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

Integrated sedimentological and micropalaeontological (benthic foraminifers and ostracods) analyses of two continuous cores (40 m and 50 m long, respectively) recovered in the lower delta plain of modem Petit Rhone Delta, along with data from the previously studied Core SF, enable detailed reconstruction of the late Pleistocene-Holocene palaeoenvironmental evolution of the Rhone 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 7 cal Icy 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 Rhone Delta mud-belt around 4 cal 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 (11.8-9.8 cal Icy 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 (9.8-82 cal Icy 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 8.2 ka climate event. (C) 2013 Elsevier B.V. All rights reserved.