Geology of the Calvin Impact Structure, Cass County Michigan

R. L. Milstein
Oregon State University, Corvallis, OR

The Calvin structure involves large-scale and intense structural deformation within a limited area of circular shape. Lithologic and structural evidence suggests the feature resulted from a single highly localized release of tremendous energy. Speculation as to possible origins of such a large-scale explosive structure include both endogenetic and exogenetic processes.

Eight characteristics of the Calvin structure lend support to origin by impact. The structure exhibits: 1) morphological characteristics of complex impact craters in sedimentary targets; 2) recognized relationships between depth, diameter, and structural uplift used to identify surface impact craters; 3) geophysical patterns analogous to those of recognized impact craters with similar lithologic components; 4) a polymictic microbreccia; 5) quartz grains exhibiting single and multiple sets of decorated shock lamellae, Bo¨hmlamellae, rhombohedral cleavage, and radiating concussion fractures; 6) no igneous or hydrothermal mineralization associated with the structure; 7) time synchronous black metallic spherules, similar to those associated with meteoritic ablation and impact ejecta, in close proximity to the structure; and 8) a necessary energy release for structural origin of at least 5.9 x 10¹⁸ J of energy, without development of magma or any igneous material.

The identification of astroblemes in the subsurface is rare. Comparison of the Calvin structure to known or suspected Complex impact craters suggests consistent structural and physical analogues. Although a considerable body of interpretive geophysical and structural data favors an impact origin for the Calvin structure, it is the identification of shock-metamorphosed quartz that confirms the structure as an Ordovician impact crater.

AAPG Search and Discovery Article #90900©2001 AAPG Eastern Section Meeting, Kalamazoo, Michigan