Study On The Cretaceous-Neogene Sedimentary Facies And Paleogeography In Hongliu Gorge,Yumen City,Gansu Province.

By the end of last lower cretaceous.From the bottom up,there are xiagou group,zhonggou group,huoshaogou group,baiyanghe group and shulehe group.It is mainly conglomerate,sandstone,siltstone,silty mudstone,mudstone and so on.The mudstone is only distributed in the upper strata of the xiagou group,Gypsum layer is distributed in the lower part of baiyanghe group.The sedimentary facies about Xiagou group mainly consists of braided river delta and lake.The Zhonggou group is mainly a fan delta sedimentary facies,and a large number of hetero-basis supporting sand conglomerate is considered as microfacies of debris flow deposition.The Huoshaogou group is mainly fan delta sedimentary facies.The xiagou group development of braided river delta facies and lake facies,braided river delta facies include braided river delta plain,braided river delta front and front subfacies,braided river delta plain subfacies including distributary channel and distributary channel between two kinds of microfacies.The zhonggou group,the huoshaogou group and the shule river group are all fan delta facies,including fan delta plain,fan delta front and fan delta front.The subfacies of plains include debris flow and swamp microfacies,and the front subfacies include subfacies of underwater distributary channels,underwater distributary channels,sheet sand and estuary.The lacustrine facies of baiyanghe group is divided into saline lake and shallow lake subfacies.Early cretaceous lower sulcus formation,The hongliu gorge is located in the braided river delta of the north rim of the qilian mountains.Drought-semi-arid climate conditions,The late deposit,Elevation of lake,The hongliu gorge area has become semi-deep lake-deep lake deposit.In late cretaceous period of the middle sulcus formation,Reduction of lake plane,Hongliu gorge becomes the edge of the lake basin.Due to the fluctuation of crustal difference,the difference between qilian mountain area and hongliu gorge area is significant.Debris flow occurs frequently,forming a fan delta in the lake.About 110 Ma years ago,the crustal activity was enhanced in the area of hongliu gorge,and the deep magma was upwelling and erupting.The late yanshan tectonic movement caused the uplift of the wine west basin,and the upper cretaceous was denuded by a large number of denudation.When the Oligocene group was deposited,the climate was drier,with less rainfall,resulting in increased salinity of the lake.Later in the deposition,the rainfall increased and the salinized lake became a freshwater lake.At the end of the Oligocene,the crust is uplifted and the hongliu gorge are exposed to the surface.The last stratigraphic unit in the hongliu gorge area is the newly formed area,and the sedimentary environment is the leading edge of the fan delta on the edge of the foreland basin.