CONODONTS AND CRATER FILL HISTORY OF THE FLYNN CREEK STRUCTURE, UPPER DEVONIAN, CENTRAL TENNESSEE, USA

The Flynn Creek structure in Jackson County, Tennessee, is an Upper Devonian impact feature filled by impact related breccias and successive layers of Frasnian clastics of the Chattanooga Shale. The crater, 3.6 km in diameter and approximately 150 m deep, characterized by steep sides and a central wave beveled uplift, is developed in Ordovician carbonates of the Nashville Group. The crater is filled by basal breccia, bedded breccia interbedded with thin black shales, and a thick black shale succession that predates the Dowelltown Member black shale that typically overlies Ordovician strata in central Tennessee outside the crater.

Conodonts from the Flynn Creek crater fill were described by Huddle (1963) from five samples in the breccias and the basal black shale. Our study includes samples from the breccia and black shale fill, as well as the overlying Dowelltown. The basal breccia and an uppermost graded interval that contains mixed Ordovician and Devonian conodonts, including Yaoxianognathus abruptus, Ancyrodella alata, and Polygnathus pennatus. This is interpreted as the fall back ejecta from a marine impact and implies a MN Zone 3 or 4 time of impact. The bedded breccia contains a similar mixed Ordovician and Devonian conodonts, interpreted as material washed into the crater from the surrounding platform. The black shale interbeds suggest generally offshore quiet water deposition and continuation of Chattanooga deposition after the impact. The black shale crater fill contains only Middle Frasnian conodonts, notably Ancyrodella gigas, Palmatolepis punctata - early form, and several other palmatolepids indicative of MN zones 5 and 6. The black shales that fill the crater are overlain by the Dowelltown Member, marked by a coarse-grained basal lag deposit. At Hurricane Bridge, approximately 20 km south of the crater, the basal Dowelltown contains Palmatolepis bohemica, indicative of MN zones 6 to 8. This indicates that pre-Dowelltown strata filled the crater before further sea level rise and the deposition of Dowelltown black shales that blanket the region.