Geomorphological control of fine sedimentation on the northern Portuguese shelf

Six cores sampled at two mud fields located on the northern Portuguese continental shelf have been studied in order to evaluate the control processes of fine sedimentation. The southern mud patch, offshore from the Douro River, is limited on the western side by some Cretaceous and Paleocene outcrops that constitute reliefs of 5-30 m in width; on the contrary, the northern mud patch, offshore from the Minho River, extends along a flat region. Granulometric distribution shows that the sediments from the Minho mud patch are coarser than those from the Douro mud patch, where the grain size diminishes towards the west. Carbonate content is also higher in the Minho mud field. The sediment organisation in fining-up sequences in the Minho mud patch and in the eastern part of the Douro mud patch indicates sediment remobilisation by storms. The Douro mud patch has higher sedimentation rates than the Minho mud patch. The highest values are probably related to the existence of reliefs, which may offer hydrodynamic protection or function as a fine sediment barrier. The elemental distribution for chemical elements was determined in different fractions of these cores. Geochemical patterns are generally quite similar for both mud fields, indicating a common continental origin for the sediments. The major differences in the chemical composition are mainly controlled by the grain-size distribution, either reflecting the presence of coarse quartz particles or a local enrichment in biogenic material.En este trabajo se han estudiado seis testigos de la zona norte de la plataforma continental portuguesa con el objetivo de evaluar los procesos que controlan la sedimentación de materiales finos en esta región. Los fangos acumulados más al sur, frente a la desembocadura del río Duero, están limitados al oeste por relieves cretácicos y paleocenos de 5 a 30 m de altura. Los fangos septentrionales, frente a la desembocadura del río Miño, se extienden sobre una región plana. La distribución granulométrica muestra que estos últimos son más gruesos que los depositados frente al Duero, en los que el tamaño de grano disminuye hacia el oeste. El contenido en carbonato es más elevado en la zona del Miño. Las secuencias granodecrecientes encontradas en los fangos del Miño y en los del sector oriental del Duero indican retrabajamiento del sedimento por tormentas. La tasa de sedimentación es mayor en los fangos situados frente al Duero, donde los valores más altos están relacionados probablemente con la existencia de relieves, los cuales ofrecerían una protección hidrodinámica o actuarían como una barrera para los sedimentos. La distribución de la concentración de diversos elementos químicos a lo largo de los testigos fue determinada para diferentes fracciones. El patrón geoquímico es bastante similar en ambas zonas y pone de manifiesto una procedencia continental de los sedimentos. Las principales diferencias en la composición química están fundamentalmente controladas por la distribución granulométrica, bien por la presencia de granos gruesos de cuarzo, bien por un enriquecimiento local en material biogénico.Instituto Español de Oceanografí

Relación entre la evolución sedimentaria de la laguna de Gallocanta (Cordillera Ibérica, NE de España) y la historia de la vegetación de su cuenca durante el Cuaternario reciente

The analysis of the facies in the sediments from Gallocanta Lake allows the identification of three stages of sedimentation. The lowest stage related with the late Pleistocene corresponds with distal alluvial areas, the second stage reflects a shallow carbonated lake probably developed from the beginning of the Holocene to the Middle Age and, the last one corresponds to a saline-carbonated lake formed in the last centuries. In the alluvial facies the pollen concentration is very low. The following stages are related with different types of pollen assemblages. The stage of a shallow carbonated lake is associated with the development of a deciduous and evergreen Quercus forest in the basin and represents the maximum water depth, 4 to 10 meters, of the Gallocanta Lake. This period The last stage of saline-carbonated lake, dated at 1900 AD, coincides with the maximum phase of Olea cultivation contemporaneous with the decrease of the lake level. The depth of the water was between 0 and 2 m. The drying out of the Gallocanta lake coincides with the lake level lowering of other lakes in the Ebro basin suggesting a regional driving-factor (climatic and/or anthropic) as the main responsible for the observed desiccation of the last century

Grands bassins fluviaux périatlantiques : Congo, Niger, Amazone

Un suivi mensuel de la granulométrie des matières en suspension (MES) des fleuves Oubangui et Congo a été effectué sur six années, de 1988 à 1993, à Bangui et Brazzaville. La médiane des sédiments transportés se situe entre 0,16 et 3 um. les MES des périodes de crues sont les plus grossières. La fraction grossière (éléments de taille > 50um) de ces MES comprend une part importante d'éléments organiques. Leur teneur augmente fortement en période d'étiage, plus légèrement au moment des crues. Les MES sont composées presque exclusivement de kaolinite et de produits amorphes. S'y ajoutent des illites et, sur le Congo à Brazzaville, des smectites qui proviennent de la cuvette congolaise. La comparaison des observations obtenues sur le bassin de l'Oubangui et sur la partie aval du Congo, à Brazzaville, montre que les variations annuelles et interannuelles des MES sont plus visibles au niveau d'un bassin de taille réduite. (Résumé d'auteur

Characterisation of river bed and suspended sediments in the Rio Madeira drainage basin (Bolivian Amazonia)

The grain size distribution of river bed sediments in the Rio Madeira drainage basin shows rapid deposition of the coarser material (greater than 10 mm) as the river flows out of the Andean range to enter the Amazon plain (Llanos). In the Andes, the suspended sediment grain size varies between 0.02 and 0.10 mm in diameter, while in the Llanos the suspended load is dominated by fine silts, between 7 and 7 micrometers in diameter. In the Llanos, neither the bed sediment nor the suspended sediment varied along a 800 km reach. (Résumé d'auteur

Characterisation of river bed and suspended sediments in the Rio Madeira drainage basin (Bolivian Amazonia)

The grain size distribution of river bed sediments in the Rio Madeira drainage basin shows rapid deposition of the coarser material (greater than 10 mm) as the river flows out of the Andean range to enter the Amazon plain (Llanos). In the Andes, the suspended sediment grain size varies between 0.02 and 0.10 mm in diameter, while in the Llanos the suspended load is dominated by fine silts, between 7 and 7 micrometers in diameter. In the Llanos, neither the bed sediment nor the suspended sediment varied along a 800 km reach. (Résumé d'auteur

Grande Vasière ou Grande Sablière ?

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