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Flora assemblages at the Triassic/Jurassic boundary in Junggar Basin, Xinjiang Northwest China

Deng S.

Volumina Jurassica

2006

Vol. 4, no. 1

278-278

The Upper Triassic and Lower Jurassic deposits in Junggar Basin, Xinjiang, Northwest China represent continuous succession and are divided into the Haojiagou Formation, Badaowan Formation and Sangonghe Formation (from base to the top). The Sangonghe Formation is definitely of the Early Jurassic age as based on plant, spore-pollen, conchostracans and bivalves. The ages of the Haojiagou and Badaowan formations are less clear. The Triassic/Jurassic boundary runs somewhere in the uppermost part of the Haojiagou Formation or in the Badaowan Formation as indicated by flora assemblages. Organic carbonaceous isotopic measurement suggests the position of the boundary in question at the base of the Badaowan Formation. The flora of the Haojiagou and the Badaowan Formations may be subdivided into the following assemblages. 1. Danaeopsis fecunda - Cladophlebis ichunensis assemblage. This assemblage is from the lower part of the Haojiagou Formation. It consists of about 20 species, characterized by Danaeopsis fecunda and numerous Cladophlebis. The main species include Danaeopsis fecunda Halle, Todites cf. shensiensis Pan, Cladophlebis ichunensis Sze, Cl. nebbensis (Brongn.) Nathorst, Cl. paralobifolia Sze, Sphenopteris chowkiawanensis Sze, Rireticopteris sp. and Cycadocarpidium sp. It is definitely of the Late Triassic age. 2. Neocalamites - Hausmannia assemblage. This assemblage is from the upper part of the Haojiagou Formation. It is composed of about 10 species with marked dominance of Neocalamites horerensis and Hausmannia xinjiangensis sp. nov. The assemblage is possibly of the Late Triassic age. 3. Todites princeps - Clathropteris elegans, assemblage. This assemblage is from the lower part of the Badaowan Formation, with about 15 species. Todites princes and Clathropteris elegans are dominant in the assemblage. It may be of earliest Early Jurassic age. 4. Coniopteris gaojiatianensis - Cladophlebis denticulata assemblage. This assemblage is abundant and of higher specific diversity. The main elements are: Selaginellites drepanoformis Zheng, Todites williamsoni (Brongniart) Seward, Coniopteris gaojiatianensis Zhang, Cladophlebis denticulata (Brongniart) Fontaine, Cl. gracilis Sze, Cl. hirta Moeller, Cl. cf. shansiensis Sze, Raphaelia diamensis Seward, Ginkgoites sp., Sphenobaiera sp., Czekanowskia rigida Heer, Samaropsis parvula Heer, S. rotundata Heer. It is noticeable that Coniopteris is recorded commonly. The assemblage occurs in the upper part of the Badaowan Formation and is of Early Jurassic age.