Clastic sedimentary rocks are formed by compaction and cementation of clasts composed of individual mineral grains or pieces of rock. Because their mineralogy varies so much, we generally classify clastic rocks based on grain size rather than composition. Grain size varies from huge clasts and boulders in gravels and conglomerates, to fine “clay size” (<0.004 mm) particles in muds and shales. The table below gives a standard classification scheme for clastic rocks based on clast/grain size. See section 7.2 (above), for more specifics about grain sizes.
|Classification of Clastic Rocks Based on Clast Size|
|clast size||sedimentary rock||major component of unconsolidated sediment||clast sizes|
|very fine||mudrocks||mudstone||claystone||clay||more than 60% clay-sized|
|siltstone||silt, generally quartz||more than 60% clay-sized|
|shale||clay||clay- and silt-sized|
|small to medium||sandstone||sand, commonly quartz||sand|
|gravel or coarser sediment||pebble|
|gravel or coarser sediment||cobble|
|gravel or coarser sediment||boulder|
In the coarsest clastic rocks, a fine-grained matrix separates large clasts that may be of many different compositions. If the clasts are angular, the rock is a breccia; if they are rounded, it is a conglomerate. Lithic fragments generally dominate the clasts in conglomerate and breccia. Figures 7.65 and 7.66 show a conglomerate and a breccia; both are from Death Valley National Park.
Rocks containing small- to medium-sized grains are generally called sandstone. The photos below in Figures 7.67, 7.68, and 7.69 show three examples. Most sandstones contain sand-sized (0.062 to 2 mm in longest dimension) quartz or feldspar grains. Coarser sandstones may contain both lithic fragments and individual detrital mineral grains. Sometimes clays or other minerals are present in a matrix between larger grains. We call a sandstone that is entirely, or almost entirely made of quartz, an arenite. The rock in Figure 7.67 is an example. We call sandstones containing significant amounts of matrix material or lithic fragments, such as the rock seen in Figure 7.68, wackes, or graywackes. Arkoses, like the one seen in Figure 7.69, are sandstones that contain significant amounts of K-feldspar.
Conglomerates and sandstones together account for 20% to 25% of all sedimentary rocks. Finer-grained clastic rocks, generally termed mudrocks, are much more common. These fine-grained rocks consist of microscopic (<0.062 mm in longest dimension) clay and quartz grains. Lithic fragments are absent.
We call the finest grained rocks shale if they exhibit fissility. Fissility, the ability to cleave into very thin layers, results from parallel alignment of clay grains. This photo (Figure 7.70) shows thin pieces of the Marcellus Shale from Pennsylvania.
We call very fine-grained rocks that are not fissile mudstones. We further divide mudstones into claystones or siltstones depending if they are clay-rich or quartz-rich, respectively.
|Minerals in Mudrocks and Sandstones|
Since clastic sediments can be derived from any preexisting rocks, they may contain a variety of minerals and rock fragments. However, only a few minerals are common. The table compares the most common minerals in mudrocks and sandstones. Clays, with subordinate quartz, dominate the mudrocks, but quartz dominates the sandstones.
Quartz, feldspar, and lithic fragments containing quartz and feldspar comprise all but the finest grained rocks because they are resistant to weathering. We call rocks dominated by quartz quartzose, while we term those containing large amounts of feldspar feldspathic, or (if they are rich in K-feldspar) arkosic. In mudstones and shales, clay minerals dominate. In all these rocks, other minerals, including micas, magnetite, rutile, ilmenite, sphene, zircon, apatite, or garnet may occasionally be significant components. Carbonate grains and organic material may be present as well. Because most minerals break down to clay and quartz if exposed at the Earth surface for sufficient time, it should be no surprise that mudstone and shale, which are composed primarily of clay and quartz, are the most abundant sedimentary rocks.
In the coarser-grained clastic rocks, the compositions of lithic fragments give clues to the origin of the sediment. In the finer-grained rocks, mineralogical composition is often difficult to determine and interpret. Suppose we look at a sandstone with a hand lens (Figure 7.71). Quartz, and perhaps K-feldspar, may be easy to find. However, lithic fragments can be confusing and small grains of chert or magnetite may be impossible to identify. For fine sandstones and finer-grained rocks, mineral identification can be problematic, even with a petrographic microscope. Distinguishing quartz from feldspar and telling clay minerals apart may be impossible without using an X-ray diffractometer or other sophisticated equipment.