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Louise Hansen, Geological Survey of Norway (Norway)
Jean-Sebastien L'Heureux, Norwegian University of Science and Technology (NTNU) (Norway)
Oddvar Longva, Geological Survey of Norway (Norway)
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Clay-rich layers in silty sediments appear in gravity cores from Trondheimsfjorden, near the outlet of the Nidelva River. These layers have a characteristic light grey colour, which makes them easily distinguishable from the otherwise brownish grey sediments. The clay-rich layers often display a clear stratification whereas the surrounding sediments are generally bioturbated. Facies indicate that the layers were deposited relatively quickly from a combination of suspension and current activity and represent 'rapidly deposited beds' or 'events beds' that accumulated during sudden increase in the supply of clay to the fjord. The clay is interpreted to originate from large quick-clay slides along the earlier Nidelva River. Such slides are documented through large, prehistoric slide scars in glacioisostatically elevated marine clay deposits on land. The clay-rich event beds have been analysed with regard to grain size, sedimentary structures, X-ray, magnetic susceptibility, gamma density and shear strength. 14C datings on land and in the fjord indicate that several of these clay slides took place in Mid to Late Holocene. The purpose of the present work is to describe the facies variations of the clay-rich event beds and to interpret their depositional processes.
Data suggest that liquefied quick clay mixed into the river and affected the density of the effluent river plume giving rise to long-lasting hyperpycnal flow conditions. Lofting and mixing with fjord water gave rise to widespread deposition of hemiturbidites further away. It is likely that damming of the river by the larger slides was followed by dam breaching. This resulted in an abrupt, strong outflow from the river giving rise to fully developed turbidity currents as documented by some sand layers in the otherwise clay-rich event beds. The beds contain information on several aspects of clay slides on land including their magnitude, frequency, possible triggers and course of events.
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