THE PERMIAN-TRIASSIC TRANSITION IN THE GREAT BASIN (CONFUSION RANGE-SPRUCE MOUNTAIN COMPOSITE): AGE CONSTRAINTS BASED ON A CONTINUOUS CARBON ISOTOPE RECORD, STRONTIUM ISOTOPES AND SEQUENCE STRATIGRAPHY

Causes of the end-Permian extinction and links to Siberian Trap volcanism remain controversial, in part because the timing of events globally is still being investigated. Although complete Permian-Triassic boundary sections are known from the Tethys, few continuous Panthalassan sections are documented. In the Confusion Range, UT, subaerial exposure is thought to have caused a Permian-Triassic hiatus between the Gerster and Thaynes Formations. Several authors have questioned this, and evidence for erosion was found to be equivocal. Fossils diagnostic of the end Permian and earliest Triassic are absent. Therefore, high-resolution δ¹³C and ⁸⁷Sr/⁸⁶Sr chemostratigraphy and sequence stratigraphy are the only currently available means to test the hypothesis.

The top of the Gerster Formation in the Confusion Range is a fossilferous packstone. δ¹³C in the Gerster is stable between 2-3‰ and corresponding ⁸⁷Sr/⁸⁶Sr of 0.7070 to 0.7072 are consistent with a late Permian age. An influx of quartz sand marks a sequence boundary ~5 meters below the top of the Gerster. Overlying this regressive event is a prominent chert bed in which δ¹³C continuously falls from 2 to 0‰. ⁸⁷Sr/⁸⁶Sr rises from 0.7073 to 0.7075, consistent with a latest Permian/earliest Triassic age. A fenestral limestone overlies the chert bed. δ¹³C continues to fall from 0 to -2‰, consistent with an earliest Triassic age. Sr isotope values of this unit are very radiogenic and likely altered. The final unit is a fossiliferous packstone, in which δ¹³C remains highly negative (-2 to -3‰). Sr values increase from 0.7076 to 0.7081 at the base of the Thaynes, consistent with a Smithian age at the top of the unit.

Continuity of the δ¹³C trend through the Gerster and fenestral interval is inconsistent with a major late Permian to early Triassic hiatus. While we consider the carbon primary, published Permian-Triassic boundary Sr values are as low as 0.7071 to 0.7073. Therefore, it is possible that the trend in seawater Sr is preserved in our section but, because Sr concentrations are low, isotopic values have become more radiogenic due to diagenesis. Early Triassic seawater Sr rise is not expected during weathering of low ⁸⁷Sr/⁸⁶Sr flood basalts. Sources of radiogenic Sr include weathering of silicates and Paleozoic carbonate at a time when seawater Sr was near a Phanerozoic minimum.