Carbon isotope stratigraphy of Upper Cambrian (Steptoean Stage) sequences of the eastern Great Basin: Record of a global oceanographic event

Abstract

A large, global positive excursion in δ 13 C (from ∼0.5 to 4.5‰) during the Late Cambrian Pterocephaliid biomere/Steptoean Stage (Aphelaspis-Elvinia zones) is documented at high stratigraphic resolution in three sections in the eastern Great Basin. The excursion, which we refer to as the Steptoean Positive Carbon Isotope Excursion, or SPICE excursion, began coincident with a world-wide extinction event. The δ 13 C data from the Great Basin reflect global seawater values in a wide range of lithologies, including oolitic grainstones, wackestones, thrombolitic boundstones, and flat-pebble conglomerates. We use a section at Shingle Pass in the southern Egan Range to divide the Pterocephaliid biomere into eight isotope steps that represent equal divisions of the δ 13 C rise and fall (±4‰). This provides a basis for recognition of a revised chronostratigraphic framework for the Pterocephaliid biomere/Steptoean Stage. Strata deposited during the beginning of the SPICE excursion record a major change in the pattern of sedimentation in the eastern Great Basin. This is reflected in a siliciclastic-carbonate transition at Shingle Pass, Nevada, and a carbonate-siliciclastic transition at the House Range and Lawson Cove sections in Utah. A regional siliciclastic influx recognized throughout the Great Basin occurs near the peak of the SPICE excursion. Carbon isotope analyses from cratonal sections in Wyoming provide independent evidence that a major sedimentary hiatus took place on the craton during the time of the SPICE excursion. The correlated changes in δ 13 C, relative sea level, and the marine biota during the SPICE excursion provide remarkably detailed records of a major paleoceanographic event. We speculate that changes in sea level, climate, or tectonics may have triggered the SPICE excursion and coeval extinction event. Subsequent burial of organic carbon caused the increase in δ 13 C and may have led to an interval of global cooling. The results of this study lend confidence to carbon-isotopic studies of pre-Mesozoic rocks.