Tectonic framework, stratigraphy, sedimentation and volcanism of the Late Precambrian Hedmark Group, Østerdalen, south Norway
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The Hedmark Group in the Imsdalen - Bjørånes - Atna - Øvre Rendal area belongs to the Osen-Røa Nappe Complex, and crops out northwest of the late-Caledonian central Østerdalen structural depression. The Brøttum Formation and the Imsdalen Conglomerate occur on the southwestern side of the Imsdalen fault (IMF), which is a synthrust high angle reverse fault. The pre-Varanger Brøttum Formation (turbidites, deltaic, fluvial) is correlated with the fluvial Rendalen Formation NE of IMF. Basin evolution prior to the Varanger glaciation also includes carbonate sedimentation, black mud deposition in stagnant secondary basins, and block faulting accompanied by basalt volcanism and
coarse-clastic, subaqueous resedimentation. The glacial Moelv Tillite rests unconformably on various units and was succeeded by post-glacial deltaic and fluvial sedimentation and early Cambrian deposition of shallow-marine quartz sands. New formations are defined: the Imsdalen Conglomerate and the
Svarttjørnkampen Basalt. The Atna Quartzite is given the rank of formation, and the Bjørånes Shale Member is referred to the Biri Formation. Parts of the Sollia Formation are correlated with well-established formations in the Hedmark Group.