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Rampino, M.R, and T. Volk, 1996: Multiple impact event in the Paleozoic: Collision with a string of comets or asteroids? Geophys. Res. Lett., 23, 49-52, doi:10.1029/95GL03605.
Eight circular geologic structures ranging from ~3 to 17 km in diameter, showing evidence of outward-directed radial deformation and intensive brecciation, lie within a linear swath ~15 km wide along a straight line stretching ~700 km across the United States from southern Illinois through Missouri to eastern Kansas. Based on their similar geological characteristics and the presence of diagnostic and/or probable evidence of shock, these structures, once classified as 'cryptovolcanic' or 'cryptoexplosion' structures, are more confidently ascribed to hypervelocity impact. No other similar occurrence of aligned features is known, and we calculate the probability of a chance alignment to be <10-9. The unusual alignment suggests that the features are coeval and related to a multiple impact event, with a best-constrained latest Mississippian-early Pennsylvanian (~330-310 Myr) age. Calculations suggest that the proposed impact-crater chain is unlikely to have been formed by an incoming impactor disrupted by terrestrial or lunar tidal effects, and may have been the result of a string of asteroidal or cometary objects produced by breakup within the inner Solar System.
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