Based on field survey,outcrop analysis and microscopic observbation,we report the basic characteristics of the carbonate gravity flow in platform margin in the lower Cambrian Xiannvdong Formation of the Northern Sichuan Basin and address its developmental mechanism in this paper.Results show that the Xiannvdong Formation typically developed an environment of diamictic carbonate platform marginal slope in shallow water. The bottom of this formation is mainly composed of three rock types, i.e., silty mudstones, bioclastic micritic limestones and algal limestones. According to the planar morphologies, two types of carbonate gravity flow deposits were classified,including nodular-like and brecciated.Based on the grain size and lithological variation of the gravity flow sediments in outcrops,five developmental stages were divided,which show an obvious superposition relationship vertically.The grain size of these sediments roughly changes from fine to coarse within sedimentary cycles,consistent with the regressive environment of the Xiannvdong period in general.In the vicinity of the platform marginal slope,slide and slump events occurred due to the triggering of geological events,e.g.,earthquake and storm.The mound and shoal mass moved down along the slope,and some of the blocks migrated and deposited here,which formed the brecciated carbonate gravity flow deposits.With the increasing injection of water,the amount of sediment-water mixture and the decomposition of the material rose,and some of the mass were broken and flowed as similar forms of debris flow along the slope,and then the debris transformed into carbonate gravity flow deposits with plastic rheological characteristics. Then,these deposits might be reworked by bottom current or experienced corrosion from seawater after deposition to form two types of nodular-like carbonate gravity flow sediments.In contrast,the brecciated carbonate gravity flow sediments were products of transportation and short-distance slumping of mega unbroken carbonate mound mass.They are co-deposited with the nodular-like calcirudites.%基于重力流剖面的野外观察、露头解剖及镜下鉴定等分析手段,对四川盆地北部下寒武统仙女洞组台缘斜坡碳酸盐岩重力流沉积及发育机制进行了研究.研究区位于仙女洞组,主要沉积粉砂质泥岩、生屑泥晶灰岩、藻灰岩等3种岩石类型;根据层面上产出形态的不同,将该区碳酸盐岩重力流沉积分为似瘤状碳酸盐岩重力流沉积和角砾型碳酸盐岩重力流沉积2类.结合露头区重力流沉积物粒度及岩性变化,将碳酸盐岩重力流沉积由下至上划分出5个发育期次,期次内部重力流沉积物由细变粗,纵向上呈现出明显的叠置关系,与仙女洞组时期所经历的海退环境大致吻合.地震、风暴浪等触发机制导致台缘斜坡上部松散沉积的灰泥丘块体发生破碎并沿斜坡发生滑塌,伴随水体注入,大块砾石在沉积物—水体混合的环境中发生破碎搅动,形成大小不等的块体,深水底流等作用对未固结或弱固结的砾屑灰岩进行溶蚀、改造,最终形成了层面形态特征不同的2类似瘤状碳酸盐岩重力流沉积.角砾型碳酸盐岩重力流沉积则主要是未破碎的巨型灰泥丘块体经搬运和短距离滑塌,最终与似瘤状砾屑灰岩共同沉积而成.
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