Wyniki wyszukiwania - BazTech

Ograniczanie wyników

1 Volumina Jurassica

1 2006

Znaleziono wyników: 1

Liczba wyników na stronie

Wyniki wyszukiwania

Sortuj według:

Ogranicz wyniki do:

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.