9.1: Clastic Sedimentary Rocks

How Clastic Sediments Become Sedimentary Rocks

Lithification (Figure 9.3) is the process of converting sediments into solid rock. Compaction is the first step. Sediments that have been deposited are buried when more and more sediments accumulate above them. The weight of the overlying sediments pushes the clasts together, closing up some of the pore spaces (the gaps between grains) and forcing them together. Pore spaces often contain water (although they can also contain air or even hydrocarbons), so the water is squeezed out.

Figure 9.3 Lithification turns sediments into solid rock. Lithification involves the compaction of sediments and then the cementation of grains by minerals that precipitate from groundwater in the spaces between these grains. Source: Karla Panchuk (2016), CC BY 4.0.

Cementation is the next step. Groundwater flowing through the remaining pore spaces contains ions, and these ions may precipitate, leaving behind minerals in the pore spaces. These minerals bind the grains together, and are referred to collectively as cement. Quartz and calcite are common cement minerals, but depending on pressure, temperature, and chemical conditions, cement might also include other minerals such as hematite and clay.

Figure 9.4 shows cemented grains in sandstone viewed under a microscope. The grains are all quartz but they appear different shades of grey because they’re being viewed through cross-polarized light. It’s difficult to tell the grains from the cement in this case because both are made of quartz, but in the image on the right the more obvious grain boundaries are marked with dashed lines. Some of the cement is marked with blue shading. Using the image on the right, see if you can pick out the grain boundaries in the image on the left.

Figure 9.4 Sandstone under a microscope. Grains and cement are quartz. Left- Original image. Right- Visible grain boundaries are marked with dashed lines, and some of the cement is shaded in blue. Source: Karla Panchuk (2018), CC BY 4.0. Modified after Woudloper, Public Domain. View source.

Concept Check: Lithification

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Types of Clastic Sedimentary Rocks

Clastic sedimentary rocks are named according to the characteristics of clasts (rock and mineral fragments) that comprise them. These characteristics include grain size, shape, and sorting. The different types of clastic sedimentary rocks are summarized in Figure 9.5.

Figure 9.5 Types of clastic sedimentary rocks. Source: Karla Panchuk (2018), CC BY-NC-SA 4.0. Photos by James St. John and R. Weller/ Cochise College. Click for more attributions.

Coarse-Grained Clastic Rocks

Clastic sedimentary rocks with a significant fraction of clasts larger than 2 mm are known as conglomerate if the clasts are well rounded, or breccia if they are angular (Figure 9.5, top row). Conglomerates form in high-energy environments, such as fast-flowing rivers, where the particles can become rounded as they bump into each other while being carried along. Breccias typically form where the particles are not transported a significant distance, such as in alluvial fans and talus slopes.

Medium-Grained Clastic Rocks

Sandstone (Figure 9.5, middle row) is a very common sedimentary rock, and there are many different kinds of sandstone. It’s worth knowing something about the different types because they’re organized according to characteristics that are useful for the detective work of figuring out what conditions led to the formation of a particular sandstone. Broadly, sandstones can be divided into two groups: arenite and wacke (rhymes with tacky).

Arenite is “clean” sandstone consisting mostly of sand-sized grains and cement, with less than 15% of fine-grained silt and clay in the matrix (the material between the sand-sized grains). Arenites are subdivided according to what the sand-sized grains are made of (Figure 9.6). If 90% or more of the grains are quartz, then the sandstone is called a quartz arenite (also called a quartz sandstone). If more than 10% of the grains are feldspar and more of the grains are feldspar than fragments of other rocks (lithic^[1] fragments) then the sandstone is called an arkosic arenite, or just arkose. If the rock has more than 10% rock fragments, and more rock fragments than feldspar, it is lithic arenite.

Figure 9.6 A compositional triangle for arenite sandstones, with the three most common components of sand-sized grains: quartz, feldspar, and rock fragments. Arenites have less than 15% silt or clay. Source: Steven Earle (2015), CC BY 4.0. View source.

Wacke is a “dirty” sandstone, containing 15-75% fine-grained particles (clay, silt) in its matrix. A wacke can have more fine-grained particles than cement in its matrix, making for a crumbly rock. Wackes are subdivided in the same way that arenites are: quartz wacke, feldspathic wacke, and lithic wacke. Another name for a lithic wacke is greywacke.

Figure 9.7 shows thin sections^[2] (microscopic views) of quartz arenite, arkose, and lithic wacke. In the images, quartz grains are marked Q, feldspar grains are marked F, and lithic fragments are marked L. Notice the relative abundances of each component in the three types of rocks.

Figure 9.7 Photos of thin sections of three types of sandstone. Some of the minerals are labelled: Q=quartz, F=feldspar and L= lithic (rock fragments). The quartz arenite and arkose have relatively little silt/clay matrix, while the lithic wacke has abundant matrix. Source: Steven Earle (2016), CC BY 4.0. View source.

Practice with Sandstone

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Now that you’re warmed up and have a completed diagram, use the diagram to help you identify the two samples in this next exercise:

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Fine-Grained Clastic Rocks

Rock composed of at least 75% silt- and clay-sized clasts is called mudrock (Figure 9.5, bottom row). If a mudrock shows evidence of fine layers (laminations) and breaks into sheets, it’s called shale. Otherwise, it’s siltstone (dominated by silt), mudstone (a mix of silt and clay), or claystone (dominated by clay). The fine-grained nature of mudrocks tells us that they form in very low energy environments, such as lakes, flood plains, and the deep ocean.

Clastic sediments are deposited in a wide range of environments, including from melting glaciers, slope failures, rivers (both fast and slow flowing), lakes, deltas, and ocean environments (both shallow and deep). Depending on the grain size in particular, they may eventually form into rocks ranging from mudstone to breccia and conglomerate. By examining clastic sedimentary rocks it is possible to translate the classification you have just learned into an interpretation of the environment in which the rocks were deposited.

Sediment Maturity

The concept of maturity is often used in discussions of sedimentary rocks, although it still refers to the maturity of the sediments themselves. Remember that a mature sediment is one that has become smaller, rounder, better sorted, and with the grains that are most resistant to physical and chemical weathering. On the spectrum of sediment maturity, quartz sandstone would be a mature sedimentary rock, and wacke or conglomerate would be immature rocks.

Practice with Sedimentary Rock Names

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If you can match sedimentary rock names to descriptions, you’re ready to try the next level and come up with the names themselves. This is more challenging, but can refer to your completed diagram above if you need help.

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1. “Lithic” means “rock.” Lithic clasts are rock fragments (multimineralic fragments), as opposed to single-mineral fragments. ↵
2. Thin sections are slivers of rock sliced thinly enough so that light can pass through them, and they can be examined under a microscope. ↵