A fine-grained rock of consolidated silt. Silt is a size range of particles between 1/16 mm and 1/256 mm; generally below visible range. At the lower size end it grades into mud (shale), and at the upper size end it grades into sandstone. Often, however, silt and clay are mixed together in varying proportions. Commonly together they form the matrix in a rock, that is the finer material among the larger particles, such as in a wacke sandstone.
Siltstone formation is very similar to that of sandstone, but individual particles are smaller, finer grained quartz. Color is variable, but usually gray to brownish.
Siltstone is a sedimentary rock which has a composition intermediate in grain size between the coarser sandstones and the finer mudstones and shales.
Siltstone is composed of silt-sized sediment grains, which are transitional in size between clay and sand grains. So, siltstone represents natural depositional environments transitional between those of shale and sandstone. For example, on the ocean floor clay dominates sediment deposition in the deep ocean far from shore, whereas sand is commonly deposited closer to shore in shallow water. Silt would therefore be deposited in between these two extremes, on the continental slope. Siltstone, when found within a sequence of sedimentary rocks, often represents such a transitional environment. Below are some images of siltstone. Note that it is very similar in appearance to shale, but it is a bit grittier.
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.
Siltstones differ significantly from sandstones due to their smaller pores and higher propensity for containing a significant clay fraction. Although often mistaken as a shale, siltstone lacks the fissility and laminations which are typical of shale. Siltstones may contain concretions.
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