| Alluvial fan is an important depositional system developed on the edge of the basin and its sedimentary characteristics and evolution processes can provide significant information in analyzing tectonic activities,changes of sediment source and climate.In the meantime,the coarse grained sedimentary bodies of alluvial fans can be an important reservoir type for hydrocarbon.However,due to the extreme complexity of sedimentary characteristics and controlling factors,the classification of alluvial fans is still internationally controversial and no uniform depositional model can be used to fully describe the detailed sedimentary facies and evolution processes of alluvial fans.Therefore,through detailed observations and analyses of outcrop sections,field measurement and grading analysis of typical samples,together with satellite images and regional geological data,this research takes modern Baiyanghe alluvial fan in the northern edge of Heshituoluogai Bain as an example to systematically study the the characteristics and types of lithofacies and microfacies,and the regional distribution of depositional zones.Based on these studies,the depositional model of alluvial fans which chould be dominated by episodic flood events in arid conditions is established and the controlling factors of evolution processes and geomorphology conformation are analyzed.Moreover,the classification scheme of alluvial fans is discussed.The Baiyanghe alluvial fan is a large scale fan under arid conditions and its forming processes can be controlled by episodic flood events caused by heavy rainfall,snowmelt or a combination of the two.Therefore,the building processes of Baiyanghe alluvial fan can be divided into flooding stage,waning flood stage and stage between recurring flood events.The whole body of Baiyanghe Fan is dominated by tractive current deposition and no debris flow deposits can be found.Thus,the Baiyanghe Fan should belong to fluvial fan type.Altogether,14 lithofacies can be identified within Baiyanghe Fan and these lithofacies can be further classified into 9 sedimentary microfacies,including incised channel flood,sheetflood,unconfined streamflood and surficial ephemeral pond,which are developed during flooding period;gravelly or sandy ribbon-shaped channels which are developed during waning flood period;erosion lag,gully,aeolian sand and river terrace,which are developed during period between recurring flood episodes.Overall,the Baiyanghe Fan was mainly constructed by deposits of microfaceis developed during flooding period,which could be interbeded with deposits of other microfacies.Four depositional zones can be subdivided from proximal fan to distal fan,namely incised channel zone,sheetflood zone,transitional zone and unconfined streamflood zone.During flooding period,the construction of the fan body is dominated by sheet-like flows and characterized by coarse-grained sediments,large deposition thickness and wide coverage area.In the meantime,water-saturated wetland can appear adjacent to the distal fan area.During waning flood period,the construction of the fan body is dominated by channelized flows,which can take the form of typical distributary channel networks.Nevertheless,the individual channels can show various morphological characteristics and can gradually disappear downstream.In contrast,during period between recurring flood events,the fan can be suffered destruction processes and surficial secondary depositional processes.These processes can be widely distributed but in relatively small scale.The wetland adjacent to the distal fan area can dry up and disappear during this period.The depositional processes of fluvial fans which are formed by episodic flood events can be controlled by the changes of water and sediment supply during single flooding period.The geological conditions of drainage basin exert great influence on the properties of sediments and depositional flows together with water and sediment supply.Additionally,neotectonic activities can lead to the destruction of the fan body.However,these activities can also influence the migration and extension processes of the incised channel,which in turn can influence the construction of the fan. |
PDF Full Text Request