T69 Impact Cratering: From the Lab to the Field; from the Earth to the Planets

Sunday, 31 October 2010
1:30 PM - 5:30 PM
Mile High Ballroom Prefunction

APPLICATION OF A PALEOSTRESS PIEZOMETER TO JEPTHA KNOB, VERSAILLES AND DYCUS STRUCTURES, ARE THEY METEORITE IMPACTS?

Paper number: T69-12
Program: Technical Sessions
Time: 4:05 PM – 4:20 PM

Authors

Abstract

In Koeberl and Anderson’s (1996) review of impact structures of the United States, they argued that Versailles (Kentucky) was a probable impact; Jeptha Knob (Kentucky) was a possible impact; and Dycus (Tennessee) was not discussed. All three structures lie in the stable craton, hundreds of kilometers from any orogenic front. Jamison and Spang’s (1976) calcite-paleostress-piezometer has been widely applied to both cratonic and tectonic settings. Craddock and van der Pluijm (1989) have measured differential stresses of 25 to 45 MPa in the flat-lying rocks in the vicinity of all three structures. Based on the calcite strains in these same rocks, they attributed the stresses to the main phase of the Alleghenian Orogeny (Pennsylvanian-Permian). At Jeptha Knob, Upper Ordovician limestones are highly deformed and are overlain by flat-lying Lower Silurian limestones and dolomites. At Dycus, deformed (vertically dipping and tightly folded) Ordovician-Devonian(?) carbonate rocks are overlain by flat-lying Devonian(?)-Mississippian carbonate rocks. Both structures predate Alleghenian deformation. Also, sampled rocks were within a few hundred meters of the surface (2.5-5.0 MPa of burial pressure) during deformation. Jamison and Spang’s (1976) paleostress-piezometer gives maximum differential stresses of >~250 MPa at both structures. Assuming Mohr-Coulomb failure of typical limestone, mean stress or burial pressure for these rocks was >~200 MPa. The only known process which would raise pressure from 2.5-5.0 MPa to >~200 MPa in a geologic instant is meteorite impact. Differential stresses of ≥200 MPa are seen at the centers of orogenic belts. At this location twins are thick, >1 µm because of high temperature, >175oC, whereas twins at Jeptha Knob and Dycus are thin »0.5 µm. Thus evidence is strong that Dycus and Jeptha Knob are impact structures.

Versailles is considered to be a probable impact because geophysical studies show that it is shallow and the near surface rocks are breccias. Differential stresses of rocks within the circular structure are 50-65 MPa. A sample from the rim of the structure gives a differential stresses of >200 MPa. The odd distribution of differential stresses, and the absence of burial depth information means the stress piezometer contributes little to understanding the Versailles structure.