FLYNN CREEK IMPACT STRUCTURE, TENNESSEE: AN OVERVIEW

Located in Jackson County, Tennessee, USA, Flynn Creek is one of the first terrestrial impact structures identified during the era of the Apollo lunar missions of the 1960s. This impact impact structure is a non-circular, slightly oval-shaped feature ~3.8 km in diameter. The impact structure is broadly similar in morphology to lunar craters -- it has a central uplift, a breccia moat, and terraced crater rim. Lithologically, however, Flynn Creek is distinct, formed when a hypervelocity impact occurred in an epicontinental shelf within a carbonate bedrock target. Biostratigraphic evidence suggests a Late Devonian impact. The target stratigraphic section was essentially flat-lying Ordovician carbonates including the Lower Ordovician Knox Group through Upper Ordovician Catheys-Leipers Formation.

Also unlike lunar craters, Flynn Creek was is a marine target crater, where the crater is filled with gravity-driven avalanche material, washed-back ejecta, and other marine resurge and settling-out deposit. From studies of drill core, we can determine that the impact-structure filling breccia is a few 10s of m thick; and there is a 300-m (or more) thick parautochthonous breccia lens below. Flynn Creek impact structure was subjected to intensive erosion after impact, which removed the original ejecta blanket. In a subsequent, post-impact phase, Upper Devonian Chattanooga Shale was deposited within the Flynn Creek impact structure and on the surrounding shelf. In turn, several hundreds of meters of other types of sediments were deposited above the Chattanooga, including the Maury Formation and Fort Payne Chert. Erosion in the Flynn Creek area due to regional uplift along the Nashville Dome, which has cut into the impact structure and thus dissected the crater rim, walls, floor, and central peak.

Direct observations shock metamorphism at Flynn Creek is limited to only shatter cones. Internal mineral deformation has not been identified, likely because the target carbonate rocks are unconducive for forming shock deformation. Recent petrographic studies have reviled irregular shaped poorly birefringent material with chemistries consistent with having been a high-T melt product. Such melts have not previously been identified and may shed light into the ongoing debate concerning melt production at small impact structures.