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:
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.
Chemical 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).
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.
Chemical 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).