Bedrock geology and Quaternary sediments in the Lista basin, S. Norway

Sedimentary rocks of U. Triassic and L. Jurassic age are suggested to occur in the submarine area between Lista and the Siregrunnen bank. This conclusion is based on shallow seismic profiling data, as well as on data from dredge- and core-samples. The boundary between the sedimentary rocks and the Precambrian area to the northeast is marked by a major fault outside the island of Hidra, which probably continues northwestwards outside the mainland, and southeastwards along the Eidsfjord. Other faults with similar direction are registered both inside and outside the threshold at the mouth of the Lista basin, which therefore seems to be structurally controlled. While the Quaternary sediment cover over the threshold area is thin and even absent, the basin inside contains sediments with a maximum thickness of about 100 m. These sediments are probably marine and glaciomarine deposits of Late Weichselian and
Holocene age. The retreat of the main ice in the basin must have taken place before 12,500 yrs B.P., possibly 13,000 yrs B.P., which gives a minimum age for the Lista substage. The age of an ice-marginal deposit at the mouth of Fedafjord, possibly representing the Spangereid substage, must be about 12,000 yrs B.P. The assumed Mesozoic rocks occurring in the Lista basin are thought to be the source for erratics found in glacial deposits in coastal areas to the northwest, e.g. coal in the 'Skagerrak moraine' at Jæren. Mesozoic sediments have probably covered the Precambrian land areas to the north of the Lista basin, and the present surface is thought to be very much the result of exhumation of a subtriassic weathered surface.