The Permian/Triassic Boundary in West-Texas (USA): What the Terrestrial World Was Like in Western Equatorial Pangea
Abstract
The Alibates and Quartermaster are extensive red-bed formations in Palo Duro basin, U.S.A. The 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 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 have previously been 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 volcanic ashes in the upper Alibates to lower Quartermaster Fm, reported by the Berkeley Geochronology Center group, yield depositional ages ranging from 251.81±0.20 and 251.54±0.16 Ma, respectively, with detrital zircons in the uppermost Quartermaster Fm indicating depositional ages of <245 Ma. This collectively suggests that the PTB is present in the Alibates Fm or the lower part of the Quartermaster Fm, and that the remainder is mostly earliest Triassic. Carbon-isotope stratigraphy exhibits a consistent pattern among the sections with a negative δ13C shift of > 7‰, likely representing the Permian extinction event, within a ~3 m interval in the upper part of the Alibates Fm. The paleomagnetic signature of these rocks is excellent, such that a magnetic polarity stratigraphy record is well-defined and unambiguous. All of the volcanic ashes lie within strata that define a normal polarity magnetozone, and in several sections we can identify the upper and lower boundaries of this magnetozone, which likely correlates with the normal polarity chron within which the the records of the Permian-Triassic boundary extinction events have been identified in the marine realm. Documentation of a large negative δ13C shift from dolomite cements and organic matter, in conjunction with the numerical age information, 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.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMEP069..11T
- Keywords:
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- 3344 Paleoclimatology;
- ATMOSPHERIC PROCESSES;
- 1030 Geochemical cycles;
- GEOCHEMISTRY;
- 1622 Earth system modeling;
- GLOBAL CHANGE;
- 4912 Biogeochemical cycles;
- processes;
- and modeling;
- PALEOCEANOGRAPHY