Impact Origin of Calvin 28 Cryptoexplosive Disturbance, Cass County, Michigan

Randall L. Milstein

The Calvin 28 cryptoexplosive disturbance is an isolated, nearly circular, subsurface structure of Late Ordovician age in southwestern Michigan. The structure, which is defined by 93 test wells, is about 7.2 km in diameter and consists of a central dome, an annular depression, and an encircling anticlinal rim. It is faulted and deformed; deformation decreases with depth and distance from the center of the structure.

Igneous or diapiric intrusion and solution collapse are rejected as possible origins for the Calvin 28 structure on the basis of stratigraphic, structural, and geophysical evidence. A volcanic origin is inconsistent with calculated energy requirements and an absence of igneous material.

Morphology, structure, and mathematical modeling favor an impact origin. Although shock-metamorphic features are absent, microbreccias occur in several wells that penetrate the structure. The evidence points to a near-surface, instantaneous shock event caused by hypervelocity impact of a cometary body.

AAPG Search and Discovery Article #91041©1987 AAPG Eastern Section Meeting, Columbus, Ohio, October 7-10, 1987.