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Direct absolute age calibration for the Upper Triassic is largely unavailab...
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 nonmarine succession, and summations of cyclostratigraphic periodicities derived from the latter. On that basis, a broad range of ages have been suggested e.g. 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 ~236 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 northwesterly throughout the British Columbia Cordillera. Two sections - Castillion Creek and (9 km to the northeast) Iron Mountain - consist of broadly similar strata of bedded, feldspar-rich siltstone-sandstone, minor mudstone, and limestone, and interlayered volcanic rocks of contrasting compositions including mafic lava flows and comparatively thin felsic ash-lapilli interbeds. At Castillion Creek highway exposure, 400+ m of calcareous siltstones and scarce impure carbonates alternate with feldspathic fine clastic beds, siliceous exhalites, and primarily mafic lava flows. Numerous steeply inclined normal faults and a few south-verging contraction faults have produced small offsets in the parallel-layered rocks. A rare rhyolitic ash-tuff layer, locally up to 20 cm thick, marks the base of conformably overlying sedimentary strata and yielded a robust U-Pb age of 224.52 ± 0.22 Ma. Limestone that depositionally overlies this tuff produced an early middle Norian conodont assemblage consisting of Epigondolella spiculata and E. tozeri. At Iron Mountain, a preliminary U-Pb age of 223.81 ± 0.78 Ma was determined from dacitic crystal-ash tuff overlain by an internally conformable 60 m section composed of limestone, lesser dacitic fragmental volcanic, and feldspar-bearing sandstone beds. Carbonate beds yield the conodonts Epigondolella quadrata and E. triangularis, diagnostic for the upper lower 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. Together, the two U-Pb dates imply that the lower-middle Norian boundary interval lies at ~224 Ma. The more robust one from Castillion Creek places a maximum age on the spiculata conodont Zone (equivalent to lower Columbianus ammonoid Zone in northeastern British Columbia) of the early middle Norian, although biogeographic and stratigraphic arguments may used to suggest that the zircons date a slightly older interval. The less robust but similar U-Pb age from Iron Mountain lies beneath fauna of the upper lower Norian triangularis conodont Zone (equivalent to the Dawsoni-Magnus ammonoid zones), but may fall within that zone. Although neither location yet provide tightly bracketed fauna or zircons, these dates appear consistent with an older base-Norian (~227 Ma) and imply the latest estimates of the L-M Norian boundary at ~216 Ma are too young.
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