Sedimentary Rocks: Features, Types and Examples of Sedimentary Rocks

Sedimentary rocks are made from layers, or strata, of mineral particles found in other rocks that have been weathered and from newly formed organic matter.  Sedimentary rocks are important because they preserve a record of ancient landscapes, climates, and mountain ranges, as well as the history of the erosion of Earth. In addition, fossils are found in abundance in sedimentary rocks younger than 600 million years and provide evidence of the evolution of life through time. Earth’s geologic time scale was worked out using this record of sedimentary rocks and fossils.

Salient Features of Sedimentary Rocks

Sedimentary rocks form at Earth’s surface by the hydrologic system. Their origin involves the weathering of pre-existing rock, transportation of the material away from the original site, deposition of the eroded material in the sea or in some other sedimentary environment, followed by compaction and cementation. Some common features are:

• They contain strata or layers. The layers are rarely horizontal and generally tilted due to lateral compressive and tensile forces. They are formed of sediments derived from the older rocks, plants and animals remain.
• Most part (around 75 percent) of the surface area of the globe is covered by Sedimentary Rocks.
• Most of the sedimentary rocks are permeable and porous.
• Sedimentary rocks are generally characterized by different sizes of joints, generally perpendicular to the bedding plains.

Types of Sedimentary Rocks

When rock minerals are weathered, their chemical composition is changed, weakening the solid rock. The rock breaks up into particles of many sizes. When these particles are transported in a fluid such as air, water, or glacial ice, we call them sediment. There are three major classes of sediment: clastic sediment, chemically precipitated sediment, and organic sediment. On this basis, three main types of sedimentary rocks are recognized viz. clastic rocks, organic rocks and chemically precipitated rocks.

• Clastic: Made up of discrete fragments or clasts of materials derived from other minerals, largely of quartz and others such as feldspar, amphiboles, clay minerals
• Organic : They contain the materials which are generated by living organisms such as corals, mollusks, and foraminifera, which cover the ocean floor with layers of calcium carbonate, which can later form limestone.
• Chemical: Formed by the Chemical & Biological Processes like limestone, rock salt, gypsum and dolostone

Clastic Sedimentary Rocks

Clastic sediment is made up of inorganic rock and mineral fragments, called clasts. These can come from igneous, sedimentary, or metamorphic rocks, and so they can include a very wide range of minerals. Quartz and feldspar usually dominate clastic sediment.  When layers of clastic sediment build up, the lower strata are pushed down by the weight of the sediments above them.

This pressure compacts the sediments, squeezing out excess water. Dissolved minerals recrystallize in the spaces between mineral particles in a process called cementation, thus giving rise to the Clastic Sedimentary Rocks. Due to the mechanical process, the clastic sedimentary rocks are also sometimes called mechanically formed Sedimentary Rocks.

Common examples of clastic / mechanically formed sedimentary rocks include Sandstone {cemented sand grains}, Siltstone {Cemented silt particles}, Congomerate {sandstone containing pebbles of hard rocks}, Mudstone {mainly silt and clay}, Claystone {mainly clay} and shale {clay and mud rock which breaks easily into flat flakes and plates}.

Chemically Precipitated and Organic Sedimentary Rocks

Chemically precipitated sediment is made of solid inorganic mineral compounds that precipitate from water solutions or are formed by organisms living in water. One of the most common sedimentary rocks formed by chemical precipitation is limestone.

The third class of sediment is organic sediment. This is made up of the tissues of plants and animals. Peat is an example of organic sediment. This soft, fibrous, brown or black substance accumulates in bogs and marshes where the water stops the plant or animal remains from decaying.

Examples of Chemically precipitated rocks are Limestone {Calcium Carbonate, formed by precipitation on sea or lake floors}, Dolomite {Magnesium and Calcium Carbontes}, Chert {a microcrystalline form of silica} and Evaporites {minerals formed by evaporation of salty solutions in shallow inland lakes or coastal lagoons}.

Limestone

Limestone is by far the most abundant chemically precipitated rock. It is composed principally of calcium carbonate (CaCO3 or calcite) and originates by both inorganic chemical and biochemical processes. Limestone has a great variety of rock textures such as skeletal limestone, oolitic limestone, and microcrystalline limestone. Marine sediments form largely by biochemical precipitation. Carbonate sediments dominate at shallow depths and in warm near-shore waters. Elsewhere, siliceous sediment, which eventually forms chert, is typical in deeper water.

Skeletal Limestone

Some marine invertebrate animals construct their shells or hard parts by extracting calcium and carbonate ions from seawater. Corals, clams, algae, snails, and many other marine organisms construct their skeletons of calcium carbonate. After the organisms die, the shells accumulate on the seafloor. Over a long period of time, they build up a deposit of limestone with a texture consisting of shells and shell fragments. These particles may then be cemented together as more calcite precipitates between the grains. This type of limestone, composed mostly of skeletal debris, can be several hundred meters thick and can extend over thousands of square kilometers.

• Chalk is a skeletal limestone in which the skeletal fragments are remains of microscopic plants and animals.

Oolitic Limestone

Other limestones are composed of small semi spherical grains of calcium carbonate known as oolites. Oolites form where small fragments of shells or other tiny grains become coated with successive thin layers of CaCO3 as they are rolled along the seafloor by waves and currents.

Microcrystalline limestone

A third important type of limestone forms in quiet waters where calcium carbonate is precipitated by algae as tiny, needle like crystals that accumulate on the seafloor as limy mud. Soon after deposition, the grains commonly are modified by compaction and recrystallization.

• Some kinds of algae produce calcium carbonate particles that accumulate to form limestone.These are found near the Kuril Islands of the north Pacific.
• Diatoms are the shells of tiny single-celled algae that are made of silica. Some deepmarine sediments are dominated by diatoms. Some accumulations convert to chert.

Dolostone / Dolomite

Dolostone or dolomite rock is a sedimentary carbonate rock that contains a high percentage of the mineral dolomite. Dolomite is a carbonate mineral composed of calcium magnesium carbonate CaMg(CO3)2. It is similar to limestone in general appearance, but reacts with acid only when powdered. Dolostone is commonly dull brownish yellow or light gray.

Chert

Chert is a common rock composed of microcrystalline quartz. In a hand specimen, it is hard, dense, and typically breaks like glass, but under a high-power microscope, it has a fibrous or granular texture. A distinctive type of deep-marine chert develops from deposits of siliceous shells of microscopic organisms, such as radiolaria and diatoms.

Rock salt

Rock salt is made of the mineral halite (NaCl). It crystallizes when evaporation concentrates sodium and chlorine ions to the point that salt is stable in the residual brine. Strong evaporation creates saline lakes in closed desert basins (for example, the Great Salt Lake and the Dead Sea). Enhanced evaporation also occurs in restricted bays along the shore of the ocean.

Gypsum

Gypsum, CaSO₄•2H₂O too originates from evaporation. It collects in layers as calcium sulphate is precipitated from water.

Hydrocarbons

Coal is an important biochemical precipitate. It forms by the decomposition of organic material buried within sedimentary rocks. Lush vegetation may form in an ancient swamp and then be converted by burial into coal. The coal beds on the left are interlayered with sandstone.

The accumulation of partially decayed vegetation is called Peat.  Peat is a compound of hydrogen, carbon, and oxygen. They formed from plant remains that built up over millions of years and were compacted under thick layers of inorganic clastic sediment. Hydrocarbons can be solid (peat and coal), liquid (petroleum), or gas (natural gas). Coal is the only hydrocarbon that is a rock. We often find natural gas and petroleum in open interconnected pores in a thick sedimentary rock layer, such as in porous sandstone.