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The writer has previously proposed a wrenching model for the Michigan basin based on the close relation of lineaments gleaned from Landsat imagery to the geometry of shear faults and related shear folds anticipated in such a model.

Recent studies by colleagues have been focused on old linear oil producing structures where (1) the density of wells permits structure maps drawn on a small contour interval, and (2) sufficient well samples allow the construction of dolomite-limestone ratio maps. Geometry of dolomite distribution patterns in Middle Devonian carbonate rocks shows close comparisons to pay zone distribution coincident with shear faults and related shear folds, cross faults, and cross folds--all related to the wrenching model. Producing well patterns follow dolomite distribution patterns. Lateral displacement can often be detected along shear faults on structure maps. Some faults not detectable on the structure map are shown on the dolomite-limestone ratio map. The application of the wrenching model to fu ure exploration for linear producing fields in the basin appears self evident; strike-slip faults in nearly horizontal rocks are elusive seismic targets. The shear faults generally show little vertical movement. Not all shear faults developed shear folds, as in the giant Albion-Scipio field (Ordovician).

Evidence suggests that movement along shear faults was episodic, with the axes of related shear folds showing some migration downward. Further testing of the shearing mechanics in this regard may assist in exploration for deeper targets.

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