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ABSTRACT: Middle to Upper Jurassic Lake Formation within the South Junggar and North Tarim Basins, Northwest China

Marc S. Hendrix

Middle to Upper Jurassic coals and organic-rich mudstones containing up to 96% TOC (total organic carbon) are exposed intermittently for hundreds of kilometers along the flanks of the south Junggar and north Terim foreland basins, northwest China. These organic-rich rocks are interpreted as being deposited largely in lakes that developed within the depocenters of the two basins. Individual coal seams within each basin are continuous for a minimum of 1-2 km and reach thicknesses of 24 m in places. Organic matter was derived dominantly from phytoplankton and, to a lesser extent, from higher plants. Samples from both basins are characterized by high hydrogen indexes (400-700) and low oxygen indexes (<30) and typically plot in the type I field. Biomarker analysis of the al phatic fraction of bitumen extracts reveals a low to medium molecular weight n-alkane distribution with a slight odd-over-even preference, attributable to algal input. The presence of such significant thicknesses of Middle to Upper Jurassic high hydrogen index coals in both Tarim and Junggar strongly suggests that little of the climatic partitioning posed today by the Tian Shan (Heavenly Mountains), which currently separate the two basins, existed during the Jurassic. Nevertheless, the presence of a Mesozoic ancestral Tian Shan is suggested by several independent lines of evidence. Paleocurrents in sandstones interbedded with the coals in the Junggar basin are northerly directed, whereas equivalent sandstones in north Tarim are southerly directed. Jurassic sandstone compositions of north Tarim are markedly more quartzolithic and are enriched in sedimentary and metamorphic rock fragments relative to the quartzofeldspathic compositions of south Junggar Jurassic sandstones, which have a lithic fraction dominated by volcanics. Thus, despite the presence of this Jurassic physiographic partitioning between Junggar and Tarim, it was apparently insufficient to restrict high rainfall and consequent lake formation to one side of the Tian Shan.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990