Towards a carbon isotope stratigraphy of the Cambrian System: potential of the Great Basin succession

The Cambrian spans a time of major biosphere change, during which the major invertebrate groups appeared and radiated. Recent discoveries of major carbon isotope excursions in carbonate rocks around the world have stimulated a working group on ‘Late Precambrian-Cambrian Event Stratigraphy’ to explore their potential for correlation (IGCP Project 303). Bioevents, sequence boundaries and trace element markers may also be used for calibration. A major impediment to this objective has been the endemicity and diachroneity of many polymeroid trilobites and early skeletal microfossil taxa associated with shallow water carbonates. The use of deeper-water carbonates, bearing more widely-distributed agnostoids and polymeroids should, however, provide an opportunity to test independently the correlative value of carbon isotope excursions on a global scale. Studies on the Cambrian of the Great Basin, western USA, show that such facies can bear similar δ 13 C signatures to contemporaneous shallow-water carbonates with more endemic trilobites. Hence ‘agnostoid facies’ do have the potential to provide a global and integrated carbon isotope and fossil stratigraphy. A test case is provided by a major positive carbon excursion in the late Dresbachian (or Steptoean) Stage, coincident with shoaling and emergence at the top of the Sauk II sequence across the Laurentian craton. If global, this distinctive marker should be traceable into other Cambrian carbonate successions since it lies between two of the most widely correlatable biohorizons: above the last occurrence of Glyptagnostus reticulatus and below the first appearance of Irvingella spp.