Wacke Identification Guide
A field guide to Wacke (greywacke), a poorly sorted muddy sandstone, and how to distinguish it from clean sandstone and arkose.
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What Wacke Looks Like
Wacke is a sandstone with more than about 15% fine clay/silt matrix between the sand grains (when very dark and hard it is called greywacke). It is a "dirty," poorly sorted sandstone.
- Color: grey, dark grey, greenish-grey to brown or black.
- Texture: poorly sorted, a jumble of grain sizes (sand grains floating in a muddy matrix); grains often angular to sub-angular.
- Composition: quartz + feldspar + abundant rock fragments (lithics) in a clay-rich matrix.
- Form: hard, dense beds, often in thick turbidite sequences with graded bedding.
Step-by-Step Field Checklist
- Confirm it is sandstone: sand-sized grains visible with a lens.
- Check sorting. Wacke is poorly sorted, with mixed grain sizes and a muddy matrix (vs the clean, even grains of pure sandstone).
- Look at color. Dark grey/green tones suggest a clay- and lithic-rich wacke (greywacke).
- Feel the hardness. Greywacke is tough and well-cemented; it does not crumble easily.
- Look for graded bedding in outcrop; fining-upward layers indicate turbidite-deposited greywacke.
Key Diagnostic Tests
- Hardness: dominated by quartz grains (~7), but the rock breaks across grains because the matrix is well-indurated.
- Grit test: rub it; sand grit confirms sand size, while the muddy matrix darkens and dulls it.
- Acid: generally no fizz unless carbonate cement is present.
- Density: ~2.6-2.8 g/cm3.
- Hand lens: look for angular quartz, feldspar and dark lithic fragments set in a fine matrix.
Common Look-Alikes
- Clean (quartz) sandstone / arenite: well sorted, lighter colored, little to no matrix, often more porous; wacke is dirty and dark with abundant matrix.
- Arkose: sandstone rich in feldspar (>25%), usually pink/red and clean-looking; arkose is feldspathic and better sorted than wacke.
- Siltstone/mudstone: finer grained overall; grains not visible and grit-free, while wacke still has obvious sand grains.
- Basalt/fine igneous rock: wacke shows clastic sand grains under a lens; igneous rock shows interlocking crystals.
Where It Is Found
Wacke/greywacke forms in deep-marine settings from rapid turbidity-current deposition off continental margins and at convergent plate boundaries. Classic exposures occur in New Zealand, the UK, the Appalachians, and many ancient orogenic belts.
Frequently asked questions
What is the difference between wacke and sandstone?
Wacke is a sandstone that contains more than about 15% fine clay/silt matrix, so it is poorly sorted and dirty, whereas clean sandstone (arenite) has little matrix and well-sorted grains.
Is wacke the same as greywacke?
Greywacke is a hard, dark, lithic-rich variety of wacke; the terms are often used interchangeably, with greywacke being the classic dark turbidite type.
How do you identify wacke in the field?
Find a hard, dark grey sandstone with mixed grain sizes, angular quartz, feldspar and rock fragments set in a muddy matrix, often in graded turbidite beds.
Wacke vs arkose, how do they differ?
Arkose is a feldspar-rich, usually pink and relatively clean sandstone, while wacke is darker, poorly sorted, and rich in clay matrix and lithic fragments.