Siltstone Rock Type, Color, Composition, Formation, Occurrence, & Uses

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Siltstone is a clastic sedimentary rock. It is characterized as mudrock because it is mostly composed of silt. A typical siltstone has a low clay mineral content.

Siltstone is one of the many rocks that don’t have a single definition. Some geologists consider siltstones to be specifically the rock that contains at least 50% clay and silt, and 2/3 of this 50% must be silt-sized particles.  Some geologists define siltstones as rocks that contain 50% or more of the silt-sized particles.

Siltstones and claystone are often confused with each other. A simple test is to chew a small sample. Claystone is smooth, and siltstone is gritty. Please note that this test is not recommended. However, many geologists use it for quick testing. Siltstones also differ from other sandstones due to smaller pores. Siltstone is often confused with shales. Siltstones don’t have the fissility and laminations that shales have. Siltstone doesn’t split along flat planes of weakness.

Siltstone sample
Siltstone sample


Siltstone is found in many colors, but the usual ones are gray, brown, and reddish brown. Other variations include white, yellow, green, red, orange, purple, and black. The color variations occur due to grain & cement composition, impurities, iron oxides, and stains on the rock due to contact with subsurface water.

Rock Composition

Siltstone is mostly composed of silt. The world silt doesn’t define a particular thing or element. It is used to define loose granular particles falling in a specific size range. The silt particles have a diameter range between 0.00015 and 0.0025 inches.

The silt in the siltstone doesn’t have a fixed composition. Its properties vary from place to place, but it is usually composed of micas, clay minerals, feldspars, and quartz.

Siltstone Formation

Siltstone is formed at silt deposit sites. Silt accumulates in sedimentary basins with a certain level of current, wave, and wind energy. The common environments include tidal, coastal, fluvial, aeolian, paludal, and shelf.

Rock Identification

Siltstone is often hard to identify due to its proximity to other mudstones like shale. It has a weathered surface that often shows sedimentary structures while none are present.

Identifying the rock involves breaking a small piece and investigating the grain size. The rock produces white effervescent powder or dislodges tiny silt grains instead of sand grains when you scrape its surface with a knife or your nails.

Where is Siltstone Found?

Siltstone usually occurs along other rocks. Its reserves are not of much interest because the rock is not economically valuable. Its notable reserves are Cheltenham Badlands, Canada, Kentucky, USA,  and ChekChau, Hong Kong.

Holtzclaw siltstone, Louisville, Kentucky
Holtzclaw siltstone, Louisville, Kentucky

Siltstone on Mars

NASA’s Curiosity Rover photographed sedimentary rock deposits on Mars. Geologists believe that these are interbeddedshales and siltstones. The rocks look like the shales and siltstones found on earth. However, scientists believe these rocks have different grains and types of cement than the rocks on earth.

Siltstone Uses

Siltstone is not an economically valuable rock. It is rarely used as a construction material or to make feedstock. Moreover, it doesn’t serve as a good underground water source because it has tiny pore spaces. Similarly, its low permeability limits its use as an oil or gas reservoir.

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