再论四川盆地须家河组的海相潮汐成因——进一步论证与层序分析

A re-discussion on the origins of tidal deposits in the Xujiahe Formation of the Sichuan Basin:Further evidences and sequence analysis

自笔者于2008年首次提出四川盆地上三叠统须家河组为海相潮汐沉积以来,通过近3年来对大量岩心、露头剖面的观察与分析,又不断收集、整理了较多的证据,有必要对该区海相潮汐成因观点作进一步的论证。须家河组为海相潮汐成因的证据包括:①古生物方面,在须一段(原小塘子组)正常海相瓣鳃类化石与侏罗系淡水瓣鳃类化石之间,存在一个半咸水—淡水化石组合带;②沉积学方面,反向交错层、潮汐成因的砂泥薄互层、双黏土层和潮汐束、水流突变特征、煤球和牛角炭、风暴沉积、间歇性陆表暴露特征、再作用面、海滩砾石与滨面砂等,同时澄清了对双黏土层和潮汐束的误解;③矿物学方面,海绿石、菱铁矿结核、硼元素含量和碳氧同位素资料也表明沉积环境由海→陆的过渡性质。在介绍Dalrymple等人河口湾沉积模式的基础上,分析了须家河组一个三级层序的纵向相序和体系域的关系。

Since we proposed that the Upper Triassic Xujiahe Formation in the Sichuan Basin is of tidal facies for the first time in 2008,more evidences have been acquired through the observation and analysis of a number of core samples and outcrops in recent 3 years.In terms of paleontology,there is a brackish-fresh water fossil combination zone between the normal marine pelecypod fossils in the first member of the Xujiahe Formation(the former Xiaotangzi Formation) and the fresh water pelecypod fossils in the Jurassic.In terms of sedimentology,the evidences include reverse corss-bedding,thin sandstone and mudstone interbeds of tidal origin,clay double layers & tidal beams,characteristics of abrupt current change,coal balls & horn carbons,storm deposits,intermittent surface exposure features,reconstruction surfaces,shoal pebbles and shoreface sands etc.In terms of mineralogy,glauconites,siderite nodules,boron contents and carbon isotopes also show that the sedimentary environment is transitional.Based on the estuarine deposition model by Dalrymple et al,this paper analyzes the relationship between vertical facies sequence and system tract of a third-order sequence in the Xujiahe Formation.