--> Abstract: Mesozoic Tectonically Driven Climatic Partitioning of the South Junggar and North Tarim Basins, Northwest China, by M. S. Hendrix, S. A. Graham, and S. C. Brassell; #91004 (1991)

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Mesozoic Tectonically Driven Climatic Partitioning of the South Junggar and North Tarim Basins, Northwest China

HENDRIX, M. S., S. A. GRAHAM, and S. C. BRASSELL, Stanford University, Stanford, CA

Detailed basin analysis of Mesozoic strata exposed in the south Junggar and north Tarim foreland basin Mesozoic depocenters, northwest China, has documented periodic structural reactivation of the intervening Tian Shan during that era. Coarse alluvial pulses, contained in an otherwise relatively fine-grained section of fluvial lacustrine sediments, were shed off both the north and south flanks of the Tian Shan during the Late Triassic, Latest Jurassic, and Late Cretaceous. Associated with each of the alluvial pulses are briefly accelerated rates of basin subsidence in each basin depocenter, suggesting periodic flexural downwarping of basin depocenters. These episodes of structural reactivation within the Tian Shan are interpreted to have been driven by the accretion onto the south Asi n margin of the Qiantang Block, the Lhasa Block, and the Kohistan-Draas arc complex, respectively.

The lofty modern Tian Shan and associated rain shadow cast over the Tarim basin to the south reflect deformation associated with the early Cenozoic Himalayan collision; so too were the two basins climatically partitioned during the Mesozoic in response to relief produced by orogenesis. Most notable is the Lower Cretaceous section that, in the south Junggar basin depocenter, consists of gray claystones and local silty carbonates containing lacustrine ostracods but little preserved organic matter. These sediments, interpreted to have been deposited in part in regionally developed oxic lakes, contrast markedly with time-equivalent strata in the north Tarim basin depocenter that consist entirely of poorly fossiliferous redbeds. Only in one locality, 200 km west of the main Tarim basin dep center, is there evidence of local saline lakes, where 20 to 30 m of laminated oil shale containing occasional evaporites occur in association with a fine-grained continental redbed sequence.

During prolonged episodes of relative tectonic quiescence (in contrast to the Lower Cretaceous climatic partitioning), it appears that Tian Shan relief was too low to confine moisture to the windward side of the range. Most notable is the Middle Jurassic section that is dominated in both basins by a relatively fine-grained series of organic-rich, meandering fluvial, and lacustrine sediments. Pyrolytic analyses of organic-rich mudstones associated with these sediments document high H.I. kerogens, suggesting algal-dominated organic matter typical of anoxic lacustrine settings. However, detailed GC-MS biomarker analysis of the alipaphatic fraction also suggests substantial input from higher land plants in addition to algal sources. Not only are n-alkane distributions typically dominated y high molecular weight compounds with an odd-over-even preference typical of higher land plants, but the abundance of specific diterpenoid biomarker compounds strongly suggests the development of widespread coniferous forests on either side of the range during the Middle Jurassic.

 

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)