Sedimentary rock

Sedimentary rocks are types of rock formed by the cementation of sediments—particles made of minerals or organic matter—that have been accumulated or deposited at Earth's surface. Sedimentation is any process that causes these particles to settle in place. Geological detritus originates from weathering and erosion of existing rocks, or from the solidification of molten lava blobs erupted by volcanoes. The geological detritus is transported to the place of deposition by water, wind, ice or mass movement, which are called agents of denudation. Biological detritus is formed by bodies and parts (mainly shells) of dead aquatic organisms, as well as their fecal mass, suspended in water and slowly piling up on the floor of water bodies (marine snow). Sedimentation may also occur when dissolved minerals precipitate from water solution.

The sedimentary rock cover of the continents of the Earth's crust is extensive (73% of the Earth's current land surface), but sedimentary rock is estimated to be only 8% of the volume of the crust. Sedimentary rocks are only a thin veneer over a crust consisting mainly of igneous and metamorphic rocks. Sedimentary rocks are deposited in layers as strata, forming a structure called bedding. Sedimentary rocks are often deposited in large structures called sedimentary basins. Sedimentary rocks have also been found on Mars.

The study of sedimentary rocks and rock strata provides information about the subsurface that is useful for civil engineering, for example in the construction of roads, houses, tunnels, canals or other structures. Sedimentary rocks are also important sources of natural resources including coal, fossil fuels, drinking water and ores.

The study of the sequence of sedimentary rock strata is the main source for an understanding of the Earth's history, including palaeogeography, paleoclimatology and the history of life. The scientific discipline that studies the properties and origin of sedimentary rocks is called sedimentology. Sedimentology is part of both geology and physical geography and overlaps partly with other disciplines in the Earth sciences, such as pedology, geomorphology, geochemistry and structural geology.

Classification based on origin

Sedimentary rocks can be grouped into four types based on how they form: clastic sedimentary rocks, biochemical sedimentary rocks, chemical sedimentary rocks, and a fourth group formed by impacts, volcanism, and other minor processes.

Clastic sedimentary rocks

Main article: Clastic rock

Clastic sedimentary rocks are made from pieces of other rocks that have been stuck together. These pieces, called clasts, are often made of quartz, feldspar, clay minerals, or mica. Clastic rocks are grouped by the size of their pieces. Big pieces make conglomerates and breccias, medium pieces make sandstones, and tiny pieces make mudrocks.

Conglomerates and breccias

Main article: Conglomerate (geology)

Breccias are made of sharp, angular pieces, while conglomerates are made of rounded pieces.

Sandstones

Main article: Sandstone

Sandstones are named based on what they are made of, like quartz or feldspar, and how much muddy material is between the grains.

Mudrocks

Main article: Mudrock

Mudrocks are made from very tiny particles carried by water or wind and settled when the water calms down. They can be siltstones, mudstones, or claystones.

Biochemical sedimentary rocks

Biochemical sedimentary rocks form when living things use materials from water or air to build their bodies. For example, limestone comes from the shells of creatures like corals and mollusks, and coal comes from old plants that were buried and changed over time.

Chemical sedimentary rocks

Chemical sedimentary rocks form when minerals in water become too concentrated and settle out on their own. Examples include oolitic limestone and rocks made of minerals like halite (rock salt) and gypsum.

Other sedimentary rocks

This group includes rocks formed by volcanic eruptions, like tuff, and rocks formed by impacts from space, like impact breccias.

Classification based on composition

Sedimentary rocks can be grouped by what they are made of.

• Siliciclastic sedimentary rocks are mostly made of minerals that contain silicon and oxygen. Examples include conglomerates, breccias, sandstone, and mudrocks.
• Carbonate sedimentary rocks are made of minerals that have carbon and oxygen. Common types are limestone and dolomite.
• Evaporite sedimentary rocks form when water evaporates and leaves behind minerals. These often include rock salt and gypsum.
• Organic-rich sedimentary rocks contain a lot of plant and animal material. Examples are coal and oil shale.
• Siliceous sedimentary rocks are mostly made of silica, like chert.
• Iron-rich sedimentary rocks have a lot of iron. The most common are banded iron formations.
• Phosphatic sedimentary rocks contain a lot of phosphorus. Examples include phosphate nodules.

Deposition and transformation

Sediment transport and deposition

Sedimentary rocks form when tiny particles are deposited by air, ice, wind, gravity, or water. These particles come from rocks that break down due to weathering and erosion. They are then transported to a new place where they settle. Some sedimentary rocks, like evaporites, form from materials that crystallize right where they are deposited. The kind of rock that forms depends on where the particles come from and the sedimentary depositional environment where they end up.

Transformation (Diagenesis)

Main article: Diagenesis

As layers of sediment pile up, older layers get buried deeper. This process, called diagenesis, changes the sediments in many ways. First, compaction squeezes the sediments together, and lithification turns them into solid rock. Early changes happen close to the surface and include small shifts in the minerals. Deeper down, more compaction happens as the weight of overlying layers presses the sediments tighter. Water is squeezed out, and minerals may dissolve and redeposit, helping to glue the grains together. This makes the rock harder and stronger. Eventually, if buried even deeper, these rocks may change further into metamorphic rock.

Properties

Sedimentary rocks are formed when small pieces of minerals, rocks, or organic matter settle and stick together. These pieces, called sediments, come from many sources like weathered rocks, broken shells, or dead plants and animals.

The color of sedimentary rocks often depends on iron. Iron can give rocks gray, greenish, red, or brown colors depending on the amount of oxygen present. Organic material, such as dead plants, can make rocks black or gray. The way these pieces fit together, called texture, affects the rock's overall properties like how much water it can hold.

Fossils are common in sedimentary rocks because they form under conditions that preserve remains better than other rock types. Fossils can be the actual remains of organisms or imprints left behind. These fossils help scientists understand past life and environments.

Sedimentary rocks often show layers, called beds, that formed one on top of the other. These layers can tell us about the environment where the rock formed, such as whether it was in a river, lake, or ocean. Structures like ripple marks and mudcracks also give clues about past water flow and conditions.

Depositional environments

The setting in which a sedimentary rock forms is called the depositional environment. Every environment has its own set of natural processes and conditions. The type of sediment that ends up in the rock depends on both the sediment that arrives in an area (provenance) and the environment itself.

A marine environment means the rock formed in a sea or ocean. There are differences between deep and shallow marine environments. Deep marine environments are more than 200 metres below the water surface (including the abyssal plain). Shallow marine environments are near coastlines and can stretch to the edges of the continental shelf. In these shallow areas, water movements are stronger because wave activity decreases with depth. This allows coarser particles like sand, clay, and silt to be moved and deposited. When sediment comes from the land, it creates layers of these materials. In warm climates, shallow marine areas far from land often form carbonate rocks because warm, shallow water is a good home for small creatures with carbonate skeletons. When these creatures die, their skeletons sink and build up layers of mud that can turn into limestone. These same warm, shallow waters are also where coral reefs form, made from the skeletons of larger creatures.

In deep marine environments, water currents are weak, so only fine particles like clay or tiny skeletons of tiny sea creatures can settle. At depths of about 4 kilometres, carbonates dissolve, so limestone cannot form below this point. However, skeletons made of silica (like those of radiolarians) remain and can form rocks such as radiolarite. When the sea floor slopes gently, sudden movements called turbidity currents can quickly bring large amounts of sediment like sand and silt down, creating layers known as turbidites.

The coast is shaped by waves. At a beach, heavier materials like sand or gravel, often mixed with pieces of shells, are left behind, while finer particles like silt and clay stay suspended in the water. Tidal flats and shoals are areas that sometimes dry out because of the tide. They often have narrow channels called gullies where stronger currents carry larger grains. Where rivers meet the sea or a lake, they can create deltas—big piles of sediment brought from the land.

On continents, environments such as lagoons, lakes, swamps, floodplains, and alluvial fans can also deposit sediments. In calm waters of swamps, lakes, and lagoons, fine sediments mix with organic material from plants and animals. In rivers, the stronger flow can carry heavier materials. Sediment can also move by wind or glaciers. Wind-carried sediment, called aeolian, is usually very evenly sized, while glacier-carried sediment, called glacial till, is mixed and unsorted.

Sedimentary facies

The type of rock that forms in a certain depositional environment is called its sedimentary facies. These environments often exist next to each other. For example, a beach with sand and gravel might be close to a deeper sea area where finer sediments settle at the same time. Behind the beach, there might be dunes with sand or a lagoon with clay and organic material. Each environment leaves its own kind of sediment behind. Over time, as layers build up, the environment can change, leading to changes in the types of rocks formed at one place. When we look at rocks side by side, the types of rocks and facies can change.

Facies can be identified by the type of rock (like limestone, siltstone, or sandstone) or by the fossil remains they contain. For instance, coral only lives in warm, shallow seas, so coral fossils tell us about shallow marine facies. Facies based on rock type are called lithofacies, and those based on fossils are biofacies.

Coastlines can move over time due to changes in sea level, tectonic forces, or large river deltas. These movements cause changes in sedimentary facies, either parallel or perpendicular to layers of the same age, following Walther's Law.

When coastlines move towards the land, deeper marine facies are placed on top of shallower ones, a pattern called onlap. When they move towards the sea, shallower facies end up on deeper ones, called offlap.

Mapping the facies of rocks of a certain age helps us understand past palaeogeography. A series of maps for different times can show how regional geography developed.

Sedimentary basins

Main article: Sedimentary basin

Places where lots of loose material settles are called sedimentary basins. How much material can collect depends on how deep the basin is, called the accommodation space. The depth, shape, and size of a basin change because of movements in Earth's lithosphere. When the lithosphere moves up, called tectonic uplift, land rises and becomes a source of new material as erosion wears it away. When the lithosphere moves down, called tectonic subsidence, a basin forms and material settles there.

One type of basin forms when two parts of a continent move apart, called a rift basin. Rift basins are long, narrow, and deep. Because the lithosphere stretches and thins, hot asthenosphere rises and heats the basin. Besides material from continents, rift basins often contain volcanic deposits. As the basin grows from stretching, the sea may flow in and add ocean material.

Influence of astronomical cycles

Changes in layers of sedimentary rock often repeat in patterns. These patterns happen because of regular changes in how material is added and the environment where it settles. Many of these changes are caused by astronomical cycles. Short cycles can be as simple as the difference between everyday tides or the stronger spring tide every two weeks. On longer time scales, changes in climate and sea level happen because of Milankovitch cycles. These are changes in Earth's tilt and orbit around the Sun, happening over 10,000 to 200,000 years.

Even small changes in Earth's tilt or seasons can greatly affect climate. For example, the ice ages of the past 2.6 million years (the Quaternary period) are thought to have been caused by these astronomical cycles. Climate changes can change sea levels and how much material flows into a region. Over time, small changes in Earth's orbit can lead to big changes in where and how material settles.

Sedimentation rates

Sediment builds up at different speeds depending on where it is. In some places, like a tidal flat, several meters of sediment can gather in just one day. But in the deep ocean, it might take a whole year for only a few millimeters to collect.

Sometimes, sediment builds up very quickly because of sudden events like landslides or big floods. These can add lots of sediment all at once. Even in places that are usually calm, most of the sediment layers might come from these quick events. In other places, sediment builds up slowly over time. The amount of sediment that turns into rock depends on how much material is added and how well it sticks together. Often, erosion removes sediment soon after it settles.

Stratigraphy

Main article: Stratigraphy

Sedimentary rocks form in layers called beds or strata. Each new layer sits on top of the older ones, following the principle of superposition. Sometimes, there are gaps in these layers called unconformities. These gaps happen when no new sediments were added, or when older layers were raised and worn away.

Sedimentary rocks hold important clues about Earth's history. They often contain fossils, which are the preserved remains of ancient plants and animals. Because these rocks form under gentle conditions, they can protect fossils better than other types of rocks. By studying these layers, we can learn about how Earth’s environment has changed over time.

Provenance

Main article: Provenance (geology)

Provenance helps us understand where sediments come from. Rocks on Earth's surface break down into smaller pieces through weather and other natural processes. These pieces can come from any type of rock—igneous, sedimentary, or metamorphic. Scientists study these sediments to trace their journey from the original rocks to where they finally settle.