Sedimentary Rock
Classification
Dana Desonie, Ph.D.
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Printed: August 20, 2013
AUTHOR
Dana Desonie, Ph.D.
www.ck12.org
C ONCEPT
Concept 1. Sedimentary Rock Classification
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Sedimentary Rock
Classification
• Describe how sedimentary rocks are classified.
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
TABLE 1.1: Sedimentary rock sizes and features.
Rock
Conglomerate
Breccia
Sandstone
Siltstone
Shale
Sediment Size
Large
Large
Sand-sized
Silt-sized, smaller than sand
Clay-sized, smallest
Other Features
Rounded
Angular
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 1.1.
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FIGURE 1.1
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 1.2).
FIGURE 1.2
Fossils in a biochemical rock, limestone,
in the Carmel Formation in Utah.
Table 1.2 shows some common types of sedimentary rocks.
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Concept 1. Sedimentary Rock Classification
TABLE 1.2: Common Sedimentary Rocks
Picture
Rock Name
Conglomerate
Type of Sedimentary Rock
Clastic (fragments of non-organic
sediments)
Breccia
Clastic
Sandstone
Clastic
Siltstone
Clastic
Shale
Clastic
Rock Salt
Chemical precipitate
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TABLE 1.2: (continued)
Picture
Rock Name
Rock Gypsum
Type of Sedimentary Rock
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.
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Concept 1. Sedimentary Rock Classification
MEDIA
Click image to the left for more content.
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?
References
1. Mark A. Wilson (Wilson44691). . Public Domain
2. Mark A. Wilson (Wilson44691). Fossils in a biochemical rock, limestone, in the Carmel Formation in Utah.
Public Domain
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