Sedimentary Petrologic Characteristics And Paleoclimate Implications Of The Danxia Formation In The Danxia Basin

The Danxia Basin of north Guangdong Province is a nearly rhombic terristrial faulted basin.The Mt.Danxiashan within the northern part of the basin is representative of the World Natural Heritage "China Danxia".Previous studies were focused on description of landforms.However,sedimentologic studies of the red beds are relatively inadequate.In this paper,the methods including field measurements of outcrop sections,pebble counting,thin section observations,grain size and composition analysis are used to study the depositional environment and paleoclimate of the Danxia Formation of Late Cretaceous.Moreover,the paleoclimatic significance of sedimentary facies changes are discussed as well.The Danxia Formation is further subdivided into three members,the Bazhai Member,the Jinshiyan Member and the Baizhaiding Member.The Bazhai Member's main characteristics are the development of bottom erosion interface,channel cut-fill structure,grain sequence bedding,parallel bedding,imbricated structure conglomerate.There are several sedimentary sequences with similar structural characteristics in the Bazhai Member of the Wumaguicao section,it is affected by multiple periods of flooding.The Jinshiyan Member develop large plate-liked and wedge-liked cross-bedding thick-layer aeolian-fine-grained sandstone,medium-thin-layer conglomerate and paleosoil.In the paleosoil,there are traces of fossil,mucous stratum-sedimentary stratum and mud-fissured structure.The Baizhaiding Member is also dominated by the basal erosion surfaces,the bedding conglomerate and the imbricated structures.The statistical results of gravel show that the average content of quartz in Zhanglao Peak section of Danxia Formation is 53.08%.The second is sandstone with an average content of 26.83%.Limestone and granite are relatively low in content.The sandstone in the Jinshiyan section of the Danxia Formation is mainly lithic feldspar sandstone,and the particles support the debris structure,mostly point-line contact.The sorting of the clastic particles is medium-preferable,and the roundness is subangular to sub-rounded.In the grading analysis,the probability accumulation curve is mainly composed of three segments and partially with two segments.The jumping component accounts for more than 50% of the content.It contains a lot of rolling components,the sorting of the rolling components is medium.The suspended component content is small.In general,the Danxia Formation has a coarse-fine-coarse variation in the vertical direction.The Bazhai Member mainly develops alluvial fan facies.The Jinshiyan Member mainly develops desert facies,which consists of the aeolian sand dune subfacies and the desert dry valley subfacies.The Baizhaiding Member also mainly develops alluvial fan facies.The conglomerate strata of the Bazhai Member may reflect relatively humid paleoclimatic conditions,rich rainfall,strong river handling capacity,and a large amount of coarse debris to be deposited downstream,reflecting the river-dominated alluvial fan deposition system.The aeolian sandstone-paleosol sequence in the Jinshiyan Member may indicate the desert sedimentary system under extremely dry and hot paleoclimate conditions,which may be related to the extensive aridification events during the Campanian Age in the mid-latitudes of East Asia.By the time of the deposition of the Baizhaiding Member,the climate gradually changed to humid,and the river-oriented alluvial fan sedimentary system once again dominated,and the formed conglomerate covered the aeolian sandstone.