Solund Basin
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| Solund Basin | |
|---|---|
 Geological map of the Solund Basin (NSD=Nordfjord-Sogn Detachment) | |
| Coordinates | 61°08′N 4°53′E / 61.14°N 4.88°E |
| Etymology | Solund Municipality |
| Region | Vestland |
| Country | Norway |
| Characteristics | |
| On/Offshore | Onshore |
| Area | 800 km2 (310 sq mi) |
| Geology | |
| Age | Devonian |
| Faults | Solund Detachment |
The Solund Basin is a sedimentary basin containing at least 6 kilometres (3.7 mi) of mainly coarse conglomerates of Devonian age. It is the southernmost of a group of basins of similar age found along the southwest coast of Norway between Sognefjord and Nordfjord, developed in the hanging-wall of the Nordfjord-Sogn Detachment. It was formed as a result of extensional tectonics during the post-orogenic collapse of crust thickened during the Caledonian orogeny, towards the end of the Silurian period. It is named for Solund Municipality in Vestland county, Norway.
During the later part of the Silurian period, the western margin of the Baltic Plate was affected by the main collisional event of the Caledonian orogeny, known as the Scandian phase. This led to large-scale thrusting 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 zones that cross-cut the Caledonian thrust pile, such as the Nordfjord-Sogn Detachment.[1]
Extent
The preserved part of the Solund Basin covers about 800 square kilometres (310 sq mi), including most of the islands of Ytre Sula, Steinsundøyna, Sula, Losna, many other smaller islands and the mainland peninsula on which the mountain of Lihesten lies in Hyllestad Municipality.[2]
Basin fill
The dominant lithology is conglomerate, with some breccia and subordinate amounts of sandstone, organised into somewhat irregular cycles of coarsening up and then fining on a scale of tens of metres up to more than 100 m.[3] The conglomerate is typically very coarse, consisting of cobble to boulder sized clasts. A strong depositional fabric is present with clast long axes showing a marked preferred orientation of northwest–southeast, often with well-developed imbrication.[2]
Hersvik landslides
In the area around Hersvikbygda, towards the north of the island of Sula, a series of large exotic lenticular bodies are found within the conglomerates towards the base of the sequence. They consist of a wide range of lithologies, including mafic and felsic volcanics, granite, diorite, gabbro and metasediments. When they were first described in 1926, they were interpreted as small thrust sheets involving the emplacement of slices of the underlying upper allochthon rocks beneath the basal unconformity into the conglomerate sequence. Later investigations failed to find any evidence for tectonic contacts at the base of the lenses and one large body of monomict brecciated gabbro was interpreted as a debris flow. At the base of one of the lenses a body of rhyolite was interpreted as a contemporaneous Devonian lava flow, the only example of volcanism of that age described from Norway. A reappraisal of these deposits, including dating the rhyolite body as Silurian, has interpreted all of these bodies as landslides, probably derived from rocks of the Solund-Stavfjord Ophiolite Complex on the basin margin to the north.[4]
Similar landslide deposits have also been recognised from the southeastern boundary of the basin near Kråkevåg, at the highest preserved stratigraphic level, demonstrating that such landslides were active throughout the deposition of the preserved part of the basin.[4]