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The Formation of Sedimentary Rocks

The observation of the rocks makes it possible to notice that some of them are made up of parallel and horizontal layers. They are sedimentary rocks; they present very varied aspects.

What are the reasons for their diversity? What is their origin ?

I. Formation process of a sedimentary rock

Sedimentary rocks are formed on the surface of the earth from particles of very varied origins.

1. The origin of particles

• The particles that make up sedimentary rocks can come from elements torn from the rocks by erosion, from the remains of living beings or from minerals present in solution (carbonate and calcium) and transforming into a solid body (precipitate) in special conditions.

2. Sedimentary deposits

• Particles are most often transported by water. However, the wind also moves them. They accumulate in the hollows of the relief or at the bottom of the water. Whatever the environment, marine, lacustrine (lakes), fluvial (rivers and rivers) or terrestrial (desert), all the particles end up being deposited in superimposed layers forming sedimentary deposits.

Sedimentary deposits therefore appear in the form of successive layers, the lowest layers corresponding to the oldest deposits.

Experiment showing the process of sedimentation and strata formation:

  1. In a beaker filled with water, pour a small amount of sand to form a first deposit. Observe sedimentation.
  2. Then pour a small amount of clay powder to form a second deposit.
  3. Then pour a small amount of sand again and observe the formation of the last stratum.

Related: How are Rocks Formed?

3. Transition from sediment to rock

• Sedimentary deposits, by their own weight, exert pressure on the particles. The layers compact (crush together) and stabilize. The water is driven out (dehydration). This set of actions called  diagenesis  gives materials a stratified (layered), compact and unified appearance. After this transformation the sedimentary deposit becomes a sedimentary rock.

• Sometimes the rock remains loose, this is the case with sand (it can flow between the fingers). But, most often, it is cemented under the effect of diagenesis, this is the case of limestones and sandstones.

II. The formation of detrital rocks (sands, clays and sandstones)

• Detrital rocks are sedimentary rocks. They may contain fossils. The materials that compose them come from the destruction of other rocks, mainly granites and gneisses. These particles (grains of variable size), torn by erosion, are carried by runoff water to the rivers. When the current decreases, they settle and form a sedimentation medium. The stack of these materials which compress under the effect of the pressure exerted by their weight constitutes a detrital rock.

1. The sands

The weathering (chemical transformation) and erosion of the pre-existing rock form a coarse sand or arena, composed of grains of quartz, feldspars and clay. Quartz grains are easily carried away by water or by the wind and their appearance depends on their mode of transport and the environment in which they are deposited.

2. Sandstones

• They are composed of quartz grains (the hardest constituent of the arena) transported by water without being degraded. These grains eventually settle. During diagenesis, water is driven out. Under the effect of the pressure exerted on the layers of deposits, a cement consisting of silica or calcium carbonate is formed. The quartz grains are thus linked to each other and form a rock called sandstone.

3. Clays

• Clay particles from the arena are easily transported by water or wind to the sea due to their fineness. They form sludge which, when dehydrated, gives a friable material.

III. The formation of limestone rocks

• The majority of sedimentary rocks are limestones composed mainly  of calcium carbonate . Calcium comes from the weathering of other rocks such as basalt or gneiss. Carbonates are present in solutions that have dissolved carbon dioxide gas.

• The rocks of the limestone massifs are altered by the chemical action of the carbon dioxide contained in the runoff water. Large quantities of limestone are thus dissolved. The minerals in solution are carried by the water towards the places of deposition.

The diversity of limestones (nature and appearance) comes from the conditions in which they were deposited, from the fossilized living beings that compose them but also from the chemical or biological origin of the deposits.

1. Deposits of chemical origin

• When water contains dissolved limestone in very large amounts, it may precipitate when carbon dioxide escapes. The limestone particles then become insoluble.

• Oolitic limestones are almost entirely composed of materials that come from the chemical precipitation of limestone. Their training conditions are very specific. In a warm sea (27 to 28°C), very shallow (less than 10 meters), agitated and rich in dissolved limestone, calcium carbonate precipitates. It is deposited around a central grain which it coats, forming concentric envelopes (called oolites). The oolites grow and then fall to the bottom of the sea. The oolitic sand ends up driving out the water and cementing itself.

In the basement of the sedimentary basins, geologists have found significant formations of oolitic limestone. The formation of this type of limestone can currently be observed on the Bahamas platform in the Atlantic Ocean.

2. Deposits of biological origin

• Many animals and plants that live in water use dissolved limestone to build their own skeletons. When they die, the calcareous skeletons they have produced accumulate in the sedimentary deposits. The fossils thus formed may constitute almost all of the limestone rock.

On distingue :

  • reef limestones containing mainly fossil corals.
  • shell limestone containing remains of shells of bivalve molluscs and gastropod molluscs.
  • the globigerine limestones composed of microscopic animals.

• The chalk is essentially made up of the accumulation of fossils. It is a very common limestone rock. It has properties that distinguish it from other limestones. It is friable, porous and permeable. The chalk is almost entirely made up of skeletons of microscopic algae called coccoliths. It is in a warm sea, rich in phytoplankton (plant plankton) and shallow (a few tens of meters) that chalk deposits are formed.

The rock also contains aligned flint kidneys. Flint is a very hard rock that appears when chalk is compacted. It is made of silica.

IV. The formation of a sedimentary basin

• For millions of years sediments have been deposited on the sea floor to form sedimentary basins.

• External factors such as climate or erosion have modified the sediment deposition conditions. The observation of a stratigraphic column (succession of sedimentary rocks represented in section) reveals many different sedimentary rocks which pile up one above the other. It can be concluded that different environments intervened successively. Marine and lacustrine deposits can follow one another and form stratigraphic columns of more than a thousand meters in places. The study of these columns makes it possible to reconstruct the conditions under which the rocks were formed.

In conclusion: the study of sedimentary rocks and their fossils makes it possible to reconstruct the climatic conditions in which the rocks were formed as well as the landscapes existing at the time.