Re-evaluating the 38th Parallel Serial Impact Hypothesis

NASA Astrophysics Data System (ADS)

The idea that the 38th-parallel structures across Kansas, Missouri, and Illinois are serial impacts has been controversial. In addition to the original eight, two other structures are proximal to the 38th parallel, Dent Branch and Silver City Dome. Only Weaubleau, Decaturville, and Crooked Creek contain quartz grains with multiple directions of planar deformational features (PDFs). Shatter cones have been found at Decaturville and Crooked Creek. Key macroscopic observations of these impacts include: (1) circular outlines and notable central uplifts, (2) remarkably intense levels of structural deformation (folding, faulting, fracturing, and brecciation), (3) deformation dying out with depth and laterally away from the central uplift, and (4) associated igneous rocks only as clasts. From field and core studies and published reports, we consider other structures along the 38th parallel to be dubious (Hazelgreen), intrusive, (Hick's Dome), or volcanic in origin (Silver City Dome, Rose Dome, Furnace Creek, Dent Branch, and Avon). The age of the Weaubleau structure is constrained biostratigraphically as middle Mississippian (latest Osagean or early Meramecian). Crooked Creek and Decaturville are deeply eroded; their ages are poorly constrained. Crooked Creek contains isolated blocks of sandstone of late Osagean age, but the stratigraphic context of the blocks is poorly known. Other investigators contend the age of Decaturville is Pennsylvanian or Permian, based on CRM paleomagnetism and occurrence of an isolated sulfide breccia body in the central uplift. The Ozark plateau experienced Missouri Valley Type (MVT) sulfide mineralization during the Ouachita orogeny, but our examination of a sample from the sulfide breccia shows it is shattered pyrite and differs from typical MVT deposits. If the breccia is not associated with the regional mineralization, a middle Mississippian age cannot be excluded. Weaubleau, Decaturville, and Crooked Creek are aligned across 199 km. A line connecting the centers of the central uplifts of Weaubleau and Crooked Creek passes 1.5 km north of the center of Decaturville. Monte Carlo simulation was used to examine the probability that three temporally unrelated impacts could be aligned (±2° angular discordance) randomly over relatively short distances in an area that approximates the continental land surface. Modeling variables included number of impacts (N=200, N=300...N=1,000) and search radii (100-600 km). Twenty repetitions of 10,000 runs provided for analysis of standard deviation. For N=200 and radius of 100 km, an average of 6.6±2.5 aligned sets were found. For N=300, the results were 25.9±3.7, yielding a probability of P=0.003. Larger sample sizes and wider search radii produced more probable results, but given the number of known impacts (~175), it is highly improbable that Weaubleau, Decaturville, and Crooked Creek structures could be aligned but not temporally related. If Weaubleau, Decaturville, and Crooked Creek were products of a serial impact, they were not analogous with the Shoemaker-Levy 9 impacts on Jupiter, where planetary rotation affected the distribution, so the impacts would have occurred within seconds. In Mississippian paleogeographic reconstructions, the direction of impact would have been WSW-ENE, arguably along the ecliptic and near the equator. The distribution of deformation at Weaubleau suggests an oblique impact from present-day WSW-ENE, an argument against the serial impact hypothesis.

Evans, K. R.; Davis, G. H.; Miao, X.; Mickus, K. L.; Miller, J. F.; Morrow, J. R.

2008-12-01