The Upper Triassic to Middle Jurassic strata and floras of the Junggar Basin, Xinjiang, Northwest China

Abstract

The Upper Triassic to Middle Jurassic strata are well exposed and basically continuous in the Junggar Basin of Xinjiang, Northwest China. The Upper Triassic strata include the Huangshanjie Formation (lacustrine facies mainly) and Haojiagou Formation (fluvial to fluvial-swamp facies). The Lower Jurassic consists of the coal-bearing Badaowan Formation (fluvial swamp facies) and Sangonghe Formation (lacustrine facies mainly). The Middle Jurassic is composed of the large coal-bearing Xishanyao Formation (fluvial-swamp facies mainly) and Toutunhe Formation (variegated lacustrine facies). The Late Triassic to early Middle Jurassic floras from the basin consist of four floristic assemblages, including the Danaeopsis-Nanzhangophyllum assemblage (Ass. I, Carnian-Norian) and Glossophyllum-Cycadocarpidium assemblage (Ass. II, Norian-Rhaetian), belonging to the Late Triassic “Northern China Flora” (Danaeopsis-Bernoullia Flora), and the Neocalamites-Marattiopsis assemblage (Ass. III, Early Jurassic) and Coniopteris-Raphaelia assemblage (Ass. IV, Middle Jurassic), belonging to the Early–Middle Jurassic “Siberian Flora” (Continental Floristic Province). The palaeoclimatic changes basically consisted of warming up and moistening from the Late Triassic to Early Jurassic, while the climate of the early Middle Jurassic might have been warm–temperate and humid, which was favourable for coal formation. The lower Middle Jurassic strata appear to be the largest sedimentary mineral resources for coal, oil and natural gas in the Junggar Basin area. Two new taxa from the Upper Jurassic are described, Danaeopsis tenuinervis Sun, Mosbrugger et Li sp. nov and Thinnfeldia? mirisecta Sun, Mosbrugger et Li sp. nov.