Submarine end moraines and associated deposits off the south coast of Norway

Abstract

A submarine end moraine of Younger Dryas age (10,600–11,000 yrs B.P.) and its proglacial deposits have been studied off the south coast of Norway, southwest of the Oslofjord. Acoustic and shallow seismic sounding, and core and grab samples formed the database. The moraine occurs as a straight, fairly shallow ridge about 60 km in length, only being interrupted by a few inlets. Where emerged parts of the moraine have been studied, it consists mainly of boulder clay. The proglacial deposits are thin, and with the exception of a central area where the sediments may attain a thickness of 60–70 m, have not filled in the troughs of the very irregular topography. The thin sediments are interpreted as glaciomarine diamictons of Late Weichselian age. An exception to this may be the thick series in the centre, which may contain various types of lithologies, including meltwater deposits. Deposition took place when local sea level was 60–120 m higher than today owing to glacioisostatic crustal depression. During the succeeding rebounds in the Holocene, reworking and transport of sediments by currents and waves occurred, resulting in the present sediment zonation: gravel and sand down to a water depth of about 40 m, sands to about 100 m, and then glaciomarine diamictons with a thin, partly reworked upper zone. Below about 250 m, pelitic sediments of Holocene age have accumulated. For comparison, the sediment distribution on the slope off the southwest coast near Lista was studied. In this area deglaciation took place about 13,000 yrs ago, with shore-level movements that were different from those in the northeastern area, revealing evidence of a Late Weichselian transgression. The occurrence of sorted sands and gravels to a greater depth, and a higher degree of roundness of clasts, is thought to be the result of this difference in shore-level history.