Studies On Sedimentary Facies And Provenance Of Waigucun Formation In Lvchun, Yunnan

Study area is located in the western of Ailaoshan orogenic belt and thesouth-eastern of Lanping-Simao Basin. The investigations such as chronologyrestriction of the detrital zircon, basin type and relation of basin-range coupling insouth part of Lanping-Simao Basin is deficient.The Waigucun Formation distributes as beddings separated by Paleozoic strata,such as Siluric, it is unconformable with the underlying Yangbazai Formation, andconformable with the overlying Sanhedong Formation. The lithologic characteristicsof Waigucun Formation are distinct between north and south part. It is an importanttransition point of the tectonic setting and deposition.This thesis indicates that the lower part of Waigucun Formation mainly depositsin delta facies, the middle and upper parts are mainly braided plain fan deposits andriver plain deposits. The sedimentary facies of Waigucun Formation located in Jiache,Baliu and Dashaba are front delta, braided plain fan and river plain, respectively.The Trassic is the developmental stage of the foreland basin, WaigucunFormation is the oldest sediments in the basin. With the Ailao Mountains orogenthrusted from east to west, the basin expanded toward the west, the sedimentarysystem of Waigucun Formation formed multiple sedimentary cycles which composedof sandstone and conglomerate.The results of the ancient curent direction,the characteristic of gravel constituent,the characteristic of debris constituent and Dickinson triangular diagram shows thatthe provenance was from orogenic zone in east.Comparative study about characteristics of detrital zircons, such as dark androse-colour zircons increasing in different degrees, shows that the sediments ofWaigucun Formation belong to the same provenance area, but some certaindifferences still exist in local, for example, Jiache area are closer to provenance area. Associated with the detrital zircon chronology results, an early-middle Triassic islandarc can be suggested in the east of study area. In addition to the ancient metamorphicstrata, Silurian to middle Triassic also provided the provenance to the basinAt the initial deposition stage of late Triassic Waigucun Formation, the orogenicbelt uplifted rapidly, and the delta deposition developed in the bottom of the forelandbasin. At the mid-late period of Waigucun Formation deposition, Ailao Mountainsorogen thrusted from east to west again, and the foreland thrust belt expanded towardforeland basin direction, as a result, multiple cycles of conglomerate or sandyconglometateâ†'pebbled sandstoneâ†'sandstone can be observed. The basin wasmainly filled with alluvial fan-braid river deposits. The sediments of fan root withinalluvial fans and central fan still remain in he middle (Guilong and Mosu area) andwestern part (Baliu area), which indicates the orogen and foreland thrust belt hadexpanded from east to west in the mid-late term of Waigucun Formation deposition.