LATEST GUADALUPIAN CARBON ISOTOPE RECORD FROM THE REEF TRAIL MEMBER OF THE BELL CANYON FORMATION, WEST TEXAS, USA

A large negative isotope shift in ∂¹³C from whole rock inorganic carbon is observed near the top of the Reef Trail Member of the Bell Canyon Formation in the Patterson Hills, west Texas. Biostratigraphically the shift begins in the upper part of the Jinogondolella altudaensis Zone, above the first appearance of the radiolarian Albaillella yamakitai, and appears to correlate with the start of a globally recognized negative excursion associated with the end-Guadalupian mass extinction. The carbonate-dominated Reef Trail Member. is the youngest unit in the basinal Bell Canyon Formation and is overlain by evaporitic strata of the Castile Formation. Samples were taken from micritic beds at intervals of 0.5 m or less from the uppermost 10 meters at the SC1 section, and also from the uppermost 14 meters at the SBR section roughly 1 km away. In addition, coarser calcarenite beds were sampled from the entire SC1 section. A baseline shift of ~-4.5‰ vPDB (from ~ 6‰ to 1.5‰) occurs in the SC1 section and -5‰ (from ~6‰ to 1‰) in the SBR section. Both fine and coarse grained lithologies show comparable shifts. The excursion coincides with a generally upward coarsening shift to more bioclastic beds. Radiolarians occur below and above the shift, and show a change in the relative abundance of taxa, further indicating that this shift records ecologic/environmental variation, and is not a diagenetic artifact. The fusulinaceans Paraboultonia splendens and Codonofusiella (Lantschichites) altudaensis (sensu Wilde and Rudine) are present at several levels in the SC1 section and persist to within 0.3 meter of the Castile/Reef Trail contact. The isotopic shift continues through unit 14 in both sections and into the Castile Formation. The introduction of the conodonts Jinogondolella granti, J. crofti, and Clarkinia postbitteri hongshuiensis in unit 14 tie the Patterson Hill sections with those throughout West Texas and to Penglaitan, China, and equate to Bed 6i in that section, just below the Guadalupian-Lopingian (Middle-Upper Permian) boundary definition.