Saturday, 15 July 2006

Red and Yellow Cambic B Horizons: Their Relationship with the Geomorphic Evolution of the Vouga Catchment (Portugal).

Virginia Pereira, Dep. Geociencias, Univ de Aveiro, Campo de Santiago, Aveiro, 3810, Portugal

The Vouga catchment, located in North Central Portugal, has an approximate area of 2500 km2. The climate is humid mesothermal; mean annual precipitation, temperature and evaporation are around 1000mm, 14oC and 1000 mm, respectively, on the littoral plain and 1900 mm, 12oC and 800 mm inland in the mountains. Two major physiographic units are recognized: the littoral platform, to the west, and the old massif, to the east. The littoral platform is a marine erosion platform where the oldest marine levels, probably Calabrian, are 160 to 200 m above the sea level. It is dominated by Ceno-Mesozoic deposits which include at the extreme west Quaternary dunes and aeolian sands, followed by Plio-Pleistocene terrace deposits. The inland mountains are dominated by a flysh-type complex of pré-ordovician scists and graywakes and by Hercynian granites. Cambisols developed on schists are widespread in the study area. They show two distinctive kinds of cambic horizons. The most extensive is characterized by a yellowish color (10YR 6/6 to 6/8) and silty loam texture while the second type, not so common, has bright red (5YR 4/6 to 5/6) colors. The present study was undertaken to investigate the genesis of these cambisols. Four pedons were selected in the study area, two of them, P1 and P2, having a yellow Bw and the other two, P3 and P4, with a red Bw. Their characterization included field morphology, physico-chemical routine analyses, clay mineralogy, heavy mineralogy and micromorphology.The data obtained indicate that the soils studied are formed from layered materials, a situation which is not always obvious from routine field observations. The complex nature of these soils required an interpretation based on the geomorphological history of the study area. The transition from the A to the cambic B horizon in soils 1 and 2 looks like a sharp erosional discordance. In many places the A horizon extends across the landscape without the B horizon because the latter is discontinuous; this field relationship, together with a sharp break in the light/heavy mineral ratio and the identification of past biological activity in the cambic B, substantiate the interpretation that the two horizons are genetically independent. The latter, easily recognized in the field by its characteristic yellowish colour and silty loam texture, is part of a fairly extensive soil probably formed under a previous hot and wet climate and severely truncated by the marine erosion dated Calabrian. This stratigraphic relationship suggests a Pliocene or older age for the paleosol. The A horizon is dated Quaternary by the erosion surface upon which it lies, i. e., the littoral platform, but it is not possible to be more precise. Soils 3 and 4, formed from schist colluvium, lie on the littoral platform. This stratigraphic relationship indicates a Quaternary age for the soil. As to the timing of rubifaction, some interglacial stages, particularly the Thyrrenian and the Wurmian, are considered to have favoured this process.

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