四川广元羊木-岩洞地区震旦纪-寒武纪界线地层序列及构造-沉积演化

Stratigraphic sequence and tectonic-sedimentary evolution of Sinian-Cambrian boundary in Yangmu-Yandong area,Guangyuan,Sichuan Province

绵阳-长宁拉张槽的形成机制及时间尚存在巨大争议,严重制约了灯影组油气勘探的部署。为更好地对拉张槽深水地层提供时代约束,深化拉张槽构造-沉积演化认识,基于露头描述、薄片观察、碳同位素测试,对广元地区4个露头剖面开展综合地层对比和沉积环境分析。岩石学及碳同位素特征表明,研究区实测4条野外露头剖面从下到上可划分为4个岩性段(Ⅰ~Ⅳ):岩性段Ⅰ为薄层泥质灰岩夹泥质白云岩条带,对应陡山沱组三段Shuram-Wonoka负偏段地层;岩性段Ⅱ以黑色页岩为代表,可对比陡山沱组四段;岩性段Ⅲ为块状白云岩-硅质岩序列,属于灯影组同期地层;岩性段Ⅳ以薄板状硅质岩为特征,相当于麦地坪组。其中,灯影组二段早期—麦地坪期,研究区从块状白云岩逐步过渡为薄板状硅质岩,见垮塌体及滑塌揉皱,指示台地转换为斜坡-盆地沉积环境。这种加深的沉积趋势与中上扬子台地相区变浅至暴露的过程完全相反,指示断裂活动影响下快速沉降的过程。

The formation mechanism and time of the Mianyang-Changniang sag are still controversial,which significantly limit the petroleum exploration of the Dengying Formation.In order to provide a better time constraint on the deep-water strata in the Mianyang-Changning intracratonic sag and deepen the understanding of its tectonic-sedimentary evolution process,this study carries out comprehensive stratigraphic correlation and sedimentary environment analysis of four sections in Guangyuan area,based on outcrop description,thin section observation and carbon isotope analysis.The characteristics of petrology and carbon isotopes show that the four outcrop profiles in the study area can be divided into four lithologic units from bottom to top(namelyⅠ,Ⅱ,ⅢandⅣ).Lithologic unitⅠis composed of thin-bedded argillaceous limestone intercalated with argillaceous dolomite band,which is equivalent to the 3rd Member of the Doushantuo Formation with the Shuram-Wonoka negativeδ13C excursion.Lithologic unitⅡis represented by black shale,corresponding to the 4rd Member of the Doushantuo Formation.Lithologic unitⅢshowing the transition from massive dolomite to chert,can be correlated with the Dengying Formation.Lithologic unitⅣis marked by platy chert,which is equivalent to the Maidiping Formation.Of these,during the early stage of the second member of Dengying to Maidiping period,massive dolomite shifted to thin-bedded chert with slump breccias and fold deposited in the study area,indicating rapid evolution from carbonate platform to slope-basin depositional environment.This deepening depositional trend is opposite to the shallowing to exposure of the whole Middle-Upper Yangtze platform,indicative of rapid subsidence induced by extensional faulting.