NEW INTERPRETATION OF THE STRUCTURE OF FLYNN CREEK IMPACT CRATER, TENNESSEE

Flynn Creek, 3.8 km in total diameter, is a marine impact structure that was formed during Late Devonian. Flynn Creek occurs at the surface, but is deeply eroded and is not particularly well exposed (e.g., there is no rim topography). Flynn Creek is best known as one of the original six proven impact craters on Earth. USGS geologist David Roddy, who completed his dissertation on Flynn Creek in 1964, was the champion of the hypothesis that Flynn Creek was formed by cosmic impact. His 1968 paper on Flynn Creek, included as a chapter in the influential monograph Shock Metamorphism of Natural Materials, reviewed Roddy’s surficial geology and early drilling results. His paper firmly established Flynn Creek as a bona fide impact crater in the days before there was an agreed set of criteria for recognizing impacts. At that time, in the absence of established crater models (simple versus complex), and without the benefit of geophysical data such as a seismic cross-section, Flynn Creek was described as an impact structure with a central uplift. Today, an impact structure with a central uplift is classified as complex. Flynn Creek is not, however, a complex impact structure. This is evident from several lines of evidence, including the remarkable and painstakingly prepared structure contour map of the base of the Chattanooga Shale made by Roddy (1968). This unique map reveals that Flynn Creek appears to be a small, simple crater that is encircled by a wide brim, which includes several slump blocks and radial resurge gullies. Flynn Creek’s deeper structure is revealed by a re-examination of drill cores from about 20 sites within Flynn Creek impact structure. In fact, Flynn Creek, which was an important impact structure in the early history of impact geology research, does not fit well as an example of either a simple or a complex impact structure. Flynn Creek’s annular brim is in essence a miniature version of the Chesapeake Bay impact structure, whose geomorphology is characterized by a nested crater in basement rocks and surrounded by a much wider outer crater rim that expanded through extensive, post-impact collapse of the poorly consolidated upper sedimentary target rocks. This paper reviews the drill core record at Flynn Creek, which reveals much about the architecture of Flynn Creek impact structure.