Sedimentary rocks are one of three main rock types, formed from the accumulation and lithification of existing materials at the Earth’s surface. These rocks are classified as clastic, chemical, or organic, based on the origin of the sediment. An organic sedimentary rock (OSR) forms from the accumulation of material derived from once-living organisms, such as the fossilized remains of plants and animals. This biological origin distinguishes OSRs from clastic rocks, made of mineral fragments, and chemical rocks, which precipitate from water solutions.
How Organic Sedimentary Rocks Form
The formation of organic sedimentary rocks begins with the rapid accumulation of biological material that must escape decay. This requires an anoxic environment, or one extremely low in free oxygen, which inhibits the bacterial decomposition of organic tissues. Terrestrial plant matter, for example, accumulates in stagnant, acidic swamps or peat bogs, where oxygen-poor water limits breakdown.
Once accumulated, this organic debris becomes buried beneath layers of overlying sediment, such as mud, sand, or volcanic ash. The increasing weight causes compaction, which squeezes out water and reduces the volume of the organic mass. Burial also leads to rising temperatures and pressures, initiating diagenesis, which transforms the loose sediment into solid rock.
For plant-derived material, this transformation involves the progressive concentration of carbon, a process called coalification. For marine organisms, hard parts like shells and skeletons made of calcium carbonate or silica settle to the ocean floor. These fragments are then compacted and cemented together, turning the biogenic sediment into a solid rock mass through lithification.
Major Categories and Composition
Organic sedimentary rocks are divided based on their original source material, leading to distinct chemical compositions. The first major category is carbonaceous rocks, derived almost entirely from terrestrial plant matter, with coal being the prime example. The grade, or rank, of coal depends directly on the heat and pressure endured during burial, which determines its final carbon content.
Carbonaceous Rocks (Coal)
The initial stage is peat, a soft accumulation of partially decayed plant debris with a low carbon percentage. Increased burial transforms peat into lignite, a soft, brown coal, which then progresses to bituminous coal, a denser black rock with a higher carbon concentration. The highest rank, anthracite coal, forms under extreme pressure and heat, possessing the highest carbon content and a distinct, shiny luster.
Biogenic Rocks (Limestone and Chert)
The second major category includes calcareous and siliceous biogenic rocks, which originate mainly from marine organisms. Biogenic limestone forms from the shells and skeletal fragments of organisms such as corals, mollusks, and microscopic plankton. These hard parts are composed primarily of calcium carbonate (CaCO3), which is cemented to form rocks like fossiliferous limestone and chalk.
Siliceous biogenic rocks, like chert, form from the accumulation of silica-based skeletal remains of marine microorganisms, specifically diatoms and radiolarians. These tiny silica (SiO2) structures settle on the seafloor and are compacted into a dense, hard rock. The compositional difference between the two biogenic types reflects the original mineral material extracted by the organisms.
Economic Utility and Global Occurrence
Organic sedimentary rocks hold practical significance due to the valuable materials they contain. Carbonaceous rocks, particularly high-rank coal, have historically served as a primary energy source, fueling industry and power generation. Although its use is declining, coal remains a globally important fossil fuel, concentrated in distinct seams and deposits.
Biogenic limestone also has widespread utility, most notably in the construction industry, where it is quarried for use as aggregate and is a primary ingredient in cement manufacture. The rock is also used in agriculture to neutralize acidic soils. These rocks are found within large sedimentary basins, which are geological depressions where sediments have accumulated.
Coal deposits are typically found in the remnants of ancient swamp environments, often associated with deltaic or coastal plain settings. Biogenic limestones and cherts are commonly found in the vast, layered rock sequences of continental interiors and platforms, indicating where warm, shallow marine environments once flourished. The distribution of these OSRs reflects the specific climate and depositional conditions of the geologic past.