"Ochoan" Quartermaster Formation of North Texas, USA, Part II: Magnetostratigraphy and geochronology

Abstract

As part of an effort to obtain a well-dated record of paleoenvironmental change (specifically in paleotemperatures and paleoatmospheric pCO2) from terrestrial strata of the latest Ochoan Series, spanning the Permian-Triassic boundary (PTB), in the southwestern United States (westernmost Pangea), we are building a detailed magnetostratigraphic record of the Ochoan Series strata represented by the Dewey Lake/Quartermaster formations (DL/QM), in west Texas. These strata are unique in that they are the 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 one of the most profound unresolved questions in earth sciences. Earlier studies showed that the red-beds of the DL/QM fms are high-fidelity recorders of the paleomagnetic field and remarkably resistant to remagnetization. Dual polarity characteristic magnetizations (ChRM) are largely carried by a mix of both specular and pigment hematite, with no discernable directional difference between the carriers, as revealed by chemical demagnetization. For these red beds, hematite cement was early acquired and is an important remanence carrier. We have collected new data from 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 the part of the Country Road 207 section that exposes 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 that the PTB lies 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 have been recognized in the latest Permian, following the termination of the Permo-Carboniferous Reverse Superchron (PCRS), we suspect that the lower DL/QM section will eventually reveal a finer polarity record.