The mesoproterozoic sub-Lifjell unconformity, central Telemark, Norway

Abstract

The sub-Lifjell unconformity subdivides the traditional Seljord group of the Telemark supracrustals, south Norway, into the Vindeggen and Lifjell groups. It is defi ned by an in situ weathering breccia and an angular unconformity above quartzites of the <1 500 -> 1155 Ma old Vindeggen group and by a volcaniclastic palaeoregolith developed above the 1155±2 Ma old porphyry of the Brunkeberg formation. Due to the complex deformation of the Vindeggen and Lifjell groups this unconformity has often been sheared or cut by faults, which impedes its use as a lithostratigraphic boundary. Dated porphyries under and above the Lifjell group defi ne the age of the sub-Lifjell unconformity between 1155±2 Ma and 1145±4 Ma indicating that the part of the unconformity developed above the Brunkeberg formation represents a relatively short time gap (<10 Ma). The part of the unconformity developed above the Vindeggen group represents a substantially larger time gap, for the Vindeggen group was folded before the extrusion of the Brunkeberg porphyry. This time duration cannot be, however, approximated more closely as the ages of the sedimentation and folding of the Vindeggen group are not known. In the terms of sequence stratigraphy, the sub-Lifjell unconformity defi nes the lower bounding surface of an extensive Mesoproterozoic beach-shallow self sequence.