Where is Greywacke found?
Greywacke
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Open the appGreywacke occurs in specific geological environments. Knowing where to look, and in what kind of rock, is the difference between going home empty-handed and finding a great specimen.
Where to find Greywacke
Appalachian Mountains, USA
Extensive greywacke deposits are found throughout the Appalachian orogen, particularly in the Taconic, Acadian, and Alleghenian foreland basins.
Scottish Highlands, UK
Significant occurrences in the Southern Uplands of Scotland, forming part of ancient accretionary prisms.
New Zealand
Widespread in New Zealand, forming a major component of the Torlesse Supergroup, which makes up much of the Southern Alps.
Himalayan Orogen
Found in various parts of the Himalayan mountain range, associated with the collision of the Indian and Eurasian plates.
California Coast Ranges, USA
Present in accretionary complexes like the Franciscan Complex, indicative of ancient subduction zones.
Collecting tips
- Look for Turbidite Sequences. Greywacke is a hallmark of turbidite deposits. Look for rhythmic bedding, graded bedding (fining upwards), sole marks (flute casts, groove casts), and interbedded shales, which are characteristic of turbidite facies.
- Examine Outcrop Color and Texture. Identify dark, dull, and gritty sandstones. Break a piece to observe the poor sorting and the presence of a fine-grained matrix that gives it a 'dirty' appearance. Angularity of grains is also a key indicator.
- Check for Rock Fragments. Under a hand lens, try to identify various rock fragments (e.g., chert, slate, volcanic rock fragments) in addition to quartz and feldspar grains. This diverse composition is typical of greywacke.
- Consider Tectonic Setting. Greywackes are common in ancient mountain belts and areas that were once active plate margins. Understanding the regional geology can help pinpoint potential locations.
Quick Check
- Color:
- Dark grey to greenish-grey
- Luster:
- Dull to earthy
- Streak:
- White to light grey (from the matrix minerals)
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