MULTIPLE IMPACT EVENT IN THE PALEOZOIC - COLLISION WITH A STRING OF COMETS OR ASTEROIDS

Eight circular geologic structures ranging from similar to 3 to 17 km in diameter, showing evidence of outward-directed radial deformation a nd intensive brecciation, lie within a linear swath similar to 15 km w ide along a straight line stretching similar to 700 km across the Unit ed States from southern Illinois through Missouri to eastern Kansas. B ased on their similar geological characteristics and the presence of d iagnostic and/or probable evidence of shock, these structures, once cl assified as 'cryptovolcanic' or 'cryptoexplosion' structures, are more confidently ascribed to hypervelocity impact. No other similar occurr ence 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 late Mississippian-early Pennsylvanian (similar to 330-310 Myr) age. Calculations suggest that the proposed impact-crater chain is unlikely to have been formed by an,incoming impactor disrupt ed by terrestrial or lunar tidal effects, and may have been the result of a string of asteroidal or cometary objects produced by breakup wit hin the inner Solar System.