The Kvamshesten Basin is a sedimentary basin containing coarse continental

clastic rock Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus,Essentials of Geology, 3rd Ed, Stephen Marshak, p. G-3 chunks, and smaller grains of rock broken off other rocks ...

s of

Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...

age. It is one of a series of basins of similar age in southwestern Norway found between Sognefjord and Nordfjord, developed in the hanging-wall of the Nordfjord-Sogn Detachment. It is named for the mountain of Kvamshesten.

Tectonic setting

During the later part of the Silurian period, the western margin of the

Baltic Plate The Baltic Plate was an ancient tectonic plate that existed from the Cambrian Period to the Carboniferous Period. The Baltic Plate collided against Siberia, to form the Ural Mountains about 280 million years ago. The Baltic Plate, however, fused on ...

was affected by the main collisional event of the Caledonian orogeny, known as the Scandian phase. This led to large-scale

thrusting Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that ...

and the development of a mountain belt similar in scale to the Himalayas. Soon after the collision finished during the Early Devonian, the thickened crust began to extend. Initially the extension took place by reactivation of Caledonian thrust faults, known as Mode 1 extension. The uplift and exhumation led to a reduction of dip in these reactivated thrusts, making them progressively less mechanically viable. At this point Mode II extension took over, with development of large extensional

shear zone In geology, a shear zone is a thin zone within the Earth's crust or upper mantle that has been strongly deformed, due to the walls of rock on either side of the zone slipping past each other. In the upper crust, where rock is brittle, the shear ...

s that cross-cut the Caledonian thrust pile, such as the Nordfjord-Sogn Detachment.

Basin fill

The northern margin of the basin is dominated by conglomerates and breccias deposited by alluvial fans. The southern margin is mainly conglomerate, while with the central axis of the basin is mainly sandstone. Between the marginal conglomerates/breccias and the axial sandstones, finer-grained sequences are locally developed consisting of

mudstone Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from '' shale'' by its lack of fissility (parallel layering).Blatt, H., and R.J. Tracy, 1996, ''Petrology. ...

siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility.Blatt ''et al.'' 1980, p ...

and fine-grained conglomerate.

Structure

The overall dip of the Devonian sequence is to the east towards the tectonic contact with the Dalsfjord Fault (Kvamshesten Detachment). The whole sequence, the basement units beneath the unconformable contacts towards the western end of the basin, the detachment and the underlying highly deformed rocks of the shear zone formed by the Nordfjord-Sogn Detachment have a synclinal geometry, indicating north–south shortening. The Devonian rocks are cut by a series of high-angle faults, most of them extensional in type with evidence of being active during sedimentation. These faults are confined to the hanging-wall of the underlying detachment fault. Some of these earlier extensional structures show evidence of reactivation as reverse faults. Both the timing and the cause of the observed north–south shortening is uncertain. Suggestions that the shortening happened at the same time as the extension along the detachment, have been countered by there being no evidence of changes in the tightness of the folding with stratigraphic level and a lack of unconformities within the basin fill.

References

{{Reflist Sedimentary basins of Europe Devonian Norway Geology of Norway

Tectonic setting

Basin fill

Structure

See also

References