Sedimentary Rock Formation and Composition

Burren walkers
Copyright the Department of the Environment, Heritage and Local Government

Formation

Sedimentary rocks form at or close to Earth’s surface. Whilst they are the most common rocks visible on Earth’s surface, they constitute only a minor part of Earth's crust. However, they contain essentially all of Earth's oil, natural gas, coal, phosphates and salt deposits. In addition, they act as a major store of groundwater reserves, and are responsible for other natural resources of economic importance. Separately, the sediment that constitutes sedimentary rock can be comprised of ancient fossils, millions of years old. They therefore act as an important record of ancient life-forms that enables an understanding of the evolution of simple and complex organisms in the plant and animal kingdoms.

Sedimentary rocks are formed when pre-existing rocks are weathered and the weathered materials are subsequently transported and deposited. In this context, weathering describes the various processes of physical wearing and chemical decomposition that arise when rocks at Earth’s surface are exposed to atmospheric conditions (primarily in the form of rainfall) (Britannica, 2018). Weathering processes produce soil, unconsolidated rock material, and particles that can dissolve in groundwater and surface runoff. Sedimentary rock can subsequently form because of deposition of grains caused by moving water or wind, from the melting of glacial ice, and from the downslope slumping (sliding) of rock and soil masses in response to gravity for example. In the context of sedimentary rock formation any unconsolidated deposit of solid weathered material constitutes sediment.

Many sedimentary rocks are formed through lithification of sediments (i.e. they are typically produced by cementing, compacting, and otherwise solidifying pre-existing unconsolidated sediments). Detrital sedimentary rock is subsequently formed through lithification when pre-existing rocks are rubbed away or mechanically weathered by forces such as water, ice and wind, leaving behind smaller rock fragments, which cement and compact together over time. Some varieties of sedimentary rock however are formed through the precipitation of minerals from water, creating chemical sedimentary rock e.g. coral reefs.

Because the processes of physical (mechanical) weathering and chemical weathering are significantly different, they create distinctly different sedimentary rock products and two fundamentally different sediment and sedimentary rock types. These can be broadly classified as

1. Clastic sedimentary rocks
2. Chemical and biochemical sedimentary rocks

Composition

• Clastic sedimentary rocks

Clastic sedimentary rock is comprised of rock and mineral grains, or clasts (i.e. rock fragments), of different size, ranging from fine-grained clay, silt, and sand to larger pebbles, cobbles, and boulder-size materials. Clasts are transported by gravity, mudflows, moving water, glaciers, and wind and are eventually deposited in various settings (e.g. in desert dunes, across continental shelves, and in river deltas). The process of transportation often sorts out discrete particles by clast size however. This enables clastic sedimentary rocks to be further subdivided on the basis of average clast diameter. Coarse pebbles, cobbles, and boulder-size gravels lithify to form conglomerate and breccia; sand lithifies to become sandstone; and silt and clay form siltstone, claystone, mudrock, and shale (Britannica, 2018).

• Chemical sedimentary rocks

Chemical sedimentary rocks are composed of minerals that dissolve in water and are then deposited as solids. This occurs most frequently when water evaporates leaving only the minerals behind e.g. stalagmites and stalactites in the Ailwee Caves, Co. Clare are examples of the formation of chemical sedimentary rocks. Stalagmites and stalactites form when groundwater saturated with calcium and carbonate drips through the ceilings of caves.

Biochemical sedimentary rocks form when organisms use materials dissolved in air or water to build their structure. Examples include many varieties of limestone which are formed from the calcareous skeletons of organisms such as corals, mollusks, and foraminifera (e.g. Burren, Co. Clare); and coal, formed from plants that have removed carbon from the atmosphere and combined it with other elements to build their structure.