Stratigraphic review of the Finnmark Caledonides, with possible tectonic implications

Abstract

The Finnmark Caledonides consist of three distinct nappes thrust south-eastwards over an autochthonous sequence resting on Archaean rocks of the Baltic Shield. The autochton is thickest in the east where a 3 km. thick sequence consists of fluvial and shallow marine sandstones and shales passing upwards into a thin carbonate unit. Unconformably above these late Riphean sediments, tillites of Vendian age are overlain by a shallow marine clastic Vendian to Tremadocian sequence. The lowest nappe containing late Riphean clastic sediments passing up into a thick stromatolitic dolomite unit is comparable to the lower part of the autochthon. The middle and upper nappes contain metasedimentary sequences at least 6 km. thick that have undergone polyphase Caledonian deformation. The middle nappe contains a tillite-bearing sequence which is thought to be of Vendian age whilst the upper nappe is of Vendian to Cambrian age. The Finnmark Caledonides are separated from the Barents Sea Caledonides, found only in the extreme north-east of Norway, by a major fault. These Barents Sea Caledonides, possessing some features in common with the Caledonides of East Greenland and East Spitsbergen, may be derived from the north-west margins of a Proto-Atlantic Ocean. The sequence in the autochthonous cover and the three north-westerly derived thrust units of the Finnmark Caledonides were developed in two distinct contemporaneous geosynclinal basins separated by a north-east-south-west trending basement ridge—the Finnmark Ridge. This ridge lies between and parallels the main Caledonian Front to the south-east, and a zone of basic and ultramafic intrusions to the north-west. The Caledonian Front swings from north-east-south-west in the south-west to east-north-east-west-south-west in the north-east. It is suggested that this region is the junction between a north-east-south-west trending Caledonian Orogen and an east-west trending Timan Orogen; the former controlled by a compressive Plate boundary and the latter by a transcurrent Plate boundary.