Fossilization Process and Four Major Types of Sedimentary Rocks

In this passage we will begin with fossilization process and end with four major types of sedimentary rocks. Fossils, among the three major types of rock, are most commonly found in sedimentary rock. Unlike most igneous and metamorphic rocks, sedimentary rocks form at temperatures and pressures that do not destroy fossil remnants.  Sedimentary rocks are formed from deposits of pre-existing rocks or pieces of once-living organism that accumulate on the Earth’s surface. If sediment is buried deeply, it becomes compacted and cemented, forming sedimentary rock.

Fossilization

Dead organisms in nature are usually quickly removed by scavengers, bacteria, rotting and erosion, but under exceptional circumstances, these natural processes are unable to take place, leading to fossilization. The chance of fossilization is higher when the sedimentation rate is high in anoxic environments or when the organism had a particularly hard skeleton. In this situation, carcass is quickly buried. Anoxic environments refer to places where little bacterial activity occurs. Larger, well-preserved fossils are relatively rare.

Organisms and Skeletons Fossils

Fossils can be both the direct remains or imprints of organisms and their skeletons. Most commonly preserved are the harder parts of organisms such as bones, shells, and the woody tissue of plants. Soft tissue has a much smaller chance of being fossilized, and the preservation of soft tissue of animals older than 40 million years is very rare. Imprints of organisms made while they were still alive are called trace fossils, examples of which are burrows, footprints, etc.

Carbonization

At high pressure and temperature, the organic material of a dead organism undergoes chemical reactions in which volatiles such as water and carbon dioxide are expulsed. The fossil, in the end, consists of a thin layer of pure carbon or its mineralized form, graphite. This form of fossilization is called carbonization. It is particularly important for plant fossils. The same process is responsible for the formation of fossil fuels like lignite or coal.

Four Major Types of Sedimentary Rocks

There are four major types of sedimentary rocks: Clastic Sedimentary Rocks, Chemical Sedimentary Rocks, Biochemical Sedimentary Rocks, and Organic Sedimentary Rocks.

Clastic Sedimentation

Rivers, oceans, winds, and rain runoff all have the ability to carry the particles washed off of eroding rocks. Such material, called detritus, consists of fragments of rocks and minerals. When the energy of the transporting current is not strong enough to carry these particles, the particles drop out in the process of sedimentation. This type of sedimentary deposition is referred to as clastic sedimentation.

Chemical Sedimentation

Another type of sedimentary deposition occurs when material is dissolved in water, and chemically precipitates from the water. This type of sedimentation is referred to as chemical sedimentation.

Biochemical Sedimentation

A third process can occur, wherein living organisms extract ions dissolved in water to make such things as shells and bones.  This type of sedimentation is called biochemical sedimentation. 

Organic Sedimentation

The accumulation of plant matter, such as at the bottom of a swamp, is referred to as organic sedimentation. 

To conclude the passage, sedimentary rock, rock formed at or near Earth’s surface by the accumulation and lithification of sediment that is detrital rock or by the precipitation from solution at normal surface temperatures that is chemical rock. Sedimentary rocks are the most common rocks exposed on Earth’s surface but are only a minor constituent of the entire crust, which is dominated by igneous and metamorphic rocks.

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Sources

en.wikipedia.org/wiki/Sedimentary_rock

www.britannica.com/science/sedimentary-rock 

www.nps.gov/subjects/geology/sedimentary.htm

www.tulane.edu/~sanelson/eens1110/sedrx.htm