Études géoarchéologiques et archéobotaniques du comblement de la canalisation du site de la Rue du Ruisseau à Grand (Vosges, Lorraine, France)

The ancient urban area of Grand (Vosges, Lorraine, France) is characterised by numerous hydraulic infrastructures including a canalisation, crossing the ‘Rue du Ruisseau’ site, which has been in the focus of multidisciplinary investigations. Stratigraphic study and characterisation (sedimentology and micromorphology) of sediments filling of this canalisation and of associated carbonated crusts show a succession of accumulation and erosive phases, driven by variations of the water level. Sequential stratigraphy allows the identification of gaps in the sedimentary sequence corresponding to cleanings and backfills of some sections of the canalisation. Combustion waste and building materials were identified in thin sections while contamination by other types of waste, especially excremental, appears to have been quite low. Several levels of carbonated crusts were identified. They indicate phases during which water was low but not stagnant, for probably several years, as shown by crusts’ laminations indicating cycles that might be seasonal. The occurrence of calcite precipitation and the lack of detrital elements in these crusts are suggesting relatively clear water, at least during the crusting phases. There is no evidence of cyanobacteria or unicellular algae associated with calcite precipitation. Together with the absence of detrital material within the crusts, this suggests that the canalisation was covered - at least at the time of the different levels of crusting. An exception was a hole built in the section 1, where algae were encrusted by calcite. Palynological tests confirmed the bad preservation of the few pollen grains present within the sediments; any further pollen study would thus be hampered by taphonomic aspects. Regarding the retrieved plant macrofossils, a typically urban assemblage was identified, mainly comprising carbonized cereal grains and associated weeds, which must be derived from waste from various domestic activities.