摘要：

四川盆地东部(文中简称“川东”)上二叠统长兴组近顶部非礁相地层尚未见暴露剥蚀现象的报道。文中以川东涪陵地区二龙口长兴组顶部非礁相地层剖面为例,综合宏微观特征、矿物学特征及地球化学特征等分析发现,在长兴组顶部存在厚3～7cm、垂向分带特征清晰的风化壳,自下而上可分为4层:泥晶生屑灰岩构成的基岩、微风化层、以富硅脱钙为特征的半风化层(由下至上分为3个微层:致密的半风化层、具晕圈的铁质夹层和疏松多孔的半风化层)和以富铁为特征的全风化层(由下至上分为3个微层:橙红色富铁质层、杂色纹层状黏土层、红色富铁质表层)。风化壳中黏土矿物几乎全是伊利石,未见到强风化阶段的次生矿物如高岭石、三水铝石,氧化铁矿物富集,总体体现出中等风化的特征,对应于风化壳演化的富铁锰阶段。进一步分析发现,该风化壳之上的飞仙关组底部存在厚约80cm的泥页岩与灰岩韵律地层,并富含珊瑚、腕足类(欧姆贝)、苔藓虫及三叶虫等原位埋藏的典型二叠系宏体生物化石,反映了研究区在二叠纪末期曾发生抬升暴露;同时研究区长兴组野外露头和钻井岩心均发育较多垂直溶沟、溶洞等岩溶组构,并为离解的碳酸盐岩砂、角砾和碳质泥等混合充填,说明川东地区长兴组非礁相地层受到了表生岩溶的改造。川东地区二叠纪末期的海退事件不仅使台地区海水变浅,甚至长时间的大面积暴露,而且使长兴组碳酸盐岩普遍受到表生岩溶改造,甚至非礁相地层也可能被改造成为岩溶型储集层。这一结果不仅对二叠纪晚期海平面升降事件研究具有借鉴意义,而且指出了台地内受到表生岩溶改造的非礁相地层也可作为勘探有利区带,因而具有重要的勘探实践意义。

关键词: 长兴组, 古风化壳, 表生岩溶, 川东地区

Abstract:

Whether there exists exposed erosion near the top of non-reef facies strata of the Upper Permian Changxing Formation in eastern Sichuan Basin has not been reported. In this paper,we take the non-reef strata at the top of Changxing Formation at Fuling area in eastern Sichuan Basin as an example,comprehensively studying the macro- and micro-features,mineralogical and geochemical elements characteristics. The result shows that there exists a weathering crust at the top of Changxing Formation. The weathering crust is 3-7cm thick and shows very clear vertical zonation, i.e.,from bottom to top,it is divided into four zones:the bedrock(micritic bioclastic limestone),saprolite layer,partially￣weathered layer which is characterized by enrichment of Si and decalcification,completely weathering layer which are characterized by enrichment of Fe.Since almost all the clay minerals available are illites, secondary minerals such as kaolinites and gibbsites of strong weathering stages have not yet been found, and iron oxide minerals are concentrated, it reflects an overall moderate weathering stage,corresponding to the ferromanganese-rich phase of weathering crust evolution stages. Further analysis reveales the presence of abundant typical Permian macroscopic fossils buried in situ,such as corals,brachiopods(Oldhamina),bryozoans and trilobites, etc., in the 80cm thick interbed of shale and limestone at the bottom of Feixianguan Formation which directly overlies the weathering crust,indicating that the study area uplifted and exposed during the Late Permian. At the same time,outcrops and cores in the study area exhibit plenty of karst fabrics such as solution grooves and karst caves, which are filled by mixture of porous carbonate sands,breccias,bauxitic mudstones and carbonaceous mudstones,indicating that the non-reef facies strata are modified by supergene karstification. The regressive event in the Late Permian was not only of seawater shallowing,but may lead to the long-term exposure of platform area,which modified reefs and beach reservoirs, and even the non-reef facies strata of the Changxing Formation by supergene karstification. This result is not only a reference for the study of the Late Permian eustatic events,but also points out that non-reef facies modified by supergene karstification inside platforms may also be a favorable exploration zone,thus exhibits important practical significance for exploration.

Key words: Changxing Formation, palaeo-weathering crust, supergene karst, eastern Sichuan Basin

中图分类号: 

• P512.1+3