Abstract The Cambrian Atlas – Ossa–Morena – North Armorican Rift extended along West Gondwana from the end of the Pan-African and Cadomian orogenies until the diachronous beginning of drift conditions related to the opening of the Rheic Ocean. The along-axis rift cross-cut the western parts of the Anti-Atlas, High Atlas and Coastal Meseta, which were linked to the Ossa–Morena Zone and the North Armorican Domain, whereas several joint tectonic branches connected with off-axis rift transects of the Central Iberian, West Asturian–Leonese and Cantabrian zones (Iberian Massif), the Central and South Armorican domains, the Occitan Domain, the Pyrenees, and southern Sardinia. The pre-rift unconformity, post-dating the orogenic collapse, is characterized by initial (half-)graben development and subsequent infill, with slope-related breccias and conglomerates controlled by the denudation of surrounding uplands. Synrift pulses show regional extension and are distinctly identifiable on the top of rift shoulders, recording episodes of carbonate production due to their association with karst and hydrothermal processes. The break-up unconformity ranges from volcanic-free angular discordances and paraconformities to generalized uplift and denudation of subaerially exposed areas, associated with the onset of granite-dominant large igneous provinces (LIPs). The Furongian–Tremadocian (Toledanian) and Ordovician (Sardic) phases have been interpreted as due to: (i) Andean-type subduction magmatism reaching the crust in an arc–back-arc setting; (ii) post-collisional decompression melting without significant mantle involvement; and (iii) partial melting of the lower continental crust affected by the underplating of hot mafic magmas linked to superplumes.

Abstract Rivers in karstic environments are known to be greatly influenced by surface water–groundwater interactions, with significant localized inflows during floods from springs, or with losses that can dry up rivers. The Middle Risle River is frequently affected by the development of sinkholes in a chalk karst area (Normandy, France). In the 2010s, two new major sinkholes in the riverbed caused a complete loss of water into the underlying phreatic aquifer, causing the river to dry up over several kilometres. The resulting changes in hydrogeological processes and surface water–groundwater interaction greatly affected water quality, water use and water-dependent ecosystems, causing a political crisis in this river-dependent touristic valley. To understand these phenomena and improve crisis management, the Middle Risle Critical Zone Observatory was set up to enhance monitoring, surveying and/or modelling of groundwater and river levels, river and spring flow, water temperature and conductivity, and ecosystem characteristics (fish, macro-invertebrates and vegetation). The results showed notable impacts on fish, macro-invertebrates and vegetation, some plants proving to be reliable indicators of surface-water–groundwater interaction. The dynamics of local hydrogeological processes were assessed and linked to the measured effects on ecosystems and water supply. Inverse modelling based on an analytical solution of the diffusive wave equation assessed lateral flow during floods, quantifying the spatial–temporal variability of surface-water and groundwater exchanges. It also highlighted the important role of karst zones in both storage and flood-peak attenuation processes, thereby protecting downstream villages against floods.

Abstract The frequent sinkholes along the temporary water streams in Saffré basin can be compared with those occurring in Upper Normandy which are commonly referred to as ‘bétoires’. These collapses are not directly linked to karst cavities. They result from a draw-off effect of fine particles contained in the Plio-Quaternary alluvium lying above the carbonate Oligocene series. The draw-off phenomenon appears through fine cracks (1 to a few millimetres) open in soft palustrine limestones broadly comparable with chalk. Based on the knowledge of backfilling work design undertaken at some ‘bétoires’ in Upper Normandy, a new project was undertaken at four sinkhole funnels, which appeared near the surface in the Saffré basin at the end of the winter in 2017. The works consisted of digging the decompressed surface and filling up the excavation pit with materials of decreasing granulometry from bottom to top in order to ease the surface water infiltration through the identified fractured zones to the bottom of the excavation. This design was also complemented by the implementation of a filtering trench in between the watercourse and the filtering bed, laid above the fractured zones, which were enlarged by the dissolution process, on the one hand, and by the construction of a pipe network on top of the filtering bed to allow for pressurized air exhaust towards the surface, on the other hand. The pressurized air comes up from the aquifer as it recharges as soon as the surface water streamflow resumes and takes part in the mechanism, leading to sinkhole collapse, which should be prevented by this backfilling design.

Abstract The implementation of new groundwater tracer tests to the chalky karst plateau of the northwestern end of the Paris Basin, combined with a critical review of previous tracer test investigations, makes it possible to characterize the role of karsts in relation to many structural features. The impact of tectonic structures and lithology on the development and evolution of the karst networks is analysed. The consequences for drinking-water supplies and its protection are examined.

Abstract The study of the temperature of two rivers in Normandy (France), the Orne and the Touques, between 2013 and 2018 allowed the main controlling factors regulating their thermal regime to be determined. The analysis was conducted by coupling different statistical treatments: linear regression between water and air temperatures, independent component analysis, principal component analysis and a multiple linear regression model. The temperature of the two rivers is mainly controlled by climatic factors but secondary regulation factors are demonstrated to play important roles: runoff for the two rivers and groundwater for the Touques. The influence of the chalk aquifer on river temperature appears to vary seasonally throughout the year, reaching its maximum in the early spring and increasing from upstream to downstream. The coupled use of the different statistical complex methods showed its validity in understanding both the temporal and spatial variations in water temperature and its correlation with the secondary factors, that could not be inferred from a simpler approach based on linear regression. These techniques could be valuable in other areas with rivers sufficiently monitored to determine the controlling water temperature factors and thus their sensitivity to climate change.

Abstract The Ordovician successions of France and neighbouring areas of Belgium and Germany are reviewed and correlated based on international chronostratigraphic and regional biostratigraphic charts. The same three megasequences related to the rift, drift and docking of Avalonia with Baltica can be tracked in Belgium and neighbouring areas (Brabant Massif and Ardenne inliers), western (Rhenish Massif) and northeastern Germany (Rügen). The remaining investigated areas were part of Gondwana in the Ordovician. The Armorican Massif shares with the Iberian Peninsula a Furongian–Early Ordovician gap (Toledanian or Norman gap), and a continuous Mid–Late Ordovician shelf sedimentation. The Occitan Domain (Montagne Noire and Mouthoumet massifs), eastern Pyrenees and northwestern Corsica share with southwestern Sardinia continuous shelf sedimentation in the Early Ordovician, and a Mid Ordovician ‘Sardic gap’. In the Ordovician, the Maures Massif probably belonged to the same Sardo-Occitan domain. The Vosges and Schwarzwald massifs display comparable, poorly preserved Ordovician successions, suggesting affinities with the Teplá-Barrandian and/or Moldanubian zones of Central Europe.