Conodont biostratigraphy and magnetic susceptibility of Upper Devonian Chattanooga Shale, eastern United States: Evidence for episodic deposition and disconformities

摘要

Recognition of stratigraphic hiatuses in fine-grained siliciclastic sedimentary rocks can be challenging but is feasible using high-resolution biostratigraphic and chemostratigraphic data within a regional correlation framework. In this case study of the Upper Devonian Chattanooga Shale in the Dupont GHS drillcore from the western margin of the Nashville Dome, the Upper Devonian Chattanooga Shale comprises several depositional units separated by intraformational hiatuses. These features are developed within a 13.3 m interval consisting of 4.2 m of Frasnian strata and 9.1 m of Famennian strata that unconformably overlie the Sellersburg Formation. Three Frasnian, seven Famennian, and one Toumaisian conodont biozones are recognized. The Frasnian-Famennian boundary is on a disconformity. To the northeast in the southern Illinois Basin, the New Albany Shale in the BCC drillcore consists of 35 m of Givetian and Frasnian strata and 30 m of Famennian strata that conformably overlie the Sellersburg Formation. One Givetian, four Frasnian, and three of the four lowest Famennian conodont zones are recognized. The Frasnian-Famennian boundary is conformable and constrained to within a 5 cm interval. Bulk magnetic susceptibility (MS) shows a long-term increase through most of the Frasnian. Shorter-term MS trends were observed in association with depositional pulses linked to global sea-level rises and highstand system tracts, characterized by total organic carbon (TOC) maxima - eight trends were resolved in the DGHS core, and six in the BCC. The high-frequency shifts in delta MS likely represent Milankovitch-band sea-level and depositional cycles at a scale that cannot be resolved based on the condensed and irregular nature of the depositional packages. Gamma ray counts in the DGHS peak in association with the eight depositional cycles, with a TOC peak at the base of each cycle.