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Geoscience seminar - Margaret Stewart & Dag Ottesen

Tunnel Valleys in the North Sea Region By Margaret Stewart, British Geological Survey & Large-scale evolution of the Mid-Norwegian Shelf during the last 3 million years By Dag Ottesen, Geological Survey of Norway

03.03.2020 | Susanne Weis Fogh

Dato ons 04 mar
Tid 15:15 17:00
Sted Institut for Geoscience, Høegh-Guldbergs Gade 2, 8000 Aarhus C, bygning 1671-137 (Auditorium)

Abstracts:

Tunnel Valleys in the North Sea Region

Margaret Stewart, British Geological Survey

Tunnel valleys are km-scale linear landforms formed subglacially and, in the North Sea, found offshore and associated with multiple glaciations by the Quaternary ice-sheets. In this talk, I present the use of seismic reflection and gravity data to map more than a thousand buried tunnel valleys in the central and northern North Sea. The tunnel valleys are generally present from the seabed to depths of around 400 metres, in a study area of 180 000 km2 from 56°N to around 62°N. Buried tunnel valleys are well imaged in seismic reflection data, particularly in horizontal timeslice in 3D seismic data. In gravity data, the tunnel valleys appear as small-scale gravity lows, likely due to their infill being less compacted and potentially more porous than the surroundings. This talk shows the most extensive study of tunnel valleys in the region to date, with more than 20% of the study area containing buried tunnel valleys, and also finds the longest tunnel valley recorded worldwide, with large meandering tunnel valleys extending for more than 160 km. We also find tunnel valleys further north than previously reported, as well as a number of isolated tunnel valleys extending towards and into the Norwegian Channel. As reported by other works in the region, the apparently extensive networks of buried valleys are found to be comprised of cross-cutting generations and the next step in the puzzle is to link them to ice sheet dynamics.

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Large-scale evolution of the Mid-Norwegian Shelf during the last 3 million years

Dag Ottesen, Geological Survey of Norway

The Quaternary Naust Formation have been mapped on the Norwegian continental shelf between Sognefjorden and Lofoten (61°N to 68°N). During the last 2.7 million years the shelf has prograded up to 150 km westwards. More than 1000 m of sediments have been deposited in large areas on the middle/outer shelf. Most of the Naust Formation on the mid-outer shelf comprises prograding wedges, commonly thinning westwards and downlapping onto the top of the underlying Kai Formation. 3D seismic data from the shelf show iceberg ploughmarks on palaeo-surfaces buried below 400 m of sediment at the present-day shelf edge. In several hydrocarbon exploration wells, abundant angular gravel fragments, interpreted as ice-rafted debris (IRD), have been found in the lower parts of the Naust Formation. The Naust Formation is subdivided into five sequences. From oldest to youngest these are named N, A, U, S and T. Naust S represents the third last glaciation, whereas Naust T is from the last and second last glaciation. An upper regional angular unconformity (URU), best developed on the inner-mid part of the shelf, marks the boundary upwards to more flat-lying sedimentary units. These units mainly comprise tills, deposited during the last extensive glaciations on the shelf.

Institut for Geoscience, Seminar