The Hornelen 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 subsidence ...

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 ...

, Norway, containing an estimated 25 km stratigraphic thickness of coarse

clastic 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 ...

sedimentary rock Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

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, w ...

age. It forms part of a group of basins of similar age along the west 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 sma ...

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 mun ...

, related to movement on the Nordfjord-Sogn Detachment. It formed as a result of

extensional tectonics Extensional tectonics is concerned with the structures formed by, and the tectonic processes associated with, the stretching of a planetary body's crust or lithosphere. Deformation styles The types of structure and the geometries formed depend ...

as part of 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 that was 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 The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleozo ...

period. It is named for the mountain Hornelen on the northern margin of the basin.

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 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 list ...

. 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 shea ...

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

Basin fill

The dominant

lithology The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lit ...

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) ...

, 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 fan An alluvial fan is an accumulation of sediments that fans outwards from a concentrated source of sediments, such as a narrow canyon emerging from an escarpment. They are characteristic of mountainous terrain in arid to semiarid climates, but a ...

s. 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 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, ...

s and

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. ...

s are developed, forming the finest-grained sediments in the basin. The

depositional environment In geology, depositional environment or sedimentary environment describes the combination of physical, chemical, and biological processes associated with the deposition of a particular type of sediment and, therefore, the rock types that will b ...

for these fine-grained rocks is thought to be lacustrine.

Age

The sequence has been dated as Devonian, based on Middle Devonian plant fossils found in the upper part of the succession.

Clast provenance

The study of clast lithology within the Hornelen sequence has been used to understand the exposed geology in the areas that provided a sediment source for the basin. The currently exposed bedrock to the north and east of the basin consist of metamorphic rocks of the Western Gneiss Region (WGR) that have been affected by

ultra-high-pressure metamorphism Ultra-high-pressure metamorphism refers to metamorphic processes at pressures high enough to stabilize coesite, the high-pressure polymorph of SiO2. It is important because the processes that form and exhume ultra-high-pressure (UHP) metamorphic r ...

, which are everywhere in faulted contact with the basin. No clasts have been found that have evidence of high-pressure metamorphism indicating that the WGR did not become exposed at the surface until after the highest preserved levels of the basin were deposited. The clasts do match well with the rock types observed within the Caledonian thrust sheets on which the Devonian sediments lie unconformably to the west of the basin.

Structure

The Devonian sequence has an overall eastward dip of 20–25°, from the unconformable contact in the west up to the tectonic contact in the east. This is modified near the northern and southern margins. Towards the northern margin the direction of dip becomes more southeasterly. Near the southern margin the basin is folded into a west–southwest trending

anticline In structural geology, an anticline is a type of fold that is an arch-like shape and has its oldest beds at its core, whereas a syncline is the inverse of an anticline. A typical anticline is convex up in which the hinge or crest is the ...

and

syncline In structural geology, a syncline is a fold with younger layers closer to the center of the structure, whereas an anticline is the inverse of a syncline. A synclinorium (plural synclinoriums or synclinoria) is a large syncline with superimposed ...

, parallel to the faulted margin. There is little evidence of internal faulting in the basin, although fracture sets are well-developed and two stages of vein development have been observed. The northern margin of the basin is formed by a steep south-dipping fault that is interpreted to be mainly post-Devonian brittle structure that has formed close to the original basin edge. The southern margin is formed by the steep north-dipping Haukå Fault, which is interpreted to increase in displacement westwards, locally cutting out the alluvial fan deposits, although it is still regarded as being close to the original basin edge. The eastern margin is formed by a low-angle extensional fault, known as the Hornelen Detachment, part of the larger scale Nordfjord-Sogn Detachment (NSD). It juxtaposes the Devonian basin fill against

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 of the NSD

that underlies the basin.

Origin

The Hornelen sequence, in common with other

Old Red Sandstone The Old Red Sandstone is an assemblage of rocks in the North Atlantic region largely of Devonian age. It extends in the east across Great Britain, Ireland and Norway, and in the west along the northeastern seaboard of North America. It also exte ...

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 In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tecton ...

pull-apart basin In geology, a basin is a region where subsidence generates accommodation space for the deposition of sediments. A pull-apart basin is a structural basin where two overlapping (en echelon) strike-slip faults or a fault bend creates an area of cru ...

model was proposed, by analogy with basins developed along the

San Andreas Fault The San Andreas Fault is a continental transform fault that extends roughly through California. It forms the tectonic boundary between the Pacific Plate and the North American Plate, and its motion is right-lateral strike-slip (horizontal). ...

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 km of displacement.

References

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