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Structure of the large glacial basins in the Northern Iberian Peninsula, a comparison study: Andorra (Eastern Pyrenees), Gállego (Central Pyrenees) and Trueba valley (Cantabric Range)

Abstract:

Valley glaciers between 16 and 50 km in length developed during Pleistocene glacial maxima in the southern part of the Pyrenean-Cantabric range (Iberian Peninsula). Glacially-overdeepened basins are a common feature of the middle and lower sections of these glaciated valleys. Three geoelectrical units can be recognized in several basins: a thick lower unit, with low resistivity values (70 - 200 Ohms per metre), interpreted as fine glaciolacustrine deposits; a thinner intermediate unit, with higher resistivity values (400 - 800 Ohms per metre), interpreted as glaciofluvial deltaic deposits; and an upper geoelectrical unit with very variable resistivity values (100 - 1500 Ohms per metre) considered to be recent alluvial deposits. Vertical electrical resistivity soundings (VES) have been complemented by seismic soundings and geotechnical tests. Seismic profiles show anomalous high velocities (higher than 3000 m/sec) for the intermediate geoelectrical unit of deltaic sands and gravels. Pressurometer tests carried out in the sediments corresponding to the upper unit also show anomalously high pre-consolidation values (higher than 2 MPa) which cannot be attribuated to the sedimentary load. As shown by Boulton & Hindmarsh (1987), Boulton & Dobbie (1993), Boulton et al. (2001) in an Icelandic glacier, and Turu (2000) in the Andorra glaciated valley, layers with high seismic velocities and high consolidation values are best explained as subglacial tills deposited in setting of a high hydraulic gradients. The new data also suggests that the intermediate geoelectrical unit is of glacial origin.