A REFINED MAGNETOSTRATIGRAPHY OF UPPER PERMIAN AND LOWER TRIASSIC STRATA ACROSS THE PERMIAN-TRIASSIC MASS EXTINCTION EVENT

The Whitehorse Group (Alibates Formation in the subsurface in Texas or the Rustler Formation in eastern New Mexico) of the upper Ochoan Series in western Texas is a continental red bed sequence exposed in the Palo Duro Basin in the Texas Panhandle and surrounding region. Principal rock types include hematitic mudstone, siltstone, and fine sandstone. The group was deposited in fluvial and lacustrine environments and has essentially no fossil record. Sandstone beds include cross bedding indicating meandering channels, while mudstones are overbank deposits. Bedded gypsum, dolomite, and shale are also present and rare volcanic ash layers allow correlation among sections and high precision age data. The Permian-Triassic Boundary (PTB) was once considered to be represented in west Texas and eastern New Mexico as a major disconformity. Recent work in the Whitehorse Group and overlying Quartermaster Fm strata has shown that a negative stable carbon isotope anomaly correlated to the PTB mass extinction event is recorded in upper Whitehorse Group strata, where ash beds yield data demonstrating a ca. 251.9 Ma age of deposition. These results indicate that the sedimentary record here is continuous across the PTB and also mark this section as a rare and important terrestrial record with the potential to be correlated with known marine records of the PTB that often provide a poor picture of the paleoenvironment. To establish a more robust magnetostratigraphy below and through the PTB interval, we have carried out high resolution sampling of key exposures of the uppermost Whitehorse Group in Palo Duro State Park and Caprock Canyons State Park, Texas. Sampling interval was dictated by competent material and ranges from a few tens of centimeters to a few meters. Thermal demagnetization indicates a characteristic remanence that is northwest with shallow positive inclination (≈ Dg=340° Ig=+30°) confirming that this negative carbon isotope anomaly, and therefore the PTB event, are within a normal polarity chron. All specimens were completely unblocked by 684° C with little or no decrease before 600° C indicating hematite carries the ChRM. This study further defines the chronology surrounding the PTB and contributes to an ongoing effort to understand the paleoenvironment and geologic conditions during the PTB environmental crisis.