The fluvio‐tidal transition of suspended sediment in terms of mineralogy and composition in the Loire River drainage basin, the largest French river basin, was investigated in the fluvial zone at Montjean and in the tidal zone at Mauves‐Thouaré, for a complete seasonal cycle. At Montjean, where the river experiences unidirectional flow, the composition and mineralogy (especially clays and clay minerals) of river suspended material (RSM) are governed by the river discharge, upstream contributions, climatic conditions and microbiological activities. However, due to reversing tidal and river currents at Thouaré, in the zone of tidal dynamics, these relations are changed. In the downstream direction sand and clay content in the RSM decrease while the silt content increases. Among clay minerals, between these two observation stations, montmorillonite remains constant, kaolinite diminishes, and the other minerals increase downstream. Combustible material (organic) and nitrate (NO3) contents in the RSM increase whereas the phosphate (PO43‐) and CaCO3 contents decline considerably during transport. At the head of the tidal zone, where the river flow encounters the tidal influence, there is a relatively stationary water mass (tidal slack) where sands, clays, phosphate, carbonate and silica are deposited by physical and chemical processes. Physical sedimentation takes place by simple gravitational deposition (sands), and by sorting and complicated differential settling (clays); chemical sedimentation takes place by precipitation (calcite‐CaCO3; apatite‐Ca5 (PO4)3 (OH,F,Cl); coagulation of dissolved silica‐SiO2) in connection with seasonal algal bloom and eutrophic events.
Statistical size distribution and scanning electron microscopic studies of suspended sand grains in the Loire River at Montjean, France were carried out over a period of a year (hydrological cycle) to discern seasonal variations. The sand fraction in suspension is better sorted during winter (average mean = 0.69 mm, median = 0.65–0.95 mm, sorting value, σ= 1.1–1.35) and is dominated by quartzo‐feldspathic minerals. During summer, on the contrary, this fraction is rich in mica minerals and is poorly sorted (average mean = 1.21 mm, median = 0.33–0.95 mm, sorting value, σ= 1.4–2.0). The grain size of the coarser fraction shows a tendency to increase with river discharge up to an optimum discharge of about 1000 m3 s‐1 and thereafter decreases. Scanning electron photomicroscopy of quartz grains from the suspended sand population indicates that both mechanical and chemical features occur. The former (conchoidal fractures, mechanical fractures and breakage) dominate in the samples collected near the water surface during winter floods and the latter (solution pits, vermicular features, silica flowers, neogene silica and diatoms) in the samples from the bottom during low summer flows. During summer, chemical action takes place on sediments prior to or upon their deposition. Sediments that are resuspended during winter floods undergo mainly physical processes (attrition/abrasion). Mixed surface features are, therefore, observed during average and low river discharge. Inheritance of these surface features from the source area is, however, not completely excluded. Thus, the history of quartz grains in suspension can be reconstructed from SEM exoscopic studies: therefore, it can be proposed that the fluvial quartz grain surface textures result from a combination of alternating chemical and physical processes.
Lithofacies, delta and reef paleoecology, southward migration of fore reef from Carboniferous to Permian, Spitsbergen
L'étude des argiles des sols et des roches mères de Grèce occidentale permet de déceler une répercussion des variations climatiques Plio-Quaternaires sur un fond hérité. Ces variations peuvent servir à une meilleure approche stratigraphique et sédimentologique de cette période. La comparaison avec les faciès homologues du pourtour méditerranéen conduit à une individualisation nette du domaine hellénique. Une confrontation avec les dépôts marins, en bordure du rivage ionien, montre une convergence remarquable. On dégage ainsi : — une phase kaolinisante (inter Mindel-Riss) — plusieurs phases à montmorillonite (en particulier inter-statiaires würmiens) dans un contexte hérité illitique. Un état de rhexistasie généralisée caractérise le Quaternaire récent.
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