UNUSUAL CIRCULAR STRUCTURES IN SOUTHERN OKLAHOMA

Near Tishomingo, OK, at least one, and maybe several, small circular depressions have recently been discovered. These features occur in an otherwise flat, wooded area with thin regolith overlying Tishomingo Granite. The largest of these depressions is approximately 6 m in diameter and 2 m deep. It is surrounded by a horseshoe-pattern of angular granitic blocks which seem to have come from the excavated region. Thus far, the most likely explanation of this circular depression is that it is a crater produced by meteoritic impact. In support of this hypothesis, two large meteorites were discovered within two miles of the depression in the 1970's.

In order to confirm that it is an impact structure, evidence of mineral shock (strain lamellae in quartz; presence of stishovite or coesite, although these are unlikely for such a small impact; shatter cones, also unlikely, because the impacted rock is granite) or meteoritic material must be found. If thus confirmed, the impact structure will be the smallest, and quite possibly the youngest, of the 174 currently-confirmed meteoritic impact craters (Earth Impact Database, http://www.unb.ca/passc/ImpactDatabase).

The crater site is currently being investigated. We are doing detailed mapping of the crater, its surroundings, and the distribution of strewn rock fragments to look for consistency with what we think was the angle of impact, based on the horseshoe-pattern of boulders on the periphery of the depression. Geophysical sensing of the main crater area, primarily magnetic profiling and electromagnetic sounding, will soon be carried out. Samples have been collected from within the structure and are being investigated petrographically for any kind of mineral shock effects.