Sedimentary Rocks

Sedimentary rocks are one of the three fundamental types of rocks found on the Earth, alongside igneous and metamorphic rocks. They are formed by the deposition and cementation of particles, which include fragments of pre-existing rocks and minerals, as well as the remains of earlier living organisms (fossils). These accumulated sediments are then transformed into solid rock through processes of compaction and cementation (lithification). Sediments are rock-derived materials that occur generally, though not always, in an unconsolidated state on or near the Earth’s surface. They are classified genetically, based on the processes responsible for their formation. Broadly, sediments fall into three major categories: (1) detrital (clastic) sediments formed primarily through physical weathering and transport; (2) organic sediments derived from the accumulation of biological matter; and (3) chemical sediments produced through physicochemical precipitation. Organic and chemical sediments are often grouped together as biochemical sediments because of their close association with biological and chemical processes.

Sedimentary rocks are vital to know the Earth’s geological and environmental history. They characteristically exhibit layering, or stratification, which is often their most evident feature. Each individual layer, known as a stratum (plural: strata), records a specific period of net sediment accumulation (sedimentation) and contains valuable information about the conditions and processes by which it is formed. Although sedimentary rocks constitute a thin veneer, covering more than 85% of the Earth’s surface, their study, known as sedimentology, is crucial for understanding the Earth’s surface processes.

Types of Sedimentary Rocks

Sedimentary rocks are divided into three major types based on how the sediment is produced, as described above, and the dominant process of rock formation (mode of origin / genetic process). The 3 types of sedimentary rock are: Clastic (or Detrital) Sedimentary Rocks, Chemical Sedimentary Rocks and Biochemical (or Organic) Sedimentary Rocks.

Table 1: Types of Sedimentary Rocks based on Mode of Origin

Type of Sedimentary Rock	Mode of Origin	Source of sediments	Example of materials	Examples
Clastic Sedimentary Rocks	Formed by physical weathering and erosion of pre-existing rocks	Broken pieces of rocks (clasts) that are transported and deposited	Sand, gravel, silt, clay	Sandstone, Shale, Conglomerate
Chemical Sedimentary Rocks	Formed by the precipitation of dissolved minerals from water	Minerals crystallising directly from solution	Dissolved ions like Ca²⁺, Na⁺, Cl⁻	Rock Salt (Halite), Gypsum, Travertine
Biochemical Sedimentary Rocks	Formed by the accumulation of shells and skeletal materials from organisms	Carbonate shells, hard parts of living organisms	CaCO₃ (calcite, aragonite) from coral, shells	Limestone, Coquina, Chalk
Organic (or Biogenic) Sedimentary Rocks	Formed by the accumulation and burial of plant or animal organic matter	Carbon-rich remains of organisms	Plant debris, organic tissues	Coal, Oil shale

Clastic (or Detrital) Sedimentary Rocks

Clastic sedimentary rocks, also referred to as detrital or terrigenous rocks, are primarily composed of rock fragments (clasts) and mineral debris. These clasts are often individual grains of quartz, feldspar, clay minerals or mica, or even fragments of multiple minerals (lithic fragments). These are produced by the weathering of other rocks at the Earth’s surface and are subsequently cemented together.

Formation of a Clastic Sedimentary Rock

The formation of a clastic sedimentary rocks involves five processes. The formation of clastic sedimentary rocks is a sequential process involving the breakdown (weathering and erosion), movement (transportation) and consolidation (lithification) of rock fragments:

1. Weathering: This is the initial stage where pre-existing rocks and minerals are broken down into smaller pieces or dissolved into ions. The process of weathering occurs at or near the Earth’s surface through physical and chemical processes.
2. Erosion: Subsequent to weathering, erosion is the process of removing these weathered sediments, soil and rock fragments from their original location by agents like water, wind, ice or gravity.
3. Transportation: Sediments are then moved by various mediums such as water (streams, rivers, ocean currents), wind, glaciers and gravity (mass movements). The energy of the transporting medium determines the size and amount of particles to be carried from the place of erosion.
4. Deposition: Deposition occurs when the transporting medium loses sufficient energy, causing the sediments to settle and accumulate in low-lying areas. Sediments are deposited in diverse environments, from lakes, channels and beaches to deep ocean floors.
5. Lithification (Diagenesis): This is the final stage where newly deposited unconsolidated sediments are transformed into a coherent, solid rock, involving processes such as cementation, compaction, desiccation and crystallisation. Diagenesis refers to all post-depositional physical, chemical and biological changes that occur to sediments after their deposition and burial. Key processes of lithification include:
   • Compaction: The process whereby fine-grained sediment is converted to consolidated rock, such as a clay lithified to a shale. The weight of overlying sediments squeezes out intergranular fluids, reducing pore space and forcing grains closer together.
   • Cementation: The diagenetic process by which coarse clastic sediments become lithified or consolidated into hard, compact rocks, usually through deposition or precipitation of minerals in the spaces among the individual grains of the sediment, binding them together. It may occur simultaneously with sedimentation, or at a later time. Common cements include quartz, calcite and hematite.

Classification of Clastic Sedimentary Rock

Clastic or detrital sedimentary rocks can be further classified based on the dominant particle size, proportion of detrital fraction etc.

Based on the dominant Particle size

Clastic sedimentary rocks are primarily subdivided based on the dominant particle size of their clasts. The Udden-Wentworth grain size scale is commonly used by geologists for this classification of clastic sedimentary rocks.

Table 2: Types of Sediments and Sedimentary Rocks based on Particle Size

Particle Size Category	Diameter Range	Subdivisions	Corresponding Rock Type
Gravel	>2 mm	Boulders (>256 mm) Cobbles (64–256 mm) Pebbles (4–64 mm) Granules (2–4 mm)	Conglomerates and Breccias
Sand	0.0625 mm – 2 mm	Very coarse (1–2 mm) Coarse (0.5–1.0 mm) Medium (0.25–0.50 mm) Fine (0.125–0.250 mm) Very Fine (0.0625–0.1250 mm)	Sandstones
Mud		Silt (0.004 mm – 0.0625 mm) Clay (	Mudrocks (Siltstones, Claystones, Mudstones, Shales)

Based on the proportion of detrital fraction

Clastic sediments and sedimentary rocks can also be classified based on the proportion of their detrital fraction like gravel (G), sand (S) and mud (M), often using triangular classification systems, such as the one presented by Folk et al. (1954).

Some of the common clastic sediments and sedimentary rocks based on the proportion of detrital fractions are:

1. Gravels and Gravelstones: These rocks contain more than 30% gravel. If they have over 80% gravel, they typically exhibit clast-supported textures, where the gravel fragments are in direct contact and support one another. If they contain less than 80% gravel, they are typically matrix-supported, with finer sand and/or mud providing support.
2. Sands and Sandstones (Arenites): These contain less than 30% gravel and a sand-to-mud ratio greater than 1:1. If they have less than 5% gravel and a sand-to-mud ratio greater than 9:1, they are classified as relatively pure sands or arenites. If the sand-to-mud ratio is less than 9:1, they are termed muddy sands or wackes.
3. Muds and Mudrocks: These contain less than 30% gravel and a sand-to-mud ratio less than 1:1. They can be further subdivided based on the ratios of clay (C), silt (Z) and sand (S) within the mud fraction.

Common types of Clastic Sedimentary Rocks

Some of the common types of clastic (or Detrital) rocks are conglomerate, sandstone and mudrock etc.

Conglomerates and Breccias

These are coarse-grained clastic rocks primarily composed of gravel-sized clasts (>2 mm). They account for about 5% of all detrital sedimentary rocks.

• Breccias: A coarse-grained clastic rock, composed of angular broken rock fragments held together by a mineral cement or in a fine-grained matrix; it differs from conglomerate in that the fragments have sharp edges and unworn corners. Breccia may originate as a result of sedimentary processes such as talus accumulation (sedimentary breccia); igneous processes, esp. explosive (igneous breccia, volcanic breccia); disturbance during sedimentation (intraclastic breccia); collapse of rock material (solution breccia, collapse breccia); or tectonic processes (fault breccia).
• Conglomerates: A coarse-grained clastic sedimentary rock, composed of rounded to subangular fragments larger than 2 mm in diameter, indicating significant transport and abrasion in high-energy environments like turbulent rivers or coastal areas, typically containing fine-grained particles (sand, silt, clay) in the interstices and commonly cemented by calcium carbonate, iron oxide, silica, or hardened clay; the consolidated equivalent of gravel both in size range and in the essential roundness and sorting of its constituent particles. They are often clast-supported, where the clasts are in contact and support each other.

Both conglomerates and breccias can be oligomictic (clasts of a single composition) or polymictic (clasts of multiple compositions).

Sandstones

Sandstone is a clastic sedimentary rock composed of abundant rounded or angular fragments of sand size particles (0.0625–2 mm) with or without a fine-grained matrix (silt or clay) and more or less firmly bound by a cementing material (commonly silica, iron oxide or calcium carbonate). The rock varies in colour, may be deposited by water or wind and may contain numerous primary features (sedimentary structures and fossils). Sandstone may be classified according to composition of particles, mineralogic or textural maturity, primary structures and type of cement. Sandstone classification schemes, such as Folk’s (1974), consider the percentages of quartz (Q), feldspar (F) and lithic fragments (L) in the sand fraction.

Mudrocks

These are fine-grained clastic rocks, making up 60-65% of all sedimentary rocks. They are composed of at least 50% silt- and clay-sized particles and typically form in low-energy depositional environments such as deep marine basins, lakes and floodplains.

Shale

Shale is a fine-grained clastic sedimentary rock composed of clay minerals, silt-sized quartz and organic matter. A fissile mudrock that splits easily into thin, parallel layers. Fissility is generally a result of the sub-parallel alignment of abundant clay and mica minerals. Shale generally, deposits in low-energy environments like the deep ocean or lakes.

Textures of Clastic Sedimentary Rocks

The texture of a clastic sediment or rock refers to the size, form and arrangement of its solid particles and the void spaces between them. Texture is a small-scale property but significantly impacts larger-scale properties such as density, porosity and permeability. It helps interpret depositional environment, transport history and energy conditions of the sediments. Textures of clastic sedimentary rocks are typically described using several key parameters, including grain size, sorting, roundness, sphericity, shape, packing and maturity.

Sorting: Sorting is a measure of the uniformity of particle sizes in a sediment. Sorting can be classified as well-sorted and poorly sorted. Sorting provides important information about the depositional medium and environmental conditions.

• Well-sorted sediments contain particles that are all roughly the same size (e.g., fine sand). This typically indicates effective sorting by transporting agents like wind.
• Poorly sorted sediments exhibit a wide range of particle sizes (e.g., boulders to clay). This is characteristic of deposition by glaciers or most mass flows.

Roundness: The degree of abrasion of a clastic particle as shown by the sharpness of its edges and corners. Roundness is defined as the ratio of the average radius of curvature of the corners of the particle image to the radius of the maximum inscribed circle. A perfectly rounded particle (such as a sphere) has a roundness value of 1.0; less-rounded particles have values less than 1.0. Visual comparison charts are commonly used to estimate grain roundness.

• Well-rounded clasts suggest extensive transport and abrasion.
• Angular clasts indicate minimal transport from their source area.

Sediment Maturity: This concept evaluates the transport and weathering history of sands and sandstones, not applying to mudrocks and gravelstones. Maturity is assessed based on two main criteria: textural maturity and compositional maturity. These criteria help geologists infer the history of sediment transport, the energy of the depositional environment and the distance from the source rock.

Textural Maturity: Textural maturity refers to the physical characteristics of sediment grains, specifically their rounding and sorting. As sediments are transported over long distances or for extended periods, they undergo abrasion and winnowing, which leads to:

• Rounding: Angular grains become increasingly rounded due to collisions and friction during transport. Well-rounded grains indicate prolonged transport and high textural maturity.
• Sorting: Well-sorted sediments contain grains of similar size, as finer and coarser particles are separated by transport processes. Poorly sorted sediments contain a wide range of grain sizes, indicating minimal transport and low maturity.

For example, beach sands are typically texturally mature because they are well-rounded and well-sorted, having been subjected to repeated wave action and long transport distances. In contrast, sediments near their source (like alluvial fans or glacial deposits) are often angular and poorly sorted, reflecting minimal transport and low textural maturity.

Compositional Maturity: Compositional maturity refers to the mineralogical composition of the sediment. As sediments are transported and weathered, unstable minerals (such as olivine, plagioclase and volcanic glass) are gradually broken down or dissolved, leaving behind only the most stable minerals (primarily quartz). This process leads to:

• Mineralogical Uniformity: Mature sediments are dominated by quartz, which is highly resistant to both physical and chemical weathering. Immature sediments contain a mix of stable and unstable minerals, reflecting limited transport and weathering.
• ZTR Index: The ZTR index (zircon, tourmaline, rutile) is sometimes used to quantify compositional maturity, as these minerals are extremely resistant and persist in highly weathered sediments.

For instance, a sediment composed mainly of well-rounded quartz grains is compositionally mature, while a sediment containing angular volcanic fragments and unstable minerals like olivine is compositionally immature.

Biochemical Sediments and Sedimentary Rocks

Biochemical sedimentary rocks are sedimentary rocks formed directly or indirectly from the chemical processes and activities of living organisms. Plants, shells, corals and microorganisms extract minerals like calcium carbonate or silica from water to build their skeletons or shells. When these organisms die, their hard parts accumulate on the seafloor and gradually get compacted into rock. Common examples include limestone formed from shells and corals and chert formed from silica-rich microscopic organisms. Organic sedimentary rocks are a specific type of biochemical sedimentary rock composed primarily of organic carbon from fossil plant matter. In other words, they form by the accumulation and lithification of biological debris (for example, plant remains or shells) rather than by mechanical (clastic) or purely chemical precipitation. Organic sedimentary rocks typically form in environments where large amounts of biological material accumulate under low‐oxygen (anoxic) conditions. In swampy, oxygen-poor environments, vegetation and organic matter are buried quickly, preventing them from undergoing complete decay. Over time, pressure from overlying layers compresses this material, squeezing out water and gases and gradually transforming it into rock.

1. Biochemical (or Fossiliferous) Limestone: These limestones primarily form from the accumulation of calcium carbonate (CaCO₃) skeletons and shells produced by marine organisms, such as corals, mollusks and foraminifera.
2. Biochemical Chert: Biochemical chert forms from the accumulation of microscopic, silica-secreting planktonic organisms like radiolarians and diatoms. Their siliceous skeletons, upon settling and lithification, recrystallise into hard biochemical chert.
3. Diatomite: A light-coloured, soft, friable, porous, siliceous sedimentary rock, consisting chiefly of opaline frustules of the diatom, a unicellular aquatic plant related to the algae. Some deposits are of lake origin but the largest are marine.
4. Coquina: Coquina is a type of sedimentary rock composed almost entirely of shell and bone fragments cemented together. It forms when these shell fragments accumulate on the seafloor and are then cemented together over time
5. Coal: This is a prominent example of an organic sedimentary rock. Coal forms from the accumulation, compaction and alteration of plant debris in lush, low-oxygen wetland environments such as swamps and bogs.

Table 3: Types of Biochemical Sedimentary Rocks

Rock Type	How It Forms	Main Material
Fossiliferous Limestone	Accumulation of shells and skeletal fragments	Calcium carbonate (CaCO₃)
Coquina	Cemented, broken shell fragments	Shell pieces (CaCO₃)
Chalk	Compaction of microscopic plankton shells	Calcite from tiny organisms
Chert (biogenic)	Silica from microscopic organisms like radiolarians	Silica (SiO₂)
Diatomite	Accumulation of diatom shells	Silica (SiO₂)

Chemical Sediments and Sedimentary Rocks

Chemical sedimentary rocks are formed through the inorganic precipitation of minerals directly from a water solution. This occurs when dissolved mineral constituents become supersaturated and precipitate out of the solution. This usually happens through evaporation or chemical reactions in lakes, seas, or groundwater. As water evaporates, minerals like halite, gypsum and calcite crystallise and accumulate to form rocks such as rock salt, gypsum and some types of carbonate rocks. These rocks indicate past conditions of high evaporation or mineral-rich waters.

1. Evaporites: These are chemical sedimentary rocks formed by the evaporation of water, which leads to the precipitation and accumulation of dissolved minerals. Common examples include halite (rock salt) and gypsum.
2. Travertine: This is a chemically precipitated limestone that forms when groundwater, supersaturated with dissolved calcium and bicarbonate, emerges at the surface and precipitates calcite. It is often found in caves and around hot springs.
3. Dolostones: These rocks are composed predominantly of the mineral dolomite. They frequently form when existing limestones are chemically altered by magnesium-rich fluids, causing calcite to recrystallise into dolomite.
4. Limestone (chemical origin): While many limestones are biochemical, some also form through direct inorganic precipitation of calcium carbonate from warm, shallow marine waters.
5. Chemical Cherts: These form when silica (SiO2) precipitates from groundwater, often replacing other minerals within a rock, rather than originating from biological activity. Varieties include flint (dark gray/black), jasper (red/yellow), petrified wood (silica replacing wood) and agate (banded chalcedony).

Table 4: Types of Chemical Sedimentary Rocks

Rock Type	How It Forms	Example Minerals
Rock Salt	Evaporation of saline (salt-rich) water	Halite (NaCl)
Gypsum Rock	Evaporation of mineral-rich water	Gypsum
Chemical Limestone	Precipitation of calcium carbonate from water	Calcite
Chert	Precipitation of silica from groundwater or seawater	Microcrystalline silica
Travertine	Rapid precipitation of calcium carbonate near hot springs	Calcite

Common Sedimentary Structures

Sedimentary structures are the physical features preserved within sedimentary rocks that record the conditions and processes active during the deposition. Because every stage of the sedimentary cycle leaves behind specific clues, these structures are among the most direct and informative indicators of past environments. They help geologists interpret transport mechanisms, flow directions, depositional settings, paleoclimate and even biological activity.

Stratification and Bedding

The formation, accumulation or deposition of material in distinct layers, known as beds or strata. It may be due to differences of texture, hardness, cohesion or cementation, colour, internal structure and mineralogic or lithologic composition. Bedding is typically the most conspicuous feature of sedimentary rocks.

A stack of beds is collectively called a strata. A group of strata that is laterally continuous, mappable on a regional scale and compositionally distinct is known as a formation, the fundamental unit of geologic mapping.

Cross Bedding

Cross bedding consists of inclined layers within a set of beds. These inclined laminae form as sediment moves up and over migrating ripples or dunes. Cross bedding is one of the best indicators of paleocurrent direction.

Cross beddings are common in beach sands, river channel deposits and aeolian (wind-blown) dunes. The inclination direction indicates the direction in which water or wind was moving during deposition. Boundaries between sets of cross beds are often erosional surfaces.

Graded Bedding

Graded bedding is a type of bedding in which each layer displays a gradual and progressive change in particle size, usually from coarse at the base of the bed to fine at the top. It may form under conditions in which the velocity of the prevailing current declined gradually, as by deposition from a single short-lived turbidity current.

Graded bedding forms when a turbulent sediment-laden flow (e.g., turbidity current) begins to slow down; then the coarse grains settle first, followed by medium grains settling next and fine grains settle last. A series of graded beds produced by repeated turbidity currents is called a turbidite sequence.

Ripple Marks

Ripple marks are a series of small ridges and troughs structure formed on the surface of sediments. When water or wind moves across loose sediment, it creates ripples, small ridges and troughs that reflect flow conditions.

It is produced superficially by wind action and sub-aqueously by currents or by the agitation of water in wave action and generally trends at right angles or obliquely to the direction of flow of wind or the moving fluid.

The shape of the ripples depends on the flow type of the transporting medium.

• Asymmetric Ripples: They form under unidirectional flow (rivers, shallow streams). Gentle slope faces upstream; steep slope faces downstream. They preserve excellent indicators of flow direction and way-up structures.
• Symmetric Ripples: It forms under oscillating wave action. Its trough-to-crest symmetry indicates shallow-water wave environments.

Mudcracks

It is an irregular fracture in a crudely polygonal pattern, formed by the shrinkage of clay, silt or mud. Mudcracks form when wet, fine-grained sediment (such as clay or silt) dries and contracts under the influence of atmospheric surface conditions. Their polygonal patterns are often preserved in ancient rocks. Mudcracks indicate subaerial exposure, periodic drying (ephemeral lakes, tidal flats, floodplains) and rapid burial after crack formation.

Sole Marks

Sole marks are features found at the base of sedimentary beds. They are produced by erosion and filling of a primary sedimentary structure, e.g. a crack, track, groove, or other depression, formed on the surface of the underlying mud agents such as currents, organisms and unequal loading and preserved as a sole cast after the underlying material had consolidated and weathered away. Sole marks provide valuable information about current direction during deposition.

Types of Sole Marks

• Flute marks: Scoop-shaped depressions carved by turbulent currents into soft sediment; later filled by overlying layers.
• Flute casts: Positive relief structures preserved on the underside of beds.

Raindrop Impressions

Raindrop marks are tiny pits formed by the impact of raindrops on soft sediment. Their presence indicates surface exposure, atmospheric conditions and rapid burial after rainfall.

Properties of sedimentary rocks

Properties of sedimentary rocks like colour, texture, grain size, composition etc. offer valuable clues about their formation, composition and the environments in which they originated.

1. Colour: The colour of a sedimentary rock often provides insights into its depositional environment and mineral composition.

• Red or reddish-brown colours typically indicate the presence of iron(III) oxide (hematite), which forms in oxygen-rich (oxidizing) conditions, often associated with non-marine red beds.
• Darker colours, such as gray or black, are commonly linked to oxygen-poor (anoxic/reducing) environments where organic matter and sulfide minerals (like pyrite) are preserved.

2. Texture: Texture is the general physical appearance or character of a rock, including the geometric aspects of and the mutual relations among, its component particles or crystals. It refers to the size, form and orientation of the constituent particles (clasts or crystals), as well as the void spaces between them. Texture is a small-scale property but significantly affects properties like density, porosity and permeability. Chemical sedimentary rocks generally have a non-clastic, crystalline texture.

3. Grain size: This is the average diameter of the particles making up the rock, typically measured using the Wentworth-Udden grade scale. Grain size is a crucial indicator of the energy of the transporting and depositional medium; coarser grains generally require higher energy and are deposited closer to their source, while finer grains are carried further into calmer environments.

4. Particle shape: This property includes various characteristics of the grains, such as their rounding, sphericity (how closely a grain resembles a sphere), surface texture and overall three-dimensional form. The degree of rounding, for instance, reflects the amount of abrasion and transport the particles have undergone.

5. Fabric: The fabric describes the three-dimensional orientation and arrangement of its constituent grains or clasts of which a sedimentary rock is composed. In crystalline chemical sedimentary rocks, it refers to the arrangement of crystals.

6. Packing: The way in which solid particles are organized or spaced in sediment or sedimentary rock, or how atoms or ions are arranged within a crystal lattice; in particular, the organisation of clastic grains, independent of any authigenic cement that may have formed between them.

7. Mineralogy: Sedimentary rocks are often characterised by a relatively limited suite of major minerals. For siliciclastic rocks, quartz is dominant, while calcite is prevalent in carbonate rocks. Minerals can be either original detrital grains derived from source rocks or new minerals that precipitated during diagenesis. The relative stability of minerals to weathering is a key factor in their presence.

8. Fossils: Fossils are the preserved remains or traces of ancient life. Fossils are most commonly found in sedimentary rocks because the lower temperatures and pressures of their formation typically allow for the preservation of organic remnants. They provide invaluable data for determining the relative age of sediments, reconstructing past climates and understanding the evolution of life on Earth. Fossilisation is often favored by rapid burial and oxygen-poor environments.

Sedimentary Environments

Depositional environments are the specific surface settings where sediments accumulate and are subsequently lithified into sedimentary rocks. Each sedimentary environment is characterised by distinct geological processes and conditions, which are reflected in the properties of the deposited sediments and the resulting sedimentary rocks. These environments are broadly classified into terrestrial, transitional (paralic) and marine settings.

Terrestrial (Non-marine) environments

These environments are located in the interior of continents, away from direct marine influence.

• Glacial: Such type of sedimentary environments are shaped by the action of glaciers. Glaciers deposit unsorted piles of sediment called glacial till. Glacial till is typically poorly sorted, contains a mix of diverse rock fragments (polymictic), can be boulder-bearing and generally lacks distinct internal stratification.
• Alluvial fans: Alluvial fans are fan-shaped deposits that form at the base of mountains where streams emerge from confined valleys onto flatter plains. Sediment transport in these environments can involve both gravity-driven mass movements and running water.
• Sand Dunes (Aeolian): These are wind-blown environments, such as deserts or coastal dunes. Wind is highly effective at sorting sediments, resulting in very well-sorted sand deposits. Cross-bedding is a common sedimentary structure in these environments.
• Mountain Streams: These are high-energy riverine environments on steep slopes, capable of transporting large particles, including boulders.
• Lakes (Lacustrine): These are quiet water environments where fine-grained sediments (mud, silt, clay) and organic material accumulate at the bottom. In some lakes, particularly those with seasonal changes, finely laminated layers called varves can form, with coarser sediment deposited in summer and finer sediment in winter.
• Rivers (Fluvial): These dynamic environments involve streams carrying large amounts of sediment. As water velocity decreases, larger particles are dropped, creating sorted deposits along riverbeds, floodplains and deltas. Fluvial environments are characterized by channelised sandstones and conglomerates interbedded with overbank mudrocks.

Marine environments

These environments encompass settings where rocks form in seas or oceans.

• Deltas: The low, nearly flat, alluvial tract of land at or near the mouth of a river, commonly forming a triangular or fan-shaped plain of considerable area, crossed by many distributaries of the main river, perhaps extending beyond the general trend of the coast and resulting from the accumulation of sediment supplied by the river in such quantities that it is not removed by tides, waves and currents. Most deltas are partly subaerial and partly below water.
• Coastal Beaches: Coastal beaches are high-energy environments dominated by wave action, which effectively sorts and rounds sediments. Beach sands are typically well-sorted and often composed of well-rounded quartz grains.
• Shallow Marine Clastics: These include coastal and continental shelf environments, generally down to about 200m depth. Wave activity is significant, leading to the transport and deposition of coarser clastic sediments, resulting in alternating layers of sand, silt and clay.
• Shallow Marine Carbonates: Found in warm, clear, shallow marine waters, usually far from continental clastic sediment sources. These environments are conducive to the growth of organisms that build carbonate skeletons (e.g., corals, mollusks), whose remains accumulate to form calcareous muds and limestones.
• Deep Marine: These environments are typically deeper than 200m, including continental slopes, submarine fans, continental rises and abyssal plains. Water energy is generally low, facilitating the deposition of fine-grained sediments such as mudrocks, deep marine limestones and cherts. Turbidity currents are a characteristic process in these environments, producing turbidites.

Provenance

Provenance is the area from which the constituent materials of a sedimentary rock or facies are derived. All types of pre-existing rocks (igneous, metamorphic and sedimentary) exposed at the Earth’s surface contribute detritus to the sedimentary cycle.

The composition of detrital sediments, particularly sandstones and gravelstones, provides critical information about their source rocks. For instance, gravelstones are especially useful for hand-specimen provenance studies because large, often unaltered, fragments of the original source rocks can be identified within them. The characteristics of these clasts—such as their composition, size, shape and mineral stability—offer clues about the climate, relief, erosion rates and tectonic setting of the source area, as well as the intensity and duration of sediment transport.