The stratigraphy of a glacial cycle: a study case of the Weichselian deposits of the North Sea Fan
Authors: Garcia Aurora , Bellwald Benjamin , Midtkandal Ivar , Planke Sverre , Anell Ingrid , Sternai Pietro , Myklebust Reidun 
Published: February 2023

To understand the dynamics of the Fennoscandian Ice Sheet, extensive studies have been done on the Norwegian Channel which was formed by intense ice streaming activity throughout the Quaternary. However, the understanding of trough mouth fans developed in front of such ice streams is still evolving, with recent research showing the importance of meltwater in the transport and deposition of sediments into the depocenter. This study focus on characterizing the deposits of the last glacial highstand and previous interglacial period at the North Sea Fan in order to unravel the predominant sedimentary processes and paleo-geographical configuration since Marine Isotope Stage 6 (last 130ka). We use over 14000km2 of high-resolution 3D seismic reflection data (vertical resolution of 2m and bin size of 6.25 x 18.75m) covering the slope and shelf break to map six surfaces and delimitate five seismic units. The stratigraphy of the section comprehends over 600 meters of sediments in the thickest part of the fan, spanning from the top of the Tampen Slide to the seabed. The lowermost units, U1 and U2, have similar seismic facies (continuous low amplitude reflections, with increasing contrast towards the head and sidewall of the Tampen Slide) but different geometries: U1 is tabular and covers the entire study area whereas U2 have a lenticular body and is limited to the northernmost part. U3 is characterized by a combination of chaotic, transparent, deformed and parallel seismic facies and is divided into four subunits by high-amplitude reflections that can be mapped throughout the dataset. Within U3, channels were described and grouped into five categories based on their morphologies and infill. Overall, the channels vary in width from tens of meters to > 2km, with depths reaching up to 105 meters. They can exceed 100km in length and have extremely low sinuosity (max. 1.05). U4 is the uppermost unit on the slope, characterized by transparent seismic facies close to the paleo-shelf, with parallel to undulating continuous reflections downslope where the thickness of the unit decreases. Two of the five types of channels are observed within this unit, as well as pockmark fields. On the shelf, U5 represents a thin wedge-shaped package with few internal reflections. Multiple iceberg ploughmarks, iceberg pits and mega-scale glacial lineations are identified, eroding into the unit. The different units represent different sedimentary processes and periods within a glacial cycle: U1 and U2 are the products of contour currents during the interglacial period after the failure of the Tampen Slide; U3 is deposited during the glacial maximum (23-19ka) mainly by meltwater turbidites and subordinately glacigenic debris flows; U4 represents the deglacial period on the slope, where the suspended load from the meltwater turbidites and ice rafted debris were deposited; U5 represents the deglacial period on the shelf, with deposition of subglacial tills. The deposits account for a big percentage of 2km thick fan, especially when considering the short deposition period of ~123 years, highlighting the variability and intensity of active processes at the North Sea Trough Mouth Fan during the Weichselian glaciation.