Study On The Sedimentary Environment And Gravity Flow Characteristics Of Longwuhe Formation In The Lower-Middle Triassic Series In Wenbadi, Qinghai Province

The Gonghe Basin is a foreland basin developed in Wenbadi area of Qinghai Province, since Indosinian period in the early Triassic. Because of lacking in data, there is little geological research in this basin. In recent years geological research has been focused on regional mineral resources and magmatic activities. In the paper excellent outcrops from a 9.5 km long field section named PM12 and several adjuvant field sections of Longwuhe Formation was described. Based on the previous research results, combining petrology, sedimentology, geochemistry and sedimentary basin analysis theories and methods, the article showed the results of analyzed the sedimentary environment and the dynamic mechanism of gravity flow. On the basis of above, we summarized the tectonic setting of the sedimentary environment evolution in the research area combining with geochemistry data.Based on the lithologic composition, texture and structure information from the protolith reconstruction, the paper was divided the lithofacies compositions into 8 main types and 13 subtypes according to different depositional events.In addition, the paper was summarized fining-upward lithofacies combination and coarsening upward lithofacies combination composed by different lithofacies representing different kinds of hydrodynamic configurations.There are four specific composite types in each combination, which represent the sedimentary system from marine-continental transitional facies to shoreface facies. A lot of gravity flow events sedimentary exist in it. So considering the characters about fluid density, support mechanism, particle characteristics, and the fluid mechanics properties, in the article, the gravity flow was divided into two gravity flows as following:1) laminar debris flow with non-Newtonian rheological properties;2) disorder turbidity current with Newtonian rheological properties;and the macro-slip massive slump deposits. In the body of my paper, the debris flow was divided into argillaceous clastic flow and sandy debris flow; and turbidity current was divided into high-density turbidity current and low-density turbidity. Slump deposits are mainly composed by the macro-slip landslide of carbonate rocks. The mechanism of the formation of the gravity flow can be described the continuous process as trigger-carry-sedimentation. And different types can be transmuted into each other caused by the energy reduction and the fluid properties changes caused by topography.Based on the combination of petrology and sedimentology, we analyzed the greywacke which is composed of barrier-free shoreface sediments and gravity flow event sediments in lower-middle Trias series in Wenbadi, Qinghai Province, mainly by geochemical research method. Finally, we can draw conclusions from the quantitative analysis of the major elements of the greywacke: 1) the lower-middle of the Longwuhe Formation was mainly located in the active continental margin(ACM), which has large deposits of flysch sedimentary formation; 2) the upper of the Longwuhe Formation transited from active continental margin(ACM) to the passive continental margin(PCM), which develops flyschoid deposits close to the shore. Due to the superposition of orogeny in geological history, the original sedimentary had undergone regional metamorphism, formed the epimetamorphic rock including schist- phyllite- metamorphic sandstone in the outcrops.