Paper No. 5
Presentation Time: 9:05 AM
“OCHOAN” QUARTERMASTER FORMATION OF NORTH TEXAS, USA, PART I: LITHO- AND CHEMOSTRATIGRAPHY
TABOR, Neil J.1, MYERS, Timothy S.
2, MACK, Greg H.
3, LOOY, Cindy V.
4, RENNE, Paul R.
5, MUNDIL, Roland
5 and GEISSMAN, J.W.
6, (1)Roy M. Huffington Department of Earth Sciences, Southern Methodist University, Dallas, TX 75275, (2)Roy M. Huffington Department of Earth Sciences, Southern Methodist University, Dallas, TX 75275-0395, (3)Geological Sciences, New Mexico State University, P.O. Box 30001/MSC 3AB, Las Cruces, NM 88003, (4)Integrative Biology & University of California Museum of Paleontology, University of California, Berkeley, 1005 Valley Life Science Building #3140, Berkeley, CA 94720, (5)Berkeley Geochronology Ctr, 2455 Ridge Rd, Berkeley, CA 94709-1211, (6)Department of Geosciences, University of Texas at Dallas, 800 West Campbell Road, Richardson, TX 75080, ntabor@smu.edu
The Alibates and Quartermaster are extensive red-bed formations in Palo Duro basin, U.S.A. The marine Alibates Fm is composed of beds of rare dolomite, laminated to massive gypsum and red mudstones, and abundant laminated to massive red quartz siltstone to fine sandstone. The Quartermaster Fm is terrestrial and up to 70 m thick; it undergoes a change from nearly equal amounts of thin laminated and ripple cross-laminated red to green fine sandstone and laminated to massive red mudstone in the lower half of the formation to fine to medium cross-bedded sandstones that define meandering channels up to 7 meters in depth with rare overbank red mudstones. Paleosols are absent in the Alibates fm and poorly developed vertic and calcic profiles occur in the Quartermaster Fm. Locally well exposed volcanic ash layers in the Alibates and Quartermaster Fms permit correlation among four sections distributed over ~150 km.
Both the Alibates and Quartermaster Fms are assigned to the Upper Permian Ochoan stage, and a major disconformity separates these strata from younger Triassic rocks such that the Permian-Triassic boundary (PTB) was assumed absent. Single-crystal zircon U-Pb ID-TIMS and sanidine and biotite 40Ar/39Ar analyses from ashes in the lower Quartermaster Fm yield depositional ages as young as 252.2±0.6 and 252.3±0.6 Ma, respectively, suggesting that the PTB is likely present in the Quartermaster fm. Carbon-isotope stratigraphy exhibits a consistent pattern among the sections. Early dolomite cements record a negative δ13C shift of 4 to 7‰ within a 6 to 12 m interval and three of four sections record multiple-negative δ13C shifts. Organic matter records an abrupt δ13C shift of -4‰ and a stratigraphically long (~40 m) +4‰ trend back toward more positive values. These patterns derived from continental strata correspond closely with marine PTB chemostragraphies from Tethyan localities. Documentation of a large negative δ13C shift from dolomite cements and organic matter, in conjunction with new geochronology, offers persuasive evidence for the preservation of a continental PTB, as well as lowermost Triassic Induan strata, in the Palo Duro basin. Dolomite δ18O values record an abrupt ~-4‰ shift through the suspected PTB interval, corresponding possibly to a substantial (>10°C) rise in surface temperatures.
