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  Åbyfjorden on the Swedish west coast; morphology, superficial deposits and gas-bearing clays
Titel: Åbyfjorden on the Swedish west coast; morphology, superficial deposits and gas-bearing clays
Auteur: Hillefors, Åke
Floden, Tom
Soderberg, Per
Verschenen in: GFF
Paginering: Jaargang 115 (1993) nr. 3 pagina's 241-261
Jaar: 1993
Inhoud: Shallow marine reflection profiling and systematic areal studies onland are used to reconstruct the evolution of the 22 km long Åbyfjorden-Berfendalen fjord valley, trending N30°E almost across the Bohus Granite area on the Swedish west coast. The asymmetric bedrock morphology, the restricted tills and the bulk infill of glacial and postglacial marine clay and gyttja are outlined along the 13 km long Åbyfjorden. The eastern 'lee'-side of the fjord valley is heavily plucked by the ice, whereas the western 'stoss'-side is rounded and striated. The basic linearity of the Bohuslan fjord system is attributed to Mesozoic fluvial erosion along a set of extensional Permian faults of unknown strike-slip displacement which offset the Subcambrian peneplain by up to 75 m vertically. The fault system has probably been recurrently active since the opening of the Oslo Graben. Åbyfjorden consists of five rock basins which successively become larger (1×0.25-2.5×0.75 km) and deeper (60-120 m) towards the inlet of the fjord. Shallow rock thresholds (c. 40 m) separate the basins which are attributed to erosion during the Quaternary glaciations, their forms may possibly be accentuated by transversal faulting, too. Glacial tills (drumlins, stoss- and lee-side moraines) are more frequent in the fjord than on land. They exceed 40 m in thickness only exceptionally. Glaciofluvial deposits seem to have the same appearance in the fjord as on land. Some glacial forms on the bottom of the fjord are not clearly defined. Varved glacial clays and postglacial clays and gyttja reach thicknesses of 120 m, smoothing out the irregular bedrock topography. A submarine channel with a seawards accentuated relief (max. 150 m wide and 10 m deep) extends along the length of the seaward sloping bottom of the fjord (max. 30 m deep at the fjord inlet). Gasses from deep levels in the fjord fracture migrate through the sediments; gas turbation structures are recorded in the varved clays and gas vacuoles occur at several levels. No pock-marks are recorded although active venting is indicated in the inner basins. Gas vents along the fjord support the concept of a tectonically active zone.
Uitgever: Taylor & Francis
Bronbestand: Elektronische Wetenschappelijke Tijdschriften

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