How Clastic Sediments Become Sedimentary Rocks
Lithification is the term used to describe the process of turning sediments into solid rock. The steps in lithification are summarized in Figure 9.3.
First, 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 between grains. The spaces often contain water, so the water is squeezed out. Forcing the grains together like this is called compaction.
Cementation is the next step. Groundwater flowing through the pore spaces contains ions, and these ions precipitate, leaving behind minerals on the surfaces of the grains. The minerals can fill in the spaces between the grains, and accumulate where two grains are touching. Over time the minerals (called cement) bind the grains together. 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 sandstone viewed under a microscope. The grains are all quartz but they appear different shades of grey because they are being viewed through polarized light. It is 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 in the left.
Something interesting about the sandstone in Figure 9.4 is that the cement is homoaxial. Notice the cement boundary marked by the red line in the image on the right. If you look closely you can see that it has regular steps in it. This is because the quartz in the cement has “discovered” the crystal structure of the quartz grain that it is forming around, and is continuing to build on it.
Types of Clastic Sedimentary Rocks
Clastic sedimentary rocks are named according to the characteristics of clasts (rock and mineral fragments) that make them up. Those characteristics include grain size, shape, and sorting. (To review grain sizes and the names for particles of different sizes, see Table 8.1. To review grain shape and sorting, see Figure 8.16.) The different types of clastic sedimentary rocks are summarized in Figure 9.5.
Coarse-Grained Clastic Rocks Are Conglomerate or Breccia
Clastic sedimentary rocks in which a significant proportion of the clasts are larger than 2 mm are known as conglomerate if the clasts are well rounded, and breccia if they are angular (top row of Figure 9.5). 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 in water, such as alluvial fans and talus slopes.
Medium-Grained Clastic Rocks Are Sandstone
Sandstone (middle row of Figure 9.5) 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 are 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 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 fragments) then the rock suffers from a surplus of names. It can be called feldspathic arenite, arkosic arenite, or just arkose. If the rock has more than 10% rock fragments, and more rock fragments than feldspar, it is lithic arenite.
Wacke is a not-so-clean sandstone, with more than 15% 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, giving quartz wacke, feldspathic wacke, and lithic wacke. Another name for a lithic wacke is greywacke.
Figure 9.7 shows thin sections (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.
Fine-Grained Clastic Rocks Are Mudrocks
Rock composed of at least 75% silt- and clay-sized fragments is called mudrock. If a mudrock shows evidence of fine layers (laminations) it is called shale, otherwise it is siltstone, mudstone, or claystone, in order of increasing abundance of clay-sized particles. The fine-grained nature of mudrocks tells us that they form in very low energy environments, such as lakes, river backwaters, and the deep ocean.
Exercise 9.1 Classifying Sandstones
The images below are magnified thin sections of sandstones. Using Figures 9.5 and 9.6, give the appropriate name for each rock.
Clastic sediments are deposited in a wide range of environments, including glaciers, slope failures, rivers both fast and slow, lakes, deltas, and ocean environments both shallow and deep. Depending on the grain size in particular, they may eventually form into rocks ranging from fine mudstone to coarse breccia and conglomerate. By examining clastic sedimentary rocks for key features it is possible to translate the classification you’ve just learned into an interpretation of the environment in which the rocks were deposited.
Maturity in sediments refers to the extent to which sediment characteristics reflect prolonged transport and weathering. Prolonged weathering and transport cause clasts to become smaller, rounder, and better sorted. It removes minerals which are more susceptible to weathering, such as feldspar and clay, leaving a sediment consisting predominantly of quartz. On the spectrum of sediment maturity, quartz sandstone would be a mature sedimentary rock, and wacke would be an immature one.
- Pore spaces are the gaps between grains. They might contain air, water, or even hydrocarbons. ↵
- Polarized light interacts with the crystal structure in a mineral so that the light passing through the crystal will look different depending on how the crystal is oriented. If you were to rotate the slide in Figure 9.4 you would see the white grains turn black, and the black ones turn white. Different minerals respond in different ways, so this is a handy property for identifying minerals under the microscope. ↵
- “Lithic” means “rock.” Lithic clasts are rock fragments, as opposed to mineral fragments. ↵
- Thin sections are slivers of rock sliced thinly enough that light can pass through them, and they can be examined under a microscope. ↵