Sedimentary rock

Sedimentary rocks are one of the three major rock groups found on Earth’s surface, formed through the accumulation, deposition and cementation of mineral particles, geological detritus and biological material. These rocks constitute a thin but extensive veneer across the continents, representing about 73% of the land surface, although they make up only a small proportion of the crust by volume. Their layered nature, occurrence in sedimentary basins and widespread distribution make them essential for understanding Earth’s geological history, past environments and the evolution of life. They also provide vital resources such as groundwater, coal, petroleum and various ores, and are central to civil engineering and environmental studies.

Processes of Sedimentation

Sedimentation involves the settling and accumulation of particles at or near Earth’s surface. Geological detritus derives from weathering and erosion of pre-existing rocks or from volcanic ejecta. These particles are transported by denudation agents such as water, wind, ice and gravity-driven mass movements. Biological detritus forms from the remains and waste of aquatic organisms, creating marine snow that builds up on lake and ocean floors. Minerals dissolved in water can also precipitate to form sedimentary deposits.
Sedimentary layers, known as strata or beds, record environmental and climatic conditions at the time of deposition. Such records make sedimentary rocks central to palaeogeographic reconstruction, climate interpretation and evolutionary studies. Sedimentary rocks are also found beyond Earth, including on Mars, providing evidence of past water activity on that planet.

Classification Based on Origin

Sedimentary rocks are traditionally grouped into four categories according to their mode of formation:

• Clastic sedimentary rocks
• Biochemical (or biogenic) sedimentary rocks
• Chemical sedimentary rocks
• Miscellaneous sedimentary rocks formed by unusual processes such as volcanism or impact events

Clastic Sedimentary Rocks

Clastic rocks are composed of fragments (clasts) of older rocks and minerals that have been transported and subsequently cemented. Clasts frequently include quartz, feldspar, mica and clay minerals, although many lithic fragments consist of multiple mineral types. Their classification largely follows particle size, as defined by the Wentworth scale:

• Gravel (> 2 mm)
• Sand (1/16 to 2 mm)
• Mud (< 1/16 mm), subdivided into silt and clay

The main clastic rock types reflect these size categories:

• Conglomerates and breccias (gravel-rich; conglomerates have rounded clasts, breccias angular)
• Sandstones (dominated by sand-sized grains)
• Mudrocks (dominated by silt and clay)

Conglomerates and Breccias

Conglomerates contain rounded gravel clasts, whereas breccias consist of angular clasts set within a finer-grained matrix. Their depositional environments range from river channels and alluvial fans to debris-flow deposits and talus slopes.

Sandstones

Sandstones vary widely in composition and texture. The Dott classification scheme is commonly used, based on the proportion of quartz, feldspar and lithic grains, and the amount of muddy matrix:

• Quartz arenites contain more than 90% quartz and little matrix.
• Feldspathic sandstones have significant feldspar content.
• Lithic sandstones contain numerous rock fragments.
• Wackes are muddy sandstones with more than 10% matrix.

Despite such classifications, traditional names such as arkose, greywacke and quartz sandstone remain widely used.

Mudrocks

Mudrocks represent the finest-grained clastic sediments, comprising at least 50% silt and clay. These sediments travel long distances in suspension before deposition. Mudrocks include:

• Siltstones (dominantly silt-sized grains)
• Mudstones (mixed silt and clay)
• Claystones (mostly clay)

Most authors use the term shale for fissile mudrocks, irrespective of grain size.

Biochemical Sedimentary Rocks

Biochemical rocks form from materials produced by organisms. These include:

• Limestones, composed largely of calcareous skeletal material from corals, molluscs and foraminifera
• Coal, formed from compressed and chemically transformed plant material
• Chert, created by the accumulation of siliceous remains from radiolarians and diatoms

These rocks often preserve fossils and provide records of ancient marine and terrestrial ecosystems.

Chemical Sedimentary Rocks

Chemical sedimentary rocks result from the inorganic precipitation of minerals from solution. Common examples include:

• Oolites, composed of concentric mineral grains
• Evaporites, formed through evaporation of water and precipitation of dissolved minerals such as halite, gypsum, sylvite and baryte

Such rocks typically form in arid, restricted basins where evaporation exceeds inflow.

Miscellaneous Sedimentary Rocks

A small but important group includes:

• Volcanic tuffs and volcanic breccias, derived from the deposition and cementation of volcanic material
• Impact breccias, formed from fragmented rock created during meteorite impacts

These deposits often record geologically sudden or catastrophic events.

Classification Based on Composition

A complementary classification system groups sedimentary rocks according to their dominant mineral content:

• Siliciclastic rocks, composed mainly of silicate grains (conglomerates, sandstones, mudrocks)
• Carbonate rocks, such as limestone and dolomite, composed of calcite, aragonite, dolomite and related minerals
• Evaporites, rich in chloride, sulfate and carbonate minerals
• Organic-rich rocks, including coal, oil shale and source rocks for hydrocarbons
• Siliceous rocks, composed chiefly of silica in forms such as chert, opal and chalcedony

This system helps geologists interpret depositional conditions, diagenesis and economic potential.

Significance of Sedimentary Rocks

Sedimentary rocks carry a detailed archive of Earth’s past environments. Through interpreting sedimentary structures, textures, fossils and mineralogy, scientists reconstruct ancient rivers, seas, deserts, glaciers and ecosystems. Sedimentology, the discipline focused on these rocks and processes, forms a key component of geology and physical geography. It intersects with geomorphology, pedology, geochemistry and structural geology, offering insights into past climates, tectonic evolution and the distribution of natural resources.