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

Abstract

The Triassic–Jurassic boundary (T–J boundary) interval is one of the most important but poorly understood intervals in the Phanerozoic. Here we fully document the ammonite and radiolarian successions at several sites in the Queen Charlotte Islands (QCI), British Columbia. Eleven ammonite taxa are described and eighteen radiolarian taxa are discussed with the radiolarian genus Tipperella described as new (type species T. kennecottensis n. sp.). There are two continuous T–J boundary sections in the QCI, 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). Radiolarian faunas include taxa from the upper Rhaetian Globolaxtorum tozeri Zone in the two continuous sections and the lower Hettangian Canoptum merum Zone in all three sections. In each section, the lowest Hettangian fossils are radiolarians and a few collections contain rare Rhaetian holdovers. These mixed faunas may equate with the lowest Hettangian ammonite zone, the Spelae Zone of the North American succession. The upper portion of section I contains a sparse ammonite fauna which indicates an early Hettangian age. This section has also provided carbon isotope data with a negative excursion spanning the T–J boundary. Section II contains a more diverse early Hettangian ammonite assemblage that permits correlation of the upper portion of this section with the lower Hettangian Minutus Zone. Section III has been previously proposed as a potential basal Hettangian Global Stratotype Section and Point (GSSP). In addition to an outstanding radiolarian fauna, the section contains moderately diverse early Hettangian ammonites that permit correlation of the lower portion of the section with the Minutus and Pacificum zones and the upper portion with the lower Hettangian Polymorphum Zone. This section has previously provided a U–Pb date of 199.6 ± 0.3 Ma from 3.6 m below the T–J boundary. Currently, there are four candidates and no ideal GSSP for the basal Hettangian. A section in New York Canyon, Nevada has also been proposed as a potential GSSP. This section is easily accessible and provides a virtually complete ammonoid succession but lacks geochronology and microfossils. The section on Kunga Island in the QCI provides geochronologic data and excellent radiolarian constraints. In essence, the sections at Kunga Island in the QCI and New York Canyon in Nevada are complementary. Close correlations between the two sections are possible using ammonite faunas. We suggest the section from Nevada be designated as holostratotype for the basal Hettangian and the QCI section be designated as a parastratotype to improve recognition of this interval.