The late glacial–Holocene seismic stratigraphy and sedimentary environment in Ranafjorden, northern Norway

In the last 14–15,000 14C years BP up to 360 m of sediments were deposited in the 70-km long Ranafjorden, northern Norway. The greatest volume of sediment relates to the Allerød ice recession period, the Younger Dryas ice advance and the succeeding ice retreat period during which three ice-marginal deposits were formed. The Holocene period constitutes 15–20 m of the sediments. Based on high-resolution seismic data and sedimentary cores, eight seismostratigraphic units are described. Focus on the Holocene unit has revealed environmental changes that can be ascribed to different events: Rapid isostatic uplift after a period of high sedimentation rate early in Holocene has triggered erosion and generated sediment failures and slumping on slopes into the fjord basins. The Norwegian offshore megaslides at Storegga (about 7200–7400 14C years BP) and Trænadjupet (about 3600–4000 14C years BP) generated tsunamis that caused debris-flow deposits and shell-bearing sand layers to be deposited in the fjord. Marked acoustic reflectors are associated with these events. Increased slump activity and marked seismic reflectors in the mid-and especially the late part of the Holocene are related to climate variability and deterioration corresponding with the climatic changes known in Norway.