Nestled in the rolling hills of Central Kentucky, the quaint little town of Versailles is a welcome respite from the rigors of city life. It is better known for its picturesque scenery of Thoroughbred horse farms and the fine bourbon whiskey produced in the region, but there is evidence of a violent catastrophe that struck the region long ago, in the Paleozoic.
The Versailles “cryptoexplosive” is an enigmatic 1.5 km circular structural feature located a few kilometers north of the town. The structure was originally referred to as ‘crypto'(ie, hidden) because an explanation for this, and a couple of other similar structures in the region, was lacking.
At the time, it was thought that a plume of water enriched magma approached the surface; out gassed, and then the structure collapsed-resulting in the features now observed at the surface(I’m not a tectonic guy, so could someone explain how a water rich magma can be found this far from a convergent boundary; are there examples??? Or, was this before the mechanisms of Plate Tectonics were understood?).
By the ’60s, geologists began to recognized the similarities of the structural features of the Versailles abnormality to other structures that were better explained as extra-terrestrial, in nature. In the ’70s, geophysical surveys were conducted in the area in an effort to determine whether, or not, the abnormality was an at-depth structure associated with mantle plumes, or whether the structure was a shallow feature that was not associated with the basement tectonic features observed in the region.
While lacking primary structures such as shatter cones or shocked quartz, the consensus among geologists, due to structural features, is that the Versailles cryptoexplosive is the eroded remains of an impact of a meteoroid.
Further reading and references:
Addendum-the stratigraphic nomenclature of the area has changed; now, the Cynthiana is recognized as part of the Lexington Limestone. The Million Shale of Nichols is now known as the basal part of the Clays Ferry Fm.