Neogene fluvial sediments in the northern Jinshaan Gorge, China: Implications for early development of the Yellow River since 8 Ma and its response to rapid subsidence of the Weihe-Shanxi Graben

Abstract

Origin of the Neogene fluvial sediments in the Jinshaan Gorge is a significant issue concerned with development of the Yellow River. Field investigation shows that the bedrock topography underneath the Neogene fluvial sediments is an up to 8–10 km-wide valley (The Tanghsien-phase valley). The Tanghsien-phase bedrock valley is made up of two flat terrace surfaces. Altitude evidence shows the Neogene fluvial sediments almost continuously distribute on a successively southward-lowering bedrock valley along the gorge. Sedimentology analysis based on roundness and gravel composition is performed and two types of Neogene gravels are classified. The first type of gravels shows the same provenance as those on the lower Quaternary terraces, namely, they were abandoned by the same river -a south-flowing river through the Jinshaan Gorge area or the ancient Yellow River. The second type of gravels interbedded with the Baode red clay could be alluvium from the Lüliang Mountain. Magnetostratigraphy indicates that the oldest fluvial sediments in the Hequ and Baode areas can be constrained to ca. 8 Ma and the sediments have a westward younger variation trend. The westward younger fluvial sediments during 8–3.7 Ma imply that the ancient Yellow River through the modern Jinshaan Gorge area might have undergone a fairly long-time lateral erosion progress and have shaped the broad valley under the circumstance of stable climate and tectonics. Owing to pulsed subsidence of the Weihe-Shanxi Graben driven by northeastward growth of the Tibetan Plateau since the Pliocene, the lateral planation progress of the Yellow River was broken up and the inset Jinshaan Gorge was finally formed.