Large river chronology along the Jinshaan Gorge on the Yellow River and its implications for initialization

Abstract

The ancient fluvial terrace is one of the keys to understanding how a large river develops in long-term landform evolution. However, surface erosion usually dissects the once extensive terrace into discontinuous segments and makes it difficult to explore the evolution of a large river worldwide. Fortunately, an exception is found in the Jinshaan Gorge of the Yellow River (JGYR), which preserves many Late Miocene-Late Quaternary fluvial terraces and thus can help understand the origin and evolution of a large river. Detailed investigation of the high terraces along the JGYR indicates a broad valley that was later superposed by a V-shaped valley due to rapid incision. To constrain the timeframe in which the broad valley persisted, as well as the onset time of the V-shaped valley formation, we select ten typical fluvial terrace profiles, apply terrestrial cosmogenic nuclide (TCN) 26Al/10Be burial dating to yield the ages of terrace deposits, and correct the results by considering both inherited and postburial nuclide production during slow red clay and loess deposition. Integrated with previous studies of magnetostratigraphic ages, our chronological results show that the lateral erosion and valley widening of the infant JGYR formed a broad valley during ca. 8–3.7 Ma; subsequent downward incision began ca. 3.7 Ma, accelerated to form a V-shaped valley and incising meanders into the initial broad valley since ca. 2.5 Ma. Moreover, most terrace ages plot in either periodic or short-lived arid conditions during ca. 8–3.7 Ma, while a distinctive downward incision has occurred under strengthened oscillating arid conditions since ca. 2.5 Ma, suggesting that climate aridification favours terrace aggradation, and tectonic uplift mainly dominated incision, which was consistent with the onset of the extension of the Shanxi Graben System ca. 8 Ma and the enhanced uplift of the Ordos Block since ca. 3.7 Ma.