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Mixed Fluvial and Loess Deposits in an Intracontinental Rift Basin, Mid-Permian (Wordian-Capitanian) Quanzijie Low-Order Cycle, Bogda Mountains, NW China

Obrist, Jonathan; Yang, Wan; Feng, Qiao

Fluvial and loess deposits coexist in some modern environments. However, their differentiation in ancient records is difficult. The mid-Permian Quanzijie low-order cycle (QZJ LC) in Bogda Mountains, NW China is studied to establish such field and petrographic criteria and to understand the controlling processes. A detailed study was carried out on 6 measured sections in Tarlong-Taodonggou half graben of 60-km2 and in Dalongkou section 70 km to the north. QZJ LC is 72-180 m thick. 201 high-order cycles (HCs) are composed of fining-upward successions of conglomerate, sandstone, and mudrock with an erosional base, representing repetitive erosion-and-deposition. Conglomerate and minor sandstone are in shallow channel forms and 0.1-8.2 m thick and 1.09 m on average with an average lateral extent of 48 m. Mudrocks are grayish brown and 0.2-13.7 m thick and 2.37 m on average. The HCs were originally interpreted as meandering stream deposits of channel-fill conglomerates and thin sandstones and thick overbank mudrocks. Close examination, however, casts doubts on this interpretation. The channels have a high width/depth ratio and contain mainly gravels and little to no point-bar sands. This suggests ephemeral braided, not meandering, channels, which could not have supplied abundant mud. Thus, the thick "overbank" mudrocks may not have a fluvial origin. Evidence for a non-fluvial origin includes massive bedding with faint pedogenic overprints and absence of sandstones common in overbank deposits. Alternatively, the mudrocks may be loess, as evidenced by uniform grain size, high silt content, massiveness, and lateral persistence. Microscopically, the mudrocks have soil fabrics, which, combined with faint field pedogenic overprints, indicate immature paleosols. The paleosols may form during moderate to rapid mud accumulation. The QZJ HCs are interpreted as ephemeral stream deposition followed by mixed overbank and wind-blown deposition. Regionally, QZJ LC is persistent with variable thickness of 20-180 m over 60,000 km2. Although regional sections contain variable amount of fluvial dominated cycles, the massive mudrocks are always present, indicating a basin-wide distribution of possible loess. This study provides insights into the dynamics of mixed fluvial-loess sedimentation and identification criteria for ancient deposits.

 

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013