefs; and (3) Devonian anticlinal traps with marked variations in porosity.

Future possibilities lie primarily with the discovery of additional dolomitized fault zones in Ordovician carbonate rocks or in Devonian stratigraphic traps. Cambrian production is a possibility, but there is inadequate control to define its potential.

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