Origin and influence of the superficial structure on the morphogenesis (Eocene plateaux of the eastern Paris Basin)

The concept of superficial structure relates to surficial deformations that are determined neither by tectonic deformation nor by surface geomorphic processes sensu stricto. This work addresses two mechanisms leading to structural surface disturbances in the Lutetian strata of the Eocene plateaux of the eastern Paris Basin. Firstly, mechanical release which is associated with stress-relief during Quaternary valley downcutting. It results in lateral spreading of the middle Lutetian limestone, and subsequent sub-erosion of underlying sands may cause valley-side cambering. Secondly, ghost-rock karstification in the phreatic zone which is leading to surficial collapse of the upper Lutetian cover. Geological structure (thickness and porosity of the middle Lutetian limestone) and valley-side slopes stratigraphy (position of the upper Ypresian sand) control the spatial distribution of these processes. Ghost-rock karstification, conditioned by tectonic joints is confined to the west; to the east, cambering processes occur where upper Ypresian sand aquifer and alluvial aquifer are connected. During the Quaternary, periglacial conditions account for cambering processes and also for a second phase of karstification within an aquifer perched in the active layer, and from mechanical release joints. Resultant morphological features are then inherited and cambering is even a fossil process where upper Ypresian sand crops out above the valley floor. Consequently, large-scale landforms, derived from the superficial structure, are essentially inactive.