Study On The Lower-Middle Jurassic Sedimentary Facies And Trace Fossils In Gaize County Of Tibet

Paper based on the 《1:50000 Renacuo region of four control union test project)), we have a study of The Lower-Middle Jurassic sedimentary facies and trace fossils in in Gaize County of Tibet. Through stratigraphic profile, the regional stratigraphic correlation, the area is divided into the Lower-Middle Jurassic Sewa group and middle Jurassic Shaqiaomu group, and the basic characteristics of each group are discussed in detail, regional distribution, and at the same time, also has carried on the detailed division of sedimentary facies.The Lower Jurassic turbidite were well developed in Gaize area of Tibet and roughly along the east-west distribution to large area. Study of the rock types, sedimentary structures, and trace fossils combination in Northern Gaize show that the Lower Jurassic Sewa Formation is a set of turbidite. It mainly consists of gray-black and gray-green sandstone, siltstone, silty shale and carbonaceous shale; There are a variety of primary sedimentary structure in the sandstone, such as Graded bedding, parallel bedding and ripple cross-stratification, channel mode and slot die is common on the lower surface. Colluvial rock is the trias collapse limestone blocks in different size, and distributed roughly along the east-west direction. Turbidite facies can subdivided into 6 subfacies (A、B、C、D、E、G), and they Belong to 3 different sedimentary environments(inner fan, middle fan and outer fan).The inner fan include A and E subfacies, represent channel fills and natural levee deposits respectively. The middle fan have all the 6 sufacies, among which, A-C sufacies show that the waterways deposited into lobes body, D and E sufacies represent the overflow deposition between the waterways. The outer fan has E and G sufacies, and shows a flat terrain with no channel development. A large number of trace fossils belonging to Nereites ichnofacies have been found in the inner and middle fan. The trace fossils in the sandstone of inner fan is of low diversity and high abundance, and are characterized by simple eating tracks and feeding traces (Chondrites, Circulichnis, etc.), were formed by the early opportunity species biological. The trace fossils from the outer fan sandstone have both high diversity and abundance. There are many complex forms of fossil, such as snake curved, spiral and grid shaped tillage track and grazing trace fossils, they are the activity remains of the balancing species. The upward complicate of ichnofossil forms and feeding means, and the increase of species type from inner fan to middle fan, demonstrate that, under such a anaerobic reductive environment, as the water energy increase and sedimentary rate reduce, the substrate stability and organic matter content gradually increase. Overall reflect the depth of 1000 meters of the bathyal environment.According to Shaqiaomu group carbonate rock and terrigenous clastic rock interaction development, the characteristics of lithology and sedimentary structure characteristics, combined with profile that belongs to the mixed shelf facies deposition, and can be further divided into two subfacies restricted shallow water and open shallow sea.According to the profile structural analysis, think that change is in early and middle Jurassic has experienced the sea water from shallow darker and lighter in a cycle.