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TitreAbsolute ages for the Norian stage: a further contribution from southern British Columbia, Canada
AuteurDiakow, L; Orchard, M J; Friedman, R
SourceCordilleran Tectonics Workshop 2012, Program and abstracts; par Pacific Section, Geological Association of Canada; 2012, 2 pages
Séries alt.Secteur des sciences de la Terre, Contribution externe 20120081
RéunionCordilleran Tectonics Workshop 2012; Victoria, BC; CA; février 24-26, 2012
Lat/Long OENS-122.0000 -121.5000 50.5000 50.0000
SujetsTrias supérieur; Norien; datations au uranium-plomb; lithologie; roches volcaniques; tufs volcaniques; rhyolites; roches sédimentaires; grès; calcaires; fossiles; biostratigraphie; zones fossilifères; Groupe de Nicola ; géochronologie; stratigraphie; Trias
Programmebassins sédimentaires du Yukon, GEM : La géocartographie de l'énergie et des minéraux
Résumé(disponible en anglais seulement)
Direct absolute age calibration for the Upper Triassic is largely unavailable and age estimates have relied on magnetostratigraphic correlation between Tethyan marine successions and the Newark basin non-marine succession, and summations of cyclostratigraphic periodicities derived from the latter. On that basis a broad range of ages have been suggested; for example, the base of the Norian at ~216 Ma or ~227 Ma, and that of the Rhaetian at ~204 or ~210 Ma. Rare chronometric ages presently provide a base Carnian age at ~237 Ma, an age of ~231 Ma within the upper Carnian, and a top Rhaetian at ~201 Ma. Upper Triassic strata of the Nicola Group in the Merritt area, 200 km east of Vancouver, form part of the southwestern margin of the Quesnel terrane, a subduction generated magmatic arc that stretches north-northwesterly throughout the B.C. Cordillera. Several U-Pb TIMS dates, which include chemically abraded grains, supported by conodont biostratigraphy obtained from two separate sections of the Nicola Group provide constraints on the lower to middle parts of the Norian. The sections located at Castillion Creek and 9 km farther northeast at Iron Mountain consist of broadly similar strata composed of bedded feldspar-rich siltstone-sandstone, lesser limestone and minor mudstone. Interlayered volcanic rocks with contrasting compositions include mafic lava flows and comparatively thin felsic lapilli-rich and rare ash tuff interbeds. At Castillion Creek a 400 m-long by 12 m-high cut adjacent to the Coquihalla highway exposes massive mafic lava separating parallel-bedded intervals dominated by feldspathic sandstone, calcareous siltstone, siliceous exhalite and limestone. Numerous steeply inclined normal faults and a few south-verging contraction faults cause small offsets. A rare rhyolitic ash-tuff layer, up to 20 cm thick, locally rests on mafic lava and comprises the base for conformably overlying sedimentary strata. This rhyolite bed yields a U-Pb age of 224.47+0.29/-0.64 Ma based on the median of a coherent group of ten 206Pb/238U dates. Limestone that depositionally overlies the rhyolite tuff produced middle Norian conodonts consisting of Epigondolella spiculata and E. tozeri. At Iron Mountain, a U-Pb age of 223.80+/-0.74 Ma, based on the weighted average of four 206Pb/238U dates, was determined from dacitic crystal-ash tuff that is depositionally overlain by an internally conformable 60 m-thick section composed of limestone, lesser dacitic fragmental volcanic and feldspathic sandstone beds. Successive carbonate beds yield the conodonts Epigondolella quadrata and E. triangularis, diagnostic for the early Norian. Waterlain rhyolitic ash forming prominent white bands in siltstone at the top of the measured section, with the potential to bracket the conodont fauna, unfortunately failed to produce zircons. However, the lower Norian ammonoid Wangoceras occurs in sandstone that is stratigraphically beneath this ash-bearing siltstone but overlying the highest conodont collection, the early Norian age of which is corroborated. The association of the two similar U-Pb dates of ~224 Ma with conodonts interpreted as both lower and middle Norian in age is anomalous and demands additional work to be undertaken. The date from Castillion Creek places a maximum age on the spiculata conodont Zone (equivalent to the North American lower Columbianus ammonoid Zone) of the early part of the middle Norian. In this case, the possibility of a non-sequence needs to be addressed. The less precise but similar U-Pb age from Iron Mountain lies beneath conodont fauna of the early Norian triangularis conodont Zone and ammonoids of the middle part of the early Norian Dawsoni Zone, although it may also fall within those zones. Although neither location yet provide tightly bracketed fauna or zircons, these dates appear consistent with an older base-Norian (~227 Ma).