Bulletin of the Geological Society of Finland (Jun 2007)
Lithostratigraphy of the Mesoproterozoic Vemork formation, central Telemark, Norway
Abstract
The Vemork formation forms a c. 2 km thick volcanic-sedimentary unit above the 1510 – 1500 Ma old Tuddal felsic volcanite formation, the oldest unit of the Mesoproterozoic Telemark supracrustals in southern Norway. It is dominated by basaltic metalavas with sedimentary interunits of variable thicknesses. Its lower contact with the major Tuddal volcanite body is considered conformable, but an angular unconformity is also possible. The change from the felsic Tuddal volcanism to the basaltic Vemork volcanism is interbedded as the flow-banded Skardfoss and Homvatnet metarhyolite members occur in the lower part of the Vemork formation. Zircon U-Pb dating of the Skardfoss metarhyolite constraints the beginning of deposition of the Vemork formation at around 1495 ± 2 Ma. In the north, the Vemork basalts are overlain by volcaniclastic arkosite and quartzites of the Vindeggen group, whereas in the south the Venutan member of diverse felsic volcanite rocks occupies the uppermost part of the Vemork group. It is mingled with the Vemork basalts, but seems to pass via a felsic vocaniclastic conglomerate and arkosites to the Gausta quartzite of the Vindeggen group. Because of the Sveconorwegian deformation and metamorphism and uneven outcrop distribution individual Vemork units cannot be followed laterally for any longer distances and vertical sections are incomplete. Consequently, the lithostratigraphy for the Vemork formation can be established only tentatively. In the Frøystaul type section, the 2 km thick sequence comprises at least 10 basaltic units separated by epiclastic units with variable amounts of both felsic and mafic volcanic material. The nature of the upper boundary of the Vemork formation with the quartzite-dominated Vindeggen group is problematic as the rocks within the contact zone are intensely foliated and mostly unexposed. Sudden dying of volcanism and input of extrabasinal epiclastic material into the lower part of the Vindeggen group indicate that a significant tectonic change took place at the Vemork/Vindeggen boundary.
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