El Bierzo. Étude sédimentologique et géomorphologique d’un bassin intramontagneux dans le NW de l’Espagne
Leidse Geologische Mededelingen , Volume 30 - Issue 1 p. 141- 181
SEDIMENTOLOGY AND GEOMORPHOLOGY OF EL BIERZO (NW SPAIN) The purpose of this investigation was to study sedimentation in an intramontane basin in its relation to the relief of the surrounding mountain area. El Bierzo, an intramontane basin in NE Spain, is partly filled by continental Tertiary sediments whose age is thought to be Vindobonian on the basis of comparison with those of the Duero basin. These deposits were analysed by sedimentological methods: determination of grain-size, grain roundness, pebble composition, mineralogy of the light and heavy fractions and of the clays (by x-ray). In some places the Tertiairy deposits overlie deeply weathered Paleozoic rocks, considered to be the C-horizon of paleosols of Tertiary age from which the red and more clayey A and B zones have disappeared. The latter, together with unweathered rocks, are thought to be the source material of the Tertiary beds. Five different facies have been distinguished in the Miocene deposits. In the SW there are red-brown conglomerates with pebbles consisting partially of shale (Las Médulas facies). The main mass of the basin sediments are mostly silts and clayey silts with some gravels, the sandy fractions again consisting mainly of shale fragments (Santalla facies). These deposits are therefore thought to derive from the the same source as those of Las Médulas and to represent the finer fractions which were transported farther. Near the borders of the basin there are some local grey calcareous deposits containing breccias that are assumed to have been formed near faults (Vega de Espinareda facies). On top of the beds in the Santalla facies there are again local conglomerates of a more yellow colour (Fresnedo facies). The Astorga-facies, lastly, forms a transition to the deposits of the Duero basin in the E; it contains red conglomerates as well as sands and silts. Among the clay minerals, illite usually predominates as in the source rocks, but in the stagnant waters of the basin centre montmorillonite was formed as well. Towards the E there is an increasing kaolinite content, and in one case a considerable amount of attapulgite was found. The heavy minerals are for the most part the common resistant species, with the addition of anatase (which occurs in lateritic soils) in the Astorga facies. These facts suggest that the Tertiary soil-forming processes were more intense (i.e. lateritic in type) in the eastern part than in the Bierzo basin proper. Sedimentation started when some parts of the Miocene relief, covered by a thick soil, began to rise and were partly eroded, and others subsided so as to form an area of sedimentation. Remains of the Early Miocene topography are preserved in various places as surfaces with low relief on which remainders of Tertiary deposits and deep weathering are found. The most important of these is the Brañuelas surface, a plateau separating the Bierzo from the Duero basin. This plateau must once have been covered by Miocene sediments, which means that the deposits of both areas were connected and that drainage took place towards the E. After the tectonic movements that affected the Bierzo basin towards the end of the Miocene, the connection was severed and the drainage direction was reversed to the W. Later, probably during the Villafranchian, pediments on the lower slopes of the uplifted mountain masses were covered by thin angular gravels(raña’s) and fanglomerates, and the erosion surfaces were remodelled. During the remainder of the Quaternary, five terrace levels were formed in the easily erocable deposits of the Bierzo, and the partial evacuation of the basin deposits was accomplished.
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Sluiter, W.J, & Pannekoek, A.J. (1964). El Bierzo. Étude sédimentologique et géomorphologique d’un bassin intramontagneux dans le NW de l’Espagne. Leidse Geologische Mededelingen, 30(1), 141–181.