The Solund Basin is a

sedimentary basin Sedimentary basins are region-scale depressions of the Earth's crust where subsidence has occurred and a thick sequence of sediments have accumulated to form a large three-dimensional body of sedimentary rock. They form when long-term subside ...

containing at least 6 km 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 The Sognefjord or Sognefjorden (, en, Sogn Fjord), nicknamed the King of the Fjords ( no, Fjordenes konge), is the largest and deepest fjord in Norway. Located in Vestland county in Western Norway, it stretches inland from the ocean to the smal ...

and

Nordfjord Nordfjord ( en, Northern fjord—in contrast to Sunnfjord) is a traditional district of Norway. Geography The region is located in the northern part of Vestland county in Western Norway. It centers on the Nordfjorden and it comprises the muni ...

, developed in the hanging-wall of the Nordfjord-Sogn Detachment. It was formed as a result of extensional tectonics during the

post-orogenic collapse In geology, orogenic collapse is the thinning and lateral spread of thickened crust. It is a broad term referring to processes which distribute material from regions of high gravitational potential energy to regions of low gravitational potential ...

of crust thickened during the

Caledonian orogeny The Caledonian orogeny was a mountain-building era recorded in the northern parts of the British Isles, the Scandinavian Mountains, Svalbard, eastern Greenland and parts of north-central Europe. The Caledonian orogeny encompasses events that ...

, towards the end of the Silurian period. It is named for the municipality of

Solund Solund is a municipality in the county of Vestland, Norway. It is located in the traditional district of Sogn. Solund is the westernmost island municipality in Norway. Holmebåen on the island of Steinsøy is the westernmost point in all of Nor ...

Vestland Vestland is a county in Norway established on 1 January 2020. The county is located in Western Norway and it is centred around the city of Bergen, Norway's second largest city. The administrative centre of the county is the city of Bergen, where t ...

Tectonic setting

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 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 sys ...

and the development of a mountain belt similar in scale to the

Himalayas The Himalayas, or Himalaya (; ; ), is a mountain range in Asia, separating the plains of the Indian subcontinent from the Tibetan Plateau. The range has some of the planet's highest peaks, including the very highest, Mount Everest. Over 10 ...

. 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.

Extent

The preserved part of the Solund Basin covers about 800 km², 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.

Basin fill

The dominant lithology is conglomerate, with some

breccia Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix. The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of ...

and subordinate amounts of

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates ...

, organised into somewhat irregular cycles of coarsening up and then fining on a scale of tens of metres up to more than 100 m. 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.

Hersvik landslides

In the area around

Hersvikbygda Hersvikbygda is a village in Solund Municipality in Vestland county, Norway Norway, officially the Kingdom of Norway, is a Nordic country in Northern Europe, the mainland territory of which comprises the western and northernmost porti ...

, 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 A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks incl ...

and felsic

volcanics Volcanic rock (often shortened to volcanics in scientific contexts) is a rock formed from lava erupted from a volcano. In other words, it differs from other igneous rock by being of volcanic origin. Like all rock types, the concept of volcanic ...

granite Granite () is a coarse-grained ( phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies under ...

diorite Diorite ( ) is an intrusive igneous rock formed by the slow cooling underground of magma (molten rock) that has a moderate content of silica and a relatively low content of alkali metals. It is intermediate in composition between low-sili ...

gabbro Gabbro () is a phaneritic (coarse-grained), mafic intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is ch ...

and

metasediment In geology, metasedimentary rock is a type of metamorphic rock. Such a rock was first formed through the deposition and solidification of sediment Sediment is a naturally occurring material that is broken down by processes of weathering and e ...

s. When they were first described in 1926, they were interpreted as small

thrust sheets In geology, a nappe or thrust sheet is a large sheetlike body of rock that has been moved more than or above a thrust fault from its original position. Nappes form in compressional tectonic settings like continental collision zones or on the ...

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. 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.

Structure

In the southeastern part of the basin, the conglomerate sequence dips 20–25° to the southeast. The main fold structure is the Lågøyfjorden anticline which runs southwest–northeast. To the northwest of the fold hinge the dip is towards the north or northwest. The southeastern margin of the basin is formed by the low-angle Solund Fault or Solund Detachment, part of the Nordfjord-Sogn Detachment. Close to this margin small down-to-the-northwest listric normal faults are developed within the conglomerates, which merge with the detachment itself. At the fault contact, the lowermost metre of the conglomerates is strongly sheared, with a

foliation In mathematics (differential geometry), a foliation is an equivalence relation on an ''n''-manifold, the equivalence classes being connected, injectively immersed submanifolds, all of the same dimension ''p'', modeled on the decomposition of ...

parallel to the fault and the clasts become elongated in a direction parallel to the lineation in the underlying mylonite. The conglomerate is also noticeably deformed with well-developed cleavage in zone about 2.5 km wide parallel to the Solund Fault. The conglomerates show evidence of very low-grade

metamorphism Metamorphism is the transformation of existing rock (the protolith) to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of , and often also at elevated pressure or in the presence of ch ...

with recognition of an assemblage of

authigenic Authigenesis is the process whereby a mineral or sedimentary rock deposit is generated where it is found or observed. Such deposits are described as authigenic. Authigenic sedimentary minerals form during sedimentation by precipitation or recrys ...

minerals that indicate temperatures in the range 230–330°C, consistent with maximum burial of up to 13 km. The

mylonite Mylonite is a fine-grained, compact metamorphic rock produced by dynamic recrystallization of the constituent minerals resulting in a reduction of the grain size of the rock. Mylonites can have many different mineralogical compositions; it is a ...

s immediately beneath the Solund Fault appear to be a highly deformed version of the mafic rocks of the upper allochthon. These mylonites contain a suite of minor structures that show slightly oblique top-down-to-the-northwest movement. The basin is cut by a prominent series of nearly north–south trending faults that are interpreted to be of latest

Paleozoic The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and ' ...

Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretace ...

age, associated with west–east extension in the North Sea rift that began during the Permo-

Triassic The Triassic ( ) is a geologic period and system (stratigraphy), system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago (Year#Abbreviations yr and ya, Mya), to the beginning of the Jurassic Period 201.36 ...

References