Home Sedimantery Rocks Clastic Sedimentary Rocks Siltstone


Siltstone is a clastic sedimentary rock that formed from grains whose sized between that of sandstone and mudstone. It can found different environmental conditions different color and textures. Siltstone generally are red and gray color with flat bedding planes. Darker colored siltstone have plant fossils and other carbon-rich matter. It is hard and durable and do not easily split into thin particles or layer. Although often mistaken as a shale, siltstone lacks the fissility and laminations which are typical of shale. Siltstones may contain concretions. Unless the siltstone is fairly shaly, stratification is likely to be obscure and it tends to weather at oblique angles unrelated to bedding. Mudstone or shale are rocks that contain mud, which is material that has a range of silt and clay. Siltstone is differentiated by having a majority silt, not clay.

Origin: Detrital/Clastic

Texture: Clastic; Fine-grained (0.004 – 0.06 mm)

Composition: Quartz, clay minerals

Color: Reddish brown

Miscellaneous: Massive; Feels slightly gritty

Depositional Environment: Flood plain, Delta, or Mid-continental Shelf

Grain size: Fine- grained

Siltstone Composition

Clastic sedimentary rocks are deposited in three ways. They are water glaciers and wind. Identifying siltstone and shale requires distinguishing between silt and clay particles.Silt and clay are both tiny particles that have weathered away from rocks and minerals. Silt is intermediate in size between the larger grains of sand and the smaller clay particles. To be classified as silt, the particles must be smaller than .06 millimeters in diameter, (.002 inches) and larger than clay-size particles, which are smaller than .004 millimeters in diameter (.0002 inches). Clay, unlike silt, also refers to several types of minerals, including montmorillonite and kaolinite

Siltstone Formation

Semi-quiet depositional environments. Coarse silt is capable of forming cross laminations in a current, while the finer-end particles generally deposit from suspension.

Thus, any depositional environment with these conditions may form silt deposits, and they range from river systems, to deltas, to shelves, to submarine fans and basins. In other words, just about anywhere.

Silt is a common component with other deposits, or is interbedded with other deposits. For example, silt beds are common in flood plain deposits above the point bar sands, and they form the TD unit in a Bouma sequence.

Where is It Found?

Siltstone is deposited in a similar environment with shale, but is usually located near the old delta, lake or sea shore, where calm currents cause less particle suspension. Siltstone is generally formed adjacent to sandstone deposits – ie close to beaches and delta edges where sand is deposited. It consists of silt, hence siltstone, sandy beaches and water adjacent to deltas. Declining currents filter sand from smaller silt particles. Siltstone rises to the shale level in deep water; where the suspended clay particles are stored further as the currents continue to lose energy. In both cases, calm water is needed to suspend and separate silt and clay. Thus, sandstone, siltstone and shale are interrelated rocks which are distinguished by particle size.

Siltstone Characteristics and Properties

  • Generally uniform, moderately consolidated to semi-lithified, silty sediment.
  • Typically blue-grey to olive green and brown; fracture surfaces are stained orangebrown to black.
  • Local intervals of colluvium.
  • No macroscopic shell, plant or fossil material.
  • Little or no evidence of bedding or other sedimentary structures.
  • Found beneath the coarse alluvium/colluvium sequence and above ‘basement’ lithologies such as granite, rhyolite and metasedimentary rocks.

Siltstone Uses

  • It is rarely mining for use as a construction material or manufacturing feedstock.
  • The pore spaces of siltstone serve as good aquifer. It is rarely porous enough or extensive enough to serve as an oil or gas reservoir.
  • Its main use is as a low-quality fill when better materials are not locally available.

Shale, Siltstone, and Mudstone

Resistance to weathering: Shale, siltstone, and mudstone units are valley formers, producing gentle slopes and subdued landforms, including broad “V”-shaped gullies having long, gentle stream gradients.

Drainage: Shale, siltstone, and mudstone units have relatively low permeability, resulting in significant surface flow. This produces a high drainage density. The low permeability of shale and clay keeps water in the vicinity of plant roots, so, in general, shale and clay will support dense vegetation. This often obscures the surface of shale and further lowers the reflectance.

Color: Shale is commonly dark because of the lithic content, dark clay minerals, and carbonaceous matter, which is frequently a constituent of shale. The dark color of shale is usually indicative of marine origin. Marine shale dominates in frequency, areal distribution, and thickness. Fluvial and lacustrine shale have higher reflectances and often contain more silt. Siltstone, an abundant sedimentary rock type that is probably more prevalent on the landscape than shale, is often misidentified and labeled as shale by engineers. The mechanical properties of siltstone make siltstone an important class from an engineering/rock strength standpoint as well.

Structure: Shale can form thick, dense deposits, but shale response to tectonic forces is quite different from sandstone. Shale responds to deformation by forming numerous lenticular shear planes having separations measured in millimeters to centimeters. Hence, shale tends to be weak, requiring support for bearing loads. In addition, depending upon the type of clay minerals present, shale may present shrink/swell problems (smectite and chlorite minerals), or squeeze/deformation problems (kaolinite and chlorite minerals).

Siltstone Key Point

  • Silt does not have a precise composition. It usually has a mix of micas, feldspar, quartz and clay minerals. The small part of the silt is mostly clay. The rough size fraction is mostly quartz and feldspar grains.
  • It is mudstone and shale, mud-containing, clay and silt. Siltstone is different because it is mostly clay and not clayy.
  • Silt stone is much less common than shale and sandstone. Rocks are generally thinner and less common.
  • Silt accumulates in sedimentary basins all over the world. A current between the sludge and the place where the sand is deposited represents the level of wind or wave energy. These include aeolian, fluvial, tidal, lakrin, coastal, glacier, paludal, deltaic and shelf environments. Sedimentary structures such as stratification, fluctuation traces, erosion, cross bed and fossils provide evidence of these environments.
  • Silt-sized particles range from 0.00015 to 0.0025 inches in diameter or in the range of 0.0039 to 0.063 millimeters in diameter. They are of medium size between the coarse clay on the small sides and the fine sand on the large side.
  • Coarse silt grains are large enough for most people to see without using a magnifying glass.
  • It can be difficult to identify without close inspection and requires a small piece to be broken to observe grain size. Scratch the surface with a knife blade or a nail, instead of changing the grains of sand, small silt grains emerge.
  • Although this test is not recommended, it is known that experienced geologists and soil scientists can detect a few female silt fragments by gently inserting them between their front teeth.
  • Siltstone has little use. If used for building, it is only due to availability, not quality. When there are no better local materials, the main use is a low quality filler.