Stepwise post-glacial transgression in the Rhône Delta area as revealed by high-resolution core data

Abstract

Integrated sedimentological and micropalaeontological (benthic foraminifers and ostracods) analyses of two continuous cores (40m and 50m long, respectively) recovered in the lower delta plain of modern Petit Rhône Delta, along with data from the previously studied Core SF, enable detailed reconstruction of the late Pleistocene–Holocene palaeoenvironmental evolution of the Rhône Delta system. Through identification of nine microfossil assemblages and with the aid of 28 radiocarbon dates, a high-resolution picture of stratigraphic architecture is offered for the first time for the entire post-glacial succession. Above late Pleistocene fluvial-channel gravels, a retrograding pattern of swamp and estuarine/bay deposits (lower transgressive systems tract — TST) is overlain by vertically stacked, late transgressive nearshore deposits (upper TST). This succession reflects the progressive landward migration of the palaeoshoreline driven by the rapid post-glacial sea-level rise, which took place in the study area between ca. 12 and 7cal ky BP. The following deceleration in sea-level rise (highstand phase) induced the onset of deltaic sedimentation, as evidenced by superposition of middle–late Holocene prodelta, delta-front and delta-plain deposits within the cored succession. At distal location the occurrence, within prodelta deposits, of a distinctive microfossil assemblage dominated by opportunistic benthic foraminifer Valvulineria bradyana marks the onset of the Rhône Delta mud-belt around 4cal ky BP. Identification and lateral tracking of six separate flooding surfaces within the TST enable the characterization of five short-term (millennial to sub millennial-scale) depositional cycles with distinctive transgressive–regressive internal architecture (T–R 1–5). The lower, transgressive portions of cycles 1–3 (11.8–9.8cal ky BP) mark abrupt shifts from freshwater to increasingly mixed, brackish-marine environments under conditions of rapid sea-level rise. In contrast, the upper, ‘regressive’ portions represent the filling of newly formed accommodation space via extensive crevasse to bay-head delta processes. Cycles 4 and 5 (9.8–8.2cal ky BP) and the uppermost transgressive deposit below the maximum flooding surface document three distinct landward shifts of the shoreline that took place under conditions of rapidly increasing sea-level rise. The vertical stacking of these millennial-scale cycles is interpreted to reflect the stratigraphic response to stepwise sea-level rise during the early Holocene. The youngest flooding surface may represent the sea-level signature of the 8.2ka climate event.