NJG77-4-04
1997
The Bjorøy Formation: a newly discovered occurrence of Jurassic sediments in the Bergen Arc System
77
4
pp. 269-287

During the construction of a subsea road tunnel across Vatlestraumen near Bergen, a ca. 10-m-wide subvertical zone of Jurassic sediments (here named the Bjorøy Formation) was encountered. The Bjorøy Formation constitutes a basal gneiss breccia, conglomerate, sandstone, coal, and unconsolidated 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 makes the Bjorøy Formation time equivalent to the offshore Sognefjord Formation farther west on the Horda Platform. The Bjorøy Formation is preserved in a pre-Jurassic fault zone. Reflection seismic data indicate 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 influence on rocks of the Bjorøy Formation is consistent with an Oxfordian marine 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 sediments on crystalline basement in the Bergen area indicates that little erosion of the basement has taken place since late Jurassic times, and that the so-called ' paleic surface' (pre-Neogene peneplain) and parts of the Norwegian strandflat may be an old (mostly Jurassic) exposition. The fault rocks in the Vatlestraumen fault zone show evidence of repeated fault activity during decreasing P-T conditions. At least two pre-depositional (pre-Oxfordian) episodes of faulting are recorded, of which the first shows evidence of crystallo-plastic deformation of quartz. Most fault rocks were formed prior to deposition of the Bjorøy Formation, but post-depositional (late or post-Jurassic) fault rocks are recognized as unconsolidated gouge zones. Late or post-Jurassic vertical movements may have been up to several hundreds of meters across the Hjeltefjord fault zone west of Bjorøy, where it also offsets the paleic surface.

H. Fossen & R. H. Gabrielsen, Geologisk institutt, Universitetet i Bergen, Allégt. 41, N-5007 Bergen, Norway;
J. Hesthammer, Statoil, N-5020 Bergen, Norway;
G. Mangerud, Norsk Hydro, Forskningssenteret, Sandsliveien 90, N-5020 Bergen, Norway;
T. Bugge, Saga Petroleum a.s., Box 490, N-1301 Sandvika, Norway.

0029-196X