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Towards the definition of the Triassic/Jurassic systems boundary

Bloos G.

Volumina Jurassica

2006

Vol. 4, no. 1

277-277

This contribution summarizes the main stratigraphic results of the T/J Boundary WG since its establishment in 1988 and of the IGCP Project 458 (2001-2005); state at end of March 2006. The T/J transition in general. It begins with the main end-Triassic extinction event which is related to the negative carbon isotope excursion 13Corg. Above follows a range of strongly impoverished Triassic survivors whose duration is different in different fossil groups. Above begins the succession of Jurassic forms, sometimes at first together with last Triassic forms. Up to now (March 2006) four candidate GSSPs have been proposed: St. Audrie's Bay (Somerset, UK), Muller Canyon (Nevada, USA), Chilingote (Utcubamba Valley, Peru), and Kunga Island (British Columbia, Canada). Further important sections are known; one, situated in the Alps, will be proposed as a fifth candidate. Three possibilities to define the T/J boundary are proposed: 1. within the succession of early psiloceratids (England, Nevada); 2. at the base of the psiloceratid succession (England, Nevada, Peru); 3. at the radiolarian turnover in the T/J transitional interval (Canada). Proposal 1. It is essentially the traditional boundary since the 19 th century, indicated by the first wide-spread occurrence of psiloceratids. Since no psiloceratid species is cosmopolitan, global correlation is possible only by regional species and thus essentially tentative. The proposed correlation Psiloceras planorbis - Ps. calliphyllum - Ps. pacificum is supported by the partly proved, partly probable occurrence of the genus Neophyllites below. Since there is no difference in the associated fossil content below and above the boundary, no proxies are known which could indicate the position of the boundary where ammonites are lacking. In this case only earlier levels are available. Proposal 2. With this proposal all psiloceratids would become Jurassic. The difficulty to distinguish between Triassic and Jurassic psiloceratids would be avoided. A global correlation by ammonites would be more difficult than in "proposal 1" because such early forms are extremely rare, belong to regionally restricted species and are not proved to be coeval. First appearances of Jurassic forms in other fossil groups, particularly in palynomorphs, could be regional proxies for correlations, also with sections where ammonites are lacking. Proposal 3. New investigations suggest that the radiolarian turnover could be a global event and potentially a suitable system boundary. However, successions of sufficiently preserved radiolarians are extremely rare and they are, moreover, poor in other fossils.Therefore, the correlation with other fossil groups is diffcult and, consequently, the exact stratigraphic position of the turnover is not yet known. No proxies of other fossil groups are known to recognize the turnover in sections without radiolarians. The strong impact on the evolution of radiolarians suggests that the radiolarian turnover is close to the main end-Triassic extinction event. In this case the turnover could be situated still in latest Triassic as defined e.g. by surviving Misikella posthernsteini in Britain.