EVIDENCE FOR A CONTINUOUS PERMIAN-TRIASSIC BOUNDARY SECTION AND LOWER TRIASSIC TERRESTRIAL STRATA IN NORTH-CENTRAL TEXAS

The Whitehorse Group and Quartermaster Formation are extensive red-bed terrestrial units representing the final episode of sedimentation in the Palo Duro Basin, north-central Texas, U.S.A. The Whitehorse Group comprises beds of rare dolomite, laminated to massive gypsum, red mudstones, and abundant laminated to massive red quartz siltstone to fine sandstone. The Quartermaster Formation undergoes a change from nearly equal amounts of thin plane- and lenticular fine sandstone and laminated to massive mudstone in its lower half to overlying strata with coarser-grained cross-bedded sandstones indicative of meandering channels up to 7 m deep and rare overbank mudstones. Paleosols are absent in the Whitehorse Group and only poorly developed in the Quartermaster Formation. Rare volcanic ash layers present in both formations permit correlation among four stratigraphic sections distributed over an ~150 km north-to-south transect.

Both the Whitehorse Group and Quartermaster Formation have traditionally been assigned to the Upper Permian Ochoan stage. However, new stable carbon isotope chemostratigraphic data from early dolomite cements, in conjunction with chrono- and magnetostratigraphic evidence, indicate that the Permian-Triassic boundary and ~5 million years of the Early Triassic are preserved in the terrestrial stratigraphic sequence of the Palo Duro Basin. The Permian-Triassic boundary is denoted by a ~7‰ negative carbon isotope shift and long-term excursion record the end-Permian extinction and Early Triassic biological recovery, respectively. Furthermore, single-crystal zircon U-Pb ID-TIMS and sanidine and biotite ⁴⁰Ar/³⁹Ar analyses from ashes in the lower Quartermaster Formation yield depositional ages as young as 252.2±0.6 and 250.0±0.6 Ma, respectively, corroborating interpretations of the chemostratigraphic and magnetostratigraphic data.