Sedimentary raw materials

From hf/iakh/sarc
Revision as of 08:19, 11 September 2009 by Sigrist@uio.no (talk | contribs) (Created page with 'While the igneous and metamorphic rocks are formed by internal processes in the earth’s crust, the sedimentary rocks are originally deposited as sediments on the earth’s surf...')

(diff) ← Older revision | Latest revision (diff) | Newer revision → (diff)
Jump to: navigation, search

While the igneous and metamorphic rocks are formed by internal processes in the earth’s crust, the sedimentary rocks are originally deposited as sediments on the earth’s surface. Later these sediments have become solid rocks by cementation. The sediments consist of particles, grain and fragments caused by decomposition and erosion of rocks. These sediments are moved to their final destination by water (rivers, sea, mud floods), glaciers, wind or by rock slides. The sediments are left at ocean floors, lakes, rivers, beaches, along valleys or infront of glaciers. Some of the sediments and the rocks they constitue are: mud/mudstone, sand/sandstone, gravel or pebbles/conglomerate or sedimentary brecchia.

In Norway, sedimentary rocks can be found in the Oslo-area. Also, thick layers of sandstone, conglomerates and sedimentary brecchia are found in the outer part of the Sognefjord and Nordfjord. Sedimentary rocks are also common at the Varangerhalvøya in Finnmark (Sigmond 1996: 13-15).


Whereas igneous and metamorphic rocks are produced by internal processes within the Earth, sedimentary rocks are formed by processes which are active at the Earth´s surface. The surface of the land is continually being attacked by agents of weathering and erosion, such as rain and rivers, wind and moving ice. These physical agents are helped by chemical decay from percolating waters, and together they break up even the toughest rocks and produce rock waste. This is transported, mainly by rivers but also by wind, and in higher latitudes by ice. Eventually this material, now referred to as sediment, is deposited at river mouths, in lakes and in the sea. It is the accumulation of this material, often in deposits many kilometers thick, which goes to make up the sedimentary rocks." (Hamilton et al 1976, 151)

Sandstone

Sandstone is a sedimentary rock made of sand (quartz, feltspar)

  • Appearance: The color of sandstone vary greatly dependent on the grain content of the rock and its matrix, but it is often red, brown, grey or yellow. The single grains in the rock can be two millimeter big. Most often the grains are rounded and sorted, that is, the coarsest are at the bottom and the finer above. In these cases the sandstone is clearly layered. It is common to find fossiles in the rock.
  • Fracture: Conchoidal fracture (but lack of elasticity often leads to unexpected fractures)
  • Grain size: Fine-medium grained (dependent on the sorting)
  • Properties: Lacks many properties which makes it perfect for percussion (knapping), however you do find axes made of sandstone.
  • Tools: Grinding slabs, medium hard hammerstones
  • Source: Known sources in Oslofeltet and Vestlandet


Colour: Very variable; frequently red, brown, greenish, yellow, grey, white.

Texture: Medium-grained. Usually well sorted, that is grains all about the same size; grains sub angular to rounded (sandstone).

Structure: Bedding usually apparent; current bedding and ripple marks common; graded bedding may occur. Concretions and fossils may be found.

Mineralogy: Quartz is the main component but is often accompanied by feldspar, mica or other minerals. The grains may be cemented by silica, calcite or iron oxides.

Field relations: Sandstones are associated with most other sedimentary rocks. Most sands accumulated either in water, usually the sea, or as wind-blown deposits in arid continental areas. Desert sandstones tend to be red, and the individual sand grains are often almost spherical and polished." (Hamilton et al 1976, 194)

Mudstone

Mudstone is a sedimentary rock made of mud.


  • Appearance: Mudstone vary in color from black, grey, white, brown and red. Mud has a very little grain size, thus the individual grains can not be seen with the naked eye.
  • Fracture: Conchoidal fracture
  • Grain size: Fine grained
  • Properties: Homogenous, brittle (the brittleness makes the rock fairly weak which does not make it suitable for heavy work tools. Mudstone can be difficult to control during production).
  • Tools: Blades, microblades



mudstone, shale and clay

Colour: Black, grey, white, brown, red, dark green or blue.

Texture: Grain size less than 1/256 mm; individual grains are too small to be distinguished with the naked eye. Mudstone and shale feel smooth, and a pure clay is not gritty when smeared between the fingers. Clays are plastic and often sticky when wet.

Structure: When consolidated and relatively massive it is known as mudstone (or claystone); if finely bedded so that it splits readily into thin layers it is called shale. When soft and uncompacted it is termed clay. Sun cracks, rain prints etc. sometimes occur on bedding surfaces; and fossils and concretions are common.

Mineralogy Too fine-grained for minerals to be distinguished with the naked eye, or usually even with the microscope. Clays consist of a mixture of clay minerals together with detrial quartz, felspar and mica. Iron oxides are usually abundant and contribute the red and yellow colours. Black shales are rich in carbonaceous matter, and pyrite and gypsum commonly occur in them, sometimes as well shaped crystals.

Field relations Clays tend only to occur in the younger geological formations, being consolidated into mudstone and shales with time. Being very fine-grained, clay is easily transported water into the sea and lakes, where it accumulates with silt, sand and calcareous organisms to form typical sequences of shales, siltstones, sandstone s and limestone s. Some clays are residual, having formed in situ as soils; such are the bauxitic clays." (Hamilton et al 1976, 196)

Breccia

  • Colour: Variable.
  • Texture: Consists of angular rock fragments (2 mm to many metres in diameter) set in a fine- to medium grained matrix. In some breccias the fragments can be seen to match along their opposed sides, indicating only modest disturbance.
  • Structure: Bedding not usual, though in some types of breccia bedding is apparent in the matrix. Fossils are rare.
  • Mineralogy: The fragments may be of any type of igneous, metamorphic or sedimentary rock. The matrix usually consists of silt or sand cemented by calcite or silica.
  • Field relations:

Colour: Variable.

Texture: Consists of angular rock fragments (2 mm to many metres in diameter) set in a fine- to medium grained matrix. In some breccias the fragments can be seen to match along their opposed sides, indicating only modest disturbance.

Structure: Bedding not usual, though in some types of breccia bedding is apparent in the matrix. Fossils rare.

Mineralogy: The fragments may be of any type of igneous, metamorphic or sedimentary rock. The matrix usually consists of silt or sand cemented by calcite or silica.

Field relations: Many breccias represent consolidated talus or scree material, that is accumulations of rock fragments formerly lying on steep hill slopes, or at the foot of cliffs. They are often found above uncomformities, and associated with conglomerate, arkose and sandstone. Other breccias are produced by the fragmentation of rocks during faulting." (Hamilton et al 1976, 192)

Arkose

Colour: Red, pink or grey.

Texture: Medium-grained, but usually nearer to the coarse end of the scale; grains angular.

Structure: Bedding may be obscure or well developed; often current bedded. Fossils rare.

Mineralogy: Contains 25% or more of feldspar, rarely more than 50%; the rest is mainly quartz, but some biotite and muscovite may occur. The cement is usually calcite or iron oxides.

Field relations: Arkoses are derived from the disintegrations of granite and granite gneisses, and because they are composed of quartz and feldspar they resemble granites, but the angular, fragmental nature of the grains serves to distinguish arkose from the closely interlocking igneous texture of granite. Arkoses occur above uniformities in the immediate vicinity of granitic terrains, or in thick deposits associated with conglomerates (containing granite boulders) derived from granites or gneisses." (Hamilton et al 1976, 194)

Dolomite

Colour: White, cream or grey, but often weathers brown or pinkish.

Texture: Coarse, medium or fine; compact, sometimes earthy.

Structure: Bedding tends to be large scale. May be massive or contain complex concretions and nodular growths. conspicuously jointed. Organic remains usually destroyed by recrystallization.

Mineralogy: Contains a high proportion of dolomite (the mineral and the rock have the same name). Detrital minerals and secondary silica (chert) may be present.

Field relations: Dolomites are usually interbedded with other limestone s and are commonly associated salt and gypsium deposits. Most dolomites are thought to be of secondary (replacement) origin; the calcite of the original limestone having been replaced in situ by dolomite, probably by percolating watery solutions. (Hamilton et al 1976, 200)

Conglomerate

Conglomerate is a sedimentary rock in which beach and/or gravel from rivers are pressed together. A conglomerate contains round pebbles and the material which glues these together is sand with some mud and chalk , often solidified with silicates. Ordinarily conglomerates consist of tough rocks which endure being transported in water over some distance.



  • Appearance: The appearance of a conglomerate vary according to which type of rock the pebbles are, in addition to the type of matrix which surrounds them. Sometimes the conglomerates can consist of several different rocks (polygen), other times of only one (monogen). A conglomerate can contain both sedimentary, igneous and metamorphic rocks.
  • Fracture: The conglomerate itself has an undetermined fracture, but many rocks it consist of can have a conchoidal fracture.
  • Grain size: Very coarse grained
  • Properties: A conglomerate can be a source of fine grained rocks for tool production.
  • Source: Over the entire country

Colour Variable.

Texture Consists of rounded pebbles (diameter greater than 2 mm), cobbles or boulders set in a fine- or medium-grained matrix.

Structure Bedding absent or only crudely developed; may be apparent from variation in the size of the pebbles. Fossils rare.

Mineralogy Pebbles, boulders etc. may consist of quartz, chert, flint or almost any igneous, metamorphic or sedimentary rock, but tougher rocks such as quartzite often predominate. The matrix usually comprises sand or silt, often cemented by silica or calcite.

Field relations Conglomerates are consolidated pebble, gravel or boulder beds which accumulate along sea and lake shores and in rivers. They are indicative of shallow water sedimentation and vigorous currents, which are required to move large rock fragments. Marine transgressions (rise of sea-level with consequent flooding of the land) are frequently marked by conglomerates which, therefore, are often found immediately above unconformities. Conglomerates are usually associated with sandstone and arkose." (Hamilton et al 1976, 192)