Fjord stratigraphy and processes – evidence from the NE Atlantic Fensfjorden system

Abstract

Fjords commonly hold sediment basins with high temporal resolution that mirror land-and marinebased processes and depositional environments during a glacial-interglacial cycle. Here, we present TOPAS seismic profiles, with a vertical resolution of <30 cm, bathymetric records and a gravity core from Fensfjorden, west Norway, which allow a detailed study of sedimentary processes and depositional environments in a NE Atlantic fjord system. Fensfjorden hosts a sediment volume of 0.70 km 3 and the sediment basins evolved reach a maximum thickness of 100 m. The basins are characterized by a lower acoustically well-laminated glacimarine unit, of Allerød and Younger Dryas age. This unit is overlain by several acoustically transparent lenses, interpreted to be slide debrites. We suggest these slide debrites to be a result of recurrent mass failures that occurred between 11 500 and 2100 cal a BP. A thin late Holocene hemipelagic unit drapes the slide debrites. During the late Holocene the sedimentation rates in Fensfjorden decreased from 0.110 to 0.040 cm a À1 and two turbidity currents occurred in the 2040-1190 cal a BP time period. This ability to map fjord processes in more detail has, furthermore, made it possible to revise the established stratigraphical framework for NE Atlantic fjords.

Detailed site survey examining the postglacial sediment succession and depositional processes within a Norwegian fjord system, Bjørnafjorden (West Norway)

“…For conversion to sediment thicknesses and depths in metres, a velocity of 1500 ms‾¹ has been applied throughout the study. This is in line with other studies of similar deposits from western Norwegian fjords (Hjelstuen et al, 2013;Bellwald et al, 2016). The seismic data were interpreted based on changes in seismic facies.…”

“…The seismic stratigraphy and patterns observed within the survey area are comparable to those observed within other western Norwegian fjords where similar seismic data have been analysed (Hjelstuen et al, 2009(Hjelstuen et al, , 2013Hjelstuen & Brendryen, 2014;Bellwald et al, 2016). In the lowermost part of Unit 2 (sub-units a and b) the deposits form a relatively thick, acoustically well laminated sequence with parallel to subparallel reflectors (Figs.…”

“…glacimarine and/or hemipelagic sedimentation (Stoker et al, 2006;Hjelstuen et al, 2009Hjelstuen et al, , 2013Lyså et al, 2010). The glacimarine deposits are overlain by lens-shaped acoustically transparent deposits (Figs.…”

“…2B). These stacked, lens shaped deposits are considered to represent slide debrites (Hjelstuen et al, 2009(Hjelstuen et al, , 2013. The lack of erosional features associated with the deposition of these slide deposits, might suggest that they consist predominantly of very soft sediments.…”

“…Mass gravity flows represent important processes within such systems and may have originated in both subaerial and submarine settings. The nature, dimension and frequency of such sediment gravity flows can vary significantly between the different fjord systems (Bøe et al, 2003;Hjelstuen et al, 2009Hjelstuen et al, , 2013Bellwald et al, 2016), representing significant geohazards to both infrastructure and populated regions.…”