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17/3/2006

Bedrock

Bedrock is the rock, usually solid, that underlies soil or other unconsolidated, superficial material (regolithes). The most common bedrock in our country is what usually is called the crystalline basement. This was formed roughly one to two billion years ago, through igneous, sedimentary and metamorphic processes. Mainly it consists of gneisses and granites, but the variation in rock types is large. After the formation the bedrock has been weathered and eroded, so what we see today at the surface are rocks that could have been formed deeper down than 10-15 km in the Earth's crust


Map bedrock

The oldest bedrock in Sweden - the crystalline basement

The most common bedrock in Sweden is what usually is called the crystalline basement or the Precambrian bedrock which is older than 570 million years. This was formed one to two billion years ago, through igneous and metamorphic processes. A history full of variety, where several mountain building periods (orogenies) have made their marks and given rise to a bedrock of varying composition. After they were formed the different rock types have been exposed to weathering and erosion, which are the main processes affecting the surface of the land. Thus, the surface we see today is composed of rocks that were formed at about 10-15 km depth in the interior of the Earth's crust. The crystalline bedrock can roughly be divided into 5 main regions or provinces with regard to age and important geological events. These provinces make up the Baltic Shield, also called the Fennoscandian Shield.

The very oldest bedrock is between two and a half to three billion years old and is found in a small area in the northernmost parts of Sweden, in the chronologically called Archaean province. It mainly consists of granites, granodiorites and diorites transformed into gneisses. Also isolated areas with greenstones and transformed sedimentary formations and basic volcanites occur.

Most of northern Sweden except for the mountain region, and the western parts of central Sweden, constitute a continuous area called the Svecofennian province. Also the expression Svecokarelian is used. The Svecofennian rocks were formed in connection to the Svecokarelian mountain building period (orogeny), with its main phase 1.8 - 1.9 billion years ago. Within this region the bedrock is very varied, including igneous rocks, for example granites, sedimentary rocks and volcanic rock that at heat and high pressure have been transformed into metamorphic rocks. Depending on their origin they are spoken about as metasediments and metavolcanites. Especially acid metavolcanite has replaced the older term leptite. Leptite was defined as a rock type or group of rocks formed from lava which was rich in silica (acid or intermediate) or volcanic tuff and marine sediments through regional metamorphism. The texture is characterized by a mean grain size between 0.5 - 0.05 mm (just visible to the eye). The term leptite has been widely used in Sweden and Finland. Hälleflinta is a related rock type with a finer texture, almost glassy.

The Svecofennian province comprises three parts: 1) the northern region with the province of Norrbotten and the ore field Skelleftefältet, 2) the central region with central and southern parts of northern Sweden, and 3) the southern region comprising parts of the provinces Dalarna, Uppland, Sörmland, Västmanland, Närke and north-eastern Östergötland. This region also encompasses the old mining district of central Sweden.

Southwest of the Svecofennian province the third large bedrock province is found, the Transscandinavian granite-porphyry province (TMB) or the Transscandinavian Igneous Belt (TIB). According to the name granites and porphyries dominate the region. The rock formation within this region started during the Paleoproterozoic era, some 1.9 billion years ago and proceeded some hundreds of million years. The main rocks are Värmland- and Småland granites, but also Dalagranites and Dalavolcanites (porphyry) are included. Late Precambrian sedimentary formations are included in this province, occurring at several places, and superimposing the older (primary) rock. Such examples are Dalasandstones constituting an odd element of older sedimentary rock in the otherwise igneous bedrock. Öjebasalt is an igneous supracrustal rock, formed during the same period as the sandstones. Similarly dolerite or diabase is frequent, for example the Åsby dolerite, found as flat sills in the formation.

The western boundary of the TMB is characterized by a marked deformation zone called the Protogine Zone. This is a zone running approximately north-south from the northern parts of the province of Värmland to the province of Skåne in which the bedrock to the east (TMB) successively change into gneiss towards the west as a result of increasing deformation. This is vizualized by an increasing schistosity. West of the Protogine Zone proper the forth major province is found, the south-western Scandinavian province, which comprises an eastern and a western segment including the western parts of central and southern Sweden, Svealand and Götaland. In this region mainly different kinds of gneisses were found, and therefore it was earlier denoted the south-western Swedish gneiss area. Within the Swedish part of this province three marked tectonic zones (fault zones) have been identified, the Mylonite zone ("the flour zone"), the fault zone of the province of Dalsland, and in between the border line along the river Göta Älv. The first extending from the central parts of the province of Värmland via Värmlandsnäs and Kollandsö in Lake Vänern to the town of Varberg, constitutes a boundary between the segments of the eastern and western parts of the region.

One of the more spectacular rock types connected to the Protogine Zone is called hyperite, a form of dolerite or diabase with a specific mineral composition of two types of pyroxene, together with olivine and iron oxidepigmented calciumrich plagioclase. These rocks are found in a band through the gneisses in the central parts of Värmland and the north-eastern parts of the province of Västergötland, and also as so called hyperite diabases (commonly called black granite, by laymen) along the Protogine Zone from Lake Vättern to the northern parts of the province of Skåne. These intrusions occurred during a very long time period extending about 600 million years.

The fifth and last province of the Precambrian bedrock is the Blekinge-Bornholm province, to some extent being in a unique position and differing from other Precambrian or Proterozoic bedrocks in Sweden. The northern limit for this province is constituted by faults. The rocks within the area are characterised by granites and transitional forms towards granite gneiss, called the Blekinge coastal gneiss.

These rough subdivisions of the Swedish bedrock give no room for the local variations that may be of great importance, for example for growth conditions. In most areas places are found where magmas have penetrated cracks (intruded) in the otherwise solid bedrock, thus forming locally different rocks. Examples are the common diabase or dolerite dykes and the hyperites in Värmland mentioned above, related to the hyperite diabases in Småland. Another example of an immense diabase dyke is Hälleforsgången, south-east of lake Hjälmaren, with a length of 40 km and overall width of 2 km.

Rock fragments and minerals from these divergent rocks may be found in regolithes in a considerably larger area than the outcropping rock itself, and thus may give rise to favourable soils.

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The younger sedimentary bedrock of Sweden

The sedimentary bedrock in Sweden has mainly developed through weathering, erosion and sedimentation (sediments like clay, silt, sand- and lime in different environments), mainly in marine environments at the time the continent still was close to the equator. These sediments have then consolidated through diagenetic processes and been transformed into shale, siltstone, sandstone, and limestone, respectively. Most of these rocks have, though, through millions of years eroded after they were formed during the Cambrian-Silurian period (early Paleozoic era) some 570 - 400 million years ago. By different reasons they have been preserved in some regions like the lake Storsjön surroundings in the province of Jämtland, the Siljan ring area in the province of Dalarna (Dalecarlia), the plain of Örebro-Kumla west of the lake Hjälmaren in the province of Närke, the Östgöta plain between Omberg-Motala and the Lake Roxen in the province of Östergötland, and the south-western part of Skåne by depressions in connection to important faults creating horsts of crystalline basement bordered by troughs or grabens of sedimentary rocks.

Other examples of sedimentary rocks that have been preserved are the table mountains in the province of Västergötland (Billingen, Mösseberg, Ålleberg, Varvsberget, Plantaberget, Gerumsberget, Kinnekulle, Lugnåsberget , Halleberg and Hunneberg). Here magma penetrating from the inner parts of the Earth formed dolerite dykes or sills. Today they constitute the rests of a sheltering bonnet on the slightly eroded sedimentary rocks. Another case, in the area surrounding lake Siljan, the deposits have been preserved in a ring structure, very likely formed by a meteorite impact at the end of the Devonian period, some 360 million years ago.

Apart from these scattered occurrences on the Swedish mainland, continuous areas with sedimentary rocks are found on the large islands in the Baltic, Öland and Gotland. In these areas the bedrock is very different from the bedrock in other parts of Sweden in so far as they are more closely connected to the bedrock in the sea area than to the bedrock of the Swedish mainland, thus belonging to the Prequaternary rocks of the continental shelf. These two islands are both part of a sedimentary sequence of strata constituting the bottom of the southern Baltic. The island of Öland consists of an earlier deposition formed during the Cambrian and the Ordovician periods with mainly sandstones and shales (e. g. alum shales) in the region of the strait of Kalmarsund and orthoceratite limestone in the eastern parts. In this area the flat lying limestones have created a plain landscape form called the "alvaret" where karst weathering morphology occurs. The island of Gotland represents a later period, the Silurian, and constitutes due to the dipping strata towards east of the sedimentary sequence a younger structure than Öland. The rocks at Gotland are characterised by marked reefs interrupted by laminated sedimentary rocks. Both limestone and marlstone occur, whereas the distribution of sandstone is fairly confined. One such area is situated at Burgsvik, where grindstones were produced in earlier days.

Rocks of similar age as those at Öland are found in a much larger area at the bottom of the sea at the Gävle Bay in the the Gulf of Bothnia. This area can in size be compared to the jutting of easternmost Svealand, an area of about 15 000 km2 (5 times the area of Gotland). Because of its location at the bottom of the sea the area is seldom marked at geological surveys. Nevertheless, the rock character has in a significant way affected soils in the north-eastern part of the province of Uppland. The rocks characterising this area are limestone, shale, and sandstone.

The province of Skåne is from south-east towards north-west intersected by the Tornqvist Zone, a fault and deformation zone that geologically split the landscape into two parts. In the north the bedrock is of Precambrian crystalline character, whereas in the south it consists of sedimentary rocks from the Cambrian-Silurian periods and even younger deposits from the Triassic, Jurassic, and Tertiary periods. The oldest rocks from the Cambrian-Silurian periods are sandstone, shale and alum shale. In the north also the Triassic and Jurassic periods are characterised by sandstone and shale.

In the south-westernmost part of Skåne, between the towns Landskrona, Lund, Ystad and Trelleborg, the bedrock was formed during the Tertiary period and characterized by limestones. The areas around Kristianstad and the bay of Båstad in the province of Halland are characterized by limestone of chalk-type and marlstone, sandstone and consolidated calcareous clay from the Creataceous period. Thus, the youngest bedrock in Sweden, with an age younger than 100 millions of years, was formed during the Cretaceous and Tertiary periods and found in the south-western part of Skåne.

The tectonic events of Skåne are very much impressed by the Tornqvist Zone, which is an intersection with faults covering a zone in a south-easterly to north-westerly direction from the Romanian coast at the Black Sea to the North Sea. The zone crosses Skåne and mark the margin between the eastern European Platform and the Baltic Shield. In Sweden it covers a 50 km wide zone in which a number of faults have caused the ridges found in Skåne, which in a strict geological sense are no ridges, but horsts. A horst is a hill extended in two directions surrounded by faults. Between the horsts of crystalline rocks are preserved areas with younger sedimentary, fossiliferous rocks like sandstone, shale and limestone. The morphologically spectacular horsts were formed through extensive movements by faulting during geologically long times. Examples of such horsts are Hallandsås, Nävlingeåsen, Linderödsåsen, Kullaberg, Söderåsen and Romeleåsen. Within this zone are also found a number of diabase or dolerite dykes formed during the Carboniferous and Permian periods, and basalts from the Jurassic-Cretaceous periods, for example at Rallate, north-west of the village of Röstånga close to Söderåsen.

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The mountain region

Our mountain chain, the Scandes, covering a roughly 2000 km long and 100-200 km wide area from The North Cape in the north to the Norwegian town of Stavanger in the south, comprises one part of the Caledonian Mountain chain, which was split up when the present North Atlantic Ocean started to emerge about 65 million years ago. The Scandinavian mountain chain was formed in a similar way as Himalaya, by plate tectonism, a still ongoing process forced through the collision between India and the Asian continental plates.

By means of plate tectonics, the Scandes or Caledonides were formed 510 to 400 million years ago, when the oceanic trench separating the two continents Baltica (present northern Europe) and Laurentia (present North America and Greenland) started contracting and finally closed. At the collision the edge zone of Baltica was forced down beneath Laurentia. By thrust deposits from the sea bottom (the Iapetus Ocean) were pushed up over Baltica. This resulted in the formation of large chunks of bedrock or nappes ("skollor" in Swedish) being transported hundreds of kilometres towards east-south-east on each other and over Baltica and also flat thrusts, separating the nappes. The different bedrock layers, nappes are today divided into lower, middle, upper and uppermost Allochthons, of which the uppermost mainly is found in Norway. Nappes high up in the sequence have been transported further relative to the lower ones.

Depending on the degree of transformation during the thrust process, the resulting rocks have received different characters. Rocks slightly transformed are arkose (sandstone containing small amounts of clay minerals or mica and large amounts of feldspar), quartzite, greywacke, slate and phyllite. More intensively metamorphosed rocks are micaschists and amphibolites, the later representing strongly transformed basalts and basic intrusive rocks, and also ultrabasic rocks in turns layered with mica schist, gneiss, quartzite and marble.

In the upper part of the Allochthon (Nappe bedrock), called the Köli Nappes, remnants of rocks deposited in the now disappeared Iapetus ocean occur. Here large amounts of sediments were deposited, that during the mountain chain folding were transformed to different kinds of phyllite, mica schist and marble. Added to this are greenschist and greenstone, representing strongly transformed volcanic rocks containing relatively little silica. Parent material for these rocks were formed during the Ordovician-Silurian period in an environment with basins and arches of islands like the ones today found in the western parts of the Pacific.

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Influence of bedrock on soil fertility

There are mainly three qualities of soil parent material that are of decisive importance for soil fertility. These are nutrient content of rocks and minerals, and their weathering ability complemented by water-holding capacities. The water-holding capacity is affected by fine texture or clay content.

The bedrock is characterised by its rock types. The rock types, igneous, sedimentary and metamorphic rocks are in turn built from combinations of rock-forming minerals. The rock may be dominated by a number of minerals or of one or two separate minerals, called major minerals. Limestone is an example of rock consisting of mainly one mineral, calcite, whereas gneiss may consist of quartz, potassium feldspar, sodium-calcium feldspar (plagioclases), amphibole, muscovite, biotite, chlorite and others (accessory minerals). The main part of the rock forming minerals are compounds of silicon and oxygen combined with other elements, like aluminium, potassium, sodium, calcium and magnesium.

Rock types formed from cooling magma through crystallisation of a number of minerals (igneous rocks), are characterised by their silica content. Geologists therefore denote igneous rocks with a silica content (SiO2-content) exceeding 65 weight-%, for acid, and rocks with silica content below 52 weight-%, for basic. However, the magmas from where these igneous rocks emanate are called ryolitic and basaltic, respectively. This convention shall not be mixed up with acidity or pH, which is a measure of hydrogen ion activity.

A measure of the potential nutrient concentration in soil is given by the base-mineral index, a quick method to assess the mineralogical soil type. This method was introduced at the beginning of the 20th century by the Swedish pioneer in soil research, Olof Tamm (1934). Base-mineral index represents the percentage of mineral content with a density exceeding 2.68 g/cm3. In practice it is the content of chemically easily weathered minerals that normally contain magnesium, calcium and iron. This type of mineral, mafic minerals, are found in the rocks gabbro, diorite, basalt, dolerite, diabase, hyperite, diorite porphyry and amphibolite. Among foresters these rocks are jointly denoted greenstone, possibly based on their greenish tint occurring at weathering. Internationally the name greenstone is used to denote amphibolite and other transformed basic rocks.

Rocks rich in quartz like mica schist, leptite, hälleflinta, gneiss, granite and porphyries mainly contain minerals with a density less than 2.68 g/cm3, thus their base-mineral index is quite low.

Weathering capacity for a rock may refer to either sensitivity to mechanical decomposition or chemical dissolution. The chemical dissolution, which results in release of mineral nutrients, always starts at the surface, or in systems of cracks of the mineral particle, and thereafter spread deeper into the particle. Weathering intensity therefore will be dependent on the surface of the mineral particles. In this regard, rocks differ depending on mineral composition and how particles are joined together.

A quality of the rock interacting with weathering capacity is the tendency to form fine-textured soils. A widely spread misunderstanding is that limestone in areas with sedimentary bedrock has been decomposed physically and given rise to the clay content of the Quaternary boulder clays or clayrich tills. More likely the limestone, for example orthoceratite limestone, contain layers with clay of varying thickness that after the limestone through millions of years leached out through chemical weathering, leave a clayey stratum that repeatedly is processed during glaciations and is worked into the other rock material, left behind by the Inland glacier as till. The fine texture supports the water-holding capacity and facilitates chemical weathering, which normally results in easy access to macro and micro nutrients of these soils.

Often qualities promoting fertility interact: easily weathered bedrock often contains a large amount of nutritious minerals and at the same time is disposed to form fine-grained soil types. It is therefore possible to classify the bedrock according to its ability to support favourable nutritional status.

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References

Loberg, B. 1999. Geologi. Material, processer och Sveriges berggrund. (In Swedish)

Lindström, M., Lundqvist, J. & Lundqvist, Th. 1991. Sveriges geologi från urtid till nutid.

SNA-Sveriges Nationalatlas: Berg och jord. 1994 (red. C. Fredén). (In Swedish)

SNV. 1994. Markens bördighet. SNV rapport 4337. (In Swedish)

Tamm, O. 1934. En snabbmetod för mineralogisk jordartsgranskning. Sv. Skogsvårdsföreningens Tidskr. 32. sid 231-250. (In Swedish)

The National Atlas of Sweden: Geology. 1994 (ed. C. Fredén). ISBN 91-87760-28-2.

Troedsson, T. & Nykvist, N. 1974. Marklära och markvård. (In Swedish)

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