THE RITLAND IMPACT STRUCTURE OF WESTERN NORWAY

The very well exposed Ritland impact structure was recently discovered (Riis, 2000; Riis et al., submitted). It is a 2.5 km in diameter circular depression, which was probably formed about 600 to 550 million years ago. It is located in Hjelmeland municipality, close to Stavanger in Western Norway. The 350 m deep depression is filled by late syn- and post-impact successions along the margins, while Cambrian alum shales and shallow marine sandstones fill parts of the central depression below thick Caledonian nappe units.

The Ritland structure can be described as a simple impact structure, most likely formed by a bolide impacting into the subaerially exposed subcambrian peneplain. Just after impact the crater was temporarily water filled, but later the sea covered the region in relation with the succeeding Cambrian transgression. During this period anoxic conditions were established in the area and black, organic rich clays (later to become alum shales) were deposited.

Along the crater margin, up to 100 m thick sedimentary breccias of syn – early post impact origin occur as scree, mass flow and suspension flow deposits. These beds carry signatures of crater modification and possible related fan delta sedimentation. The successions are covering monomictic breccias of crushed Precambrian, gneiss-dominated basement and a thin sequence of impact-related melt-rocks. Several slivers of melt material with clear-cut signs of ballistic transportation and some few shock metamorphic quartz grains have been found, so far.

An international project group, financed by the Scientific Council of Norway, has initiated a project aiming at giving better models of crater configuration, formation and timing of the Ritland event. This three year the project is first of all aiming at more detailed geological mapping and sedimentological/structural analyses along with geophysical, geochemical and mineralogical investigations of the structure. In combination with numerical modeling this hopefully will supply us with new insights and much needed documentation of the third impact structure of Norway.