by: Charles O’Dale

  • Type: Complex
  • Age ma: 450 ± 10 a
  • Diameter: 7.24 b km
  • Location: N 41° 50 W 85° 57
  • Shock Metamorphism: shock metamorphosed quartz (Milstein 1995) c

a Dating Method: Geological – older than Early Sulurian and after the deposition of the youngest rocks of the Late Ordovician Richmond Group (see text).

b Defined by gas test wells.

Microbreccias occur in deep wells that penetrate the structure. Morphology and structural parameters support an impact origin.

Article images are from this paper.

The Calvin impact crater, Cass County, Michigan : identification and analysis of a subsurface ordovician astrobleme

Milstein, Randall L.


The Calvin impact crater is an isolated, nearly circular subsurface structure of Late Ordovician age in southwestern Michigan. The crater is defined by gas test wells, has a rim to rim diameter of 6.2 km, an overall dimensional diameter of 8.5 km and consists of a central dome, an annular depression, and an encircling anticlinal rim. Due to the severity of subsurface disruption observed at the Calvin structure the Michigan Geological Survey considers it the most anomalous and enigmatic subsurface geologic feature in the Michigan Basin. This study models the gross morphology of the subsurface Calvin structure using multiple tools and compares the results to known impact craters. The data presented show that consistent structural and physical analogs exist between the subsurface Calvin structure and known or suspected impact craters. Combined results of seismic, gravity, magnetic, and resistivity data, suggest the Calvin structure’s morphology results in structural patterns similar to those of recognized impact craters in sedimentary targets.

The Calvin structure exhibits recognized relationships between stratigraphic displacement and structural diameters observed in impact craters. During microscopic investigations of sample cuttings from wells in close proximity to the Calvin structure a collection of metallic microspherules of Ordovician age were recovered. No microspherules of similar composition have been reported from other occurrences of later geologic ages. A strong body of circumstantial evidence links these stratigraphically isochronous microspherules with the formation of the Calvin structure. One other possibility for the microspherules’ occurrence is that they represent an extraterrestrial component rich-layer recording a multiple Ordovician impact event on the North America mid-continent. While a considerable body of interpretive data favors an impact origin for the Calvin structure, it is the identification of shock metamorphosed quartz, a characteristic indicative of impact cratering events, that most strongly confirms the Calvin structure as an Ordovician buried complex impact crater.

Milstein (1986) identifies eight characteristics of the Calvin 28 cryptoexplosive disturbance that strongly favor origin by impact.

(1) Terrestrial surface impact structures with central uplifts (complex craters) show a modelable relationship between stratigraphic displacement in the uplift and the crater form (Grieve and others, 1981). The Calvin 28 structure exhibits this relationship.

(2) The waning of structural deformation beneath Calvin 28 is indicated by seismic profiles and dipmeter readings from deep test wells. The lessening of derangement with depth would not be expected from a tectonic or volcanic origin, but would be consistent for structural deformation incurred from a downward-projected shock envelope (Shoemaker, 1960; Lindsay, 1976).

(3) The structural pattern of Calvin 28 indicates that a large amount of near-surface energy was involved in the formation of the structure and that the energy was released as a single nearly instantaneous event.

(4) No igneous material has been recovered from well cuttings or identified in petrographic studies involving the structure.

(5) Calvin 28 is an apparently isolated circular structure of intense deformation in otherwise flay-lying strata.

(6) The lack of any magnetic or gravity anomalies associated with the circular structural pattern is a trait commonly identified with impact craters in sedimentary targets.

(7) The presence of a microbreccia consisting of fractured and unfractured floating quartz grains in a carbonate matrix is similar to microbreccia associated with impact craters in sedimentary targets (Short and Bunch, 1968).

(8) The energy required to produce the 4.5 mi (7.24 km) diameter structure, the apparent structural relief, the missing strata, and the intense structural deformation is at least 1 x 1026 ergs. While this value exceeds energy estimates for known singular explosive endogenetic events, it would be considered a conservative value for energy released by a hypervelocity impact in a sedimentary target (Shoemaker and Wolfe, 1982).

I took this image of the south edge of the Calvin Impact Structure, <100 m from the point of impact. The surface relief of the structure is illustrated by the “dip” in the highway. Milstein comments that although the visible geology at the Calvin 28 cryptoexplosive disturbance is subtle and not particularly exciting, the subsurface geology is of much interest; many researchers consider the structure to be the most anamolous geologic feature in the Michigan Basin.


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