LON-CAPA alb21-3

ABOUT THE TOP OF THE TRIAS

G. Warrington

Though of obvious interest to STS members, the upper limit of the Trias is the responsibility of the Triassic/Jurassic Boundary Working Group (TJBWG) of the International Subcommission on Jurassic Stratigraphy (ISJS). The STS Secretary, who is also Secretary of the TJBWG, briefly reviewed recent TJBWG activity during the STS business meeting in Halle (see Abbreviated Minutes of that meeting elsewhere in this issue of Albertiana). The work of the TJBWG will be reported in more detail as progress is made towards the selection of a GSSP for the base of the Hettangian Stage, marking the base of the Jurassic and, inter alia, the top of the Trias.

Some recent advances are noted here, for the benefit of STS members interested in the upper boundary of the System. Important discoveries, which are very relevant to discussions on the definition of the Triassic/Jurassic (T/J) boundary, have recently been made during the study of the lowest ammonite faunas in the Lias Group in the UK (Bloos & Page 1997; Page & Bloos 1998). These faunas include Neophyllites and Psiloceras erugatum (Phillips), found below the level of the appearance of Psiloceras planorbis, the index fossil for the lowest zone of the Hettangian; P. erugatum has been found in situ for the first time. Occurrences of Neophyllites have been discussed further by Bloos (1999). Further information on the ammonoid succession in the base-Jurassic candidate GSSP area of New York Canyon, in the Gabbs Valley Range, Nevada, USA, has been published by Guex et al. (1998), and further studies of the candidate GSSP on Kunga Island, Queen Charlotte Islands, British Columbia, Canada, have been published by Carter et al. (1998). The programme of the 5th International Symposium on the Jurassic System, held in Vancouver, Canada, in August 1998, included the following talks and poster presentations relevant to the T/J boundary:

Bloos & Page - The ammonite sequence in the early Hettangian of north-west Europe.
Carter - Extinction and recovery of radiolarians at the Triassic-Jurassic boundary in Queen Charlotte Islands.
Cohen & Coe - Re-Os dating of organic-rich mudrocks and the Os isotope composition of seawater.
Copestake & Partington - Stratigraphic sequences in the Jurassic-lowermost Cretaceous (Hettangian to Ryazanian) of the North Sea Basin and adjacent areas of north west Europe.
Damboranea - Hispanic corridor: its evolution and the biogeography of bivalve molluscs.
Embry - Large-scale sequence boundaries in the Jurassic succession of the Sverdrup Basin, Arctic Canada.
Erlström & Guy-Ohlson - Distribution and evolution of the Triassic-Jurassic boundary sedimentation in Scania, southern Sweden.
Haggart - Fossils and faults: unravelling the history of Queen Charlotte Islands, British Columbia.
Hirsch & Katz - Mesozoic radiation and Tr/J extinction events.
Iglesia Llanos & Riccardi - Bio-magnetostratigraphic scale of the Lower Jurassic of the Neuquen Basin, Argentina.
Keisuke - Microfaunas in Triasso-Jurassic pelagic successions of the Chichibu Super Belt in East Shikoku, SW Japan.
McRoberts - The Triassic-Jurassic boundary and end-Triassic mass extinction.
Orchard - Decline and fall of conodonts at the Triassic-Jurassic boundary
Pálfry & Dosztály - A new marine Triassic-Jurassic boundary section in Hungary: preliminary results.
Pálfry & Smith - Timing of Early Jurassic recoveries and spacing of mass extinctions.
Pálfry, Smith & Mortensen - A U-Pb and ⁴⁰Ar-³⁹Ar time scale for the Jurassic.
Posen, Hounslow & Warrington - Magnetostratigraphy of the Hettangian/Rhaetian boundary section, St Audries Bay, UK
Taylor - The Triassic/Jurassic system boundary in the Gabbs Formation, Nevada
Tripathi - Palynological characteristics during Late Triassic-Early Jurassic time slot in India.
Warrington - St Audrie's Bay, Somerset, England: a candidate Global Stratotype Section and Point for the base of the Jurassic System.
Whalen, Carter & Orchard - Rhaetian radiolarians and conodonts near the Triassic-Jurassic boundary in Baja California Sur.

Of particular interest for the geochronology of the Triassic was the report of a U-Pb zircon dating of 199.6 ± 0.7 Ma from a tuff bed close below the proposed level of the base of the Jurassic (Pálfry et al. 1998). The publication of the proceedings of this conference will, therefore, be of interest to STS members. References

Bloos, G. 1999. Neophyllites (Ammonoidea, Psiloceratidae) in the earliest Jurassic of South Germany. N. Jb. Geol. Paläont. Abh., 211: 7-29.
Bloos, G. & Page, K.N. 1997. New observations on the ammonite faunas near the base of the Jurassic in Britain - a preliminary note. Int. Subcomm. Jurassic Stratigraphy
    Newsl., 25: 25-30 (also in: JMG Newsletter, 6: 18-24; Jurassic Microfossil group, Int. Subcomm. Jurassic Stratigraphy).
Carter, E.S., Whalen, P.A. & Guex, J. 1998. Biochronology and paleontology of Lower Jurassic (Hettangian and Sinemurian) radiolarians, Queen Charlotte Islands, British
    Columbia. Geol. Surv. Can. Bull., 496.
Guex, J., Taylor, D., Rakus, M. & Bucher, H. 1998. Deux nouveaux genres et quatres nouvelles espèces d'ammonites (Cephalopoda) du Lias inférieur. Bull. Soc.
    Vaudoise Sci. Nat., 86: 73-85 (also in: Bull. Géol. Lausanne, 339: 73-85).
Page, K.N. & Bloos, G. 1998. The base of the Jurassic System in west Somerset, south-west England - new observations on the succession of ammonite faunas of the
    lowest Hettangian Stage. Geoscience in south-west England: Proc. Ussher Soc., 9: 231-235.
Pálfry, J., Smith, P L. & Mortensen, K. 1998. A U-Pb and 40Ar-39Ar time scale for the Jurassic (Abstract). Abstracts and Program, 5th Int. Symp. on the Jurassic
    System, Vancouver, p. 72.

© ALBERTIANA, February 1999