Sedimentary Rocks

Sedimentary rocks are generally found in the form of layers. They are formed from the accumulation and compaction of material at or near Earth’s surface. The process begins when existing rocks are broken down by weathering and erosion into fragments, minerals, or dissolved substances, which are then carried by water, wind, or ice and deposited in places like riverbeds, lakes, deserts, and ocean floors. Over time, as more layers pile up, the buried sediments are squeezed together (compaction) and bound by minerals that precipitate between the grains (cementation), a process known as lithification. Sedimentary rocks fall into three main types: clastic rocks made of rock fragments, such as sandstone and shale; chemical rocks formed when dissolved minerals precipitate out of water, such as rock salt and some limestone; and organic (biochemical) rocks made from the remains of living things, such as coal and fossil-rich limestone. They are easily recognized by their visible layers, or strata, and are especially important because they often preserve fossils, offering a window into Earth’s past environments and life.

Examples of Sedimentary Rocks

Clastic (made of rock and mineral fragments)

• • Conglomerate: rounded pebbles cemented together
  • Breccia: angular fragments cemented together
  • Sandstone: sand-sized grains, often quartz
  • Siltstone: very fine grains, between sand and clay in size
  • Shale: thin layers of compacted clay and mud
  • Mudstone: similar to shale but without the thin layering
  • Claystone: made almost entirely of clay particles
  • Arkose: sandstone rich in feldspar
  • Greywacke: dark, poorly sorted sandstone

Chemical (minerals precipitated from water)

• • Limestone: mostly calcite
  • Dolostone: made of the mineral dolomite
  • Rock salt (halite): formed by evaporating seawater or salt lakes
  • Gypsum: a soft sulfate mineral from evaporation
  • Rock gypsum and anhydrite: other evaporite sulfates
  • Chert: dense microcrystalline silica
  • Travertine: calcite deposited by springs and caves
  • Oolitic limestone: made of tiny rounded calcite grains
  • Banded iron formation: alternating iron-rich and silica-rich layers

Organic or biochemical (from the remains of living things)

• • Coal: compressed plant material
  • Chalk: soft limestone made of microscopic marine shells
  • Coquina: loosely cemented shell fragments
  • Fossiliferous limestone: limestone full of visible fossils
  • Diatomite: made of the silica shells of diatoms (algae)
  • Coral limestone (reef limestone): built from coral and reef organisms
  • Oil shale: shale rich in organic matter

Classification of Sedimentary Rocks

Sedimentary rocks are classified into three main categories. This classification is made based on their formation.

01. Clastic (detrital) Rocks

Clastic rocks, also called detrital rocks, are sedimentary rocks made of fragments (clasts) of pre-existing rocks and minerals that were broken down by weathering, carried by water, wind, ice, or gravity, and eventually deposited in layers in places like river channels, beaches, deltas, lakes, and ocean floors. As more sediment piles up, the deeper layers are compacted, and the spaces between grains are filled with natural cements such as silica, calcite, or iron oxide, turning loose sediment into solid rock. The sedimentary rocks are further classified by grain size, then further refined by grain shape, composition and sorting.

1. By grain size (Udden-Wentworth scale)

2. Subdivisions within each group

• Rudaceous: conglomerate has rounded clasts, which indicates long transport. Breccia has angular clasts, which indicates short transport from a nearby source.
• Arenaceous: sandstones are subdivided by composition.
  • Quartz arenite is almost pure quartz and very mature.
  • Arkose is rich in feldspar (at least 25%) and usually comes from granitic sources.
  • Greywacke is dark, poorly sorted sandstone with a muddy matrix, often found in deep-marine settings.
  • Lithic sandstone contains many rock fragments.
• Argillaceous:
  • Siltstone is made of silt-sized grains and feels slightly gritty.
  • Shale is fine-grained and splits into thin layers (fissile).
  • Mudstone is a mix of silt and clay without the thin layering.
  • Claystone is made mostly of clay particles.

3. Other criteria used to describe clastic rocks

• Sorting: well sorted rocks have uniform grain size, and poorly sorted rocks have a mix of sizes.
• Roundness: grains range from angular to well rounded, depending on how far they traveled.
• Maturity: texturally and compositionally mature rocks are well sorted, well rounded, and rich in resistant minerals like quartz.
• Cement: silica, calcite, or iron oxide, which affects hardness and colour.
• Matrix: the fine material filling the spaces between larger grains.

02. Chemical Rocks (sedimentary)

Chemical sedimentary rocks form when minerals dissolved in water, mostly from the weathering of other rocks, precipitate out of solution and accumulate, rather than being built from transported rock fragments. This usually happens when water evaporates, when its temperature, pressure, or chemistry changes, or when it becomes oversaturated with dissolved ions, so the minerals crystallize and settle in lakes, lagoons, shallow seas, hot springs, and caves. They are commonly grouped by composition: carbonates such as limestone (calcite), dolostone (dolomite), and travertine; evaporites such as rock salt (halite), gypsum, and anhydrite, which form as seawater or salty lakes dry up; siliceous rocks such as chert, made of microcrystalline silica; and iron-rich rocks such as banded iron formations. Unlike clastic rocks, they typically have an interlocking crystalline texture, and they are often softer and may dissolve in water or weak acid, as limestone does. They are valuable as sources of salt, gypsum for plaster and drywall, limestone for cement and construction, and iron ore, and they help geologists reconstruct ancient climates and water conditions, since evaporites point to dry, hot environments and carbonates to warm, shallow seas. The line between chemical and organic rocks is not always sharp, because many limestones and cherts form through a mix of direct precipitation and biological activity.

How are the Chemical Rocks formed?

Chemical sedimentary rocks form when minerals dissolved in water come out of solution and build up as solid crystals. The process happens in a few stages.

1. Minerals get dissolved: Rainwater and groundwater slowly dissolve minerals from rocks on land, such as calcium, sodium, magnesium, silica, and iron. Rivers and groundwater carry these ions into lakes, lagoons, seas, and underground cavities.

2. The water becomes oversaturated: The water can only hold so much dissolved material. Once it holds more than it can, the minerals begin to crystallize. This happens in a few main ways:

• Evaporation: in hot, dry climates, water leaves a lake or restricted sea arm and the dissolved salts become concentrated until they crystallize. This forms evaporites like rock salt (halite) and gypsum.
• Changes in temperature or pressure: warm, shallow seawater loses dissolved carbon dioxide, which makes calcium carbonate precipitate and form limestone. The same thing happens at springs and in caves, forming travertine and stalactites.
• Changes in chemistry: shifts in acidity or the mixing of different waters can force minerals out. Silica can precipitate to form chert, and iron can precipitate to form banded iron formations.

3. Crystals accumulate: The precipitated crystals settle on the bottom of the basin, forming layers of mineral deposits. Because the minerals crystallize directly from solution, the resulting rock usually has an interlocking crystal texture rather than rounded grains.

4. Burial and lithification: As more layers pile up, the deposits are buried and compacted. Water squeezed through the sediment can deposit more minerals between the crystals, cementing them into solid rock. Some deposits also recrystallize over time, for example when limestone slowly changes into dolostone.

Classification of Chemical Rocks (Sedimentary)

Chemical sedimentary rocks are classified mainly by their mineral composition, and secondarily by how they formed.

1. Carbonate rocks (carbonate minerals)

2. Evaporites (formed by evaporation of water)

Evaporites usually form in a predictable order as water evaporates: carbonates first, then gypsum and anhydrite, then halite, then potash salts.

3. Siliceous rocks (silica)

4. Iron-rich rocks (iron minerals)

5. Other chemical deposits

• Phosphorites: rocks rich in phosphate minerals, used for fertilizer.
• Manganese deposits: precipitated from ocean water, as in manganese nodules.

Notes:

• The groups are based on chemistry, not appearance, so rocks that look different can sit in the same group.
• Some rocks sit between categories. Many limestones and cherts form partly through biological activity, so books may place them under chemical or organic rocks.
• The environment helps with identification. Evaporites point to arid conditions, carbonates to warm, shallow water, and travertine and sinter to springs.

03. Organic Rocks (Sedimentary)

Organic sedimentary rocks, also called biochemical or bioclastic rocks, are formed from the accumulated remains of living things, such as shells, skeletons, plant material, and microscopic organisms. When these organisms die, their hard parts or tissues settle on the floor of a sea, lake, or swamp and build up in layers, and over time the burial, compaction, and cementation turn them into solid rock.

Classification of Organic Rocks

They fall into three main groups: calcium carbonate rocks, such as chalk (made of microscopic plankton shells), coquina (loosely cemented shell fragments), fossiliferous limestone, and coral or reef limestone; silica-rich rocks, such as diatomite, made of the glassy shells of diatoms; and carbon-rich rocks, such as coal, which forms when plant matter in swamps is buried and compressed over millions of years, and oil shale, which contains organic matter that can yield oil. They are often rich in visible fossils, which makes them valuable for reconstructing ancient life and environments, and they are economically important as fuel (coal), as raw material for cement and construction (limestone), and for industrial uses such as filtration and abrasives (diatomite). As with chemical rocks, the boundary is not always sharp, since many limestones form through a mix of biological activity and direct chemical precipitation.

How are the Organic Rocks formed?

Organic sedimentary rocks form when the remains of living things accumulate in large quantities and are later turned into solid rock. When marine organisms such as plankton, corals, mollusks, and diatoms die, their shells and skeletons, made of calcium carbonate or silica, sink and pile up on the floor of a sea or lake, while in swamps and bogs dead plant material collects in oxygen-poor water that slows decay. Over time, new layers bury the older ones, and the weight of the overlying sediment compacts the material and squeezes out water. In shell and skeleton deposits, dissolved minerals such as calcite or silica then crystallize between the fragments and cement them together, forming rocks like chalk, coquina, coral limestone, and diatomite. In plant deposits, buried material is first converted to peat and then, as heat and pressure increase, loses water and gases and becomes progressively richer in carbon, passing through lignite and bituminous coal to anthracite. In oil shale, organic matter mixed with mud is preserved and slowly transformed into kerogen. This entire process of burial, compaction, and cementation, known as lithification, usually takes thousands to millions of years.