AGE CONSTRAINTS ON THE FORMATION OF THE DEVONIAN FLYNN CREEK STRUCTURE, TENNESSEE

The Flynn Creek crater in north-central Tennessee was produced by a meteorite that struck a flat lying succession of Ordovician carbonates. The crater is filled by a basal breccia and a thick succession (55 m) of Late Devonian black shales. Lower Frasnian conodonts in shallow water lag deposits that overly the Ordovician succession in the region indicate that the Devonian sea had flooded the area by that point in time. The impact occurred in shallow water and marine sedimentation commenced immediately after settling of impact-related deposits. A specimen of Ancyrodella alata from the initial deposits above the crater fill breccia indicates a lower Frasnian age of MN Zone 3 thru 4 for the oldest post-impact sediments. Specimens of other polygnathid conodonts (Po. pennatus, Po. dubius), as well as the report of Ancyrodella rotundiloba (Huddle, 1963) are in agreement with that assessment.

The post-impact fill of the crater consists of black shales that were long thought to be equivalent to the Late Devonian Chattanooga Shale. Only the upper third of the black shale succession, however, is correlative to the Chattanooga Shale. Most of the black shales in the crater are older, and are separated from the overlying Chattanooga Shale by an erosional truncation. The upper portion of this “lower” black shale package contains conodonts that imply deposition during upper MN 5 to 6. The basal shale package of the overlying Chattanooga Shale (separated by erosional truncation) contains conodonts that indicate deposition during MN 6 to 7.

Based on condont ages and sedimentological constraints it appears therefore that the Flynn Creek impact post-dates the Amönau Event (base of MN Zone 1), and pre-dates the Alamo Impact (MN Zone 5). Thus, within the limitations of biostratigraphic age determination available for the Late Devonian, the Flynn Creek impact can not be linked to other known Devonian impact events.