How is radioactive decay used to date sedimentary rocks?

Sedimentary rocks can be dated using radioactive carbon, but since carbon decays  pretty fast, this only works for rocks younger than around 50 thousand years. So to date most older fossils, scientists look for layers of igneous rock or volcanic ash above and below the fossil. Hope that helps.

By measuring the amounts of nitrogen and carbon-14 in the sedimentary rocks.

Explanation:

Distinct radioactive isotopes exhibit distinct half-lives, that is, half of the total time consumed for half of the total isotopes in the substance to decay. Thus, in order to find the age of the sedimentary rock in the radiometric dating, there is a need to find the initial amount of the isotopes (in the given case of nitrogen and carbon-14), and then that decayed one is predicted. The determination of the half-lives of the elements is then utilized to find the age of the fossil or the rock.

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