Sedimentary rocks are rocks which are formed as a result of sedimentation of material over the course of time - either on the land or within water bodies on the planet. Sedimentary rocks are second most common of the three types of rocks found on the planet; while igneous rocks happen to be the most common rock type. On the basis of their composition, sedimentary rocks are categorized into three different types - 'clastic sedimentary rocks', 'chemical sedimentary rocks' and 'organic sedimentary rocks'.
What are Organic Sedimentary Rocks?
As their name suggests, these rocks are sedimentary rocks which are formed as a result of layer by layer sedimentation of organic matter - in form of plant and animal remains, on the surface of the Earth. The organic matter in this case includes plant matter such as leaves, roots, grass, etc., as well as animal remains such as bones, teeth and shells. In geological sense, any sedimentary rock type that is characterized by presence of more than 3 percent of organic carbon is referred to as organic-rich sedimentary rock. It is the presence of this organic matter which gives these rocks their characteristic dark color. More importantly, most of the organic sedimentary rocks found on the planet are source rocks i.e., the rocks from which hydrocarbons are obtained.
How are these Rocks Formed?
The process of formation begins with the transportation of sediments in the form of organic matter by water, wind or glacier. When plants or animals die on land, their remains are transported to the water bodies by surface run-off or by glacier. On the other hand, leaves and other plant matter - which is relatively light in weight, is deposited to the sedimentation site by wind. Similarly, the remains of marine organisms - i.e., their bones, teeth, shells, etc., and aquatic plants are deposited at the sea floor or river bed when they die. As this organic matter continues to accumulate layer over layer, the layers at the bottom are subjected to immense pressure; and this pressure along with the prevailing temperature makes the matter compact and results in formation of these rocks.
Types of Organic Sedimentary Rocks
The organic matter - along with the pressure and temperature that the layer of sediments are subjected to, determines which type of organic sedimentary rock is formed. When planktons die, their calcium carbonate or silica rich shells start accumulating at the sea floor and form limestone, which is one of the best examples of organic sedimentary rocks, and chert. Similarly, layer by layer accumulation of dead plant matter first results in formation of peat, and eventually - when it is subjected to high degree of pressure and a lot of heat over a course of several thousand years, results in formation of coal. One also needs to take into consideration the fact that the formation of anthracite coal from peat is preceded by the formation of lignite and bituminous coal.
Organic sedimentary rocks are also important as they can give us a detailed account of what happened in the region from where they are obtained in the past. Though simple, the entire process takes several thousand years. In fact, the formation of coal which is being mined in various parts of the world today began millions of years ago. More importantly, the process of sedimentary rock formation is just a small part of the broad concept of rock cycle which describes the transition between the three major rock types on the Earth.