豫西伊川地区中元古界兵马沟组下段沉积相分析及环境演化

Sedimentary facies analysis and environmental evolution of lower part of Mesoproterozoic Bingmagou formation in Yichuan,Western Henan

兵马沟组作为华北克拉通南缘太古宙结晶基底至上覆中-新元古代沉积盖层过渡的关键层,对该区中元古代初期古地理背景研究有重要的指示意义。在野外剖面实测基础上运用岩性岩相分析、镜下薄片观察、粒度参数分析及古流向分析等方法对兵马沟组下段沉积环境演化进行分析与讨论。结果显示:兵马沟组下段为一套砾岩-含砾砂岩-砂岩-粉砂岩和泥岩的陆源碎屑岩沉积,概率累积曲线多呈一段式,萨胡判别函数均指向重力流特征,砂岩结构和成分成熟度普遍较低,形成于冲积扇向扇三角洲过渡的沉积环境,发育扇根、扇中和扇三角洲前缘亚相,同时发育泥石流沉积、河道充填沉积、辫状河道沉积、漫流沉积、水下分流河道沉积与河道间沉积微相;该时期区域地势北高南低,南部水体较深,古流向为SSW向。研究结果可为熊耳群到五佛山群时期华北克拉通南缘海陆格局及陆相-海相沉积水动力条件变化等古环境恢复提供一定依据。

Bingmagou formation is the key layer of transition from the Archean crystalline basement to Meso-Neoproterozoic sedimentary cover in the southern margin of the North China Craton(NCC).It has an important indicative values for analyzing palaeogeographic background of the early Mesoproterozoic in this area.The sedimentary evolution of the lower part of the Bingmagou formation was analyzed and discussed based on field observation.Lithology and lithofacies analysis,thin section observation,grain size analysis,and palaeocurrent analysis were used for data analysis.The results show that the lower part of the Bingmagou formation is a set of terrigenous clastic deposition of medium coarse conglomerate,conglomerate bearing sandstone,sandstone,siltstone and mudstone.The probability accumulation curve is mostly a segment and the Sahu discriminant function indicates the characteristics of gravity flow.The maturity and composition maturity of the sandstone structure are generally low.It was formed in the sedimentary environment of the transition from alluvial fan to fan delta,and fan root,fan and fan delta front subfacies occured.Meanwhile,the microfacies of debris flow,channel filling,braided channel,diffuse flow,underwater distributary channel and interchannel sedimentation also occurred.During the deposition of Bingmagou formation,the regional topography appeared high in the north and low in the south.The palaeocurrent was SSW.The results provide a basis for the rebuilding of palaeoenvironment including the occurrence of sea and land in the southern margin of NCC,and the changes in hydrodynamic conditions of the continental and marine facies during the deposition in the transition from Xiong'er group to Wufushan group.