Palaeoenvironmental evolution of the Seine River estuary during the Holocene.
Résumé
Quaternary climato-eustatic oscillations and tectonic movements result in base-level changes that modify the shape of fluvial systems. At the onset of the Holocene, NW European fluvial systems were deeply incised and comprised several terraces. Some terraces now lie beneath the estuary that formed in response to the Holocene sea-level rise. The macrotidal estuary of the Seine River is one example of that NW European fluvial valley evolution. Although the system has been extensively studied over the last 40 years, no study has drawn together all the research on its Holocene evolution. We gathered available data on five sites distributed across the system, in order to discuss that evolution and the related forcing factors. We have identified four main phases. (1) Prior to 6000 BC, there was a retrogradational shifting of the shoreline, where pre-Holocene topography and the sea level were the main forcing factors. Only depositional environments of low-elevated areas were affected by the influence of the sea-level rise, while the others were affected by climatic changes. (2) Around 6000-5000 BC, there was a maximum flooding event, where all depositional sites were affected by the influence of the sea-level rise. (3) Between 5000 and 1100 BC, there was an aggrading sequence, where tide and local hydrological conditions were the main forcing factors. The deposition record indicates that there were open tidal environments toward the sea, while landward peat accumulated, and was sometimes disturbed by tidal channel formation. (4) From 1100 BC, there was a progradational shifting of the shoreline, where human activity was the main forcing factor. Seaward environments at this time still indicate open-tidal conditions, while landward ones show a return to clastic deposits related to floodplain deposition. All the 14C data accumulated in the area should allow us to present a regional sea-level curve in the near future.