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Making sense of sedimentary rocks

The huge variety of sedimentary rocks is due to the many ways that sediment can be derived, and the wide range of environments in which it may be deposited. Features common to most sedimentary rocks are their occurrence in layers (beds or strata), and the occasional occurrence within them of fossils. Sedimentary rocks are usually classified on the basis of their mineral composition and grain size, which in turn relates to the origin of the sediment:

Sediment origin:

Fragmental (or clastic) sediments are made up of particles of weathered rock, including rock fragments, residual mineral grains (especially quartz, which is both hard and chemically inert) and clay minerals. Examples include: conglomerate, sandstone, mudstone & shale.

Organic sediments are, as the name suggests, formed largely or entirely of sediment generated by living organisms (e.g. shell or plant material). The resulting rocks include many limestones (e.g. shelly limestone, chalk); and also coal.

sediments, (or evaporites) are formed by direct chemical precipitation, typically in hot climates. These include some limestones (e.g. oolitic limestone), gypsum, and halite (rock salt).

Grain size:

A simple classification of coarse-, medium- and fine-grained is commonly used, with boundaries at 2mm and 1/16mm, thus effectively defining all sanstones as medium-grained, even though it is very hard to see grains in fine sandstones with the unaided eye. Note that different boundary sizes (2mm & 5mm) apply to crystalline (igneous & metamorphic) rocks. Limestones are often re-crystallised and it is frequently hard to decide on the original grain size of the sediment.

Mineral composition:

The mineral composition of sedimentary rocks (both grains and cement) is immensely variable, and of most use in sub-classification of the broader rock groups described above. Some terms, such as calcareous for any rock having a significant content of calcium carbonate (grains or cement) are useful but detailed classification is beyond the scope of these web pages.
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