Sedimentation And Diagenesis Of The Conglomerate Of Anning System In Lanzhou

The conglomerate of the Anning system is mainly outcropping in Dashagou and Caojiawan around the study area.It contains rich information of regional tectonic movement,palaeoclimate and diagenetic environment.And it plays an important role in the formation of the palatial Danxia morphology.The petrological,sedimentary and diagenetic characteristics and genetic mechanism of the Anning conglomerate have been studied systematically and comprehensively.The main points are as following:Petrological characteristics:(1)field features: The strata of Anning system is horizontal looks like palatial Danxia landforms.The conglomerate sandstone is hard due to calcite cementing.The particles are coarse-mega grains.Vertical joints were developed well.The conglomerate sandstones are columnar or mushroom due to weathering and erosion.The gravels of the conglomerate have a poor roundness and poor sorting.The conglomerate is the protective cover of underlying red loose sand layers.(2)The microcosmic characteristics of conglomerate sandstone: The main clastic particles of the conglomerates consist of quartz,feldspar,lithic debris.They have a medium-coarse grain structure.Feldspar particles have different slight degrees of weathered alteration.It was found that feldspars were replaced by iron calcite and kaolinite.The cements are mainly calcite,which are bright or muddy crystals shape.The main cementation pattern is of pore.Contact cementation occurred in someplace.Cathodoluminescence displayed that calcite is the main cement of the conglomerates.Sedimentary evolution process of the conglomerates: The red sand sediment of the Anning system is mainly formed in a vast basin.The gravels debris were transformed into the basin as delta or fluvial alluvial deposits around the basin to form conglomerate.Later,they all were uplifted as terrestrial landforms.And the Neogene red beds were strongly weathered and eroded by tectonic uplift under arid-hot climate.Formation mechanism of cement: The cement of the conglomerates is caused by original sedimentation,compaction pressure dissolution and leaching of meteoric water.The conglomerates have a good porosity and permeability.It is conducive to the rapid dissolution,evaporation and condensation of carbonate.When the climate was humid,the river carries particles into the lake and a lot of minerals were deposited in the sand and gravels layers.While,when the climate was arid,the carbonate minerals which were dissolved in gravel pore solution are supersaturated,precipitated,and crystallized at last.With the increase of the thickness of the sediment overlying the gravel layer,the compaction pressure dissolution process causes partial dissolution of feldspar,and the dissolution and recrystallization of primary carbonate.Later,tectonic uplift lifted the gravel layer close to the surface.The leaching of meteoric water brought abundant cementation materials into the sand-gravel layers.The formation of the conglomerate cement reflected the tectonic activity under the climate cycles from arid to humid and from humid to arid in Lanzhou area since Cenozoic.The controls of conglomerate on regional geomorphology.The Anning system conglomerate is the top boundary of Lanzhou Danxia landform.It is cemented hard and formed a hard cover,which is the key to the formation of palatial Danxia landform.Once the cover is eroded away,this kind of Danxia landform will also disappear.Danxia landform is an important geological relic resource.It is an important aspect to further develop tourism in Gansu Province to publicize "Tianfushagong Palace",the Danxia landform tourism resources,which has important humanistic value and economic significance.