上扬子西北缘晚震旦世古环境演化

Paleo-environmental evolution of late Sinian Period in thenorthwestern margin of Upper Yangtze

上扬子地区震旦系灯影组不仅记录了埃迪卡拉纪晚期环境与生命的协同演化,而且大量发育的丘滩体沉积正成为四川盆地深层古老碳酸盐岩油气勘探的重要目标。因此分析灯影组沉积环境演化不仅可以约束埃迪卡拉纪晚期全球海平面变化、古海洋环境演化,而且对于四川盆地深层油气勘探具有重要意义。前人对于上扬子地区震旦系灯影组的沉积环境演化、相对海平面变化仍缺乏较为精细的刻画和解释,严重制约了以上研究的进展。本文通过对上扬子西北缘绵竹清平剖面灯影组详细的野外沉积学解剖及室内微相分析解释,识别出以各类颗粒云岩、微生物云岩为主的21类岩石微相;根据微相组合关系分析,认为发育多种向上变浅的潮下—潮间—潮上沉积序列以及若干颗粒滩、微生物丘沉积构成的米级沉积旋回;以此为基础详细刻画出灯一段—灯二段下部、灯二段中上部—灯二段顶部、灯三段底部—灯四段下部、灯四段中上部—灯四段顶部构成的四次海侵—海退次级旋回以及灯二段和灯四段顶部暴露侵蚀的沉积环境演化及海平面变化历程,并建立了扬子台地西缘绵竹地区埃迪卡拉纪晚期以潮坪相、微生物丘滩体、颗粒滩为典型沉积特征的浅水碳酸盐岩台地沉积模式。综合沉积微相特征、微相组合特征及剖面沉积相演化规律认为,该地区沉积环境演化主要受次级海平面波动影响,海侵—海退次级旋回的中上部可能为灯影组优质储层发育的主要层位,其内部的两次暴露不整合可能与冰期事件导致的海平面下降密切相关。

The Ediacaran Dengying Formation in the Upper Yangtze area not only recorded the co-evolution of environment and life at the end of the Precambrian,but the massively developed mound-shoal deposits are currently becoming an important target for oil and gas exploration in deep ancient carbonate rocks in the Sichuan basin.Therefore,analyzing the evolution of the Dengying Formation sedimentary environment can not only constrain the global sea level change and the evolution of the paleo-ocean environment in the late Ediacaran,but also has important value for deep oil and gas exploration in the Sichuan basin.Previous studies lack detailed description and explanation of the sedimentary environment evolution and relative sea level changes of the Dengying Formation at the end of the Ediacaran in the Upper Yangtze area,which severely limited the progress of the above studies.Based on detailed field sedimentology and microfacies analysis and interpretation of Dengying Formation in Qingping section of Mianzhu in the northwestern margin of the Upper Yangtze,21 types of rock microfacies were identified,mainly composed of granular and microbial dolomites.According to the analysis of microfacies assemblage relationship,it is considered that there are several shallower subtidal-intertidal-supratactic sedimentary sequences and meter-level sedimentary cycles composed of several grain banks and microbe mounds.On this basis,the sedimentary environment evolution and sea level change process of four transgression-regressive secondary cycles and two exposed erosions are described in detail.The sedimentary model of shallow water carbonate platform in the Mianzhu area in the western margin of Yangtze Platform in Late Ediacaran is established,which is characterized by tidal flat facies,microbial mound bank body and grain bank.Based on the characteristics of sedimentary microfacies,combination of microfacies and vertical evolution of sedimentary facies,it is believed that the evolution of the sedimentary environment in this area is mainly affected by secondary sea level fluctuations,and the middle and upper part of the transgression-regression secondary cycle may be high-quality reservoirs in the Dengying Formation.The two exposure unconformities in the main development horizon may be closely related to the sea level drops caused by ice age events.