The Calvin impact crater, Cass County, Michigan: Identification and analysis of a subsurface Ordovician astrobleme.

摘要

The Calvin impact crater is an isolated, nearly circular subsurface structure of Late Ordovician age in southwestern Michigan. The crater is defined by 110 oil and 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.;During microscopic investigations of sample cuttings from wells in close proximity to the Calvin structure a collection of metallic microspherules were recovered. I report here, to my knowledge, for the first time, the recovery and analysis of metallic microspherules of Ordovician age. 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 astrombleme. Based on the available data, I conclude the subsurface Calvin structure is a buried complex impact crater and suggest it be formally recognized as the Calvin Impact Crater.;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.