Greywacke or
Graywacke (
GermanGerman is a West Germanic language, thus related to and classified alongside English and Dutch. It is one of the world's major languages and the most widely spoken first language in the European Union. Around the world, German is spoken by approximately 105 million native speakers and also by...
grauwacke, signifying a grey, earthy rock) is a variety of
sandstoneSandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains. Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any color, but the most common colors are tan, brown, yellow,...
generally characterized by its hardness, dark color, and poorly-sorted, angular grains of
quartzQuartz is the second most abundant mineral in the Earth's continental crust . It is made up of a continuous framework of SiO
4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO
2.There are many different varieties of...
,
feldsparFeldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
, and small rock fragments or
lithic fragmentsLithic fragments, or lithics, are pieces of other rocks that have been eroded down to sand size and now are sand grains in a sedimentary rock. Lithic fragments can be derived from sedimentary, igneous or metamorphic rocks). A lithic fragment is defined using the Gazzi-Dickinson point-counting...
set in a compact,
clayClay is a naturally occurring material composed primarily of fine-grained minerals, which show plasticity through a variable range of water content, and which can be hardened when dried and/or fired...
-fine matrix. It is a texturally-immature
sedimentary rockSedimentary rock is the type of rock that is formed by sedimentation of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....
generally found in Palaeozoic
strataIn geology and related fields, a stratum is a layer of rock or soil with internally consistent characteristics that distinguishes it from contiguous layers. Each layer is generally one of a number of parallel layers that lie one upon another, laid down by natural forces. They may extend over...
. The larger
grainParticle size or grain size refers to the diameter of a grain of granular material, such as sediment or the lithified particles in clastic rock. Granular material can range from very small colloidal particles, through clay, silt, sand, and gravel, to boulders.In contrast, crystallite size is the...
s can be sand-to-gravel-sized, and
matrixThe matrix or groundmass of rock is the fine-grained mass of material in which larger grains or crystals are embedded.The matrix of an igneous rock consists of fine-grained, often microscopic, crystals in which larger crystals are embedded. This porphyritic texture is indicative of multi-stage...
materials generally constitute more than 15% of the rock by volume.
Greywacke or
Graywacke (
GermanGerman is a West Germanic language, thus related to and classified alongside English and Dutch. It is one of the world's major languages and the most widely spoken first language in the European Union. Around the world, German is spoken by approximately 105 million native speakers and also by...
grauwacke, signifying a grey, earthy rock) is a variety of
sandstoneSandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains. Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any color, but the most common colors are tan, brown, yellow,...
generally characterized by its hardness, dark color, and poorly-sorted, angular grains of
quartzQuartz is the second most abundant mineral in the Earth's continental crust . It is made up of a continuous framework of SiO
4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO
2.There are many different varieties of...
,
feldsparFeldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
, and small rock fragments or
lithic fragmentsLithic fragments, or lithics, are pieces of other rocks that have been eroded down to sand size and now are sand grains in a sedimentary rock. Lithic fragments can be derived from sedimentary, igneous or metamorphic rocks). A lithic fragment is defined using the Gazzi-Dickinson point-counting...
set in a compact,
clayClay is a naturally occurring material composed primarily of fine-grained minerals, which show plasticity through a variable range of water content, and which can be hardened when dried and/or fired...
-fine matrix. It is a texturally-immature
sedimentary rockSedimentary rock is the type of rock that is formed by sedimentation of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....
generally found in Palaeozoic
strataIn geology and related fields, a stratum is a layer of rock or soil with internally consistent characteristics that distinguishes it from contiguous layers. Each layer is generally one of a number of parallel layers that lie one upon another, laid down by natural forces. They may extend over...
. The larger
grainParticle size or grain size refers to the diameter of a grain of granular material, such as sediment or the lithified particles in clastic rock. Granular material can range from very small colloidal particles, through clay, silt, sand, and gravel, to boulders.In contrast, crystallite size is the...
s can be sand-to-gravel-sized, and
matrixThe matrix or groundmass of rock is the fine-grained mass of material in which larger grains or crystals are embedded.The matrix of an igneous rock consists of fine-grained, often microscopic, crystals in which larger crystals are embedded. This porphyritic texture is indicative of multi-stage...
materials generally constitute more than 15% of the rock by volume. The term 'Greywacke' can be confusing, since it can refer to either the immature (rock fragment) aspect of the rock or the fine-grained (clay) component of the rock.
The origin of greywacke was problematic prior to the understanding of
turbidity currentA turbidity current or density current is a current of rapidly moving, sediment-laden water moving down a slope through air, water, or another fluid...
s and
turbiditeTurbidite geological formations have their origins in turbidity current deposits, which are deposits from a form of underwater avalanche that are responsible for distributing vast amounts of clastic sediment into the deep ocean.-The ideal turbidite sequence:...
s since, according to the normal laws of
sedimentationSedimentation is the tendency for particles in suspension or molecules in solution to settle out of the fluid in which they are entrained, and come to rest against a wall...
,
gravelGravel is rock that is of a specific particle size range. Specifically, it is any loose rock that is larger than two millimeters in its smallest dimension and no more than . The next smaller size class in geology is sand, which is > in size. The next larger size is cobble, which is >. Gravel can...
,
sandSand is a naturally occurring granular material composed of finely divided rock and mineral particles.As the term is used by geologists, sand particles range in diameter from 0.0625 to 2 millimeters. An individual particle in this range size is termed a sand grain...
and
mudMud is a liquid or semi-liquid mixture of water and some combination of soil, silt, and clay. Ancient mud deposits harden over geological time to form sedimentary rock such as siltstone or solid, mudrock lutites. When geological deposits of mud are formed in estuaries the resultant layers are...
should not be laid down together. Currently
geologistFor other uses, see Geologist .A geologist is a scientist who studies the solid and liquid matter that constitutes the Earth and terrestrial planets...
s attribute its formation to submarine
avalancheAn avalanche is a rapid flow of snow down a slope, from either natural triggers or human activity. Typically occurring in mountainous terrain, an avalanche can mix air and water with the descending snow...
s or strong turbidity currents. These actions churn
sedimentSediment is any particulate matter that can be transported by fluid flow, and which eventually is deposited.Sediments are most often transported by water transported by wind and glaciers...
and cause mixed-sediment slurries to occur. When this is the case, the rocks may exhibit a variety of sedimentary features. Support for the turbidity current origin is the fact that deposits of greywacke are found on the edges of the
continental shelvesThe continental shelf is the extended perimeter of each continent and associated coastal plain, and was part of the continent during the glacial periods, but is undersea during interglacial periods such as the current epoch by relatively shallow seas and gulfs. The continental rise is below the...
, at the bottoms of
oceanic trenchThe oceanic trenches are hemispheric-scale long but narrow topographic depressions of the sea floor. They are also the deepest parts of the ocean floor....
es, and at the bases of mountain formational areas. It also occurs in association with black
shaleShale is a fine-grained, clastic sedimentary rock composed of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable...
s of deep sea origin.
Greywackes are mostly grey, brown, yellow or black, dull-colored, sandy rocks which may occur in thick or thin beds along with
slateSlate is a fine-grained, foliated, homogeneous metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low grade regional metamorphism. The result is a foliated rock in which the foliation may not correspond to the original sedimentary...
s and
limestoneLimestone is a sedimentary rock composed largely of the mineral calcite . The deposition of limestone strata is often a by-product and indicator of biological activity in the geologic record...
s. They are abundant in
WalesWales is a country that is part of the United Kingdom, bordered by England to its east, and the Atlantic Ocean and Irish Sea to its west. It is also an elective region of the European Union...
, the south of
ScotlandScotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...
, the Longford Massif in
IrelandIreland is the third-largest island in Europe and the twentieth-largest island in the world. It lies to the north-west of continental Europe and is surrounded by hundreds of islands and islets. To the east of Ireland, separated by the Irish Sea, is the island of Great Britain...
and the Lake District National Park of
EnglandEngland is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west and the North Sea to the east, with the English Channel to the south separating it from continental...
, and compose the majority of the main alps that make up the back bone of
New ZealandNew Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses , and numerous smaller islands, most notably Stewart Island/Rakiura and the Chatham Islands. The indigenous Māori named New Zealand Aotearoa, commonly translated as The Land of the Long White Cloud...
. They can contain a very great variety of
mineralA mineral is a naturally occurring solid formed through geological processes that has a characteristic chemical composition, a highly ordered atomic structure, and specific physical properties. A rock, by comparison, is an aggregate of minerals and/or mineraloids, and need not have a specific...
s, the principal ones being quartz,
orthoclaseOrthoclase is an important tectosilicate mineral which forms igneous rock. The name is from the Greek for "straight fracture," because its two cleavage planes are at right angles to each other. An alternate name is alkali feldspar...
and
plagioclasePlagioclase is a very important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...
feldspars,
calciteCalcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 470°C, and vaterite is even less stable....
,
iron oxideIron oxides are chemical compounds composed of iron and oxygen. Altogether, there are sixteen known iron oxides and oxyhydroxides.-Oxides:* FeO, iron oxide, * Fe3O4, iron oxide,...
s and graphitic, carbonaceous matters, together with (in the coarser kinds) fragments of such rocks as
felsiteFelsite is a very fine grained volcanic rock that may or may not contain larger crystals. Felsite is a field term for a light colored rock that typically requires petrographic examination or chemical analysis for more precise definition. Color is generally white through light gray, or red to tan...
,
chertChert is a fine-grained silica-rich microcrystalline, cryptocrystalline or microfibrous sedimentary rock that may contain small fossils. It varies greatly in color , but most often manifests as gray, brown, grayish brown and light green to rusty red; its color is an expression of trace elements...
,
slateSlate is a fine-grained, foliated, homogeneous metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low grade regional metamorphism. The result is a foliated rock in which the foliation may not correspond to the original sedimentary...
,
gneissGneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks. Gneissic rocks are usually medium to coarse foliated and largely recrystallized but do not carry large...
, various
schistThe schists form a group of medium-grade metamorphic rocks, chiefly notable for the preponderance of lamellar minerals such as micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is produced...
s, and
quartziteQuartzite is a hard metamorphic rock which was originally sandstone. Sandstone is converted into quartzite through heating and pressure usually related to tectonic compression within orogenic belts...
. Among other minerals found in them are
biotiteBiotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula K3AlSi3O102. More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the...
and
chloriteThe chlorites are a group of phyllosilicate minerals. Chlorites can be described by the following four endmembers based on their chemistry via substitution of the following four elements in the silicate lattice; Mg, Fe, Ni, and Mn....
,
tourmalineTourmaline is a crystal silicate mineral compounded with elements such as aluminium, iron, magnesium, sodium, lithium, or potassium. Tourmaline is classed as a semi-precious stone and the gem comes in a wide variety of colors...
,
epidoteEpidote is a calcium aluminium iron sorosilicate mineral, Ca
2Al
2O, crystallizing in the monoclinic system. Well-developed crystals are of frequent occurrence: they are commonly prismatic in habit, the direction of elongation being perpendicular to the single plane of symmetry...
,
apatiteApatite is a group of phosphate minerals, usually referring to hydroxyapatite, fluorapatite, chlorapatite and bromapatite, named for high concentrations of OH
−, F
−, Cl
− or...
,
garnetThe garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives. The name "garnet" may come from either the Middle English word gernet meaning 'dark red', or the Latin granatus , possibly a reference to the Punica granatum , a plant with red seeds...
,
hornblendeHornblende is a complex inosilicate series of minerals . Hornblende is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole. It is an isomorphous mixture of three molecules; a calcium-iron-magnesium silicate, an...
and
augiteAugite is a single chain inosilicate mineral described chemically as SiO
3 or calcium magnesium iron silicate. The crystals are monoclinic and prismatic. Augite has two prominent prismatic cleavages, meeting at angles near 90°....
, sphene,
pyriteThe mineral pyrite, or iron pyrite, is an iron sulfide with the formula FeS
2. This mineral's metallic luster and pale-to-normal, brass-yellow hue have earned it the nickname fool's gold due to its resemblance to gold...
s. The cementing material may be siliceous or
argillaceousArgillaceous minerals appear silvery upon optical reflection and are minerals containing substantial amounts of clay-like components . Argillaceous components are fine-grained aluminosilicates, and more particularly clay minerals such as kaolinite, montmorillonite-smectite, illite, and chlorite...
, and is sometimes calcareous.
As a rule greywackes are not
fossilFossils are the preserved remains or traces of animals, plants, and other organisms from the remote past. The totality of fossils, both discovered and undiscovered, and their placement in fossiliferous rock formations and sedimentary layers is known as the fossil record...
iferous, but organic remains may be common in the finer beds associated with them. Their component particles are usually not very rounded or polished, and the rocks have often been considerably indurated by
recrystallizationRecrystallization may refer to:*Recrystallization *Recrystallization *Recrystallization...
, such as the introduction of interstitial silica. In some districts the greywackes are cleaved, but they show phenomena of this kind much less perfectly than the slates. Some varieties include
feldspathic greywacke, which is rich in feldspar, and
lithic greywacke, which is rich in tiny rock fragments.
Although the group is so diverse that it is difficult to characterize mineralogically, it has a well-established place in
petrographicalPetrology is the branch of geology that studies rocks, and the conditions in which rocks form...
classifications because these peculiar composite arenaceous deposits are very frequent among
SilurianThe Silurian is a geologic period and system that extends from the end of the Ordovician period, about 443.7 ± 1.5 Ma , to the beginning of the Devonian period, about 416.0 ± 2.8 Ma . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
and
CambrianThe Cambrian is the first geological period of the Paleozoic era, lasting from ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux...
rocks, and are less common in
MesozoicThe Mesozoic Era is one of three geologic eras of the Phanerozoic eon. The division of time into eras dates back to Giovanni Arduino, in the 18th century, although his original name for the era now called the "Mesozoic" was "Secondary" The Mesozoic Era is one of three geologic eras of the...
or
CenozoicThe Cenozoic Era The Cenozoic (also Cænozoic or Cainozoic) Era The Cenozoic (also Cænozoic or Cainozoic) Era (meaning "new life" (Greek (kainos), "new", and (zoe), "life"), is the most recent of the three classic geological eras and covers the period from 65.5 million years ago to the...
strata. Their essential features are their gritty character and their complex composition. By increasing
metamorphismMetamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Both mineralogical, chemical and crystallographic changes can occur during this process.Three types...
, greywackes frequently pass into mica-
schistThe schists form a group of medium-grade metamorphic rocks, chiefly notable for the preponderance of lamellar minerals such as micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is produced...
s, chloritic schists and sedimentary
gneissGneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks. Gneissic rocks are usually medium to coarse foliated and largely recrystallized but do not carry large...
es.
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