Abstract
AbstractThere is limited knowledge about the preservation of aggradation phases in Quaternary fluvial records. Previous numerical modelling of erosion and deposition in Late Quaternary Allier River (France) generated the prediction that this river has reach‐specific fluvial dynamics related to climate‐driven tributary sediment‐flux dynamics. To test this prediction, new optically stimulated luminescence (OSL) samples were collected of the Late Quaternary Fx terrace at five locations along a stretch of 60 km. OSL dates of both quartz and feldspar sand grains indicate that all relatively basalt‐poor sediments display significantly different ages for each reach (ranging from 36.3 ± 2.0 to 21.1 ± 2.3 ka). The more basalt‐rich terrace body consistently yields ages in the range 21.1 ± 1.7 to 16.1 ± 1.5 ka, suggesting contemporaneous aggradation along the whole studied Allier reach during this interval. Our own new OSL date of a Tartaret eruption around 16.8 ± 2.5 ka also fits this time window, suggesting a direct link with volcanic activity. However, there are many more dated volcanic events that coincide with the older basalt‐poor units, making it less likely that a direct link between terrace‐sediment basalt content and volcanic activity exists. The timings of the dated depositional events in MIS 3 and 2 all match with simulated climate drivers and published landscape erosion rates. Counterintuitively, the volcanic Chaîne des Puys area supplied more sediment during the cold and dry Last Glacial Maximum. Basalt content in the Allier terrace sediments reflects climate‐related sediment‐flux dynamics upstream. The scarcity of older basalt‐poor sediment bodies from MIS 4 and 3 in the Fx terrace suggests that less sediment was supplied and/or the intermittent erosional phases in the Allier were very effective at removing them. We hypothesize that this observation of predominant preservation of the last aggradation phase could be a common phenomenon in most climate‐driven terraces. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd
Highlights
All around the world, sediments from MIS 2 are commonly found in the youngest fluvial terraces (Bridgland, 2000; Veldkamp et al, 2007; Counts et al, 2015; Winsemann et al, 2015; Olszak, 2017)
We describe all sampled sections and their locations, together with the optically stimulated luminescence (OSL) sample results
The fluvial well‐sorted sandy layers were sampled for OSL and specific attention was paid to the top basalt‐rich sandy unit embedded in more localized fluvial and slope deposits
Summary
Sediments from MIS 2 are commonly found in the youngest fluvial terraces (Bridgland, 2000; Veldkamp et al, 2007; Counts et al, 2015; Winsemann et al, 2015; Olszak, 2017). Attempts to extract more detailed climate information from these terrace records is often biased towards reconstructing the timing of incisional phases (Cordier et al, 2014; Vandenberghe, 2015; Kolb et al, 2017). These changes are often linked to a change in fluvial style, typically a shift from cold‐stage braided rivers to warm‐stage meandering rivers. A study of Tana River in Kenya demonstrated that in an equatorial fluvial system, without major shifts in its fluvial style, more sediments accumulated within the floodplain during a cold stage compared to a warm stage (Schoorl et al, 2019). The approach of pattern‐oriented sampling has been advocated by Briant et al (2018) to bridge this gap
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