Simulation experiment of sedimentary process and sedimentary architecture of alluvial fan under the control of normal faults

Normal fault structures are widely developed in basins and orogenic belts,which control the accommodation space and the distribution of sediments and thus affecting the morphology of alluvial fans. A flume tank experiment was carried to simulate and clarify the control of normal faults on the sedimentary process and internal architecture of alluvial fans.The results show that the large amount of sediments carried by debris flow tend to be unloaded near the hanging wall of faults and are subsequently reworked by traction current,which result in a triangular distributary gravel bar grows vertically on fault plane with the tip pointing to the source area. When the hydrodynamic force is strong,debris flow goes across distributary gravel bar and forms over-bar lobe at the tail of the distributary gravel bar. When the hydrodynamic force is weak,debris flow forms fault plane-dominated lobe along fault plane and is located on both sides of the distributary gravel bar. Under the control of normal faults and the barrier of distributary gravel bar,the unloading process of sediments varies greatly at different positions on the surface of alluvial fan. The particle size varies greatly among different facies,with coarsest grains developed on the fans of hanging wall,finer grained on over-bar lobe and finest sediments on fault plane-dominated lobe. The development process of alluvial fan can be divided into three stages,according to the sandbody thickness and fault throw of distributary gravel bar. The fault throw also affects the sedimentary architecture of alluvial fan,with larger the fault throw generating larger the accommodation space of hanging wall,longer development time of distributary gravel bar and more complex of the superposition pattern of the sand bodies inside the fan. The internal architecture of alluvial fan that is controlled by normal faults includes longitudinal sandbar,distributary gravel bar and debris flow lobe in the profile vertical perpendicular to the sediment source direction from the proximal to the distal end. Along sediment longitudinal section,composite channel,superimposed distributary gravel bar complex and superimposed bodies of multi-phased lobes are dominant facies.