Paleoenvironment Implication Of The Jurassic Trace Fossils In Gamba Of Tibet

Neo-Tethys is an ocean which has disappeared developed between the northern side of Gondwana and the southern edge of Eurasia during early Mesozoic.Reconstructing the Tethys and its evolutionary process through exploring and studying the ocean floor and the sediments of this ancient ocean has been great concerned all over the world.During Jurassic,the Gamba area of southern Tibet was located is on the northern passive continental margin of the Indian plate,belongings to the southern Tethys Himalayan sub-belt.In a relatively stable tectonic environment,the sediments of which are mainly composed of terrigenous debris and carbonate deposition.There are few body fossils but great number of trace fossils in the Jurassic strata in the study area.Because of the characteristics of in-situ preservation,slow evolution and wide range,the trace fossils can be a good indicator of the paleoenvironment.There was little research on the trace fossils in south Tibet,so it is very important to study the Mesozoic traces fossils in this study area.Based on stratigraphic division,we can establish the stratigraphic sequence of the Early-Middle Jurassic in the study area and then explore the law of relative sea level change,which can provide supports for correct understanding of the evolution of Neo-Tethys.In this thesis,the methods of petrology,sedimentology and palaeoichnology are used to analyze and study the lower Jurassic Ridang Formation?J₁r?of the middle Jurassic Zhela Formation(J_2-3z)of the Longzhong area in Gamba,and the following results are gained:1.According to the characteristics of lithology and sedimentary structure,the sedimentary environment evolution of the study area in the Jurassic can be divided into three stages:?1?the abyssal to hemipelagic facies in early stage?lower Ridang Formation?;?2?the shallow-sea shelf facies in middle period?middle and upper Ridang Formation?;?3?the bathypelagic facies in late stage?lower Zhela Formation?.2.There are a large number of well-preserved trace fossils in study area.A total of25 ichnospecies from18 ichnogenus were identified.Which were classified into 3ichnofacies according to their stratigraphic distribution:Nereites ichnofacies,Cruziana ichnofacies and Zoophycos-Nereites ichnofacies.According to their association features,which were divided into three assemblage zones:Phycosiphon-Paleodictyon assemblage zones,Beaconites-Palaeophycus assemblage zones,and Granularia-Neonereites assemblage zones;3.The sedimentary facies and ichnofacies reflect the evolution process of early-middle Jurassic sedimentary environment in the study area from the deep-sea to semi-deep sea?early-middle Early Jurassic?to the shallow sea?middle-late Early Jurassic?and then to the semi-deep sea?middle Jurassic?.