四川盆地东部中二叠统茅口组顶部钙结壳的发现及其发育模式

对四川盆地东部涪陵地区角帮沟剖面中二叠统茅口组顶部钙结壳进行的岩石学和矿物学系统分析发现,该剖面中钙结壳的矿物组成和含量呈规律变化,可分为基岩带、过渡带及钙结带。岩石学分析表明,基岩带为灰白色中厚层泥晶生物灰岩,其内发育碳酸盐岩砂和岩溶角砾充填的岩溶系统;过渡带为黄白色、黄褐色、褐红色含燧石条带或团块状泥晶生物灰岩,其内发育钙质胶结的原地角砾充填的侵蚀漏斗;钙结带为局部钙结岩化的灰色亮晶生屑灰岩叠覆土黄色多孔块状钙结岩,其内发育多期裂缝、根模、肺泡结构和次生碳酸钙胶结物等典型钙结壳结构。矿物学分析表明,自过渡带至钙结带Ca CO3含量呈逐渐上升的趋势,推测内部次生碳酸钙逐步胶结富集是钙结岩形成的主要原因。结合四川盆地茅口组顶部以发育铝土质泥岩和铝土岩的岩石组合为特征,且其与中二叠世上扬子地区位于赤道附近的古地理背景相吻合,表明四川盆地东部地区茅口组顶部钙结壳形成于湿热气候条件下;并通过与经典钙结壳特征的比较,总结出湿热气候条件下的钙结壳发育模式。该研究成果不仅是对四川盆地中二叠统茅口组表生成岩作用产物的补充,而且还可为国内外同类型钙结壳研究提供详实的材料。

Discovery of caliches at top of the Middle Permian Maokou Formation,eastern Sichuan Basin and their developmental model

This paper aims at petrological and mineralogical study on the top of the Middle Permian Maokou Formation where caliches occurred in eastern Chongqing City. It has found that the caliches can be divided into three lithological zones consisting of mother-rock zone,transitional zone and calcrete zone from bottom to top,and that mineral composition and content vary in a regular pattern. Petrological study indicates that,the mother-rock zone consists of medium- to thick-layered grey-white micritic bioclastic limestone,where epigenetic karst is identified by carbonate sands and karstificated breccia filling. The transitional zone is composed of yellowish-white,yellowish-brown and brown-red micritic biolithite limestone,with chert nodules and cherty bandings,while erosional funnels filled by in-situ breccias of calcite cementation are pervasive. The calcrete zone is mainly composed of grey crystalline bioclastic limestone,locally calcretized,but overlaid by khaki porous block calcretes,where representative micro-structures of multi-stage fissures,rhizoliths,alveolar structure and secondary carbonate cements are recorded. Mineralogical study shows that,CaCO₃ content inclines to increase gently from the transitional zone to the calcrete zone,from which it infers that the enrichment of secondary calcite interior cementation is the main cause for calcrete developing. The rock assemblage of bauxitic mudstone and bauxite occurred at the top of Maokou Formation in Sichuan Basin is consistent to the palaeogeographic settings during the Middle Permian in Upper Yangtze region,and it suggests that caliche may develop and be preserved afterwards in such a warm-wet climate. Then comparing with the classic caliches,a developmental model of the caliches that occurred in a warm-wet climate has been summarized. The results may not only provide supplements for epidiagenesis produced within Maokou Formation during the Middle Permian in Sichuan Basin,but also offer specific materials for the study of the same types of caliche home and abroad.