Sedimentation in fossilization:
Sedimentary rocks, formed on or close to the Earthâ€™s surface, contain most of the common fossils. The fragmentation of pre-existing rock and accumulation of particles results in the formation of sediments. Climatic change or continent movement over time is evident by the presence of thousands of sedimentary layers.
The most common condition in fossilization is the presence of a dead organismâ€™s skeleton or shell (hard parts), followed by rapid burial.
Before the organism decays or disintegrates, it must settle in a position where it gets a good chance of being buried.
If it is unsuccessful, then aerobic bacteria start degrading the organismâ€™s soft tissue and leave the rubble, i.e., the exoskeleton.
The exoskeleton is then preserved by the steady supply of sediments from lakes, delta rivers, and earth parts below the sea level as it contains a high percentage of minerals
Sedimentation at the lake or sea bottom serves as an ideal place for fossilization as it protects the rubble from damage and rapidly covers it over a long time. It also helps to eliminate the predators and solvent water to protect the tissues and debris.
Steno’s law or the Law of Superposition says that the oldest sedimentary layer (bed) lies at the bottom and the newest one on the top in an undisturbed manner.
Sedimentary rock formation:
Sedimentation on or close to the Earthâ€™s surface due to compression or other geological processes creates three different types of rocks: (i) Sedimentary rocks, (ii) Igneous rocks, and (iii) Metamorphic rocks.
Weathering, erosion, precipitation, dissolution, and lithification are the geological processes that lead to the formation of a sedimentary rock near the surface of the Earth.
The raw materials for sedimentary rock formation are created by breaking down the large rocks into minor ones because of rain and wind. Weathering and erosion then convert the mountains and boulders into sand or mud sediments. Thirdly, chemical weathering makes the water slightly acidic and slowly removes the stone.
Two other geological processes, precipitation and lithification, build minerals or new rocks. Chemicals from the acidic water are precipitated out, which leads to the formation of minerals and rocks.
At last, sand, clay, and other sediments are compacted slowly into rocks due to the lithification process under the ocean or other water bodies.
Categories and structures of sedimentary rocks:
Sedimentary rocks are categorized into detrital, chemical, and organic rocks.
1. Detrital rock: This kind of rock is formed mechanically due to erosion and rock fragments accumulation. The sediments and organic matter from the decayed plants and animals are compressed over a long time and grouped as detritus. Inorganic detrital rocks, such as sandstone, are formed from the broken pieces of other rocks.
2. Chemical rock: This kind of rock is formed due to the precipitation of certain chemicals such as calcium carbonate and is found in caves, deserts, and oceans. Limestone, stalagmite, and stalactite are the different types of chemical rocks formed.
3. Organic rocks: Organic rocks are produced due to the accumulation of biological debris in bogs and swamps, as well as from an organismâ€™s remains.
The sedimentary structure provides information on how and where sedimentation occurs and provides the geologic history of an area. It is produced during the deposition of sediments or just after that. Bedding is the most common sedimentary structure that contains a series of layers visible within a rock. Crossbedding is a kind of sedimentary structure that is common in sandstones. It consists of a thin, inclined layer within a horizontal rock bed. Ripple marks are the sediment layers formed by moving water or wind.