The Ritland crater - An early Cambrian impact structure in West Norway

Category Tectonic & Seismotectonic
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Riis, Fridtjof۱; Dypvik, Henning۲; Krّgli, Svein Olav۲
Holding Date 11 October 2008

The Ritland crater is a 2 km wide and 350 m deep circular depression in the sub-Cambrian peneplain. It is located in the community of Hjelmeland, county of Rogaland (West Norway). The crater is situated in a mountainous area with excellent three dimensional exposures of the sedimentary rocks filling the structure and the deformed basement rocks along the crater rim.
The sub-Cambrian peneplain is exposed as a conspicuous flat to slightly undulating surface in the mountainous areas east of Ritland. Outside the crater, the planated basement surface (heterogeneous gneiss complex) is covered by a fine-grained, bioturbated less than 1 m thick sandstone bed. The sandstone is covered by dark grey, graphite rich phyllites and shales. Within the crater the equivalent marine shales carry a Middle Cambrian fauna described originally by Henningsmoen and later by Bruton. These shales and overlying shallow marine sandstones were overridden by overthrust nappes of the frontal Caledonian thrusts.
Within the crater, the crushed basement rocks are covered by sedimentary, post-impact breccias with fragments reaching up to 2-3 m. In the deeper part of the crater depression, the sedimentary breccia is intercalated with crossbedded sandstones, deposited in water. The succeeding coarse-grained sedimentary fill forms wedges of variable thickness along the margin of the crater. The maximum stratigraphic thickness observed is almost 200 m along the rim of the crater. Following an ?early Cambrian marine transgression the crater was dominated by fine-grained sedimentation, to day seen as shales onlapping the crater walls. The level of erosion in the central part of the crater does not reach the basement, and about one third of the interior is covered by Cambrian shales and sandstones and overlying thrust nappes. The basement gneisses exposed along the crater walls are intensely fractured and brecciated, commonly with a characteristic dark grey to black fracture fill. The quartz and feldspar grains typically have dark grey to black colour and individual quartz grains commonly display a mosaic fracture pattern and strongly undulating extinction.
Lack of basement exposures in the central crater has made it difficult to find samples containing shocked minerals. In 2007, however, samples taken in a small area of exposed basement rocks in the crater interior revealed outcrops of a fine-grained rock with suevitic texture containing quartz grains with good planar structures of two directions. These thin sections and some new samples will be further analysed, and hopefully contribute to a confirmation of our hypothesis of the formation of the Ritland crater as an impact structure.