BIOSTRATIGRAPHY, SEA LEVEL CHANGE, AND DISCONFORMITIES IN THE UPPER DEVONIAN CHATTANOOGA SHALE OF WESTERN TENNESSEE BASED ON CONODONTS AND MAGNETIC SUSCEPTIBILITY

A drill core in Humphreys County, western Tennessee on the western side of the Nashville Dome in the Illinois Basin contained 14 m of predominantly organic-rich Chattanooga Shale that lies unconformably on the Middle Devonian Sellersburg (Pegram) Limestone and is overlain by carbonates equivalent to the Sunbury Shale in the Appalachian Basin. Conodonts were recovered from 78 horizons; bulk magnetic susceptibility was measured at 5 cm intervals. Conodonts indicate that the base of the Chattanooga is Frasnian Zone (FZ) 3; three Frasnian conodont zone intervals are discernable. The Frasnian-Famennian boundary is disconformable, characterized by thin sandstones and a lag bed, where crepida Zone strata lie on FZ 13 strata. Famennian strata are divisible into five conodont zone intervals, the Protosalvinia interval is recognized above the first occurrence of Palmatolepis glabra distorta and Palmatolepis grossi, which indicate the utahensis Zone through granulosus Zone; the Devonian-Carboniferous boundary seems to be disconformable, however this interval of the core had poor recovery. A medium gray muddy carbonate interval above the last Famennian fauna yielded a rich siphonodellid fauna typical of the Sunbury Shale and assigned to the sandbergi Zone. Magnetic susceptibility shows six major and 27 minor trends. Abrupt shifts are attributed to eight significant disconformities in the strata which correspond to increases in organic content and decreased MS values over the disconformity interpreted as deepening/flooding events.