Wacke
Sandstone with abundant fine matrix (>15% clay/silt matrix)
A poorly sorted, muddy sandstone with abundant clay matrix between its grains, typically dark and deposited by turbidity currents.
- Mohs hardness
- 6-7 (component grains)
- Color
- gray, dark gray to greenish
- Type
- sedimentary
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Overview
Wacke is a poorly sorted, muddy sandstone in which the sand grains are set in an abundant fine matrix of clay and silt (generally more than about 15 percent matrix). This contrasts with clean arenites, which have little matrix. The best-known variety is greywacke (graywacke), a tough, dark, lithic-rich wacke.
The abundant matrix and mix of angular grains make wacke look dark, dirty, and chaotic compared with well-washed sandstones. It indicates rapid deposition that buried mud and sand together without time for sorting.
Wacke is a workhorse term in sedimentary classification, signaling immature, rapidly deposited sediment.
Formation & geology
Wacke typically forms from turbidity currents — dense, sediment-laden underwater flows that surge down continental slopes and deposit a chaotic mix of sand and mud on the deep-sea floor. The rapid settling traps fine matrix among the sand grains, producing the characteristic poor sorting.
Such deposits accumulate in deep marine basins, trenches, and foreland basins next to active mountain belts and volcanic arcs, where erosion is fast and sediment is dumped quickly.
Greywacke beds are common in ancient accretionary complexes and orogenic belts worldwide, often forming thick, monotonous successions of graded sandstone-mudstone layers (Bouma sequences).
How to identify it
Look for a hard, dark gray to greenish, dirty-looking sandstone in which sand grains are clearly set in an abundant fine clay-silt matrix, giving a poorly sorted, almost concrete-like texture. Grains are often angular and varied (quartz, feldspar, and rock fragments).
Its dark color, toughness, and muddy matrix distinguish it from clean, light, well-sorted quartz arenite and from feldspar-rich arkose. Graded bedding (coarse base fining upward) is a strong clue to a turbidite origin.
It does not fizz in acid (unless carbonate cement is present) and is hard and gritty, reflecting its quartz and rock-fragment content.
Uses & significance
Wacke and greywacke are strong, dense rocks used widely as crushed aggregate, road base, railway ballast, and rough building and dimension stone. Their toughness makes them durable in construction, and they have been used for paving and walls in many regions.
Geologically they are very important as records of ancient mountain belts and turbidite systems, helping reconstruct plate-tectonic settings and sediment provenance.
They have no gem or metaphysical use. Their main value is as construction material and as a key to interpreting deep-marine and orogenic geology.
Frequently asked questions
What is the difference between a wacke and an arenite?
A wacke has abundant fine clay-silt matrix (over about 15 percent) between its sand grains, while an arenite is a clean sandstone with little matrix.
Is wacke the same as greywacke?
Greywacke is the most common variety of wacke: a tough, dark, lithic-rich, matrix-bearing sandstone typically deposited by turbidity currents.
How does wacke form?
It usually forms from turbidity currents that rapidly deposit a poorly sorted mix of sand and mud on the deep-sea floor, trapping fine matrix among the grains.
How can I identify wacke in the field?
Look for a hard, dark, dirty-looking, poorly sorted sandstone with sand grains in an abundant clay-silt matrix, often showing graded bedding.
What is wacke used for?
It is used as crushed aggregate, road base, ballast, and rough building stone, and is valuable to geologists for interpreting ancient mountain belts.
Wacke guides
In-depth guides for identifying, valuing, and understanding Wacke.
