The topics covered in this chapter can be summarized as follows:
9.1 Clastic Sedimentary Rocks
Sedimentary clasts are classified based on their size, and variations in clast size have important implications for transportation and deposition. Clastic sedimentary rocks range from conglomerate to mudstone. Clast size, sorting, composition, and shape are important features that allow us to differentiate clastic rocks and understand the processes that took place during their deposition.
9.2 Chemical and Biochemical Sedimentary Rocks
Chemical and biochemical sedimentary rocks form from ions that were transported in solution, and then converted into minerals by chemical and/or biological processes. The most common chemical rock, limestone, typically forms in shallow tropical marine environments, where biological activity is a very important factor. Chert and banded iron formation are deep-ocean sedimentary rocks. Evaporites form where the water of lakes and inland seas becomes supersaturated due to evaporation.
9.3 Organic Sedimentary Rocks
Organic sedimentary rocks contain abundant organic carbon molecules (molecules with carbon-hydrogen bonds). An example is coal, which forms when dead plant material is preserved in stagnant swamp water, and later compressed and heated.
9.4 Depositional Environments and Sedimentary Basins
There is a wide range of depositional environments, both on land (glaciers, lakes, rivers, etc.) and in the ocean (deltas, reefs, shelves, and the deep-ocean floor). In order to be preserved, sediments must accumulate in long-lasting sedimentary basins, most of which form through plate-tectonic processes.
9.5 Sedimentary Structures and Fossils
The deposition of sedimentary rocks takes place according to a series of important principles, including original horizontality, superposition, and faunal succession. Sedimentary rocks can also have distinctive structures that are important in determining their depositional environments. Fossils are useful for determining the age of a rock, the depositional environment, and the climate at the time of deposition.
9.6 Groups, Formations, and Members
Sedimentary sequences are classified into groups, formations, and members so that they can be referred to easily and without confusion.
Questions for Review
- What are the minimum and maximum sizes of sand grains?
- The material that makes up a rock such as conglomerate cannot be deposited by a slow-flowing river. Why not?
- Describe the two main processes of lithification.
- What is the difference between a lithic arenite and a lithic wacke?
- How does a feldspathic arenite differ from a quartz arenite?
- What can we say about the source area lithology and the weathering and transportation history of a sandstone that is primarily composed of rounded quartz grains?
- What is the original source of the carbon that is present within carbonate deposits such as limestone?
- What long-term environmental change on Earth led to the deposition of banded iron formations?
- Name two important terrestrial depositional environments and two important marine ones.
- What is the origin of a foreland basin, and how does it differ from a forearc basin?
- Explain the origin of (a) bedding, (b) cross-bedding, (c) graded bedding, and (d) mud cracks.
- Under what conditions is reverse graded bedding likely to form?
- What are the criteria for the application of a formation name to a series of sedimentary rocks?
- Explain why some of the Nanaimo Group formations have been divided into members, while others have not.