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Geology of the Bergen area

The Bergen Arc System

The Bergen Arc System is the name given to the series of arcuate metamorphic rocks that swings around a gneiss massive just west of the city of Bergen.


The Bergen Arcs is a folded stack of Proterozoic and Lower Paleozoic rock units that define a large fold structure that is clearly visible from topographic maps, geologic maps and satellite images. The rocs, strongly influenced by Caledonian deformation and metamorphism, generally show a pronounced foliation or metamorphic layering that dictate the location of valleys and ridges, which makes the arcuate structure quite conspicuous from topographic maps and satelite images alike.

Two of the arcs, known as the Minor and Major Bergen Arc, contain Upper Cambrian to Ordovician greenstone, metagabbro and metasediments interpreted as dismembered ophiolite fragments, overlain by sheared Upper Ordovician – Silurian metasediments. The Minor Bergen Arc (in which the city of Bergen is located) is narrower and contains more intensely strained rocks than the wider and longer Major Bergen Arc. It also contains a higher number of mylonitic basement slivers(gneisses).

Between the Major and Minor Bergen Arcs are the Proterozoic Blåmanen and Lindås nappes, the latter containing Caledonian shear zones where Sveconorwegian (Grenvillan) granulites, many of anorthositic composition, are transformed into eclogite. A number of publications can be found from this area, many of them coauthored by Håkon Austrheim at the University of Oslo. The Blåmanen Nappe consists of a basement complex of more than 1.4 billion years old migmatitic gneisses, overlain by a (probably) late Proterozoic metasedimentary sequence (Rundemanen Formation). Conglomerates of this unit show amazing strain gradients and variations in strain geometry.

Bergen as seen from the north, with the main lithologic units indicated.


Locality 1: Mylonites, Verftet

This locality contains a clean cut through beautifully sheared and folded mylonitic gneisses belonging to one of several basement slices that were tectonically shuffeled together with Paleozoic rocks of the Minor Bergen Arc. These rocks are exposed at Verftet, Nordnes, north of downtown Bergen (loc. 1 on map).

The geology of this area is described in Publication 7 in my publication list.

 

 

 

Photo: Mylonitic gneiss, where the mylonitic foliation is folded and affected by shear zones and fractures (5.308756E 60.39608N).


Locality 2: Strained conglomerate, Sandviksfjellet

If you are interested in strained conglomerates, this one is a must: Hike the steep mountain slope N of the downtown area, from Sandviken Sykehus (hospital) up to Sandvikshytten. Below this cottage the strained and folded conglomerate described in publication 2 is located. Following the conglomerate through a tight map-scale fold we can observe a change in state of strain from flattening through constriction and back to flattening.

This locality is referred to in my Structural Geology book. A gescription of these rocks and their structure is found in Publication 3, and strain analyses are presented in Publication 2 in my publications list.

Strained quartz conglomerate, Sandviksfjellet (upper picture: 5.329188°E 60.423431°N, lower picture: 5.328071°E 60.424691°N).


Locality 3: Krossneset Granite

To get a flavor of the island-arc granitoids go to Krokeide (ferry place; loc. 3), where the Krossnes Granite is well exposed, containing abundant xenoliths in places. This granite is (one of) the youngest element(s) and the northern portion of the Ordovician Sunnhordland Batolith.

More information about this granite is given in Publications 1 and 5 in my publications list.

Photo: Krossnes Granite with xenoliths (60°13'33"N 5°17'21"E).

 

 

 

 

 


Locality 4: Ulven syncline

The youngest (and least deformed) metasediments in the arcs themselves are found in the Ulven area (loc. 4) near the town of Os. Phyllites containing (rare) fossils are overlain by quartzite and quartz conglomerate (Ulven conglomerate), all intruded by lamprophyric dikes; see publication 67.

The best exposures are found just N of the road intersection near Ulven Leir. Here we find cleaved phyllite overlain by quartzite and quartz conglomerate with preserved sedimentary structures, indicating that we are located on the steep limb of a syncline, the Ulven Synline.

Literature: Publication 67 in my Publications list, and Færseth et al., 2011.

Top left: Interbedded conglomerate and metasandstone (quartzite) (60°11'54.80"N 5°25'53,35"E).

Bottom: Lamprophyric dike cross-cutting the layering in the quartzite. New road section (60°11'53.24"N 5°26'12,13"E).


Locality 5: Eclogite in the Lindås Nappe

The classic area for Bergen Arcs eclogites is the northern part of Holsnøy, Meland, and island N of Bergen. A good starting point for a visit is Sætre.

Eclogitized anorthositic rock, Holsnøy. Blocks of "dry" granulite (anorthosite gabbro) in eclogitized matrix (5.007533°E 60.588482°N).

Close-up of eclogite with green omphacite and red-brown garnet (ca. 4.961537°E 60.600944°N).


Locality 6: Sheared gneisses of the Øygarden Complex

There are many places to study the gneisses along the North Atlantic coastline to the west of the arcs. Toftøy (loc. 6) north of Sotra is a particularly good one. In this area there are well-exposed post-Caledonian brittle faults, such as the road section toward Turøy, and there are beautiful folds in the heterogeneous gneisses that are worth a visit.

The fabrics so nicely displayed by these mylonitic gneisses show a consistent top-to-W(NW) sense of shear. This is best interpreted as an expression of the intense extensional ductile deformation that is found in SW Norway, including the Nordfjord-Sogn detachment to the North.

Descriptions of these rocks and structures are presented in Publications 8 and 11 in my publications list.

(Localities of photos: 4.93059°E 60.479922°N and 4.931267°E 60.482456°N).

 

Geologists in the Bergen Arcs

Hans Reusch (1852-1922) was a pioner geologists who mapped and interpreted large areas in South Norway, including the Bergen Arcs. He was the first to find fossils in the Silurian phyllites in the Os area.

Later Carl Fredrik Kolderup (1831–1913) and his son Niels Henrik Kolderup (1898-1971), both professors at Bergens Museum, did much more detailed work. They explored the geology of the Bergen area and published a quite important monograph in 1940 called The Bergen Arc System, in which their observations and interpretations were presented.

With this publication came a new and very useful map of the area.Naturally, radiometric dating techniques were not available at the time, so the age relations and genesis were not all correct. However, they provided descriptions and maps that formed the basis for further exploration throughout the decades to come.


Interpretations of the Bergen Arcs

The interpretation of the rock units of the Bergen Arcs has changed over time, but the general modern understanding is that they represent a stack of Caledonian thrust nappes resting on parautochthonous basement: the Øygarden Complex in the west, and the Western Gneiss Region to the east and northeast.The Lindås Nappe appears similar to the Jotun Nappe to the east. However, in the Bergen Arcs it is resting on rocks of the Major and Minor Bergen Arcs that contain oceanic/ophiolitic rocks, thus giving the Lindås Nappe at a higher position in the nappe stratigraphy than the Jotun Nappe. Also, the presence of Caledonian eclogites in the Lindås Nappe is interesting.

The figure (above) from the northern (Lindås) part of the Bergen Arc shows how the Caledonian nappe units occupy a synformal structure, resting on reworked basement known as the Western Gneiss Region in the north-east and the Øygarden Complex in the west. This synform tightens up to the south.

Selected literature

Austrheim, H., and Griffin, W.L., 1985, Shear deformation and eclogite formation within granulite-facies anorthosites of the Bergen Arcs.: Chemical Geology, v. 50, p. 267-281.

Bingen, B., Austrheim, H., Whitehouse, M.J., and Davis, W.J., 2004, Trace element signature and U–Pb geochronology of eclogite-facies zircon, Bergen Arcs, Caledonides of W Norway: Contributions to Mineralogy and Petrology, v. 147, p. 671-683.

Fossen, H., 1988, The Ulriken Gneiss Complex and the Rundemanen Formation: a basement-cover relationship in the Bergen Arcs, West Norway: Norges Geologiske Undersøkelse, v. 412, p. 67-86.

Fossen, H., 1989, Geology of the Minor Bergen Arc, West Norway: Norges Geologiske Undersøkelse, v. 416, p. 47-62.

Færseth, R.B., Thon, A., Larsen, S.G., Sivertsen, A., and Elvestad, L., 1977, Geology of the Lower Palaeozoic rocks in the Samnanger-Osterøy area, Major Bergen Arc, Western Norway: Nor. geol. unders., v. 334, p. 19-58.

Kolderup, C.F., 1902, Studier over bergartene ved Bergen.

Kolderup, C.F., and Kolderup, N.H., 1940, Geology of the Bergen Arc System: Bergen Museums Skrifter, v. 20, p. 137p.