COCORP (Consortium for Continental Reflection Profiling) surveys across the mid-Michigan gravity anomaly, near the McClure-Sparks No. 1 deep well, delineate a thick layered sequence of relatively strong reflectors that define a structural trough within the Precambrian beneath the Michigan Basin. These deep reflections, which correlate spatially with the mid-Michigan gravity and magnetic high, are interpreted to be a pile of basaltic extrusive rocks intercalated with clastic rocks and correlative with Keweenawan volcanics exposed in the Lake Superior region to the northwest. A reflection-poor zone between the base of the Paleozoic, as found in the deep well, and the top of the deep layered sequence probably corresponds to the upper Keweenawan clastic assemblage. Direct evidence of faulting within the inferred volcanic sequence is lacking, although sharp flexures, structural benches, and dips of as much as 10° suggest subsidence controlled by fault-block motions. Reflections from beneath the inferred volcanic pile are rare, the deepest clear event being at about 18 km. There is no unequivocal evidence on these sections to indicate whether Precambrian structures were reactivated to affect the overlying Paleozoic strata of the Michigan Basin proper, although data quality may be insufficient to detect such effects.

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