NJG91-1/2-04
2011
Provenance and correlation of sediments in Telemark, South Norway: status of the Lifjell Group and implications for early Sveconorwegian fault tectonics
91
1, 2
pp. 57-75
978-82-92394-66-3

The Telemark Supracrustals are a thick, well preserved, greenschist facies, Mesoproterozoic sediment and volcanic rock sequence exposed in the Telemark area in South Norway. From 1500 to 1347 Ma intracontinental rifting, followed by a long period of tectonic quiescence left a bimodal volcanic sequence, the Rjukan Group, overlain by a mainly quartzitic sedimentary succession, the Vindeggen Group. From c.1170 to 1000 Ma, a dynamic, probably largely strike-slip, tectonic regime leading to the Sveconorwegian Orogeny resulted in the deposition of laterally discontinuous groups of sediments, commonly conglomeratic, and bimodal volcanic to sub-volcanic magmatic rocks above a major unconformity, called the Subsvinsaga unconformity. Here, we report detrital zircon U-Pb data from three sandstone units that have been alternatively placed below and above this unconformity. The Lifjell Group was regarded as correlative to the Vindeggen Group by J.A. Dons, as part of his classical Seljord Group (now obsolete). Then it was reassigned to a much higher level, overlying the 1155 ± 2 Ma Brunkeberg porphyry. It includes prominent c. 1780 and 1500 Ma detrital zircon age modes and lacks any zircons younger than 1350 Ma. the age distribution is similar to the Brattefjell Formation in the Vindeggen Group, supporting the original correlation between these units.

Another set of samples was taken from an elongated sandstone-conglomerate unit near Høydalsmo. Detrital zircon shows typical Palaeoproterozoic and c.1500 Ma ages, but 1300-1200 Ma and c.1130 Ma ages are also abundant. This unit is coeval with the Eidsborg Formation and confirms for the first time that the Eidsborg Formation is separated from underlying deposits by an angular unconformity. Depositional contacts with the Rjukan, Oftefjell and Høydalsmo groups indicate that the Eidsborg Formation once covered large areas in Telemark, and that all the earlier formations were already exposed to erosion. A survey of different conglomerates in the Sveconorwegian succession in Telemark reveals a trend from quartzite clasts in the oldest Svinsaga conglomerates to polymictic conglomerates in the younger formations and a general lack of granitic clasts until the very uppermost parts of the succession. This can be explained by exhumation of a thick quartzite cover and repeated burial, uplift and recycling of earlier deposits, until the basement was exposed. The activity of basin-bordering faults is clearly demonstrated by the changing conglomerate clast lithologies. The Sveconorwegian succession in Telemark was probably deposited in a strike-slip tectonic environment. The stratigraphic status of the Lifjell Group still remains enigmatic, but most likely it is part of the Vindeggen Group.

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