--> Abstract: Depositional Environments, Cyclostratigraphy, and Paleoclimatic Signals of Lower to Middle Jurassic Fluvial-Lacustrine Deposit; #90063 (2007)

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Depositional Environments, Cyclostratigraphy, and Paleoclimatic Signals of Lower to Middle Jurassic Fluvial-Lacustrine Deposits, Northern Bogda Mountains, Southeastern Junggar Basin, Northwestern China

 

Yang, Wan1, Qiang Guan2, Jihong Li3, Xiaohui Zhang3, Jun Han2, Wei Guan3, Fan Song3, Wenqiang Yang3 (1) Wichita State University, Wichita, KS (2) East Junggar Division, Xinjiang Petroleum Bureau, PetroChina, Fukang, China (3) Department of Geology, Northwestern University, Xian, China

 

A 2112-m Lower-Middle Jurassic section was measured at a cm-dm scale in northwestern Bogda Mountains, NW China. It contains fluvial-lacustrine rocks of upper Lower-Jurassic upper Badaowan and Sangonghe, and Middle-Jurassic Xishanyao and Toutunhe formations. Five types of cycles were identified. 357 lakeplain-littoral cycles contain upward carbonaceous shale/coal, tabular sandstone, interbedded siltstone and shale, and paleosols. They are 1-3 m thick, interpreted as lake transgression/expansion-regression/contraction cycles. 306 thin deltaic cycles consists of upward thin carbonaceous shale/coal, tabular sandstone, an upward-coarsening succession of shale, siltstone, and sandstone, and paleosols. They are1-5 m thick, as lake expansion-embayment fill cycles. These two cycle types suggest high-frequency lake-level oscillation, indicating high-frequency precipitation seasonality. 83 thick deltaic cycles contain pronounced upward-coarsening-and-thickening successions of shale, siltstone, sandstone, and conglomerate. They are 20-30 m thick, stacked into prominent sandstone-rich intervals. Five braided-stream cycles of conglomerate and sandstone stack at basal Sangonghe Formation. 16 meandering-stream cycles of conglomerate, sandstone, shale, and paleosols scatter throughout the section. Lakeplain-littoral and thin deltaic cycles thin upsection. They alternate with braided-stream and thick deltaic cycles, forming 15 intermediate-order cycles suggesting episodes of increased coarse clastic influx, subsidence, source uplift, and/or precipitation that punctuated prolonged lakeplain-littoral sedimentation. Paleosols change from thin well-developed Histosols and rare immature Calcisols in Badaowan-Sangonghe cycles, thick coal-rich Histosols and minor Spodosols in Xishanyao cycles, to variegated Argillisols and poorly-developed Histosols in Toutunhe cycles. A long-term subhumid-humid, humid, to subhumid-semi-arid climate change is inferred, probably caused by waning Tethys-Laurasia megamonsoon due to decreased continentality during Pangea breakup.

 

AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California