Trilobite extinctions, facies changes and the ROECE carbon isotope excursion at the Cambrian Series 2-3 boundary, Great Basin, western USA

The mass extinction of the olenellid trilobites occurred around the Cambrian Series 2-Series 3 boundary. Like many other crises, it coincided with a negative carbon isotope excursion but the associated palaeoenvironmental changes remain unclear. To investigate the causal mechanism for this event, we report facies changes, pyrite framboid petrography and carbon isotope values from Cambrian Series 2-Series 3 (traditionally Early-Middle Cambrian) boundary strata of the Carrara Formation (Death Valley region, California) and Pioche Formation (Nevada). These data reveal regionally changing water depths from high-energy, nearshore fades (oolitic grainstone) to more offshore silty marl and finer-grained carbonate mudstone. In the Carrara Formation, the series boundary occurs within a deepening succession, transitioning from high-energy, nearshore facies (oolitic grainstone and oncolitic packstone) to offshore marl, the latter of which contains pyrite framboid populations indicative of low-oxygen (dysoxic) depositional conditions. Intermittent dysoxia persisted below sub-wave base settings throughout the early and middle Cambrian, but did not intensify at the time of extinction, arguing against anoxia as a primary cause in the olenellid trilobite extinction. Within both field areas, the extinction interval coincided with a minimum in delta C-13(carb) values, which we interpret as the regional manifestation of the Redlichiid-Olenellid Extinction Carbon isotope Excursion (ROECE). The Series 2-Series 3 boundary is reported to closely coincide with a large-amplitude sea-level fall that produced the Sauk I/II sequence boundary, but the placement of the Series 2-Series 3 boundary within a transgressive interval of the Carrara Formation shows that this is not the case. The main sequence boundary in the succession occurs much lower in the succession (at the top of the Zabriskie Quartzite) and therefore precedes the extinction of the olenellids and ROECE. (C) 2017 Elsevier B.V. All rights reserved.