Study On The Simulation Experiment Of The Sedimentary Process And Sedimentary Architecture Of The Alluvial Fan Under The Control Of Normal Faults

Alluvial fan is a type of debris deposit that commonly occurs at basin margin.It often represents as a cone that points to mountain pass and extends toward plain.During the sedimentary process of basins,fault activities are common and often affects the sedimentary process and sedimentary architecture of alluvial fans.In order to further understand the control of normal faults on the sedimentary process and internal architecture of alluvial fans and discuss the differences in the sedimentary characteristics of alluvial fans,the sedimentary process of alluvial fans under the control of normal faults was simulated by flume tank experiment.Through experiment,it was observed that the sedimentary process of alluvial fan was divided into three stages under the impact of normal faults.At the first stage,the debris flowed through the footwall and then quickly unloaded and accumulated in hanging wall.The coarse particles accumulated as a set of near–triangle gravel bar on fault plane(distributary gravel bar).The bar continuously climbed along fault plane.During this process,the debris flowed through distributary gravel bar and its flow velocity decelerated due to blocking.The debris particles deposited at the tip and both flanks of distributary gravel bar.At the second stage,the sedimentary thickness of distributary gravel bar was equivalent to fault thrown.The height of the fan bodies in hanging wall was similar to that of the base level of footwall.Debris flow flowed toward fan margin along the fan plane with large dip angle in hanging wall.The deposition was gradually prominent in footwall.Vertical sand bars develped on the both sides of water channels and extended horizontally.Large area of overflow and cross–ditch phenomenon occurred on the surface of sand bars.At the third stage,the sandbodies in hanging wall and footwall were well connected.The distributary gravel bar disappeared.The control of fault on alluvial fans gradually decreased.With the constant supply of debris materials,the shape and sedimentary process of fan bodies gradually approached to general alluvial fans.Through the research on alluvial fan sandbodies,it was found that the alluvial fans had three special genetic units under the control of normal faults.(1)distributary gravel bar,it occurred near hanging wall.Coarse particles accumulated rapidly on fault plane and developed vertically along fault plane,which affected the direction and velocity of debris flow;(2)Fault plane–dominated lobe,it occurred in the two flanks of distributary gravel bar in hanging wall.Debris materials were hindered and decelerated after flowing through distributary gravel bar.They developed toward fan margin along fault plane after they were divergent at the end of distributary gravel bar,so fan bodies widened horizontally.(3)Over–bar lobe,distributary gravel bar extensively developed at its end when its height was closed to fault throw.After the main channel passed through footwall,strong water flowed through and eroded distributary shoal.Then,it rapidly accumulated toward fan margin after crossing distributary gravel bar,so fanbodies developed vertically.The differences in fault throw and fault dip angle affected the accommodation space of hanging wall and the hydrodynamic characteristics of hanging wall and footwall,thereby affecting the entire sedimentary process of alluvial fans.The greater the fault throw or fault dip angle,the more obvious the impact of fault on alluvial fans and the poorer the particle differentiation.Compared with general alluvial fans,the alluvial fans under the control of normal faults had the following characteristics in sedimentary process:(1)It had specific“distributary gravel bar” genetic unit.The existence of distributary gravel bar made the main water channel divergent on fault plane.As a result,water channels rarely migrated.(2)Affected by the large accommodation space of hanging wall,alluvial fan mainly extended vertically during sedimentary process and its extension degree was much larger than that of general alluvial fan.(3)Coarse particles mainly concentrated near faults.Due to the large dip angle of fan plane,the flow velocity of debris flow accelerated and more coarse particles deposited at fan margin.(4)Experimental analyses revealed that the alluvial fan under the control of normal fault structures had quite different internal architecture from general alluvial fans.In vertical,the former developed sand bar,distributary shoal and debris flow lobe,but the latter was dominated by debris flow lobe.Transversely,the former was mainly controlled by composite water channel,superimposed bodies of distributary gravel bar and superimposed bodies of multi–phase water channels,but the latter was mainly controlled by debris flow lobe and composite water channel.