ORIGIN OF THE SERPENT MOUND CRYPTOEXPLOSION STRUCTURE: EVIDENCE FROM X-RAY DIFFRACTION STUDIES

The Serpent Mound disturbance is a nearly circular geologic feature approximately 8 km in diameter located in Adams County, Ohio near the intersection of Adams, Highland, and Pike Counties. The rocks at the Serpent Mound disturbance exhibit complex faulting in contrast to the fairly flat-lying sedimentary rock beds of the surrounding country. The Serpent Mound disturbance is referred to as a cryptoexplosion structure and is though to have originated from some powerful explosive event. Two primary hypotheses have been put forth to explain the structure. One hypothesis explains the disturbance through a "cryptovolcanic" event. Such an event would involve an eruption of explosive gases from molten rock originating in the earth's mantle, a dry volcanism since no igneous rock is present in the disturbance (Hansen 1994). Another hypothesis suggests that the disturbance is an impact structure, crediting its origin to the impact of a large extraterrestrial body. We are testing the two hypotheses by x-ray diffraction analysis of quartz grains from the Serpent Mound cryptoexplosion structure. The presence in the quartz grains of the high-pressure quartz polymorph coesite would favor the impact hypothesis. The very high pressures required to form coesite can be achieved in impact events but volcanic explosions are not believed to generate such high pressure. Coesite has not been found in association with volcanic eruptions. Preliminary data from our analyses permit the presence of coesite in quartz from the Serpent Mound area, which, if confirmed, would support the hypothesis of an impact origin. Preliminary x-ray powder diffraction analysis and single-peak profiling of shocked calcite from limestone in the disturbance reveal that the values of peak half-width with respect to 2q are comparable to those of samples from two other impact structures, the Kara Crater in Russia, and the Steinheim Basin in Germany (Skála and Jakes 1999).