Here's why:
* Biogenic sedimentary rocks are formed from the accumulation of organic matter, such as the remains of plants and animals. These remains can include shells, bones, teeth, wood, and even microscopic organisms.
* Other types of sedimentary rocks, like clastic sedimentary rocks (sandstone, shale, conglomerate), chemical sedimentary rocks (limestone, rock salt), and evaporites (rock gypsum), are formed through different processes and do not involve organic matter.
Examples of biogenic sedimentary rocks:
* Coal: Formed from the accumulation and compression of plant matter.
* Chalk: Formed from the accumulation of microscopic marine organisms called coccolithophores.
* Limestone: Can be formed both chemically and biogenically. Biogenic limestone often contains the remains of marine organisms like corals, shells, and foraminifera.
So, while other sedimentary rocks may contain organic matter, only biogenic sedimentary rocks are formed *exclusively* from things that were once living.
