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

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 Sr-87/Sr-86 measurements of well-preserved carbonate shell material from Nevada to the well-known and biostratigraphically constrained Sr-87/Sr-86 record from Tethys, where a negative excursion in Sr-87/Sr-86 is noted across the Norian-Rhaetian boundary. Our new Sr-87/Sr-86 data from the Luning and Gabbs formations reveal a comparable trend, with a sharp drop in Sr-87/Sr-86 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.