Chapter 9. Sedimentary Rocks

Adapted by Karla Panchuk from Physical Geology by Steven Earle

Figure 9.1 Cretaceous sedimentary rocks exposed along a road near Drumheller, Alberta, Canada. Sedimentary rocks form in layers called beds, and the planar boundaries that separate each bed are called contacts. Each bed tells a story about the conditions in which it formed. In this picture the beds are indicating that sea level repeatedly rose and fell. The black layer about halfway up the picture is a coal seam. It tells us that the environment at that time was swampy. Source: Karla Panchuk (2008) CC BY 4.0

Learning Objectives

After reading this chapter and answering the review questions at the end, you should be able to:

  • Explain the differences between the four kinds of sedimentary rocks: clastic, chemical, biochemical, and organic.
  • Describe some of the specific kinds of rocks in each of the four categories, and the depositional environments in which they form.
  • Describe the various terrestrial and marine sedimentary depositional environments, and explain how the formation of sedimentary basins is related to plate tectonic processes.
  • Apply your understanding of the features of sedimentary rocks, including grain characteristics, sedimentary structures, and fossils, to the interpretation of past depositional environments and climates.
  • Explain what groups, formations, and members are in sedimentary rocks, and why such terminology is used.

Sedimentary Rocks Form From the Products of Weathering and Erosion

Weathering and erosion (Chapter 8) are the first two steps in the transformation of pre-existing rocks into sedimentary rocks. The remaining steps in the formation of sedimentary rocks are transportation, deposition, burial, and lithification. These steps are shown on the right-hand side of the rock cycle diagram in Figure 9.2.

Figure 9.2 The rock cycle. Processes related to sedimentary rocks are shown on the right-hand side. Source: Steven Earle (2015) CC BY 4.0 view source

Transportation is the movement of sediments or dissolved ions from the site of erosion to a site of deposition. This can be by wind, flowing water, glacial ice, or mass movement down a slope. Deposition takes place where the conditions change enough so that the sediments can no longer be transported. This could happen if the current slows down.

Burial occurs when sediments are deposited upon existing sediments, covering and compacting them. Lithification is what happens when those compacted sediments become cemented together to form solid sedimentary rock. Lithification occurs at depths of hundreds to thousands of metres within Earth.

Four Types of Sedimentary Rocks

Sedimentary rocks can be divided into four main types: clastic, chemical, biochemical, and organic. Clastic sedimentary rocks are composed mainly of material that is transported as solid fragments (called clasts), and then cemented together by minerals that precipitated from solution. Chemical sedimentary rocks are composed mainly of material that is transported as ions in solution. Biochemical sedimentary rocks also form from ions in solution, but organisms play an important role in converting those ions into calcium carbonate or silica body parts. Organic sedimentary rocks contain large amounts of organic matter, such as from plant leaves and tree bark.