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A northeast-trending graben was hypothesized to extend southwest of Saginaw Bay to the Mid-Michigan Gravity High, based on interpretation of Landsat 1 imagery, stream drainage maps, and sparse well-log data. The edges of the graben were thought t o extend along and southwest of the Pinconning oil field on the northwest side, and the Quanicassee River on the southeast side. Subsequent analysis of digital terrain, magnetic, gravity, seismic, and well-log data showed that no unequivocal evidence for a discrete, simple graben within the originally defined limits could be found. However, the new data indicated that the proposed edges of the “graben” correspond to structural lineaments (monoclines and anticlines) expressed within the Paleozoic section and on the bedrock surface. These structural features correlate with basement contacts and/or fault zones inferred from interpretation of magnetic and gravity images. These possibly basement-controlled structural lineaments influenced depositional patterns intermittently during the Paleozoic, as evidenced by the presence of northeast-trending highs within the limits of the “graben” on isopach maps of Pennsylvanian, Mississippian, and Devonian stratigraphic units. Rapid thinning and facies changes in Middle and Lower Ordovician units across the southeastern edge of the “graben,” coupled with its correlation with northeast-trending positive gravity and magnetic anomalies, suggest that this is a significant structural feature, possibly controlled or influenced by the Grenville Front.

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