Greywacke is variation of sandstone that saperate from other to hardness, dark color, and poorly sorted angular grains of quartz and feldspar.. It is a textural immature sedimentary rock found in the Paleozoic layers. Larger grains can be from sand to pebble length, and matrix materials are in the order of 15% by volume of rocks. A deep color characteristic of deep seas in low oxygen environment.
The grains of chert, mudrock, limesone, polycrystalline quartz, and volcanic rocks are also quite common. Clasts of detrital muscovite, biotite and chert occour in accossory amount.
Deep-water lifeless and bath body fossils, pelagic fauna and flora, and re-transferred shallow – waterrorganic remains are all found in wacke sandstone sequences. large, flat and smooth flat surfaces
Name origin: From German Grauwacke, from grau (signifying a grey)+ wacke
Colour: Grey to black; often with white quartz veins
Group: Clastic Sedimentary Rock
Texture: An immature sandstone
Grain size – < 0.06 – 2mm, clasts typically angular, visible to the naked eye.
Hardness – hard.
Colour – grey to black; often with white quartz veins.
Other features – gritty to touch (like sandpaper), often veined, non-vesicular.
Major minerals of Greywacke: Grains of quartz, feldspar, and small rock fragments or lithic fragments set in a compact, clay-fine matrix.
Accessory minerals of Greywacke: Clasts of detrital muscovite, biotite and chert occour in accossory amount.
For this course we will use a classification of sandstones that is partially based on Blatt and Tracey (p. 257) and partially based on Williams, Turner, and Gilbert (p. 326).
A feldspar-rich sandstone is called an arkose. Lithic rich sandstones are called litharenites. Further subdivisions are shown in the diagram. If the rock has between 10 and 50% clay matrix, the rock is called a wacke. Quartz wackes have predominantly quartz surrounded by a mud or clay matrix. In a feldspathic wacke, feldspar is more abundant, and in a lithic wacke, lithic fragments are more abundant. The term graywacke is seldom used today, but was originally used to describe a lithic-rich sandstone with between 10 and 50% mica, clay, or chlorite matrix. Rocks with greater than 50% clay matrix are called sandy mudstones, and will be discussed in the lecture on mudrocks.
Greywacke is formed form the deposition of muddy fine sand in deep water, like out in the ocean.
The sand and mud is the sediment (unconsolidated loose particles) that has been transported by rivers and glaciers down to the sea.
The sediments have come from the erosion of exposed rock. Weather breaks off pieces of rock which are transported. Particles that are very fine can be transported further out to sea before they settle onto the sea floor. These are the sediments that form greywacke.
Greywacke is thought to be formed from muds and sands that flow down the continental slope and out onto the deep sea floor where they accumulate great thicknesses over a long period of time. These sediments become deeply buried and undergo change by being compressed and cemented together. There layers and fossils also suggested that they were formed from deposits in the bottom of the sea.
This diagram shows how the Greywacke formed off the coast of Gondwana by erosion and transportation by rivers to the ocean floor.
Plate tectonics cause continents and ocean floor to move and Erosion is an important geological process of the rock cycle and produces rock fragments that are transported by rivers. The larger the fragments the faster the river has to go to carry them along. Rivers carried sediment from land out to sea on the continental shelf, where it was then transported down channels as turbidity currents onto huge submarine fans on the deep ocean floor.
Formation of Greywacke Sandstone
Sandstone is a type of sedimentary rock wihich is made from particles that have beed glued together. In this case ,the sand is mixed with mud and clay and squeezed so that all the water disappears and the particles are pushed close together .In summary, the formaton of greywacke rock is a result of the rocks cycle processes of erosion,transport of eroded material by rivers, deposition onto the sea floor and then pushing up by plate tectonic movement. Graywacke sandstone is a sedimentary rock that is made up mosttly of sand-size grains that were rapidly deposited very near the source rock grom which they were weathered. Greywacke is deposited in deep ocean water near volcanic mountain ranges ,where unrerwater landslides and density currents called turbidites quickly transport sediment short distances into a subduction zone or ocean trench. This type of sandstone contains fewer grains made of quartz and more of feldspars, volcanic rock fragments ,as well as silt and clay than most sanstone. It is therefore also known as “dirty sandstone”. The volcanic rock fragments give graywacke a greenish-gray color.
What makes the beds in Graywacke ?
Graywacke sandstone deposits display flat – lying beds,each composed of sedimentary particles of differemt sizes. The sandstone beds can be from inches to many feet thick and are often separated by thin ,dark shale beds. Each sandstone bed was formed furing a single turbidite or submarine landslide event and was deposited over a short period of time from hours to days. The thin shale beds formed between turbidite evetns,when mud particles slowly settled to the sea floor,and may represent thousands of years. Turbidites dislay graded bedding,that is ,the grain size decreases upwards in the bed. During a turbidite event ,the larger and heavier grains settle out first. As the energy in the landslide event decreases, finer and finer particles settle out to the sea floor
Where is it found?
Greywackes are mostly grey, brown, yellow or black, dull-colored sandy rocks which may occur in thick or thin beds along with shales and limestones. They are abundant in Wales, the south of Scotland, the Longford Massif in Ireland and the Lake District National Park of England; they compose the majority of the main alps that make up the backbone of New Zealand; sandstones classified as feldspathic and lithic greywacke have been recognized in Ecca Group in South Africa. They can contain a very great variety of minerals, the principal ones being quartz, orthoclase and plagioclase feldspars, calcite, iron oxides and graphitic, carbonaceous matters, together with (in the coarser kinds) fragments of such rocks as felsite, chert, slate, gneiss, various schists, and quartzite. Among other minerals found in them are biotite, chlorite, tourmaline, epidote, apatite, garnet, hornblende, augite, sphene and pyrites. The cementing material may be siliceous or argillaceous and is sometimes calcareous.
Characteristics and Properties
Greywacke sequences (Begg & Mazengarb 1996) consist of interbeds of:
• Sandstone – coarse to medium grained, and medium to dark grey. Individual grains are poorly sorted angular quartz and feldspar, plus fragments of metamorphic and igneous rocks. The intergranular filling is clay minerals formed during induration or slight metamorphism.
• Mudstone – layers of clay, silt or mud, generally dark grey to black, sometimes red from iron minerals. Proportions of mudstone to sandstone vary between localities.
- Decorative Aggregates, Floor Tiles, Flooring, Homes, Interior Decoration
- As Building Stone, As Facing Stone, Garden Decoration, Office Buildings, Paving Stone
- Curbing, Whetstones
- Building houses or walls, Cement Manufacture, Construction Aggregate, for Road Aggregate, Raw material for the manufacture of mortar
- Artifacts, Sculpture, Small Figurines
- As armour rock for sea walls, Petroleum reservoirs, Sea Defence, Tombstones