H. Fossen et al., THE BJOROY FORMATION - A NEWLY DISCOVERED OCCURRENCE OF JURASSIC SEDIMENTS IN THE BERGEN ARC SYSTEM, Norsk geologisk tidsskrift, 77(4), 1997, pp. 269-287
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.