Architecture and evolution of a modern braided fluvial system: The Weihe River in northern China

Modern sedimentation plays a crucial role for understanding ancient sedimentation in the stratigraphic record. This study reveals 3D architecture and evolution of a modern braided fluvial system. The Weihe River is the largest tributary of the Yellow River in northern China. Recent archaeological excavations revealed sets of ancient bridges in the Xi'an segment over this waterway, which provided an opportunity to understand the inner architecture and evolution of this river. Excavated pits and adjacent natural gullies were analysed using the architectural elements methods, revealing nine lithofacies types, and seven distinct architectural elements. Architectural elements typical of braided fluvial deposits including downstream accretion, lateral accretion, and channel fill elements can be identified in the studied sections. The results suggest that the Xi'an segment of the Weihe River was deposited as a braided fluvial system during the Western Han Dynasty (ca. 2,000 years ago). The sandy sediments which filled in the abandoned riverbed were mainly transported by the main stream from the west, whereas the gravel lithofacies packages were supplied by near-source tributaries that originated from the southern Qinling Mountains. Calculations based on empirical equations suggest that at the time of the Western Han Dynasty, the width of the river course was between 400 m and 500 m, and the river depth was between 2.7 and 3.4 m, similar to the dimensions recorded in historic documents. The Weihe River has migrated approximately 4.3 km to the north since the time of the Western Han Dynasty. Considering the regional geological setting, the data suggest that this northward migration was controlled by the continuous uplift of the Qinling Mountains as well as movement of syn-sedimentary normal faults to the south of the Weihe Basin. These tectonic movements controlled the evolution of the basin and maintained the geomorphologic characteristics of such higher topography in the south and lower in the north over a long period of time.