Study on the Simulation Experiment of the Sedimentary Process and Sedimentary Architecture of the Alluvial Fan Under the Control of Normal Faults

Abstract

During the sedimentary process of basins, fault activities 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. 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. Debris flow flowed toward fan margin along the fan plane with large dip angle in hanging wall. At the third stage, the distributary gravel bar disappeared. 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 fan bodies developed vertically.

Compared with general alluvial fans, the alluvial fans under the control of normal faults had the following characteristics in sedimentary architecture. In vertical, the former developed debris flow lobe, but the latter was dominated by sand bar, distributary gravel bar and debris flow lobe. Transversely, the former was mainly controlled by debris flow lobe and composite water channel, but the latter was mainly controlled by composite water channel, superimposed bodies of distributary gravel bar and superimposed bodies of multi–phase water channels.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019