The Calvin 28 structure is an isolated, nearly circular subsurface structure of Late Ordovician age in southwestern Michigan. The structure is defined by 107 test wells, is 7.24 km in diameter, and consists of a central dome, an annular depression, and an encircling anticlinal rim.Seismic and geophysical well-log data confirm an intricate system of faults and structural derangement exists within the structure. Deformation decreases with depth and distance from the structure. United States Geological Survey topographic maps and aerial imagery show the structure is reflected as a surface topographic rise controlling local drainage.Igneous or diapiric intrusion and solution collapse are rejected as possible origins for Calvin 28 on the basis of stratigraphic, structural, and geophysical evidence. A volcanic origin is rejected because of an absence of igneous material.Although shock-metamorphic features are unidentified, microbreccias are found in deep wells that penetrate the structure. Morphology and structural parameters suggest an impact origin.

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