Strontium isotope stratigraphy of the Gabbs Formation (Nevada): implications for global Norian–Rhaetian correlations and faunal turnover

Abstract

The Luning and Gabbs formations in west-central Nevada, USA represent a Late Triassic shallow marine sedimentary succession with global significance (the Gabbs Formation was a candidate for the basal Jurassic GSSP). Typically, the Norian–Rhaetian stage boundary is placed at the contact between the formations, and the Rhaetian–Hettangian boundary (the Triassic–Jurassic boundary) is within the Muller Canyon Member of the Gabbs Formation. However, the use of different biostratigraphical index-species schemes in Norian–Rhaetian successions between Tethys and Panthalassa, the two largest ocean basins at the time, makes precise correlation problematic. Here, we compare 87Sr/86Sr measurements of well-preserved carbonate shell material from Nevada to the well-known and biostratigraphically constrained 87Sr/86Sr record from Tethys, where a negative excursion in 87Sr/86Sr is noted across the Norian–Rhaetian boundary. Our new 87Sr/86Sr data from the Luning and Gabbs formations reveal a comparable trend, with a sharp drop in 87Sr/86Sr within the Nun Mine Member of the Gabbs Formation, suggesting the position of the Norian–Rhaetian boundary is higher in the succession, and not between the Luning and Gabbs formations as previously defined. Relating the stage boundary using global isotopic signals is a useful tool for biostratigraphical correlation of successions between Tethys and Panthalassa, and for estimating the rate of faunal turnover at the Norian–Rhaetian stage boundary in comparison with the succeeding Late Triassic mass extinction. If correct, this biostratigraphical–chemostratigraphical correlation suggests that the current index groups for the Panthalassic stage boundary should be changed.