Classification of Siliciclastic Sediments and Sedimentary Rocks

We can now use our knowledge of sedimentary processes to classify sediments and their lithified counterparts, sedimentary rocks. As we have seen, the major divisions are the siliciclastic sediments and sedimentary rocks and the chemical and biological sediments and sedimentary rocks. Siliciclastic sediments and rocks constitute more than three-fourths of the total mass of all types of sediments and sedimentary rocks in Earth’s crust (Figure 5.18). We therefore begin with them.

Figure 5.18: The relative abundances of the major sedimentary rock types. In comparison with these three types, all other sedimentary rock types—including evaporites, cherts, and other chemical sedimentary rocks—exist in only minor amounts.

Siliciclastic sediments and rocks are categorized primarily by particle size (Table 5.3):

• Coarse-grained: gravel and conglomerate
• Medium-grained: sand and sandstone
• Fine-grained: silt and siltstone; mud, mudstone, and shale; clay and claystone

We classify siliciclastic sediments and rocks on the basis of their particle size because it distinguishes them by one of the most important conditions of sedimentation: current strength. As we have seen, the larger the particle, the stronger the current needed to transport and deposit it. This relationship between current strength and particle size is the reason like-sized particles tend to accumulate in sorted beds. In other words, most sand beds do not contain pebbles or mud, and most muds consist only of particles finer than sand.

Of the various types of siliciclastic sediments and sedimentary rocks, the fine-grained siliciclastics are by far the most abundant—about three times more common than the coarser-grained siliciclastics (see Figure 5.18). The abundance of the fine-grained siliciclastics, which contain large amounts of clay minerals, is due to the chemical weathering of the large quantities of feldspar and other silicate minerals in Earth’s crust into clay minerals. We turn now to a consideration of each of the three major classes of siliciclastic sediments and sedimentary rocks in more detail.

Coarse-Grained Siliciclastics: Gravel and Conglomerate

Gravel is the coarsest siliciclastic sediment, consisting of particles larger than 2 mm in diameter and including pebbles, cobbles, and boulders. Conglomerate is the lithified equivalent of gravel (Figure 5.19a). Pebbles, cobbles, and boulders are easy to study and identify because of their large size, which tells us the strength of the currents that transported them. In addition, their composition can tell us about the nature of the distant terrain where they were produced.

Figure 5.19: Examples of the three major classes of siliciclastic sedimentary rocks.

There are relatively few sedimentary environments—mountain streams, rocky beaches with high waves, and glacier meltwaters—in which currents are strong enough to transport gravel. Strong currents also carry sand, and we almost always find sand between gravel particles. Some of it is deposited with the gravel, and some infiltrates the spaces between particles after the gravel is deposited.

Medium-Grained Siliciclastics: Sand and Sandstone

Sand consists of medium-sized particles, ranging from 0.062 to 2 mm in diameter. These particles can be moved by moderate currents, such as those of rivers, waves at shorelines, and the winds that blow sand into dunes. Sand grains are large enough to be seen with the naked eye, and many of their features are easily discerned with a low-power magnifying glass. The lithified equivalent of sand is sandstone (Figure 5.19b).

Both groundwater geologists and petroleum geologists have a special interest in sandstones. Groundwater geologists study the origin of sandstones to predict possible supplies of water in areas of porous sandstone, such as those found in the western plains of North America. Petroleum geologists must understand the porosity and cementation of sandstones because much of the oil and natural gas discovered in the past 150 years has been found in buried sandstones. In addition, much of the uranium used for nuclear power plants and weapons has come from uranium deposits precipitated in sandstones.

Sizes and Shapes of Sand Grains

Medium-sized siliciclastic particles—sand grains—are subdivided into fine, medium, and coarse grains. The average size of the grains in any one sandstone can be an important clue to both the strength of the current that carried them and the sizes of the crystals eroded from the parent rock. The range of grain sizes and their relative abundances are also significant. If all the grains are close to the average size, the sand is well sorted. If many grains are much larger or smaller than the average, the sand is poorly sorted (see Figure 5.6). The degree of sorting can help us distinguish, for example, between sands deposited on beaches (which tend to be well sorted) and sands deposited by glaciers (which tend to be muddy and poorly sorted). The shapes of sand grains can also be important clues to their origin. Sand grains, like pebbles and cobbles, are abraded and rounded during transportation. Angular grains imply short transport distances; rounded ones indicate long journeys down a large river system (see Figure 5.7).

Mineralogy of Sands and Sandstones

Siliciclastics can be further subdivided by their mineralogy, which can help identify the parent rocks. Thus, there are quartz-rich sandstones and feldspar-rich sandstones. Some sands are bioclastics, rather than siliciclastics; they are formed from materials such as carbonate minerals that were originally precipitated as shells, but then broken up and transported by currents. Thus, the mineralogy of sands and sandstones indicates the source areas and materials that were eroded to produce the sand grains. Sodium- and potassium-rich feldspars with abundant quartz, for example, might indicate that the sediments were eroded from a granitic terrain. Other minerals, as we will see in Chapter 6, might indicate metamorphic parent rocks.

The mineral content of sands and sandstones also indicates the plate tectonic setting of the parent rock. Sandstones containing abundant fragments of mafic volcanic rock, for example, might indicate that the sand grains were derived from a volcanic mountain belt at a subduction zone.

Major Kinds of Sandstones

Sandstones can be divided into four major groups on the basis of their mineralogy and texture (Figure 5.20):

Figure 5.20: The mineralogy of four major groups of sandstones and the sedimentary environments where they are most likely to be found.

• Quartz arenites are made up almost entirely of quartz grains, usually well sorted and rounded. Pure quartz sands result from extensive weathering before and during transportation that removed everything but quartz, the most stable silicate mineral.
• Arkoses are more than 25 percent feldspar. Their grains tend to be more angular and less well sorted than those of quartz arenites. These feldspar-rich sandstones come from rapidly eroding granitic and metamorphic terrains where chemical weathering is subordinate to physical weathering.
• Lithic sandstones contain many particles derived from fine-grained rocks, mostly shales, volcanic rocks, and fine-grained metamorphic rocks.
• Graywacke is a heterogeneous mixture of rock fragments and angular grains of quartz and feldspar in which the sand grains are surrounded by a fine-grained clay matrix. Much of this matrix is formed from fragments of relatively soft rock, such as shale and some volcanic rocks, that are chemically altered and physically compacted after deep burial of the sandstone formation.

Fine-Grained Siliciclastics

The finest-grained siliciclastic sediments and sedimentary rocks are the silts and siltstones; the muds, mudstones, and shales; and the clays and claystones. All of them consist of particles that are less than 0.062 mm in diameter, but they vary widely in their ranges of grain sizes and in their mineral compositions. Fine-grained sediments are deposited by the gentlest currents, which allow the finest sediment particles to settle slowly to the bottom in quiet waves.

Silt and Siltstone

Siltstone is the lithified equivalent of silt, a siliciclastic sediment in which most of the grains are between 0.0039 and 0.062 mm in diameter. Siltstone looks similar to mudstone or very fine grained sandstone.

Mud, Mudstone, and Shale

Mud is a siliciclastic sediment containing water in which most of the particles are less than 0.062 mm in diameter. Thus, mud can be made of silt- or clay-sized sediment particles or varying quantities of both. The general term “mud” is very useful in fieldwork because it is often difficult to distinguish between silt- and clay-sized particles without a microscope.

Muds are deposited by rivers and tides. As a river recedes after flooding, the current slows, and mud, some of it containing abundant organic matter, settles on the floodplain. This mud contributes to the fertility of river floodplains. Muds are also left behind by ebbing tides along many tidal flats where wave action is mild. Much of the deep seafloor, where currents are weak or absent, is blanketed by muds.

The fine-grained rock equivalents of muds are mudstones and shales. Mudstones are blocky and show poor or no bedding. Distinct beds may have been present when the sediments were first deposited but then lost through bioturbation. Shales (Figure 5.19c) are composed of silt plus a significant component of clay, which causes them to break readily along bedding planes. Many muds contain more than 10 percent calcium carbonate sediments, forming calcareous mudstones and shales. Black, or organic, shales contain abundant organic matter. Some, called oil shales, contain large quantities of oily organic material, which makes them a potentially important source of oil.

Hydraulic fracturing, also known as “fracking,” is caused by the injection of highly pressurized fluids into shale. This creates new channels (fractures) in the rock, which link together tiny pores filled with oil and natural gas to create a flow of larger scale that is economically viable. The Marcellus Formation, found in the northeastern United States (see Figure 5.21), was named for Marcellus, New York. It is a unit of shale that had previously untapped natural gas reserves. In 2007, the Marcellus Shale was first drilled into, and using fracking methods, the extraction of natural gas became economically viable. The environmental impacts of fracking are debated, though, due to the effects of the chemicals used, the water supply, and the safety of drilling.

Figure 5.21: The Marcellus Formation, found in the northeastern United States, has previously untapped natural gas reserves. The shaded area of the map indicates the most economically promising parts of the Marcellus shale.

Clay and Claystone

Clay is the most abundant component of fine-grained sediments and sedimentary rocks and consists largely of clay minerals. Clay-sized particles are less than 0.0039 mm in diameter. Rocks made up exclusively of clay-sized particles are called claystones.