INTEGRATED MAGNETOSTRATIGRAPHY AND GEOCHRONOLOGY OF THE "OCHOAN" QUARTERMASTER FORMATION OF NORTH TEXAS

To obtain a well-dated record of paleoenvironmental change preserved in terrestrial strata of the latest Ochoan Series, which spans the Permian-Triassic boundary (PTB), in the SW United States (westernmost Pangea), we are building a detailed magnetostratigraphic and geochronologic record of the Ochoan Series strata represented by the Dewey Lake/Quartermaster formations (DL/QM), in west Texas. These strata arethe only Upper Permian sedimentary rocks from western Pangea containing volcanic ash fall deposits. They offer a unique and critical paleogeographic vantage on events across the PTB, which encompassed the most severe biotic crisis known since the emergence of metazoan life on Earth and whose cause(s) remain an unresolved question. Earlier studies showed that the red beds of the DL/QM fms are high-fidelity paleofield recorders and are remarkably resistant to remagnetization. Dual polarity characteristic magnetizations (ChRM) are carried by a mix of both specular and pigment hematite, with no discernable directional difference between carriers, as revealed by chemical demagnetization. For these red beds, hematite cement was early acquired. We collected ten sites over a 3 meter interval below and above the lowest ash in the DL/QM exposed at the South Prong, Caprock Canyon, and 14 sites through part of the Country Road 207 section with a probably correlative ash. Polarity data show that the DL/QM section immediately above and below these two ash occurrences is entirely normal polarity, consistent with previous work suggesting the PTB within a normal polarity chron. The interval containing another ash about 20 m above the lower ash at Caprock Canyon, is of reverse polarity. Ar/Ar and U-Pb data for these ashes are consistent with this placement for the PTB, i.e. within error of the U-Pb age of 252.4 Ma for the GSSP at Meishan, China. Given that some 14 normal polarity chrons are recognized in the latest Permian, following the termination of the Permo-Carboniferous Reverse Superchron, we suspect that the lower DL/QM section will eventually reveal a finer polarity record. The available magnetostratigraphic data across the PTB from studies at additional sections throughout the world are summarized and reveal the need for further refinement of the polarity time scale during this important interval in earth history.