Which one of the following sedimentary rocks is organically formed ?

Shale

Chert

Halite

Chalk

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This question was previously asked in

UPSC NDA-2 – 2019

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Sedimentary rocks are formed from the accumulation or deposition of mineral or organic particles. Organically formed sedimentary rocks are composed primarily of the remains of organisms (plants or animals). Chalk is a type of limestone composed mainly of the microscopic calcium carbonate shells (coccoliths) of marine algae called coccolithophores. Shale is a clastic sedimentary rock made of clay and silt. Chert is a hard, fine-grained rock composed of microcrystalline quartz, typically formed chemically or biochemically (though often grouped with chemical rocks). Halite (rock salt) is a chemical sedimentary rock formed by the evaporation of saline water. Therefore, Chalk is the only rock among the options that is definitively and primarily formed from organic remains.

– Organically formed sedimentary rocks are composed of organic remains.
– Chalk is made of fossilized microscopic marine organisms.
– Shale is clastic, Chert is chemical/biochemical, Halite is chemical.

Other examples of organically formed sedimentary rocks include coal (formed from accumulated plant remains) and some types of limestone composed of larger shell fragments or coral reefs. The classification of some rocks like Chert can be debated depending on the origin of the silica (chemical vs. biogenic). However, chalk is a classic example of an organically formed rock.

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