ORIGINS OF THE 38th PARALLEL STRUCTURES

The 1994 collision of more than twenty fragments of Shoemaker-Levy 9 comet on Jupiter prompted a review of possible serial impacts on Earth. The 38^th parallel structures, which stretch from southeastern Kansas to southern Illinois, were proposed as the product of a serial impact, but other arguments have been made for their igneous origin. Over the last six years, the structures have been studied in detail, and most are clearly igneous. We recognize only three structures as having an impact origin: Weaubleau, Decaturville, and Crooked Creek. These structures occur in a nearly straight line over a distance of 199 km, but poor resolution of the ages of the Decaturville and Crooked Creek impacts prevents confirmation of a serial impact hypothesis. There are four key arguments.

The strongest case for serial impact is based on close proximity and alignment. Computer simulations, with 10,000 runs each, used model parameters for the present-day land surface area, 200 and 300 random impacts, search radii of 100 km, 200 km, and 300 km, and angular variance of 5°. For 200 randomly generated impacts and 100 km search radius, the probability of random alignment was 6 in 10,000; a highly improbable outcome. Probability of random alignment increases significantly if the number of impacts or search radius is increased. We did not consider possible tectonic re-configuration of random patterns.

The known ages of the structures overlap. Based on biostratigraphy, the Weaubleau structure is middle Mississippian in age (latest Osagean or early Meramecian). Post-impact mineralization and subsidence at Decaturville means paleomagnetic ages need revision; previously these had excluded the possibility of overlapping ages with Weaubleau. Float blocks with middle Mississippian fauna occur at Crooked Creek, but their stratigraphic context is uncertain.

Countervailing arguments to the serial impact hypothesis include the inferred angle of impact at Weaubleau (southwest to northeast) from structural interpretation of deformation. A second argument hinges on the preservation of Upper Ordovician clasts in the Decaturville structure. Elsewhere across southwest and central Missouri, the sub-Mississippian unconformity has removed most post-Lower Ordovician strata, with the exception of a few Devonian karst fills.