INTEGRATING TRILOBITE BIOSTRATIGRAPHY AND CARBON ISOTOPE STRATIGRAPHY OF THE CAMBRIAN (PAIBIAN; STEPTOEAN) SPICE EVENT IN CENTRAL TEXAS AND WESTERN UTAH

The Steptoean Positive Carbon Isotope Excursion (SPICE) has been identified worldwide and is a significant feature for global correlation. Although the SPICE has been documented from several localities in Laurentian North America, biostratigraphic characterization has relied on paleontological work done decades earlier. A new program of sampling in Texas and Utah allowed us to analyze trilobite-bearing collections to provide direct links between the faunal succession and carbon isotope stratigraphy. The Steptoean (Paibian) Riley Formation of Texas is strongly condensed. Increasingly positive values of the rising limb of the SPICE are recorded in the skeletal carbonates of the Cap Mountain Member. Peak positive values reaching 2.5–2.99‰ (VPDB) occur in correlatives of the Dunderbergia Zone in the overlying lowstand sandstone succession of the Lion Mountain Member. The upper portion of the SPICE is not preserved due to the absence of skeletal carbonate lags in trough cross-bedding towards the top of the member. The Candland Shale Member of the Orr Formation at its type locality at Orr Ridge, Utah, is broadly correlative with the Texas succession. Our data from trilobite-rich skeletal carbonates forms a curve that closely matches one published by Saltzman et al. (1998) from the same locality. A steady rise to about 2.5‰ occurs in the Aphelaspis Zone and stabilizes in the Dicanthopyge and lower Prehousia zones. Values then rise rapidly in the upper Prehousia Zone, peaking between 4.5–5‰ in the overlying Dunderbergia Zone. The base of the lowstand succession in Texas correlates into the upper Candland Shale, an interval that has been variously interpreted by previous workers as either transgressive or regressive. The extinction interval at the base of the Steptoean Stage is exposed at both sections. The onset of extinctions (base of the Coosella perplexa Zone) is characterized by an unremarkable shift in the carbon isotope curve of no more than about 0.5‰. This suggests that, although physical environmental change might possibly act as a trigger for the extinction, there may be a significant role for ecological impact of biological factors, such as immigration of invasive species.