Intensified pedogenesis caused the increase in the fine particle content of late Cenozoic fluvial and lacustrine deposits in the NE Tibetan Plateau

Abstract

The grain-size composition of fluvial-lacustrine deposits in terrestrial basins is commonly used to reconstruct temporal variations in sedimentary processes, tectonic activities and paleoclimatic conditions. However, previous studies have generally ignored possible grain-size alteration induced by post-depositional weathering and pedogenesis, which undermines the use of grain-size analysis for reconstructing depositional environments and paleoclimatic changes. In the present study, the grain-size distributions of late Cenozoic fluvial-lacustrine deposits in the Xining Basin (XNB) in the NE Tibetan Plateau (TP) were analyzed. The results demonstrate that the contents of the ultrafine (<1 μm), clay (<4 μm) and fine silt (<10 μm) fractions of bulk siltstone/mudstone samples show an upward-increasing trend, with the depositional environment changing from a shallow lake to a fluvial and finally an alluvial fan environment. Combined with the results of detailed geochemical and rock magnetic analyses of size-fractionated samples, a positive relationship was found between the content of fine-grained sediments and the weathering/pedogenic intensity in the fluvial and lacustrine sedimentary sequences. These results indicate that, under suitable conditions, chemical weathering can significantly affect the grain-size composition of fluvial-lacustrine deposits in terrestrial basins. The results are therefore very important for understanding the processes of formation of fine-grained sediments in complex natural environments.