Two major unconformities beneath the Neoproterozoic Murchisonfjorden Supergroup in the Caledonides of central Nordaustlandet, Svalbard

Abstract

Two unconformities have been found in central Nordaustlandet. New mapping has located a major unconformity at the base of the Neoproterozoic Murchisonfjorden Supergroup, with quartzites and basal conglomerates of the Djevleflota Formation unconformably overlying dark phyllites of the Helvetesflya Formation and metavolcanic rocks of the Svartrabbane Formation. A second unconformity separates the Helvetesflya from the Svartrabbane formations. These rocks were isoclinally folded, metamorphosed in lower greenschist facies, and, apparently, syntectonically intruded by Grenville-age granites, prior to uplift, erosion and Neoproterozoic deposition. Caledonian tectonothermal activity, as recorded in the Neoproterozoic strata, appears to vary very little across Svalbard's Eastern Terrane from Ny Friesland, in the west, to Murchisonfjorden in western Nordaustlandet and, via Wahlenbergfjorden, to the central Nordaustlandet area, described here. Upright folds with associated high angle, usually E-dipping cleavages, characterise the Caledonian deformation over an east-west distance of about 100 km. This evidence allows the possibility that the pre-Devonian basement, to the east of Nordaustlandet, beneath the northern Barents Sea (Barentsia), may be composed of Grenville-age complexes little influenced by Caledonian tectonothermal activity. Alternatively, Barentsia is dominated by Caledonian hinterland tectonics, with extensive middle Paleozoic tectonothermal reworking of a Precambrian basement.