The geological evolution of Bjørnøya, Arctic Norway: Implications for the Barents Shelf

The small island of Bjørnøya ("Bear Island"), situated in the Barents Sea almost midway between northern Norway and Spitsbergen, displays a Precambrian 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 the development of analogous structures along the major lineaments that subsequently contributed to the formation of both the Norwegian-Greenland Sea and the Arctic Ocean. Precambrian to Ordovician dolomites, limestones, quartzites and shales form the basement on which the Upper Palaeozoic succession of Bjørnøya was deposited. In latest Devonian and early Carboniferous times the area subsided asymmetrically, probably in response to NE-SW extension; a southwestwards downtilted half-graben developed over the present-day island, with the basinal axis dipping gently NNW. 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 renewed rifting and the same western hinterland again 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, perhaps also with decreasing subsidence rates and only intermittent tectonism, 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 stabilized and platform carbonate deposition resumed, with the development of paleoaplysinid carbonate build-ups. Early Permian flexuring, uplift and peneplanation followed, probably with some transpressive movements. The highly condensed midto Upper Permian marine succession of mixed siliciclastics and carbonates oversteps all older strata. The StapNORWEGIAN JOURNAL OF GEOLOGY The geological evolution of Bjørnøya 195