Sedimentary Facies And Trace Fossils Of The Rongmanan Mugagangri Formation In Nima County,Tibet

The paper relies on the "1:50,000 regional geological survey-Rongma South" project,through the field survey and indoor rock and fossil identification,the age attribute,sedimentary facies of the Nanmuganggang group of Rongma Township,Nima County,Tibet.The sedimentary environment and trace fossils have been studied in detail and some new insights have been made.Biostratigraphic research: The Muqigangri Formation is a set of deep-water sediments dominated by dark fine clastic rocks,characterized by gray thin-layered fine sandstone,siltstone and gray-black thin mudstone,and silty mudstone rhythm interbed.The deposition thickness is large,containing no or containing rare fossils.For the attributes of the times,the predecessors were mostly constrained by the era of overlying strata and underlying strata,and the group itself had few fossil evidence.In this actual survey section,four ancient plant fossils were collected from the 20 th layer of sandstone in the upper part of the Mudugang Formation.All of them were identified as the same genus,Neocalamites nathorsti Erdtman,according to which the age of the Muqigang group was Early-Middle Jurassic.Study on the sedimentary facies of the section: According to the lithology,sedimentary structure and the combination of fossils in the Muqigang Formation,it is considered that this group is a set of submarine fan deposits,and further divided into five rock combination types,three sedimentary subfamilies.Phases(internal,medium and external)and several types of sedimentary microfacies.According to the phase sequence structure of the section and the variation of the internal fan-middle fan retraction sequence and the external fan-middle fan's product sequence in the Muqigangri group,it is considered that the Muqigang day group has the characteristics of multi-layer fan superposition.Relics research: A large number of trace fossils collected in the wild in the Muqigang Group,based on their characteristics,branches,filling structures and braided structures,identified 10 relics of 11 relics,including Chondrites,Fucusopsis,Planetolites,Phycosiphon,Ovatiichnum,Megagrapton,Palaeophycus,Circulichnis,Macanopsis,Paleodictyon.These trace fossils are almost all developed on the rock level or obliquely intersected with the small angle of the layer.Therefore,they are classified into the Nereites relics separated by Seilacher(1967),and the environmental significance such as paleo-depth and oxygen content is discussed through extensive comparison.Provide further evidence for sedimentary environment analysis.