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1 2006

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Early Hettangian ammonites and radiolarians from the Queen Charlotte Islands (British Columbia, Canada) and their bearing on the definition of the Triassic/Jurassic boundary

Longridge L., Carter E., Smith P., Tipper H.

Volumina Jurassica

2006

Vol. 4, no. 1

290-290

The extinction that marks the Triassic/Jurassic boundary (T/J boundary) is one of the so-called "big five" that punctuate the Phanerozoic. Two sequences spanning the boundary occur in the Queen Charlotte Islands (QCI) of British Columbia; one is located at Kennecott Point on Graham Island (section I), the other on the southeastern shore of Kunga Island (section III). A second section at Kennecott Point contains fossils of Early Hettangian age only (section II). Eleven ammonite taxa are present in the Lower Hettangian sections of the QCI. Ammonite faunas indicate an Early Hettangian age for the upper portion of section I and permit correlation of the upper portion of section II with the middle Lower Hettangian Minutum Zone. Section III contains moderately diverse Early Hettangian ammonites that allow correlation of the lower portion of the section with the Minutum and Pacificum zones and the upper portion with the upper Lower Hettangian Polymorphum Zone. Lower Hettangian radiolarians from the Canoptum merum Zone are present in all three sections; a few Upper Rhaetian holdovers from the Globolaxtorum tozeri Zone are also present in sections I and II. The T/J boundary radiolarian faunas correlate closely with those in the Inuyama area of Japan indicating that radiolarians were globally distributed at that time. Although ammonite preservation is poor, radiolarian preservation is excellent and the turnover combined with continuous deposition and lack of facies changes over this interval marks the most distinct boundary level currently recognized worldwide. Section I has also produced a carbon isotope curve which records a prolonged negative excursion spanning the T/J boundary while section III has provided a U-Pb date of 199.6±0.3 Ma which constrains the boundary. The latter section has already been proposed as a potential basal Hettangian Global Stratotype Section and Point (GSSP). Although all three QCI sections lack ammonites from the basal Hettangian Spelae ammonite Zone, sections I and III have both yielded earliest Hettangian radiolarian collections which contain Rhaetian holdovers suggesting an approximate correlation of the lower portion of these sections with the Spelae Zone. A section in New York Canyon, Nevada has also been proposed as a potential GSSP. This section provides a virtually complete ammonoid succession but lacks geochronology and microfossils. In essence, the sections at Kunga Island and New York Canyon are complementary. Close correlations between the two sections are possible using ammonite faunas as well as the negative carbon isotope anomalies which span the T/J boundary in Nevada and at Kennecott Point. We suggest the section from Nevada be designated as holostratotype (and datum) for the basal Hettangian and the QCI section be designated as a parastratotype to improve recognition of this interval.