River avulsions and sedimentary evolution of the Luanhe fan-delta system (North China) since the late Pleistocene

Abstract

The Luanhe Fan-Delta (LFD) is one of the most important coastal deltas in North China. Numerous investigations on the late Quaternary evolution of this fan-delta system have been implemented during the 1980s. However, few detailed sedimentologic studies have been undertaken. Based on comprehensive analysis of sedimentary facies, grain size, microfossils, radiocarbon dates (AMS14C) and optically stimulated luminescence (OSL) ages from three ~30 m-long cores, along with recent reports from a series of cores and seismic profiles from the coastal areas, we reconstructed the detailed late Quaternary sedimentary evolution of the northern Bohai coast, which evolved from a coastal barrier-lagoon system during MIS5 to an alluvial plain, with river channels and floodplains between MIS4 and the Early Holocene. A variety of transgressive coastal depositional environments (freshwater marshes, beach ridges, sandy barriers, shallow seas) developed during the early Holocene. These environments were followed by delta progradation. The delta system then gradually turned into a coastal barrier-lagoon system under the combined impact of prevailing wave-driven currents and local subsidence. Two dramatic 90-degree diversions of the lower Luanhe River course likely influenced the sedimentary evolution of the LFD in its inner portion. The first avulsion took place in Qianxi, probably at the end of the Late Pleistocene (~15,000 cal a BP). It led to the abandonment of the Luanhe alluvial fan in the western part of the northern Bohai coastal plain (NBCP), and to the formation of a new alluvial fan in the eastern NBCP. This alluvial fan evolved into a fan-delta system (7000–3500 cal a BP) during the Holocene transgression. The second avulsion occurred at Zhuacun during the Late Holocene, probably at 3600 ± 100 cal a BP. Following this latter avulsion, the Luanhe River shifted toward the east and formed a new fan-delta system (~3700 cal a BP–present), with its apex in Luanzhou. Abrupt tectonic uplift in this highly seismic region likely controlled the two avulsions and the dramatic shift of the alluvial-fan complexes. Climate and sea-level changes may also have partly influenced the development of the LFD.