Earth Materials: Sedimentary rock classification, occurrence, and plate tectonic significance

Sedimentary rocks are formed from the lithification of sediment (Fig. 11.1). They are classified into three main groups based on the type of sediment from which they are formed. Those formed from the detritus of weathered rocks are called siliciclastic because most of their minerals are silicates. Those formed from sediment derived from organisms are described as biogenic , and those formed by chemical precipitation are referred to as chemical sedimentary rocks . Each of these categories is subdivided on the basis of grain size, composition, or mode of deposition. Siliciclastic sedimentary rocks are divided into mudrocks , sandstones , and conglomerates on the basis of grain size. The biogenic rocks are divided into carbonates , cherts , and coals on the basis of composition. And chemical sedimentary rocks are subdivided into evaporites , carbonates , phosphorites , and banded iron formation on the basis of composition and mode of deposition. Each of these divisions is further subdivided into the main sedimentary rocks. Some rocks may belong to more than one of the main groups. For example, some limestones are formed from both biogenic sediment and chemical precipitates, and some organic-rich rocks such as oil shales contain considerable amounts of siliciclastic sediment. Despite these overlaps, the tripartite division provides a convenient framework in which to discuss the main types of sedimentary rock. In this chapter, we discuss the main types of sedimentary rocks, their depositional sites, and plate tectonic significance.