Evidence of a Continuous Continental Permian-Triassic Boundary Section in western Equatorial Pangea, Palo Duro Basin, Northwest Texas, U.S.A.

The Whitehorse Group and Quartermaster Formation are extensive red-bed terrestrial sequences representing the final episode of sedimentation in Palo Duro Basin north-central Texas, U.S.A. Regionally, these strata record culmination a long-term regression sequence beginning middle to late Permian. includes beds abundant laminated massive red quartz siltstone fine sandstone rare dolomite, gypsum, claystones, as well diagenetic gypsum. exhibits change from nearly equal amounts thin planar lenticular mudstone its lower half overlying with coarser-grained, cross-bedded sandstones indicative meandering channels up 7 m deep overbank mudstones. Paleosols absent Upper only poorly developed Formation. Volcanic ash-fall deposits (tuffs) present uppermost permit correlation among five stratigraphic sections distributed over ∼150 km provide geochronologic age information for rocks. Both have traditionally been assigned Permian Ochoan (Changhsingian) stage, workers assumed that Permian-Triassic boundary is characterized by regionally significant unconformity. Chemostratigraphic or biostratigraphic evidence this assignment, however, lacking date. Single zircon U-Pb CA-TIMS analyses at least two distinct volcanic ash fall layers Formation, which were identified collected different localities across Basin, yield interpreted depositional ages ranging 252.19 ± 0.30 251.74 0.28 Ma. detrital zircons located few meters beneath top range dates Mesoproterozoic (1418 Ma) Middle Triassic (244.5 Ma; Anisian), latter maximum age, no older than Anisian, thus indicating lie somewhere within Formation/upper succession. Stable carbon isotope data 180 samples early-burial dolomicrite cements preserve chemostratigraphic signal similar sections, large ∼−8‰ negative excursion ∼20 Group-Quartermaster boundary. This be same associated end-Permian extinction concert new high precision radioisotopic presented fact lies normal polarity magnetozone. Dolomite cement δ 13 C values remain less (between about −5 −8 permil) into part before becoming more positive toward section. long interval represent earliest (Induan) inception biotic ecosystem “recovery.” Oxygen show progressive trend through interval, suggesting substantially warmer conditions around event cooler after Triassic. Our observations on paleoenvironment paleoclimate western, sub-equatorial Pangea was systems not conducive plant preservation.