Analysis of dynamic sedimentary environments in alluvial fans of some tributaries of the upper Yellow River of China based on ground penetrating radar (GPR) and sediment cores

摘要

There are ten ephemeral tributaries of the Yellow River in the Inner Mongolia of China. As their catchments are covered by loose materials and are dominated by an intense interaction between wind and water erosion, these tributaries have had a high sediment yield and alluvial fans have been formed at the confluences of the tributaries with the Yellow River. However, there is a lack of understanding in the deposition model of the alluvial fans and the roles of fluvial and aeolian processes in sediment accumulation. In this paper, we used Ground Penetrating Radar (100 MHz) and sediment cores to investigate the sediments in channels and alluvial fans of four out of the ten tributaries. Radar reflection profiles of sedimentary strata are dominated by horizontal or sub-horizontal layers with a large scale, showing that the fluvial deposition on the alluvial fans has been dominated by a vertical accretion pattern. The reflection features like reflection free, inclined reflections, discontinuous, undulate, chaotic, sinuous reflections and channel fill exist to a lesser extent. A few GPR profiles send signs of intense erosion, and the one near the apex of an alluvial fan reveals that erosion had been active likely during the early Holocene and the channel position should be relatively fixed. The information extracted from this profile and the widely exposed late Pleistocene strata in the upper parts of the alluvial fans according to the geological map of the study area indicates that sediment accumulation has been slow in the upper alluvial fans since the early Holocene. The sediment cores indicate that sedimentary strata under the alluvial fans are mainly composed by sand and silt deposited in a fluvial environment and rarely reworked by wind actions. Also according to the core data, the sediment deposits in the west fans were coarser than those in the east fans, likely suggesting that the hydrodynamic force of the western tributaries was stronger than that of the east ones. The hydrodynamic environment of the tributaries was usually unstable historically and hyperconcentrated flows might occur with a higher frequency in one of the tributaries in history.