Earth Parts #30 - Detrital Clastic Sedimentary Rocks
Channel: Earth.Parts
Date: 2018-03-20

Sediments that form from the weathering and erosion of pre-existing rocks are called detrital clastic sediments, where detrital refers to the particles being detritus eroded from elsewhere, and clastic means a sediment or sedimentary rock is made of lots of small bits, or clasts.

Detrital sediment derives from weathering of exposed rock, usually rock that is part of a mountain range, such as the Andes mountains, Alps, Rockies or Himalayas. Mountain rock is exposed to harsh weather; wind, rain, freeze/thaw processes, avalanches, river outwash, etc. Material eroded from mountains travels by water or wind to collect elsewhere as sediment; typically on the sea floor but also in lake basins and river floodplains, for example.

Sediment that becomes buried can harden into solid rock, which geologists describe according to its features, such as grain size. The size of grains in a sedimentary rock reveals information about how and where the grains formed. Large grains like pebbles can only be carried by great force, such as fast-moving water or intense wind, while silt or tiny clay particles the size of dust can be easily lifted and carried over large distances by wind or water, where they can collect as very fine-grained silt, mud and clay, which can become compacted to form shale, siltstone and mudstone.

Sediment of coarser grain size, such as sand, can be moved by wind or water but tend to drop out of river or sea water that moves slowly. As a result, rivers tend to deposit sand closer to shore and silt further out, while mud/clay particles may drift for weeks and accumulate in deep, quiet water far from shore. 

Geologists sort sedimentary clastic particles by grain size. Particles larger than 2 mm are consider very course sediment such as granules, pebbles and cobbles at the larger sizes. Sedimentary particles between 2 mm and 0.06 mm are considered to be sand regardless of composition. 

Particles of sediment smaller than 0.06 mm but larger than 0.004 mm are called silt, while mud/clay particles are smaller than 0.004 mm in diameter and take a very long time to settle, and only then in slow, quiet water.