Record of the Permo-Triassic Crisis in Oceanic Environments: New Targets in Oman, Nevada, and British Columbia

International audienceData from the great Permo-Triassic biotic crisis are restricted to a limited number of geological sections, oftencharacterized by an unconformity within continental platform successions (Wignall et al.; 1996). In order tosolve this problem (global regression event), the study of oceanic successions emerges as one alternative. Recentattempts confirm this potential: for instance, pelagites of the Mino-Tamba terrane from Japan include theboundary within a 15 m interval of black shales intercalated within radiolarian chert of respectively late Permian(Changxingian) and early Triassic (Dienerian) ages (Isozaki; 1997). This succession has been interpreted asrecording a significant decrease of siliceous bioproductivity in the late Permian, predating a major anoxia eventaround the boundary, followed by progressive recovery of siliceous planktic productivity in the early Triassic(Isozaki, ibid.). Present debate also includes the patterns of selectivity of extinctions and the search for suitablepaleooceanographic causes (Knoll et al.; 1996) as well as mechanisms of survival and recovery specific to eachfaunal group. For planktic and nektonic organisms around the P-T boundary, these mechanisms are not alwayswell documented.In this perspective, we thought of testing the quality of the sedimentary record from various oceanicsuccessions, as well as investigate the paleogeographic range and timing of this anoxia on a global scale. For thispurpose, we selected two domains: the Oman Mountains and the North American Cordillera, identified as twocomplementary areas from two major domains of the world ocean at the end of the Permian: Tethys andPanthalassa. Our multidisciplinary approach is based on: (1) biochronologic, paleobiogeographic andevolutionary study of radiolarian, conodont, ostracod, ammonite and pseudoplanktic bivalve faunas; (2) faciesanalysis and stable isotopes geochemistry. We identified and sampled several Permo-Triassic oceanicsuccessions of the Hawasina series (Oman), the Candelaria flysch and the Black Rock terrane (Nevada), and theCache Creek terrane (British Columbia). Four preliminary observations can be drawn from our first phase offield work: (1) most localities display radiolarian chert as the dominant type strata in the late Permian; (2) upsection, successions grade into "boundary shales" and/or black shales or various thicknesses devoid of apparentunconformities; (3) earliest Triassic successions usually consist of shales and/or platy carbonates; (4) siliceoussedimentation reappears progressively up section. This typology is consistent from one locality to another, alongwith close lithologic resemblance between remote localities, for instance the Hawasina series (Oman) and theBlack Rock terrane (Nevada). Differences of sedimentation rates related to the amount of carbonate input withinstarved basins seems a significant factor for some local variations. Biochronology and geochemistry in progresswill be critical in support of these preliminary correlations.Isozaki, Y., Science, 276, 235-238, (1997).Knoll AH, Bambach RK, Canfield DE & Grotzinger JP, Science, 273, 453-457, (1996).Wignall PB, Kozur, H, & Hallam, A, Historical Biology, 12, 39-62, (1996)