Development of the submarine channel in front of the Nidelva River, Trondheimsfjorden, Norway

The Nidelva submarine channel is a 4 km long and up to 100 m deep erosional feature seaward of a former outlet of the river Nidelva, in Trondheimsfjorden, Norway. Mechanisms of channel evolution and sedimentary processes associated to its development are revealed by analyzing the sea bed morphology and the stratigraphy of the fjord sediments, based on high resolution seismic data, swath bathymetry imaging and gravity cores. Seismic data suggest that the submarine channel is eroded into Late Pleistocene clays filling a bedrock depression. A thick Holocene deltaic succession overlay the glacio-marine clays. Seismic facies suggest that turbidity currents likely initiated downcutting of the channel in the Early-Holocene. These flows were guided by sea bed morphology and bedrock exposures, favouring confluence and acceleration. The area was subjected to glacio–isostatic rebound and rapid fall of relative sea-level in the Holocene. This resulted in a fast delta progradation likely with an enhanced activity of delta derived sediment gravity flows promoting incision. In Mid- to Late-Holocene, such flows, including hyperpycnal flows, were responsible for deep incision of the channel. Laminated clay-rich beds are interpreted as deposited by the overspill of turbiditic flows along the channel and likely originate from large terrestrial quick clay slides. In the last 2000 years, erosive flows initiated by mass wasting along the coast controlled the evolution of the channel after the mouth of the Nidelva River moved to a more easterly position in the embayment. Present slope failure along the channel flanks seems to be controlled by the distribution of the laminated clay-rich layers in the stratigraphy.