User:Mikenorton/Hornelen Basin

The Hornelen Basin is a sedimentary basin containing an estimated 25 km stratigraphic thickness of coarse clastic sedimentary rocks of Devonian age. It forms part of a group of basins of similar age along the west coast of Norway between Sognefjord and Nordfjord, related to movement on the Nordfjord-Sogn Detachment. It formed a a result of extensional tectonics as part of the post-orogenic collapse of crust that was thickened during the Caledonian Orogeny towards the end of the Silurian period. It is named for the mountain Hornelen on the northern margin of the basin.

Basin fill
The dominant lithology is sandstone, which is organised into a series of coarsening-upward cycles, laid down by a west-draining axial river system. At the basin margins the sandstones interfinger with conglomerates deposited by alluvial fans. The northern conglomerates were deposited by mainly debris flow fans, while those to the south are mainly streamflow in type. Near the northern margin there is a zone where siltstone is developed, forming the finest-grained sediments in the basin.

Origin
The Hornelen sequence, in common with other Old Red Sandstone sequences in Norway, Scotland and Greenland was originally described as deposited in an intermontane depression with most faulting being post-depositional. The first tectonic model for the basin was developed in 1964, with an eastward migrating depocentre proposed in a downthrown basin between two high-angle normal faults with sedimentation keeping pace with eastward propagation of the faults. In 1977 a strike-slip pull-apart basin model was proposed, by analogy with basins developed along the San Andreas Fault system, with the cyclicity explained as a result of periodic fault activity. In 1984 another extensional model was proposed and this was supported in 1986 by the recognition of the underlying Nordfjord-Sogn Detachment, a major extensional structure with tens of kms of displacement.