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3.2.2.1. Sedimentary rocks

Sedimentary rocks

Sedimentary rocks are formed by the accumulation of sediments, materials of different sizes and composition that are transported by water, ice or wind, and that are subjected to physical and chemical processes that consolidate them (diagenesis), transforming them into a rock.

When the means of transport (water, ice, wind) does not have enough energy to transport the sediment, they are deposited on the bottom of seaslakes,... forming overlapping horizontal strata in areas called sedimentary basins.

Diagenesis is the set of processes by which unconsolidated sediments are transformed into sedimentary rock.

Sediments accumulate in sedimentation basins and, under their own weight, compaction occurs. The volume is reduced by loss of the pores that remain between the sediments, and the water leaves these spaces. When water is released, the salts that were dissolved in it can form crystals and cause cementationThus, the sediment is united by a cement that gives rise to sedimentary rock .

Sedimentary rocks are those that cover most of the earth's crust.

According to the process that has originated the rock and the types of sediments of which they are composed, several types of sedimentary rocks are distinguished, although this classification is not entirely correct because some rock can be classified into more than one group:

Detrital rocks

Detrital sedimentary rocks come from sediments that come from other rocks. The eroded rock fragments are transported by water, ice or wind, until they are deposited when the geological agent that transports them does not have enough energy. They are then compacted and give rise to detrital rock.

The structure of a detrital rock is formed by the following components:

  • Skeleton: formed by clasts or rock fragments.
  • Pasta: material that is between the clasts and joins them.
    • Matrix: formed by clasts much smaller than those that form the skeleton.
    • Cement: crystals formed by chemical precipitation in the pores.
  • Pores: holes that remain unoccupied inside the detrital rock.

Detrital sedimentary rocks are classified according to the size of the sediments that constitute them.

sediment type
detrital rock
> 2mm
gravels
conglomerate
2mm - 1/16mm
sands
sandstone
< 1/16mm
clays
shales

Thus, we distinguish the following detrital sedimentary rocks :

  • Conglomerates: They are rocks formed from large rock fragments (> 2 mm) that are called gravel. The conglomerates, if they have rounded clasts, are called puddings, and if they are angular, they are called breccias.
  • Sandstones: They are rocks formed from grains of sand (between 2 mm and 1/16 mm) which, although quartz predominates , may have another mineralogical composition. Sandstone is made up of compacted grains of sand joined by different types of cement, and its feel is rough.
  • Shales: These are rocks made up of very fine grains, clay-sizedwith a very soft touch. These grains have a varied composition, their diameter less than1/16 mm, and are joined by a generally calcareous cement. Since they have almost no pores, they are very waterproof.

Interactive Activity: Order the types of sediment by their size.

Interactive Activity: Order each type of rock according to the size of the sediment.

There is another type of rockhybrid rocks, such as marl, which are rocks made up of a mixture of clay and limestone in variable proportions. Therefore, they would be placed in an intermediate group between detrital rocks and rocks produced by chemical precipitation. (It has already been said that this classification was not entirely correct).

Rocks of chemical and biochemical origin

These rocks are not formed by fragments from other rocks, but by sediments from mineral salts or the remains of living beings.

Sedimentary rocks of chemical origin are formed by the accumulation of materials on the earth's surface as a result of some chemical reaction that causes dissolved salts in the water to precipitate to the bottom.

Rocks formed by non-evaporite precipitation

Carbonated rocks: limestone and dolomite

Carbonate rocks have more than 50% calcium carbonate or other carbonate minerals.

Although carbonate rocks can also be of detrital origin, they are produced by chemical reactions that cause calcium carbonate to precipitate.

The most abundant carbonated rocks are limestone (consisting of calcium carbonate) and dolomite (calcium and magnesium carbonate). They are easy to identify, since an effervescence reaction occurs when they come into contact with hydrochloric acid (or salfumán, its commercial name).

Many limestones are organic in origin, and may contain fossils that can sometimes make up the majority of the rock. The shells and shells of marine animals are made of calcium carbonate and, when they die, their remains accumulate at the bottom forming limestone rocks.

Siliceous rocks

Siliceous rocks, such as flint or agate, are formed by precipitation of silica (SiO2).

Siliceous rocks can also be formed by the accumulation of siliceous skeletal remains of some organisms such as diatoms, radiolarians and sponges.

Rocks formed by evaporite precipitation

Evaporitic rocks are formed by chemical precipitation from a brine. If the climate is arid, the surface waters evaporate and the salts precipitate, generating salt-type rocks.

Examples of evaporite rocks are gypsum and halite (or rock salt, common salt used in cooking).

Organogenic rocks

Organogenic rocks were formed from the remains of organisms that, when they died, accumulated at the bottom of a lake or the sea, were covered by other sediments, and the action of certain bacteria caused them to become enriched in carbon or hydrocarbons . Organogenic rocks have the ability to release energy when burned, which is why they are called fossil fuels.

Some examples of organogenic rocks are:

  • Coal: It is formed by the accumulation of plant remains in continental areas, in environments without oxygen (anaerobes), and bacteria cause them to become enriched in carbon. There are several types of coal, which, ordered from the lowest to the highest carbon content, are:
    • Peat.
    • Lignite.
    • Bituminous coal.
    • Anthracite coal.

El lignito es un tipo de carbón

Lignite is a type of coal

  • Petroleum: It is formed from the remains of marine plankton at the bottom of the oceans. Afterwards, the right conditions must be given (quiet and anaerobic environment), and they must be covered with sediment, so that the bacteria can act.

Petroleum is not a mineral because it does not meet the conditions seen in its definition. Yes, it is a rock, since it is a natural deposit that is part of the earth's crust, although in this case, its liquid state may seem strange to us.