Devonian–Carboniferous slivers within the basement area of north-east Oscar II Land, Spitsbergen
Formed during an early compressional period in the opening of North Atlantic Ocean, a Tertiary fold-thrust belt extends along the mid-to- southern part of the western coast of Spitsbergen. Complex thrust structures involve the basement (Caledonian and older) and many shallow dipping thrust faults dissect the overlying cover rocks (Devonian and younger) in Oscar II Land in the northern part of the belt. Some of these faults occur within the basement rocks with slivers or fault blocks of the cover rocks from south-western Broggerhalvoya to innermost St. Jonsfjorden in north-eastern Oscar II Land. Six of the slivers contain Carboniferous rocks and one is a fault-bounded block with Devonian rocks. These steeply west-dipping faults form a complex fault system- EOFC (Engelskbukta-Osbornbreen Fault Complex) - within the basement area. The lithological units of the basement are separated by faults within the EOFC, which is structurally continuous with the Broggerhalvoya fold-thrust zone to the north and is thought to continue to the fold-thrust zone on the south-eastern coast of St. Jonsfjorden. Some previous authors considered that the two lithologically contrasting Vendian diamictites and intervening Moefjellet Formation are stratigraphically continuous and defined two separate tilloid successions in the present area. This interpretation has been extended over the whole of western Spitsbergen. However, the present study indicates that these two tilloid formations and the Moefjellet Formation are separated by the faults, probably thrusts, within the EOFC and are not in a continuous stratigraphic relation. Therefore, the two-stage history of Vendian glaciation seems questionable.