Unraveling the deposition and incision paces of alluvial fan-river system by using single grain K-feldspar luminescence dating

Abstract

Alluvial river in the piedmont area deposits and incises cyclically, in response to both exogenetic and endogenetic forcing across a range of timescales. The paces of these processes unveil the mechanisms governing the evolution of alluvial fan-river system and affect the policy for river management. Numerical simulation suggests that the incision of alluvial river is much faster than the deposition process. However, the field observations are rare. Alluvial rivers entrench up to 300 m into the alluvial fans built pre-Holocene on the northern piedmont of Chinese Tianshan, which provide ideal sites to unveil the pace of alluvial fan-river evolution. Jingou River is one of these rivers, with four levels of terraces identified at the downstream area. These terrace deposits are characterized by a complex of upper very coarse gravels and cobble (VCGC) unit and lower medium and coarse gravels (MCG) unit. In this study, potassium-feldspar (K-feldspar) single grain pIR₁₁₀IR₁₇₀ luminescence dating procedure was employed to date the samples taken from sandy lens of both upper VCGC and lower MCG units, for which the depositional ages are supposed to be associated with the fan-building and river incision processes, respectively. The luminescence ages of the lower MCG unit suggests a gradual fan building phase between 16 ± 1.9 ka and 8.1 ± 0.9 ka, while the ages of the upper VCGC unit cluster around ∼5.2 ka rather than a monotonic decrease towards low level terraces. Several scenarios are discussed with respect to the attained age sequences to unravel the pace of deposition and incision of Jingou River, of which the asymmetric aggradation and incision is more preferred and echoes to the finding of previous numerical investigation.