Sedimentary Characteristics Of The Lower-middle Jurassic Sewa Formation In Rongma Area,Tibet

Based on the stratigraphic section?fossil collection and microscopic observation of the of the Sewa Formation in southern Rongma township,Nima county,Tibet,the auther analsised the sedimentary sequence and sedimentary tectonic environment of the strata and then divided the sedimentary facies,sedimentary subfacies and sedimentary microfacies of each period.the analysis and research of biostratigraphic characteristics is also a key point of this paper,thus trying to understand the Jurassic strata in temporal and spatial distribution.The main rock types of the Sewa Formation in the study area are clastic rocks,and carbonate rocks are mostly isolated and rootless.Clastic rocks contains sandstone and mudstone.According to the lithologic characteristics,sedimentary structure and other environmental indicators of the profile,the Sewa Formation in the study area is mainly a steep slope type submarine fan deposit including two subfacies of middle fan and outer fan,in which the outer fan is mainly gray and gray-black thin layer siltstone,silty mudstone and mudstone rhythmically alternation,containing a small amount of small end cut channel siltstone lens bodies.the main lithology of the middle fan is medium thin layer siltstone,siltstone and silty mudstone,forming an unequal thickness rhythmically interbedded layer.the subfacies contain more channel siltstone lens bodies and gradually change from the bottom to the top in vertical direction.The middle fan mainly develops two microfacies,branch channel microfacies and foliar microfacies,while the deposition at the end of the branch channel and overflow deposition constitute the main sedimentary units of the subfacies of the outer fan.There is no obvious difference between Chinese and foreign fans,reflecting that the sea floor fan was of steep slope type at that time.Isolated limestone blocks are scattered in the middle and outer fans of the profile,and the block size is relatively large.based on remote sensing maps and regional geological data,isolated limestone can be found in the middle-lower Jurassic zewa Formation in the study area.the isolated limestone is distributed intermittently alongthe east-west direction from the easternmost to the rongma area and the westernmost to the gaize county.Coral fossils were collected from collapsed limestone on the section of the Sewa Formation in the study area.after laboratory slicing and identification,the collected coral fossils were all hexagons,which were dated to the late Triassic noli period.Based on the analysis of stratigraphic tectonic background and stratigraphic correlation,the age of this stratum is limited to after the late Triassic.