Analysis of a drill core from the central uplift of Flynn Creek impact structure, Tennessee

Abstract

Flynn Creek impact structure is a small, asymmetric marine impact structure located in north-central Tennessee that formed when a hypervelocity impact occurred in an ancient shallow marine environment, with the target strata ranging from Lower to Upper Ordovician carbonates. Like other, similarly sized marine-target impact craters, Flynn Creek’s structure-filling deposits consist of gravity-driven avalanche material, washed-back ejecta, and aqueous settling deposits. Sedimentological and petrographic analysis of drill core FC77-1, located on the western flank of the central uplift, led to the distinction of three sedimentological units, a generally fining-upward sequence from 109 to 32 m depth bounded by generally coarsening-upward sequences, within the upper 175-m interval of the core. Line-logging and thin-section analysis of selected drill core samples also show that the Flynn Creek impact breccia consists almost entirely of dolostone clasts (90%), with minor components of cryptocrystalline melt clasts, chert and shale fragments, and clastic grains. Cryptocrystalline melt clasts, the first melt clasts of any kind to be reported from Flynn Creek impact structure, appear isotropic in thin section, are in fact made of exceedingly fine quartz crystals. Resurge gullies and an ‘inverted sombrero,’ an annular, sloping surface, are two geomorphic features sometimes referred to in studies of other marine-target impact structures, but have yet to be described at Flynn Creek. In this report, these features are related to marine crater formation and Flynn Creek is compared to similarly sized marine-target impact structures.