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Mikrokonkrecje syderytowe w holoceńskich osadach ujścia rzeki Gwadiana w Portugalii : raport wstępny

Wilamowski A., Boski T., Moura D.

Przegląd Geologiczny

2013

Vol. 61, nr 10

568--575

The authors described micro-scale carbonate concretions occurring at a depth of about 41 m within Holocene sandy sediments filling the Guadiana River paleovalley, representing the transitional fluvial- -estuarine facies. Despite their minute sizes, the micro-concretions show a complex internal structure. Regular distribution of Mg, Mn and Ca can be assigned to a chemical zoning, pointing to precipitation under varying chemical conditions. Based on morphological and chemical peculiarities, four types of concretions can be distinguished. Detrital Fe-Mg-carbonates form nuclei. At first, Mn-rich carbonate precipitated as a coating, which is coherent with the sequence of precipitation of carbonates associated with bacteria-mediated oxidation of organic matter. Subsequently, the main bodies of concretions precipitated as Fe-carbonate (siderite). The last precipitation episode is marked by enrichment in Ca. The XRD patterns show the presence of a mixture of discrete carbonate phases.

Natural versus anthropic causes in variations of sand export from river basins: an example from the Guadiana River Mouth (Southwestern Iberia)

Dias J. M. A., Gonzales R., Ferreira Ó.

Polish Geological Institute Special Papers

2004

Vol. 11

95-102

Flood events in many river basins with highly variable discharge values remove accumulated sediments from the riverbed and estuaries. These sediments are exported to the shelf and the adjacent coastlines. Data for rainfall and river discharge for the Guadiana River basin in southwestern Iberia show a strong link with North Atlantic Oscillation (NAO) index patterns. A negative NAO index usually results in more rainfall, and subsequent flooding in the river basin during winter months. During the second half of the 20th century, the flow regime of the Guadiana River and its tributaries have been increasingly constrained by the construction of dams. The consequences were a reduction of coarse-grained sediment export from the upper river basin to the estuary, and a reduction in the number and type of floods.