Fluvial or aeolian? Unravelling the origin of the silty clayey sediment cover of terraces in the Hanzhong Basin (Qinling Mountains, central China)

Abstract

This study is focused on a silty clayey sedimentary sequence on a terrace in the intramontane Hanzhong Basin, located in the Qinling Mountains (QLM), central China. Traditionally, the QLM are considered to have blocked dust transport from northwest to southeast China. However, in recent years, geo-archaeological studies have documented loess-palaeosol sequences at numerous locations in and surrounding the QLM. In the loess deposits overlying the terraces of the Hanjiang River in the Hanzhong-, Ankang- and Yunxian basins, abundant artefacts, flakes, stone tools (e.g., scrapers and choppers) and cores are commonly found. The loess deposits have been deposited with lower sedimentation rates, and they are finer grained and more intensely weathered compared to the loess deposits on the Central Loess Plateau (CLP). The loess deposits overly coarse sandy and gravely fluvial deposits (terraces). Silty fluvial deposits are situated in between them. Discrimination between these two types of deposits could prove difficult because both deposits are fine grained (silt and clay) and can have similar grain size distribution characteristics. This is, however, crucial for palaeo-environmental interpretations during hominin occupation, understanding fluvial morphodynamics, and for pedostratigraphic correlation with the typical loess-palaeosol sequences on the CLP. The aim of this research is to determine and characterize the transition of the fluvial to aeolian depositional environment in a fine grained sequence, based on field observations, organic matter and carbonate content, grain size and shape analyses, mineral content (mica's) and end-member modelling of the grain size dataset. In addition, terrestrial cosmogenic nuclides (TCN) burial dating is used to determine the age of the basal, coarse grained fluvial deposits. The determined age, 0.6 ± 0.14 Ma, allows for a chronological correlation of the deposits to the loess-palaeosol sequence on the CLP independent from the pedostratigraphic correlation. This age also gives insight in terrace abandonment and the fluvial morphodynamics of the Hanjiang River.The result indicates a clear distinction between sediments deposited in a fluvial environment and those formed in an aeolian depositional environment. However, the aeolian (loess) deposits show some atypical characteristics. For example, the end-member model results show a coarsening in the five palaeosol layers. This is in contrast with the fine grained nature of palaeosols on the CLP. The coarsening observed in the studied palaeosol layers is interpreted as the result of local surface runoff processes, eroding fine sediment and/or depositing relatively coarse material during interglacial periods.Because of the known depth of the fluvial-aeolian transition and the absolute age of the TCN burial dated terrace deposits, pedostratigraphic correlation of the palaeosol layers with the Central Loess Plateau is possible. The oldest palaeosol is correlated with S5 (0.625–0.503 Ma). The transition from a fluvial to aeolian environment takes place in L6, between 0.625 and 0.693 Ma. This is consistent with the TCN age of 0.6 ± 0.14 Ma. This age also marks the abandonment of the terrace caused by incision of the Hanjiang River, which is possibly related to an uplift phase of the QLM.