Geologiske Guider

Geologiske Guider

Bjørnøya- an Upper Paleozoic-Triassic window into the Barents Shelf.
Guide NR
Guide 1-2014
Bjørnøya- an Upper Paleozoic-Triassic window into the Barents Shelf.
Levels geological guides
Level 3
Front cover illustrations
Atle Mørk
Editorial committee

The small island of Bjørnøya ("Bear Island"), situated in the Barents Sea almost midway between northern Norway and Spitsbergen, displays a Pre?cambrian to Triassic succession in a continuous series of spectacular cliff exposures. These exposures provide a key not only to the evolution of the Stappen High (on which Bjørnøya rests) but also to a better understanding of the Barents Shelf.  

Precambrian to Ordovician dolomites, limestones, quartzites and shales form the basement (Hecla Hoek) on which the upper Paleozoic succession of Bjørnøya was deposited. In the latest Devonian and early Carboniferous times a southwestwards downtilted half?graben developed over the present?day island. Some 600 m of sandstones, coals and shales are preserved in two upward coarsening sequences. These represent the repeated progradations of sandy fan systems over floodplains with lakes and northward meandering river channels. Mid?Carboniferous (Serpukhovian) uplift was followed by rifting and the western hinterland shed debris over its faulted eastern margins. A shift from humid to a semi?arid climate is reflected by the predominantly red colouration of the resultant 200 m thick succession of conglomerates, sandstones and shales, with caliche horizons. Penecontemporaneous regional sea level rise resulted in the gradual replacement of the alluvial floodbasin deposits by shallow marine siliciclastics and carbonates of shoreline, tidal flat and shallow shelf origin. Continued transgression through the Moscovian is indicated by the gradual change to a marine carbonate?dominated succession, with cherty biomicrites reflecting the establishment of an open carbonate shelf over the entire area.  

A marked rejuvenation of tectonic activity in the late Moscovian established a different depositional mosaic ? faulting affected exposures on the present island along N?S to NE?SW lineaments, with differential subsidence down to the west. This produced erosion of earlier deposits over the eastern part of Bjørnøya and deposition of conglomerates, sandstones, shales and dolomites in alluvial gully, coastal and shallow shelf environments to the west. A 200 m thick succession is preserved in western areas and eroded remnants are also preserved as outliers elsewhere on the island. Conglomerate clasts indicate derivation by successive stripping and redeposition of mid?Carboniferous to uppermost Devonian and then basement strata. By the latest Carboniferous the region had again stabilised and platform carbonate deposition resumed, with the development of palaeoaplysinid carbonate build?ups. Early Permian flexuring, uplift and peneplanation followed. The highly condensed mid? to Upper Permian marine succession of mixed siliciclastics and carbonates oversteps all older strata. The Stappen High then remained a positive feature through to the late Triassic, the youngest beds preserved being of Carnian age. The high subsequently subsided significantly during the Mesozoic, but it again became a positive feature as a result of one or more phases of uplift during the Cenozoic.

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