Geology of Bear Island, Norway

This document is a brief illustrated description of the geology of Bear Island, which is found half way between north Norway and the south cape of Spitsbergen. The work reported here was done in collaboration with geologists at the University of Troms¿ (Alvar Braathen and Tora Haabet) and Saga Petroleum (Dave Worsely, Stein Erikson Kristensen, Bj¿rn T¿rudbakken) in the summer of 1994. The island is about 16 by 20 km in size and exposes a faulted hinge zone within a Upper Carboniferous half graben, with a master border fault submerged to the west. This island provides an isolated exposure in the west central part of the Barents Shelf, and thus the geology here is of interest in interpreting geophysical and other data from adjacent areas of the submerged shelf. Cliff exposures around the entirety of the island provide excellent exposures.

The above geologic map of Bear Island (modified from Dallmann & Krasilscikov, 1995, and Worsley and others, in preparation) shows several important patterns. Devonian through Triassic platform cover strata overlie metamorphic basement that consists of carbonates and phyllites. Permian Miseryfjellet Fm. strata overlap faults and other tilted strata, and thus constrain the timing of tilting and faulting. Underlying Carboniferous units (Roedvika to Nordkapp Fm.) are generally younger to the west, due to overall tilt in that direction. More than one trend of faults can be seen on the map, suggesting different Carboniferous faulting phases exist. The numbers on the map consecutively refer to the approximate location the 5 photos below were taken from.

Caledonian basement deformation: A thick unit of shallowly dipping and competent carbonates within the basement sequence that form sea cliffs several hundred meters high can give the impression that the basement rocks are only mildly deformed. However, a closer look at other exposures demonstrates that they are involved in a WNW verging fold and thrust stack. The adjacent photo looks northward and illustrates a westward verging fold within interbedded quartzites and phyllites. This deformation is Caledonian in age since Ordovician rocks are involved. These Caledonian structures likely influenced development of the Carboniferous rift since they are subparallel. In one case a normal fault reacivated an earlier thrust fault. A more detailed description of the Caledonian deformation is in progress (Braathen et al.).

Half graben tilting: This is a view of the Landn¿rdingvika Fm. dipping about 30 degrees to the west. A zone of steeper western dips extends northward from here across the island. A cross section (below) indicates the dip is most likely due to half-graben development. To the southeast these dipping strata are truncated by horizontal Miseryfjellet Fm. strata, a relationship seen elsewhere and documented below.

Normal faults: This photo shows carbonates that tend to be higher in the Kapp Kare Fm. sequence, juxtaposed against reddish clastics lower in the sequence by an east-dipping normal fault. Normal faults of this type are fairly abundant (only some of the larger ones are shown on the map). The sea cliffs are approximately 10 meters tall for scale. While normal faults predominate, conjugate sets of strike-slip faults also exist (Lepvrier and others, 1989).

Constraints on timing of faulting: This photo shows a synsedimentary fault within the Kape Hanna Fm.. Undisturbed overlying beds and distinctive deposits on the down-thrown side clearly indicate the synsedimentary character of the faulting. The fault surface is well exposed and even has some striae on it (interesting in that they must have formed essentially at the surface). To the right on the down-dropped side is a trough with a carbonate turbidite fill, and not evident in this photo, even further right is synsedimentary folding. This fault, oblique to the regional trend, at least locally directed and captured turbidites. This feature constrains the timing of one phase of faulting.

Angular unconformity on the graben hinge. This photo from the N cape of the island is taken further east in the half graben structure, where erosion and uplift associated with hinge development has created an angular unconformity where much of the Carboniferous fill seen to the west is missing. The view is to the south and the dip of the older strata to the west. Both erosional truncation and thinning of units likely account for the missing strata. The hinge zone acted as a structural high and sediment source during basin development. Nearby normal faults in the underlying Nordkapp Fm. are clearly truncated by the Permian Miseryfjellet Fm. strata, a relationship again clearly constraining the age of the majority of faulting seen in this area. Older strata show evidence of a source to the west (Worsley and others, in prep.).

Above are two offset cross sections through the island showing the half-graben geometry. Together the sections are about 10 km long and there is no vertical exaggeration. Units are colored as they are in the map above. The blue line represents sea level, and the green line the topographic profile. Similar half grabens to this exist on Svalbard, but many of these have been modified by subsequent Tertiary tectonism.

References cited.

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