Decaturville Crater | Decaturville. Missouri
The Decaturville crater, also referred to as the Decaturville Dome, is an impact crater near the town of Decaturville, Missouri, United States. It is one of the 38th parallel structures, a series of circular geophysical features stretching across the central United States which have been hypothesized to be the remnants of an ancient serial impact event.
The first report of anything unusual at the Decaturville site was by Shumard, 1873, who, reporting on the geology of LaClede County, reported an igneous outcrop that produced small quantities of ore. The structure was initially interpreted as volcanic in origin, and an understanding of its impact origins emerged only slowly. The first attribution of the Decaturville structure to an impact seems to have been J. D. Boon and C. C. Albrittton in their 1936 work, 'Meteorite craters and their possible relationship to "cryptovolcanic structures".' Boon and Albritton's suggestions did not convince the scientific community at large, however, and the volcanic vs impact debate lasted several more decades, well into the 1960s.
The beginning of a real, quantified, understanding of the structure's impact origins depended on the work of Shoemaker and Eggleton in 1961 and Dietz in 1963. Between about 1960 and 1965, more or less, the Decaturville structure and the nearby Crooked Creek structure became significant battlefields in a war of paradigms. Prior to this time, impact craters were largely interpreted as volcanic or igneous structures; phreatic explosions, ring dikes, cryptovolcanic, or simply 'cryptoexplosive' features.
Through the pioneering work of Dietz and others, and not without opposition, the field of impact crater science emerged from the reinterpretation of existing evidence and from the addition of new and refined petrological and morphological distinctions. Until the sheer mounting weight of evidence bore out in their favor, these scientists received significant argument from equally skilled professionals (eg Amstutz) that held with earlier views. This battle, in large part, is what drove the development of the empirical tool kit of crater science. Unambiguous confirmation of impacts required diagnostic indicators of shock metamorphism. The result of this battle, in the scientific literature, is the suite of techniques we use today in order to differentiate impact craters from objects of similar morphology.
From 1968 to 1970, understanding of the Decaturville structure improved significantly due to its study as a possible analog for the geology of the lunar surface. Initial abstracts of this work were published by Offield and Pohn in 1970 and 1971. In these inital summaries, they firmly established the impact origin for the structure, based on geometry of deformation, which they described in a general sense, and upon evidence of shock metamorphism in several types of minerals. They also made early observations regarding impactite variation, noticing the geographical distribution of monomict vs polymict breccias and noted the presence of remnant bedding in monomict breccias. An in depth summary of their cumulative work, the most detailed yet written for this site, was written by Offield and Pohn, in 1979, under NASA sponsership. A brief summary of early investigations of the site (between 1873 and 1979) are offered on page 2 and 3 of their 1979 report.
The overall morphology of the Decaturville structure is consistent with a complex impact crater with a central uplift. The slightly dished dome of the central uplift, in which rebounded material is raised far above its original position, is surrounded by a large depression. Surrounding this basin is a periferal raised crater rim, which forms an outer circular boundary of the impacted rocks.
The Decaturville crater is 6 kilometres (3.7 mi) in diameter and is estimated to be less than 300 million years old (dating to the Permian or younger). The crater is exposed at the surface. The effect that the impact had on the surrounding bedrock can be seen in a roadcut that runs along Highway 5 about 16 miles north of Lebanon.
The crater was first described and mapped in detail by T. W. Offield and H. A. Pohn under NASA funding in the 1970s. Their work was reported in a 1979 U.S. Geological Survey Professional Paper that provides the most detailed description to date. The report is available here: https://pubs.usgs.gov/pp/1042/report.pdf