4.12 Sedimentary Rock Classification

How do you know that this is a sedimentary rock?

If you look closely at the rock you will see that it is made of sand-sized particles that have been lithified to create sandstone. The rock is eroding into very unique shapes, but these shapes are more likely to form from a rock made of small cemented together grains than from an igneous or metamorphic rock.

Types of Sedimentary Rocks

When sediments settle out of calmer water, they form horizontal layers. One layer is deposited first, and another layer is deposited on top of it. So each layer is younger than the layer beneath it. When the sediments harden, the layers are preserved. Sedimentary rocks formed by the crystallization of chemical precipitates are called chemical sedimentary rocks. As discussed in the concepts on minerals, dissolved ions in fluids precipitate out of the fluid and settle out, just like the halite in Figure below.

Biochemical sedimentary rocks form in the ocean or a salt lake. Living creatures remove ions, such as calcium, magnesium, and potassium, from the water to make shells or soft tissue. When the organism dies, it sinks to the ocean floor to become a biochemical sediment, which may then become compacted and cemented into solid rock (Figure below).

Table below shows some common types of sedimentary rocks.

Common Sedimentary Rocks

Picture	Rock Name	Type of Sedimentary Rock
[Figure 4]	Conglomerate	Clastic (fragments of non-organic sediments)
[Figure 5]	Breccia	Clastic
[Figure 6]	Sandstone	Clastic
[Figure 7]	Siltstone	Clastic
[Figure 8]	Shale	Clastic
[Figure 9]	Rock Salt	Chemical precipitate
[Figure 10]	Rock Gypsum	Chemical precipitate
[Figure 11]	Dolostone	Chemical precipitate
[Figure 12]	Limestone	Bioclastic (sediments from organic materials, or plant or animal remains)
[Figure 13]	Coal	Organic

Summary

• Sediments settle out of water in horizontal layers.
• Sedimentary rocks are classified based on how they form and on the size of the sediments, if they are clastic.
• Clastic sedimentary rocks are formed from rock fragments, or clasts; chemical sedimentary rocks precipitate from fluids; and biochemical sedimentary rocks form as precipitation from living organisms.

Review

1. How does an organism become a sedimentary rock?
2. How do chemical sedimentary rocks differ from clastic sedimentary rocks?
3. What are the different sedimentary rock types based on grain size, from small to large?

Explore More

Use this resource to answer the questions that follow.

1. What are the three types of sedimentary rocks.
2. What is the first type he talks about and what are they made of?
3. Name and describe the first type of clastic sedimentary rocks that he talks about.
4. Name and describe the second type of clastic sedimentary rocks that he talks about.
5. Name and describe the third type of clastic sedimentary rocks that he talks about.
6. What is the 2nd type of sedimentary rock that he talks about? How does he make one of these types of rocks?
7. What are two other chemical sedimentary rocks?
8. What is the third type of sedimentary rock? What is their feature?
9. How does lignite form? How about bituminous and anthracite coal?
10. What is limestone that is formed by marine skeletons or coral called? Although it is chemical, it is also what?