Terminology and delimitation
In Germany, Grauwacke is also an outdated stratigraphic name for rocks of the lower carbon of the Kulm facies . Grauwacken can be found in Central Europe, for example, in the old Rump Mountains such as the Harz, Rhenish and Thuringian Slate Mountains . About the same age Grauwacken appear in the Alps as a narrow strip north of the Central Alps, whereby this so-called Grauwackenzone predominantly contains sandstones that cannot be referred to as Grauwacken in the actual sense.
From a sediment petrographic aspect, especially with regard to the mineral stock, the term Grauwacke is considered poorly defined (“field name”). The mineral composition of the sand fraction of a greywacke hardly differs from that of an arkose , apart from the feldspar content . The main distinguishing features between the two rocks are the clay minerals of the matrix , in Arkosen mainly kaolinite , in Grauwacken mainly chlorite and mica . However, diagenetic processes can lead to kaolinite being converted into chlorite and the feldspars of an arkosis to mica. Therefore, the entire geological context is important for the distinction between arkose and greywacke: greywacke are marine rocks and part of flysch series , while arkoses are typical continental molasse deposits .
Grauwacken are marine, clastic sediments , which are preferentially deposited in sedimentary basins, which are in front of a fold mountains that are in the process of being formed (so-called mountain depths ). The raw material comes from a relatively small, but geologically very varied delivery area . It is transported by rivers into the sea and initially deposited on the continental shelf . By exceeding the stable slope angle or by earthquakes, the not yet solidified deposits can become unstable and slide down the shelf slope in a turbid flow, whereby the slipped material can cover more than 100 kilometers. Sediments deposited by turbid currents or the rocks resulting from them are generally referred to as turbidites . A single Grauwackenbank represents a proximal turbidite, i.e. H. Material that was deposited relatively close to the starting point of the turbidity flow. As is generally typical for turbiditic deposits, a Grauwackenbank often shows a graded stratification in vertical section . However, it should be noted that not every proximal turbidite is a greywacke, but only those that were deposited in a mountain range. Turbidite series rich in greywacke are also summarized under the name Flysch .
Composition, texture and structure
The strongly consolidated, mostly dark rocks form a subgroup of sandstones . Their sand fraction consists mainly of quartz and feldspar (mostly plagioclase ), with a not inconsiderable part of these minerals occurring within fragments of volcanic rock the size of grains of sand. Other rock fragments come from pebble and clay slate . The sand fraction is medium to fine-grained, sometimes also coarse-grained, whereby the sorting is generally poor. The grains are also usually poorly rounded.
The mineral grains and rock fragments are embedded in a fine-grained base mass ( matrix ), which corresponds to a clay stone and mostly consists of chlorites and mica . There is no uniform regulation of how high the matrix proportion must be in order for a greywacke to be present. The German geologist Hans Füchtbauer suggested a matrix share of more than 15%. The matrix is mainly responsible for the mostly dark (green) gray color of the rock.
Due to the very heterogeneous mineral inventory and the poor grain sorting and rounding, including the relatively high matrix content, a sandstone is also referred to as a sandstone of very low compositional and structural maturity . In contrast to this, sandstones in turbidite series of passive continental margins have a significantly higher maturity. As is generally typical for Turbidite, the larger gray-wacke benches are often graded and finely laminated in their upper part.
According to Hans Gerhard Huckenholz (1963) Grauwacken consist of:
Grauwacken can be divided into quartz wacke (dominated by quartz), feldspar wacke (with quartz, feldspar and small amounts of rock fragments) and lithic wacke (high proportion of rock fragments).
In addition , the term greywacke slate is used for tectonically stressed, fine-sand, dark marine mudstones whose sand fraction is similarly heterogeneous as that of a greywacke .
Natural stone types
- Rhenish Slate Mountains
- Upper Harz Greywacke (Lower Carbon)
- Südharzer Grauwacke
- Selker Greywacke
- Lausitzer Grauwacke ( Neoproterozoic )
- Hans Füchtbauer: Sediments and sedimentary rocks . 4th edition. E. Schweizerbartsche Verlagbuchhandlung, Stuttgart 1988, ISBN 3-510-65138-3 .
- Francis J. Pettijohn, Paul Edwin Potter, Raymond Siever: Sand and Sandstone . 2nd Edition. Springer Verlag, New York 1987, ISBN 3-540-96350-2 .
- Heinrich Bahlburg, Christoph Breitkreuz: Fundamentals of geology . 2nd Edition. Elsevier Verlag, Munich 2004, ISBN 3-8274-1394-X .
- Lindlarer Grauwacke , about Grauwacke from the Bergisches Land
- Hans Murawski, Wilhelm Meyer: Geological dictionary. 12th edition. Spektrum Akademischer Verlag, 2010, ISBN 978-3-8274-1810-4 , p. 66 f.
- Hans Gerhard Huckenholz: The current status in the sandstone classification. Advances in mineralogy. Vol. 40, 1963, pp. 151-192 - also published in English under the title A Contribution to the Classification of Sandstones. Geologiska Föreningen i Stockholm Förhandlingar. Vol. 85, No. 1, 1963, pp. 156-172, doi: 10.1080 / 11035896309448877 -, quoted in Hans Murawski, Wilhelm Meyer: Geologisches Handbuch . 12th edition. Spektrum Akademischer Verlag, 2010, ISBN 978-3-8274-1810-4 , p. 66 f.