Chapter 9. Sedimentary Rocks

Introduction

Learning Objectives

After carefully reading this chapter, completing the exercises within it, and answering the 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 can be 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 they are used

Sedimentary Rocks Form From the Products of Weathering and Erosion

Cretaceous sedimentary rocks exposed along a road near Drumheller, Alberta, Canada. Sedimentary rocks form in layers called beds. Each bed tells a story about the conditions in which it formed. In this picture the beds are telling about sea level rising and falling repeatedly. The black line about half way up the picture is a coal seam. It tells us that the environment was once swampy. [KP]

Figure 9.1 Cretaceous sedimentary rocks exposed along a road near Drumheller, Alberta, Canada. Sedimentary rocks form in layers called beds. Each bed tells a story about the conditions in which it formed. In this picture the beds are telling about sea level rising and falling repeatedly. The black line about half way up the picture is a coal seam. It tells us that the environment was once swampy. [KP]

In Chapter 8 we talked about weathering and erosion, which are the first two steps in transforming 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.

The rock cycle, showing the processes related to sedimentary rocks on the right-hand side. [SE]

Figure 9.2 The rock cycle, showing the processes related to sedimentary rocks on the right-hand side. [SE]

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 sediments being transported can no longer be transported. This could happen if the current slows down. Burial occurs when more sediments are piled onto existing sediments, and layers formed earlier are covered and compacted. Lithification is what happens when those compacted sediments become cemented together to form solid sedimentary rock. Lithification happens at depths of hundreds to thousands of metres within the Earth.

There Are Four Types of Sedimentary Rocks

In this textbook we divide sedimentary rocks into four main types: clastic, chemical, biochemical, and organic. Clastic sedimentary rocks are composed mainly of material that has been transported as solid fragments (clasts), and then cemented together by minerals that form from ions. Chemical sedimentary rocks are composed mainly of material that has been transported as ions in solution. Biochemical sedimentary rocks also form from ions in solution, but organisms play an important role in turning 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.

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