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Proposal of the Muller Canyon section (New York Canyon area, Nevada, USA) as stratotype for the Triassic/Jurassic boundary

Guex J., Taylor D., Rakús M., Bartolini A., Atudorei V., Carter E., Lucas S.

Volumina Jurassica

2006

Vol. 4, no. 1

283-285

Completeness of the sedimentary and biostratigraphic record is the most important characteristic of a global stratotype intended to record the boundary between two geological stages. Such a reference section must also contain fossils with proven chronological value. In the Jurassic, ammonites are usually considered the ideal chronological marker and all high-resolution zonations concerning that period are based on this fossil group. Figure 1 demonstrates that the Muller Canyon section (New York Canyon area, Nevada, USA) provides by far the most complete biochronological ammonite sequence known in the world for this interval of time. The oldest Jurassic ammonites found in the area are the smooth Psiloceras tilmanni and P. spelae, which are characterized by nodes in their earliest stage of development (Knötchenstadium). Odoghertyceras, a genus found also in Peru and Canada, occurs just above that fauna. Other smooth ammonites found between the first occurrence of P. spelae and the last occurrence of Choristoceras crickmayi belong to a group that we call "intermediate phylloceratids". Such forms lack the nodose juvenile stage and are geometrically intermediate between rhacophyllitids and psiloceratids. In our region, C. crickmayi (younger than C. marshi) co-occurs with the last Arcestes, Placites and abundant Rhacophyllites. In our sections we have found the first Jurassic-type bivalves below the first occurrence of Psiloceras, in beds that we assign to the topmost Rhaetian. In our view, the occurrence of bivalves is strongly controlled by the nature of the sediments and by local ecological conditions and this group does not seem suitable for use as index for the base of the Jurassic. The absence of Triassic ammonoids in the classical NW European sections precludes the possibility of defining the stratotype for the Triassic/Jurassic boundary (TJB) in that region and the ammonite faunas are not well diversified in the classical Tethyan Alpine sections. At present, there are three potentially good GSSP candidates: Chilingote (Peru), Kunga Island (BC, Canada) and Muller Canyon (Nevada). As far as ammonite record is concerned, Nevada ranks foremost. However, an interesting solution would be to propose the Kunga Island section (British Columbia) as parastratotype because it records a complete sequence of radiolarians across this interval, and this group is not preserved around the TJB in the New York Canyon area.

Pliensbachian to Aalenian radiolarian biochronology and global correlation

Carter E., Gorican S., Guex J., De Wever P., Dumitrica P., Hori R., Matsuoka A., O'Dogherty L., Whalen P.

Volumina Jurassica

2006

Vol. 4, no. 1

151-152

A new UA radiolarian zonation for the Pliensbachian to Aalenian interval is established using 145 distinctive, widely-distributed species. The data are from biostratigraphic sections in: Queen Charlotte Islands, NE British Columbia, Baja California Sur, Japan, Oman, Turkey, Slovenia and Austria. A catalogue of 280 species (with revised taxonomy) is completed. For the zonation, about half these species were eliminated from the total dataset, because they are either rare (e.g. Danubea, Farcus, Pseudopoulpus, Rolumbus), long-ranging (e.g. Pseudocrucella, Orbiculiformella, Paronaella) or non-diagnostic with wide limits of variability (e.g. some species of Bagotum, Droltus, Parahsuum). Rich well-preserved radiolarians from thick continuous stratigraphic sections in Queen Charlotte Islands provide the most detailed record for this stratigraphic interval, and all collections are tied with North American ammonite zones or assemblages. An initial sequence of 25 UAs (including ammonite data) was determined from this material only. Subsequently, data from other areas were added and a global sequence of nine radiolarian zones was obtained. These zones can be correlated worldwide and link previously established UA zonations for the Hettangian-Sinemurian (Carter et al. 1998) and the Middle to Upper Jurassic (Baumgartner et al. 1995).

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.