Sedimentary processes and depositional environments in glaciated fjord systems — Evidence from Nordfjord, Norway

Abstract

Fjords situated in past, or present, glacial regimes represent important repositories between land masses (source systems) and the open oceans (sink systems). Newly collected TOPAS high-resolution seismic profiles from the previously glaciated Nordfjord system, on the west coast of Norway, reveal sedimentary processes and depositional environments within fjords in more detail than before. The seismic data show that the fjord basins are characterised by a well-laminated lower unit that is overlain by acoustic transparent lensoidal bodies. We infer the lower unit, which is up to 350 m thick, to be composed of glacimarine sediments. The transparent bodies represent up to 0.25 km 3 large slide debrites that comprise < 5% of the total sediment volume in the fjord. The Nordfjord basin well-laminated sediments were deposited from the fjord mouth to the fjord head as the Fennoscandian Ice Sheet started to retreat from the outer coast at c. 12 700 14C yr BP (c. 15 000 cal. yr BP), during the last deglaciation. The entire Nordfjord system was ice free for a short time period before the ∼ 1000 year long Younger Dryas readvance halted the main Fennoscandian deglaciation and over-ran the eastern part of Nordfjord. In contrast to the Last Glacial Maximum, we suggest that the sediments in the ice-covered fjord basins were preserved during Younger Dryas. The rapid isostatic adjustments accompanying the withdrawal of the Younger Dryas ice sheet promoted conditions for failures. We relate the identified Nordfjord slide bodies to this paraglacial environment. This implies that the acoustic well-laminated glacimarine fjord unit has been deposited in about 2000 yr, and that sediment rates as high as 20 cm/yr have existed during deposition.