Paper No. 7
Presentation Time: 9:30 AM
MAGNETIC POLARITY STRATIGRAPHY OF THE QUARTERMASTER (DEWEY LAKE) FORMATION (UPPER PERMIAN-LOWERMOST TRIASSIC), PALO DURO BASIN, NORTHWEST TEXAS, USA
Upper Permian to lowermost Triassic hematite-cemented detrital sedimentary rocks, which include a small number of regionally extensive ash beds, in west Texas were deposited during the time interval of one of the greatest mass extinction event sequences in Earth history. The magnetic polarity stratigraphy , as well as key rock magnetic properties, of the Quartermaster (QM) (=Dewey Lake) Formation in the Palo Duro Basin, northwest Texas have been determined using thermal, alternating field, and chemical demagnetization methods, anisotropy of magnetic susceptibility (AMS), acquisition of isothermal remanent magnetization (IRM) and backfield demagnetization, and thermal demagnetization of three component IRM. We sampled five sections in a general NNW-SSE transect across the basin starting in the north at Palo Duro Canyon State Park. Our collection includes 490 samples as drilled cores and 81 as block samples. Demagnetization results show that the QM contains a primary/near-primary latest Permian characteristic remanent magnetization (ChRM) at each level sampled and thus the magnetic polarity stratigraphy for each section can be compared with existing polarity time scales across the Permian-Triassic boundary (PTB). The magnetostratigraphy provides the basis for chemostratigraphic correlations across the basin and integration with work to obtain high precision geochronologic information on all of the ash beds exposed in the QM. Data from the sampled sections show 4 magnetozones (R-N-R-N). AMS data show that these rocks retain a strong depositional fabric. The principal magnetic carrier of the ChRM is hematite, and the onset of laboratory unblocking temperatures varied for the ChRM in different localities between the NRM and 550°C; completely unblocking at or slightly above 683°C. Chemical demagnetization results in the removal of over 60-90 percent of the NRM in a total of 97-144 hours of leaching, demonstrating the importance of pigment hematite to the ChRM. The grand mean paleomagnetic direction for our collection is Decl.= 333.2, Incl.= 20.1, a95= 3.3, k= 7.9, N= 42 sites and results in a paleomagnetic pole for the latest Permian for North America of 55.8, 131.0. This estimate accounts for the effects of inclination shallowing due to sediment compaction after deposition and during diagenesis.