THE BJOROY FORMATION - A NEWLY DISCOVERED OCCURRENCE OF JURASSIC SEDIMENTS IN THE BERGEN ARC SYSTEM

During the construction of a subsea road tunnel across Vatlestraumen n ear Bergen, a ca. 10-m-wide subvertical zone of Jurassic sediments (he re named the Bjoroy Formation) was encountered. The Bjoroy Formation c onstitutes a basal gneiss breccia, conglomerate, sandstone, coal, and unconsoiidated sand. Palynological analyses suggest an early to middle Oxfordian (early Late Jurassic) age for the sandstone, and a similar age is indicated for coal fragments in the unconsolidated sand. This m akes the Bjoroy Formation time equivalent to the offshore Sognefjord F ormation farther west on the Horda Platform. The Bjoroy Formation is p reserved in a pre-Jurassic fault zone. Reflection seismic data indicat e that Jurassic sediments also may occur in thicknesses of up to 50-60 m above the tunnel as S(SW)-dipping strata. Evidence of marine influe nce on rocks of the Bjoroy Formation is consistent with an Oxfordian m arine transgression of the eastern margin of the (northern) North Sea, and indicates that at least parts of southwestern Norway were covered by the sea in the latest Jurassic. The deposition of Late Jurassic se diments on crystalline basement in the Bergen area indicates that litt le erosion of the basement has taken place since late Jurassic times, and that the so-called 'paleic surface' (pre-Neogene peneplain) and pa rts of the Norwegian strandflat may be an old (mostly Jurassic) exposi tion. The fault rocks in the Vatlestraumen fault zone show evidence of repeated fault activity during decreasing P-T conditions. Ar least tw o pre-depositional (pre-Oxfordian) episodes of faulting are recorded, of which the first shows evidence of crystallo-piastic deformation of quartz. Most Fault rocks were formed prior to deposition of the Bjoroy Formation, but post-depositional (late or post-Jurassic) fault rocks are recognized as unconsolidated gouge zones. Late or post-Jurassic ve rtical movements may have been up to several hundreds of meters across the Hjeltefjord fault zone west of Bjoroy, where it also offsets the paleic surface.