The Upper Rhone Delta Sedimentary Record in the Arles-Piton Core: Analysis of Delta-Plain Subenvironments, Avulsion Frequency, Aggradation Rate and Origin of Sediment Yield

摘要

Results from analyses of the Arles-Piton sediment core, retrieved from the apex of the Rhone Delta, highlight processes of Holocene deltaic construction controlled mainly by hydrosedimentary variability and channel avulsions. The alluvial suite was investigated for grain size, sedimentary structures, CaCO_3, organic matter, heavy minerals and chrono-stratigraphy (~(14)C and archaeological/historical dates). The study shows the succession of six facies associations: a distributary channel (before 6157-5843 BC), a swamp (5719-5530/4796-4463 BC), a distal flood plain (5719-5530/4796-4463 BC), a distributary channel (4796-4463/2900-2503 BC), a proximal flood plain (2900-2503 BC/AD 270-290), and a crevasse splay (after AD 270-290). Substantial changes in hydrodynamics are strongly linked to three channel avulsions (before 6157-5843 BC, after 4796-4463 BC and after 2900-2503 BC). A correlation with the whole channel avulsion history of the Rhone Delta allowed us to propose an average rhythm of channel avulsion of c. 1450 years. From 5719-5530 BC to AD 270-290, the flood plain aggraded at the average rate of 2.5 mm/a. The aggradation rates were higher both in the proximal and distal flood plains, where sedimentation process is continuous. They were lower both in the active distributary channels, because of frequent truncation of the alluvial suite, and the abandoned channels where detritic inputs are minimum. The sediment supply arriving to the upper Rhone Delta was derived mainly from proximal source areas (Massif Central, Southern Alps) during the last 8000 years, except during the hydrological changes of Roman antiquity during which detritic inputs were derived firstly from the Northern Alps and Southern Alps, and secondly from the Massif Central.