How are sedimentary rocks classified by geologists?

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chemistry of sedimentary rocks — sedimentary rock Figure 1: Chemical composition of sedimentary rocks.

Geologists classify sedimentary rocks into three main categories: terrigenous clastic rocks, carbonates, and noncarbonate chemical sedimentary rocks. Terrigenous clastic rocks are composed of fragments of pre-existing rocks and minerals, and are further subdivided by grain diameter into conglomerate, breccia, sandstone, siltstone, and mudrock. Carbonates, such as limestone and dolomite, are chemical sedimentary rocks formed from precipitated carbonate minerals. Noncarbonate chemical sedimentary rocks include less common types like cherts, phosphorites, evaporites, iron formations, and organic-rich deposits such as coal and oil shale. This classification is based on physical characteristics and composition, though no single scheme has achieved universal acceptance.12