Prospects for correlation of the Triassic/Jurassic boundary between marine and non-marine facies using carbon-isotope stratigraphy: a case study from Xinjiang, northwest China

• Autorzy: Hesselbo S. , Deng S. , Lu Y.
• Konferencja: International Congress on the Jurassic System (7 ; 06-18.09.2006 ; Kraków, Poland)
• Języki publikacji: EN

Abstrakty

EN

An apparently global pattern of carbon-isotope change in carbonates and organic matter across the T/J boundary has increasingly been recognized from marine sections, and may be useful as a basis for correlation. Evidence from other geological events indicates that this isotopic pattern should also have the power to resolve correlations between marine and non-marine sections where organic matter is preserved. In the present study we apply this method to the non-marine succession in the Junggar Basin, Xinjiang, northwest China. The Haojiagou Section there includes an excellent ˜700 m thick exposure of the Haojiagou Formation and lower member of the Badaowan Formation, from which fossils indicative of the T/J boundary have been described. However, different horizons have been suggested on the basis of different fossil groups. The T/J boundary has previously been placed at the lowest level in the section (at the base of Bed 25, Haojiagou Formation) on the basis of megaspores, and Bed 40, higher in the Haojiagou Formation, has been suggested to be Early Jurassic in age based on the occurrence of conchostracan taxa and the bivalve Ferganochonca. In contrast, on the basis of palynology, the T/J boundary has been placed higher in the section, at the base of Bed 45 (also the base of the Badaowan Formation), where there is an absence of pre-Jurassic index fossils such as Taeniaesporites, and occurrences of the post-Triassic index fossils, such as Cyathidites. Macrofossil plants suggests that the boundary lies between Beds 37 and 51 based on Hausmannia sp. in Bed 37, and Todites princeps and Clathopteris elegans in Bed 51 and higher. Carbon-isotope data have been obtained from both bulk sedimentary organic matter and from fossil wood fragments that were individually selected. Delta 13CWood carbon isotope values show a systematic stratigraphic trend from -21 per mil in Bed 25, to -26 per mil in Bed 52, although some samples around bed 25 also show very light isotopic values. Bulk organic isotope data show a similar overall trend, but show considerably less variance. Although the data are noisy, they suggest that the "main" negative isotope excursion of marine sections corresponds to the formational boundary and the "T/J boundary" recognized on the basis of plant macrofossils and microfossils, whilst the "initial" isotope excursion of marine sections may be expressed in wood values around beds 23-25. Depending on GSSP decisions either of these horizons may be regarded as the T/J boundary.

Słowa kluczowe

EN

carbon isotopes; Triassic/Jurassic boundary; non-marine strata

• Wydawca: Państwowy Instytut Geologiczny - Państwowy Instytut Badawczy
• Czasopismo: Volumina Jurassica
• Rocznik: 2006
• Tom: Vol. 4, no. 1
• Strony: 286--287

Twórcy

autor

Hesselbo S.

autor

Deng S.

autor

Lu Y.

• Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK,