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Sedimentary Rock Classification

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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

Sedimentary rock sizes and features.
Rock Sediment Size Other Features
Conglomerate Large Rounded
Breccia Large Angular
Sandstone Sand-sized
Siltstone Silt-sized, smaller than sand
Shale Clay-sized, smallest

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 "Minerals" lessons, dissolved ions in fluids precipitate out of the fluid and settle out, just like the halite in Figure below.

The evaporite, halite, on a cobble from the Dead Sea, Israel.

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).

Fossils in a biochemical rock, limestone, in the Carmel Formation in Utah

Fossils in a biochemical rock, limestone, in the Carmel Formation in Utah.

Table below shows some common types of sedimentary rocks.

Common Sedimentary Rocks
Picture Rock Name Type of Sedimentary Rock
Conglomerate Clastic (fragments of non-organic sediments)
Breccia Clastic
Sandstone Clastic
Siltstone Clastic
Shale Clastic
Rock Salt Chemical precipitate
Rock Gypsum Chemical precipitate
Dolostone Chemical precipitate
Limestone Bioclastic (sediments from organic materials, or plant or animal remains)
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.

Practice

Use this resource to answer the questions that follow.

1. List the three types of sedimentary rocks.

2. List the characteristics of clastic rocks.

3. How do clastic rocks form?

4. Contrast conglomerates and breccia rocks.

5. What can be found in clastic rocks?

6. Explain the difference between layers and bands.

7. What can we learn from sedimentary rocks?

8. How do chemical rocks form?

9. What are bioclastic rocks?

10. List the two types of biocalstic rocks.

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?

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