vidéo : les conditions de sédimentation dans les milieux continentaux les milieux fluviatiles (شرح بالعربية)

Chapitre 1 : How to create a paleogeographic map of a region Study of the Earth Sciences Activity: Sedimentation conditions in the main current sedimentation environments -Sedimentation conditions in continental environments, fluvial environments After diagenesis, fluvial sediments form conglomerates, sandstones, siltstones and claystones. Many ancient series are made up of these facies: are they of fluvial origin? Coastal deposits have the same facies. The pebbles are worn and flattened; it is nevertheless admitted that their flattening is less than in the case of coastal pebbles. The sands are poorly or well sorted, depending on the length of transport; the grains are little worn or angular and show traces of shocks. The proportion of clay is always significant. Conglomerates and sandstones contain a matrix fraction. Siltstones and claystones often show paleosol horizons with root traces, sometimes in the form of calcareous crusts (calcretes). The facies are arranged vertically in a sequence that constitutes a sequence. Fluvial sequences are generally grano-decreasing (they are said to be positive), with a level of pebbles at the base and silts at the top. Among the sedimentary structures, there are in particular linguoid rides (in the shape of a tongue) and large oblique bedding when the network is meandering. The oriented structures (oblique bedding, imbrications) show a current on average unidirectional. Fossils are generally absent; only traces of organic activity are observed in the silts (burrows, roots). The large sandstone formations of the Devonian and Triassic of Europe (Old and New Red Sandstones) are fluvial deposits of braided or meandering networks. The alternation of periods of erosion and sedimentation in the history of a river produces stepped or nested fluvial levels called terraces. In an old river, fluvial sediments can be found in the form of terraces, either stepped or nested: Stepped terraces: in this case erosion is always greater than deposition, the terraces are stepped one above the other. Nested terraces: the erosion that follows deposition is less than that, so the course of the river during the erosion period does not clear all the sediments deposited previously, the terraces will be deposited one on top of the other, the lowest terrace is always the most recent. The formation of the fluvial meander reflects the dynamics of the water current in a river, because the arrival of water at high speed on one bank causes its erosion, and we obtain a concave bank: erosion bank and opposite this, we have a convex bank due to the strong sedimentation due to the low speed of the water current. The more this dynamic is active, the more the meander widens, thus increasing the width of the river bed. Meanders are the sinuosities described by the courses of water, each meander is formed by a steep concave bank which corresponds to the erosion zone and a convex bank in gentle slope which corresponds to the deposition zone. Fluvial terraces are figures that characterize sedimentation in fluvial environments, their formation is linked to the alternation of periods of deposition and erosion. When the erosive phases are very important compared to the depositional phases (resulting from erosion that dominates sedimentation) in areas with high rainfall), we speak of stepped terraces. In the opposite case we speak of nested terraces (resulting from sedimentation that dominates erosion in areas with low rainfall).