10: Sedimentary Rocks

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Learning Objectives

After completing this chapter, you should be able to:

Describe how erosion and weathering relate to the formation of sedimentary rocks.
Identify sedimentary rocks and their features.
Describe the formation and history of different types of sedimentary rocks.
Describe the formation of different sedimentary structures.
Discuss the distribution of sedimentary rocks, sedimentary structures, and types of weathering in varying sedimentary environments.

10.1: Introduction
We are particularly interested in the history and events that occur on the surface of the Earth both because it is easier to directly observe and test, and has direct relevance to our lives and our own history. Sedimentary rocks are the pages in which history is written since they contain powerful environmental indicators, traces of life, and chemical signatures that can inform us about a wealth of subjects from the occurrence of ancient catastrophes to the productivity of life.
10.2: Weathering and Erosion
Sedimentary rocks are formed by the weathering, erosion, deposition, and lithification of sediments. Basically, sedimentary rocks are composed of the broken pieces of other rocks. The obvious place to start this chapter is a discussion of how rocks are broken down, which is a process called weathering. There are two basic ways that weathering occurs in nature, mechanical and chemical.
10.3: Identifying Sedimentary Rocks
The classification of sedimentary rocks is largely based on differentiating the processes that lead to their formation. The biggest division in types of sedimentary rocks types is based on the primary type of weathering that leads to the material building the sedimentary rock. If the rock is largely made from broken pieces (called clasts) of rock that have been mechanically weathered the rocks are referred to as Detrital or Clastic Sedimentary Rocks.
10.4: Lab Exercise (Part A)
Start identifying the rocks by separating out the organic sedimentary rocks from the chemical and biological sedimentary rocks from the clastic sedimentary rocks. Make sure to use all of the tools available including the glass plate and the diluted HCl to identify the chemical and biochemical sedimentary rocks. The streak plate can be helpful in identifying coal, which will produce a dark gray streak. Finally, use the hand lens to examine the size of the grains in the clastic sedimentary rocks.
10.5: Sedimentary Structures
Sedimentary rocks often show distinctive patterns that are unrelated to their type of rock, yet reflect events or conditions during deposition and are called sedimentary structures. These patterns in the rocks can be very informative to geologists attempting to reconstruct the environment in which a sedimentary rock was formed. Imagine the wind blowing steadily along a beach; this wind pushes the sand into dunes that can be preserved in the rock record, informing us about the wind.
10.6: Depositional Environments
A depositional environment is the accumulation of chemical, biological, and physical properties and processes associated with the deposition of sediments that lead to a distinctive suite of sedimentary rocks. Sedimentary environments are interpreted by geologists based on clues within such as rock types, sedimentary structures, trace fossils, and fossils. We can then compare these clues to modern environments to reconstruct ancient environments.
10.7: Lab Exercise (Part B)
The following exercises use Google Earth to explore the depositional environments that are the source for sedimentary rocks. For each locality think about the types of sediments that are accumulating, the types of weathering that would occur, as well as the presence of any sedimentary structures.
10.8: Student Responses
The following is a summary of the questions in this lab for ease in submitting answers online.

Thumbnail: Limestone is a sedimentary rock that is often composed of the skeletal fragments of marine organisms. Much of the pyramid in Egypt were constructed with limestonre including the pyramids at Giza as shown from the plateau to the south of the complex. From left to right, the three largest are: the Pyramid of Menkaure, the Pyramid of Khafre and the Great Pyramid of Khufu. The three smaller pyramids in the foreground are subsidiary structures associated with Menkaure's pyramid. (CC BY-SA 2.0; Ricardo Liberato via wikipedia).