Characterization and quantification of Holocene colluvial and alluvial sediments in the Valdaine Region (southern France)

Abstract

This study presents a Holocene sediment budget for the Valdaine Region, located at the edge of the southern French Pre-Alps. Holocene colluvial and alluvial deposition are quantified based on existing and new field data. Average sediment thickness values were calculated for different landscape units, and available chronological data were used to make a time-differentiated sediment budget. Total Holocene colluvial deposition in the Valdaine (334 km2) amounts 167 × 106 m3, while alluvial deposition in the Roubion and Jabron catchments (in total 610 km2, including their catchment upstream the Valdaine) amounts 177 × 106 m3. Especially, colluvial deposition is high (0.75 × 106 g/m2) compared with other catchments. Three major deposition periods (8500–2000 bc, 700 bc–ad 800 and ad 1200–2000) and two periods of relative hillslope stability with local fluvial incision (2000–700 bc and ad 800–1200) set the framework for a time-differentiated sediment budget. Results show that depositional phases relate to intense land use and hillslope stability and fluvial incision to land abandonment. Catchment averaged colluvial deposition increases from 13 × 10−6 m3/m2/yr for 8500–2000 bc to 355 × 10−6 m3/m2/yr for ad 1200–2000, while alluvial deposition increases from 16 to 147 × 10−6 m3/m2/yr between the same time periods. A relationship with climatic fluctuations was not found because of the limited temporal resolution of the sediment budget. The derived sediment cascade model shows how alluvial sediments change from fine (silt and clay) to coarse (sand and gravel) after ad 1200. This went along with the establishment of a braided river pattern, indicating that the main source of sediment shifted to the mountainous headwaters. Further research including fingerprinting and modeling would be necessary to further understand the sediment budget and to more accurately quantify the different source areas and the export from the catchment.