Search

The distribution of Mn, V, Th, Pb and isotopes of Pb in the labile fraction of sediments from a channel infill in the Middle Loire alluvial plain are used to highlight some aspects of the basin evolution over the period from 0 to 10 ka BP. The acid extractable matter (AEM) in the sediment samples is variable in amount and in trace element contents. Iron-Mn oxyhydroxides are the principle trace element carrying phase in the labile fraction and carbonates are a secondary carrier. Vanadium and Pb originate from the weathering of silicates and are used as a silicate erosion rate index in the fluviatile record. Most of the AEM data plot along a general trend between 3 endmembers (basalts, Cretaceous carbonate rocks and granites) in the relationship between 207Pb/206Pb and 206Pb/206Pb. These endmembers have been mixed in various proportions depending on natural Holocene inputs (erosion, volcanic events) or human influences (mining and smelting of ore).

The purpose of the geophysical campaign carried out in the Val d'Avaray was to contribute to a reconstruction of fluvial dynamics in the middle reaches of the River Loire. The campaign used of three classical geophysical methods (electromagnetic profiling supplemented by electrical soundings, electrical resistivity tomography and ground penetrating radar [GPR]) to define alluvial bodies. In addition, it aimed to detect the presence of clayey–peaty paleochannels and estimate the depth of incision of the Loire into the autochthonous calcareous bedrock. Paleochannels represent the only datable sediments that can be used to define a reliable chronological and stratigraphic framework. The electromagnetic profiling survey enabled the construction of conductivity maps that gave a general plan view of the various alluvial deposits and the positions of the paleochannels. The campaign identified three sedimentary bodies of different apparent resistivity deposited at three different periods; Body A of sandy composition overlain by thick clayey loam, Body B of rather sandy composition overlain in certain areas by a thin layer of clayey loam and Body D which represents the actual sandy deposits of the River Loire formed during flood stages. A fourth body (C), between B and D, was detected only by morphological analysis and could not be identified with EM mapping because the resistivity contrast with Body B was too low. The deposits rest on a relatively conductive ( ρ ∼60 Ω m) marly-limestone bedrock, which is at shallower depth beneath Body A than Body B. The survey also successfully highlighted conductive features corresponding to paleochannels made up of peat or pre-existing beds of the River Loire. The resistivity cross-sections obtained by electrical resistivity tomography and the GPR profiles showed the vertical geometry of the deposits. The data from these methods can be combined to produce precise 3D models that are of considerable value to geologists.

Diagenetic mineral sequences in Triassic and Lower Lias sedimentary successions studied in the Balazuc borehole are seen to have modified the initial reservoir qualities of the formations. The upper part of the Triassic Lower Sandstone unit and the Hettangian Lower and Upper Calcarenite units were initially good quality reservoirs, whereas the Triassic Upper Sandstone unit and the Rhaetian Sandstone unit were initially average quality reservoirs, temporarily improved by pedogenesis. Early burial induced partial plugging of the initially porous bodies as a result of fluid exudation from proximal sediment bodies; the circulation of these fluids brought about alteration, dissolution and mineral deposition. Tectonic events associated with the formation of the Southeast Basin palaeomargin caused movement along the Uzer Fault and imparted a fracture porosity to the buried sedimentary formations. However, increasing subsidence and burial reactivated fluid circulation, dissolution and mineralization with further plugging of pore spaces. Present reservoir quality, as assessed from measured water porosity and neutron density well logs shows a changed reservoir distribution. Apart from the main Uzer Fault and one secondary fault (1644–1647 m) that today form drainage channels, the Triassic Lower Sandstone unit and its topmost breccia are now non-reservoirs, the Triassic Upper Sandstone, Rhaetian Sandstone and Hettangian Calcarenite units form a succession of residual-matrix-porosity reservoirs isolated by non-porous units.