Architecture of braided river alluvial fan under episodic flood conditions: a case study of modern Baiyang River alluvial fan in northwestern margin of Junggar Basin
The Baiyang River alluvial fan is a composite alluvial fan composed of the debris-flow deposits during the episodic flood period and the braided river sediments during the intermittent flood period. Its architectures are very different from that of debris-flow fans and fluvial fans. The current study selected the modern Baiyang River alluvial fan developed in the arid climate of the northwestern margin of the Junggar Basin as an anatomical example, and conducted detailed measurements on 103 natural profiles and 8 artificial large-scale exploration trenches. On the basis of the observation and analysis, the evolutionary process and sedimentary configuration characteristics of the geomorphic unit of the intermittent braided river alluvial fan under the condition of paroxysmal flood were analyzed, and the paroxysmal flood of different flow states was studied. Various construction and reconstruction mechanisms under different flow regime were analyzed,and the sedimentary characteristics of intermittent braided river alluvial fans controlled by episodic floods were clarified,and their sedimentary architecture models were established. Observations from the current study suggest that in the flood period, the episodic flood formed sheet-like sheet flow(or sheet flood)deposition, and that during the flood retreat period, with the weakening of the flood intensity,it was transformed into braided flow deposition, and that during the inter-flood period, there are still continuous braided fluids(braided rivers)flowing in the restricted channels, resulting in restricted(banded)braided channel deposits. The more complex depositional architectures were seen to occurrence in which the sediments of the two periods were frequently superimposed in space and time. This model has a certain reference value for genetic identification, prediction and comparison of alluvial fan sand-gravel reservoirs in oil fields.