川西马尔康—金川地区晚三叠世松潘甘孜残留洋盆复理石建造沉积特征与沉积模式

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

西康群是松潘甘孜复理石沉积盆地的主体,分布范围广、岩性单调,强烈的构造变形和褶皱叠加导致地层原始的沉积序列、沉积构造破坏殆尽,基于野外露头的沉积学研究较少,巨量复理石的沉积模式也没有合理的解释。详细研究了川西马尔康金川县一带西康群典型剖面的沉积构造、砂板比、古流向、砂岩粒度及矿物组成,讨论了西康群的沉积环境及沉积模式。认为该区复理石沉积环境属于海底扇扇中扇端,局部存在砂质碎屑流或等深流的参与和叠加改造;存在两期次的海底扇,其触发机制均为海平面下降,导致三角洲向陆架边缘进积形成海底扇,而随着冲积平原及三角洲复合体的逐渐消耗,沉积物供应量逐渐减小,海底扇的规模及影响范围逐渐缩减,形成两期向岸退积的海底扇沉积。松潘甘孜盆地复理石建造属于受周缘造山带提供多源或线源,多期次叠加的,以细粒浊积体为主的海底扇,海岸冲积平原及三角洲复合体经过二次搬运使得砾级粗粒沉积物进一步破碎,形成中细粒砂岩及泥岩为主的扇体沉积。

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 de⁃formation 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 environ⁃ment 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 subma⁃rine 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 sub⁃marine fan gradually decreased until finally the second submarine fan was formed by deposition of shore retrograda⁃tional material.The flysch formation in the Songpan⁃Ganzi Basin probably originated as a submarine fan deposit domi⁃nated 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.