Earth Materials Introduction to Mineralogy and Petrology

In the previous chapter, we looked at how sediment is formed, transported, deposited, and lithified to form rock. In this chapter, we discuss the classification of sedimentary rocks, which fall into three main groups: (1) the siliciclastic sedimentary rocks, which include mudrocks (shale), sandstones, and conglomerates; (2) the biogenic sedimentary rocks, which include limestones, dolostones, cherts, and coals; and (3) the chemical sedimentary rocks, which include evaporites, phosphorites, and iron-formations. We learn how each type can be identified and examine the textures of the most important types in photomicrographs of petrographic thin sections. Correct identification of sedimentary rocks is the first step to working out the geologic history of an area, because these rocks preserve a record of the source of sediment, its mode of transport, and its site of deposition, which in many cases is determined by plate tectonic processes. We examine the geologic record preserved in some specific sedimentary rock sequences. Sedimentary rocks are the source of all fossil fuels, most iron ore, material used for fertilizers, much of the world's gold ore, and many other placer mineral deposits. Most of the world's construction material comes from sediments and sedimentary rocks in the form of building stones and raw material (lime and aggregate) for making cement. Porous sedimentary rocks form important aquifers both for agricultural use and for sustaining urban areas. In this chapter, we indicate with which sedimentary rocks each of these economically important resources is associated. We also discuss the plate tectonic setting in which each sedimentary rock is formed.