PETROPHYSICAL ANALYSIS OF IMPACT BRECCIA IN THE FLYNN CREEK CRATER, JACKSON COUNTY, TENNESSEE

The Flynn Creek crater is located 8 km south of the community of Gainesboro, in Jackson County, Tennessee. This 200 m deep impact structure has a diameter of 3.8 km and formed in what was then a shallow marine environment (10-20 m water depth) approximately 360 million years ago. The timing of this impact event is closely associated with deposition of the Chattanooga Shale Formation during the Devonian Period, with the pre-impact conditions of units in the area being consolidated and flat lying. The units impacted include Ordovician limestones and dolomites that overlie crystalline basement and, putatively, earliest sediments of the Chattanooga Shale. The resulting impact breccia weathers to a light gray with granule to boulder-sized clasts throughout. The current hypothesis for the formation history of this area is that the impact event that formed the Flynn Creek Crater occurred just before the Chattanooga Shale was deposited.

This study serves to test two hypotheses: (1) that impact breccia of the Flynn Creek crater may contain clasts of Chattanooga Shale; and (2) clast sizes in the breccia decrease in size up section from the base of the structure. For the first hypothesis, a Gamma Surveyor II hand-held scintillometer was used to conduct a spectral analysis of the impact breccia. Forty-two measurements determining the amount of potassium (%), uranium (ppm), thorium (ppm), and overall gamma ray dose rate (nGy/h) were collected from three outcrops of the Flynn Creek crater: two impact breccia outcrops and one Chattanooga Shale outcrop. The measurements were used to determine if traces of radioactive elements indicative of Chattanooga Shale occur in the impact breccia. Averaged gamma ray dose rates for the two breccia outcrops were 12.26 and 65.58 nGy/h, an order of magnitude lower than the mean dose rate in the Chattanooga Shale outcrop, 150.385 nGy/h. These results suggest that fragments of Chattanooga Shale do not occur in the impact breccia, or if present, they are so small that they make a negligible contribution to radioactivity of the breccia. The next step is to examine breccia clast sizes to identify patterns of clast size distribution within the deposit. Likewise, thin sections cut from breccia samples will provide information about clast compositions as well as the presence/absence of very small Chattanooga Shale clasts.