Fossil Assemblage And Paleoenvironment Analysis Of The Lower Jurassic Ziliujing Formation In Wanzhou Area, Chongqing

Wanzhou District is located in the northeastern part of Chongqing,Da'anzhai Member of lower Jurassic Ziliujing formation have good outcrops and fossils.4 sections are observed in this dissertation,Da'anzhai Member of lower Jurassic Ziliujing formation is a suite of coquina and shale with unequal thickness interbedding.It can be divided into upper and lower parts.The upper part is a thick layer of coquina with mudstone and siltstone,The lower part is gray-yellow coquina and shale with unequal thickness interbedding.There are abundant bivalve fossils in the study area.5 genera and 10 species of bivalve fossils have been found,including Cuneopsis johannisboehmi?Cuneopsis sichuanensis?Cuneopsis wanxianensis?Pseudocardiniacarinata?Pseudocardiniahupehensis?Psilunio henanensis?Sibireconchasichuanensis?Pseudocardiniakweichouensis?Pseudocardinia elliptica?Tutuella rotunda.In addition,the trace fossil have been found.By looking up literature and comparing it with known index,it is found that the fossil size,morphology,and output state are different from those of other fossils.So,a new species of trace fossil have been built——Chongqingichnus wanchouensis ichnogen.et ichnosp.nov.It is the first appeared and it is the largest Jurassic trace fossils in Chongqing.Trace fossils are straight or curved,some are S-shaped,no branching,stacked one on another,substantially parallel to stratum.The coarsest part is about 30cm,the thinnest is about 3cm,the longest is up to 5 meters.Trace fossils have be found in the junction between the grey-deep gray shale and coquina,its characteristics are shown at the bottom of the coquina.The fossils of bivalves are deposited on the trace fossils,and it can be found in the whole Da'anzhai Member,which are closely related to them.On the other hand,it is rare that trace fossils and body fossils are well developed in the same stratum,so we try to build a special fossil assemblage,the combination of trace fossils and body fossils,Chongqingichnus-Pseudocardinia.According to judgment,the fossils are likely to be a biological trace.After discussion we think that the trace maker is a Gastropoda.Based on the analysis of paleontological characteristics and lithology of Wanzhou area,various sedimentary phenomena found by contrast,we think that Da'anzhai Member of the early Jurassic period,Wanzhou is a typical sedimentary environment of large inland lakes,it is alternating facies belt between shallow lake and-semi deep lake.