Here's why:

* Clastic rocks are formed from fragments of other rocks. Chalk is not made from rock fragments.

* Chemical rocks are formed by precipitation from water. While some chemical processes are involved in the formation of chalk, it's primarily the accumulation of biological material.

* Biochemical rocks are formed from the accumulation of organic remains. Chalk is formed primarily from the accumulation of microscopic marine organisms called coccolithophores. These organisms have hard, calcium carbonate shells that accumulate on the ocean floor after they die. Over time, these shells are cemented together to form chalk.

So, the primary origin of chalk is the accumulation of the biochemical remains of coccolithophores.