| Popis: |
Summary Sandy layers of the sedimentary Roupar Basin (Galicia, Spain) contain ferruginous accumulations that can be classified into three groups on the basis of their morphological and analytical characteristics. Hard laminar crusts of goethitecemented sand are thought to have been formed in some cases by chemical diffusion of dissolved Fe 2+ to the capillary fringe of a former water table, and in other cases by oxidation and precipitation of iron oxides at the interface between sediment layers with different textures and different aeration conditions. Globular accumulations consist of an indurated coat of goethite surrounding blackish, acid, reduced material containing iron sulphides and sulphates; their origin is related to gases formed by the decomposition of organic matter in a lacustrine environment. Tubular goethite precipitates are the most striking features of these sediments and are attributed to redistribution of iron by channelled water flow (‘piping’). |
