What Environment Is Likely to Form Siltstone or Shale?

Properties of Composition

• Silt and clay granules are, respectively, the precursors to the rocks siltstone and shale. Silt and clay sediments are found in soil, along with other sediments like sand and gravel. The sizes of the grains of sediment in siltstone are slightly larger than the sediment grains comprising shale. Both types of particulates are generally so small as to render them invisible to the human eye.

Mudrock Formation

• As mudrocks, siltstone and shale form in similar environments. The presence of their precursor sediments, silt and clay, are required for their formation. These sediments accumulate over time, deposited by water, a crucial element to the formation process. The water in which mudrocks take shape must be relatively placid. In an oceanic environment, mudrocks tend to form at depths below the level that waves could reach. Often the seafloor offshore of continents provides an ideal environment. Mudrocks also can form in freshwater lakes or in slow-moving rivers, as well as in deltas and lagoons. This doesn̵7;t mean that mudrocks are only found exclusively underwater -- ancient bodies of water shifted or dried up, leaving mudrocks exposed on land in many locales.

Siltstone Formation Specifics

• Siltstone is commonly interspersed with other sedimentary rock types, such as sandstone and shale. It often forms above layers of shale. In oceanic environments, it tends to form closer to the shore than shale. Frequently, siltstone forms along a continental shelf, a sloping underwater environment bordering a large landmass.

Shale Formation Specifics

• Shale tends to form in deeper water than siltstone. In an oceanic environment, this means that shale is found further from shore. Some amount of compression is also required for the formation of shale. This compression often stems from additional clay or other sediments piling on top of the future shale.