Sedimentary Characteristics and Model of the Triassic Songpan-Ganzi Flysch Basin,in Maerkang-Jinchuan Area,Western Sichuan

Key words: 
• Late Triassic /
• Songpan-Ganzi Basin /
• flysch /
• sedimentary characteristics /
• sedimentary model /

Abstract: The Xikang Group is the main body of Songpan-Ganzi flysch sedimentary basin. It has a single lithology and is widely distributed. The original stratigraphic sequence and sedimentary structure was destroyed by tectonic deformation and widespread folding. Few sedimentological studies have been done on the outcrop,and sedimentation models do not give a reasonable explanation for the huge amount of flysch observed. In this study,the sedimentary structure,sand plate ratio,paleocurrent direction,sandstone grain size and mineral composition of typical sections in the Maerkang-Jinchuan area in western Sichuan Province were researched in detail,and the sedimentary environment and sedimentary model of the Xikang Group is discussed. It is inferred that the flysch sedimentary environment was a middle-to terminal submarine fan,with sand debris flow and deep flow action and transportation. Two submarine fans were formed triggered by falling sea levels,causing progradation of the delta to the edge of the shelf. As the alluvial plain and delta complex were gradually consumed,the sediment supply and the scale and extent of the submarine fan gradually decreased until finally the second submarine fan was formed by deposition of shore retrogradational material. The flysch formation in the Songpan-Ganzi Basin probably originated as a submarine fan deposit dominated by fine-grained turbidite with multi-period superposition and multi-or single provenance. Coarse sediments were further broken down after the coastal alluvial plain and delta complex depositions were moved for the second time, and finally the main fan was deposited, consisting of middle-to fine-grained sandstone and mudstone.