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Investigation of a glacial outwash sequence on the swiss plateau using high resolution seismic reflection... [Etude d'une séquence fluvioglaciaire sur le plateau suisse à l'aide de sismique réflexion à haute résolution]

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Published in Quaternaire. 2000, vol. 11, no. 3, p. 207-217
Abstract High resolution reflection seismic investigations were carried out on the Swiss Plateau south of Fribourg to delineate the geometry of glacial and penglacial, up to 100 m thick, Quaternary sedimentary bodies. Five main sedimentary units/subunits associated with two major glacial cycles are infilling a network of glacial palaeovalleys cut into the Tertiary Molasse bedrock. These units consist mainly of fluvial, fluvioglacial and glaciolacustrine deposits, with two intervals of diamictons. Among those, a major outwash sequence formed by a sandur system is of particular interest, because its lower part contains a significant aquifer. Some 5 km of high resolution seismic data were acquired with a hammer source and processed using PC-based softwares. Seismic energy was sufficient to penetrate up to ca. 100 m of glacial sediments. Seismic profiles were calibrated with information from unlogged boreholes and nearby outcrops. Glacial palaeovalleys can be clearly identified. In the Quaternary sequence, three to four units can normally be distinguished. The laterally discontinuous, compacted, subglacial till sequences may be identified by high amplitude reflectors. The base of the laterally more continuous outwash sequence is usually marked by a good reflector. The top of this sequence is quite reflective when capped by poorly permeable diamictons, but poorly reflective when the latter have been eroded and porous fluvio-glacial deposits directly overlie the outwash sequence. Consequently, this change in reflectivity helps to outline the zones where the aquifer is exposed to surface pollution risks (« hydraulic windows »). Although patchy, the data acquired in this study demonstrate how high resolution seismic can contribute to delineate glacial and periglacial sedimentary bodies in the subsurface. In particular, the vertical and lateral extension of the aquifer-bearing outwash sequence can be mapped. Seismic information also helps to identify the nature and geometry of the rocks encasing the porous reservoir (e.g. the capping sediments, or the flanks of the palaeovalleys), thereby giving clues of possible pathways for the water to reach the reservoir.
Keywords AquiferGlacial and fluvio-glacial sedimentsHigh resolution seismicOutwash sequenceSwiss Plateau
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Structures
Research group Limnology and Environmental Geology
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SIGNER, Claude et al. Investigation of a glacial outwash sequence on the swiss plateau using high resolution seismic reflection... [Etude d'une séquence fluvioglaciaire sur le plateau suisse à l'aide de sismique réflexion à haute résolution]. In: Quaternaire, 2000, vol. 11, n° 3, p. 207-217. https://archive-ouverte.unige.ch/unige:90825

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Deposited on : 2017-01-04

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