1.4.2 Sedimentary processes
Sedimentary grains are formed when the rocks at the Earth's surface are slowly broken up physically by exposure to wind and frost, and decomposed (chemically) by rainwater or biological action. These processes are collectively termed weathering. Once a rock has been broken up by weathering, the small rock fragments and individual mineral grains can be eroded from their place of origin by water, wind or glaciers and transported to be deposited elsewhere as roughly horizontal layers of sediment. The resulting sediment reflects the original rock types that were weathered, the efficiency of erosion and transport, the extents of chemical and physical degradation of the sediment grains during transport, and the conditions under which the grains were deposited from the transporting water, wind or ice. For example, sand-sized grains of quartz are one of the main constituents of sandstone, but those grains may have been transported by water in a river, carried by waves on a sea-shore, or blown around in hot desert sandstorms (to give just three possibilities). How might we distinguish which of the many possibilities is the most likely in any given case?
One approach is to use the size and shape of the grains in a sediment or sedimentary rock to reveal quite a lot about the origin of the sediment. For example, a vigorous river transports much larger grains than a gentle current in a lake, so the size of the grains gives an indication of the strength of the currents that could have transported and deposited the grains. In other words, the grain size depends on the energy of the environment in which the sediment was deposited. The general shape of the grains will tell you about the nature of the transporting medium; for example, was it water or air? (See Box 2, A story in a grain of sand).
Are all sediments composed of fragments of rock and minerals eroded from preexisting rocks?
No, some sedimentary rocks also contain the remains of dead organisms, i.e. fossils of plants or animals that were living at the time the sedimentary material was deposited.
Any record of ancient life preserved in a rock is known as a fossil – sometimes fossils are rare, whereas a few rocks are composed of virtually nothing else but fossils. In particular, many limestones were formed by the accumulation of the calcite (calcium carbonate – CaCO3) shells and skeletons of certain marine organisms. Chalk is a well-known type of limestone that outcrops extensively across southern England; it is almost pure calcite, and consists largely of minute calcite plates of countless planktonic algae (phytoplankton) fossils. Other limestones, such as those found in the Peak District of northern England, contain abundant fossils of reef-building corals. Another example of a biologically formed sedimentary rock is coal, which is formed from compressed layers of woody plants.
Fossils are important when reconstructing the geological past because they are records of the environment at the time and place the fossil organisms were living. For instance, limestones rich in corals typically indicate warm shallow seas – the conditions needed for a coral reef ecosystem to thrive.
It is important that as many lines of evidence as possible are used to give a consistent interpretation of a rock's origin. No single feature should be taken as unequivocally diagnostic. For instance, think of a desert sand, composed of well-rounded, red oxide-coated, well-sorted quartz grains. Now imagine that climatic conditions change and these sand grains are swept away by flowing rivers and re-deposited elsewhere. The new sand deposit will be produced by the action of flowing water, but the sand grains may retain many of the characteristics of wind-deposition inherited from their previous history. A misleading interpretation can be reached if other lines of evidence are ignored. Such supplementary evidence could come from any fossils in the rock and the nature of adjacent sedimentary layers. Getting all of the necessary information involves a mixture of making observations and measurements at rock exposures in the field as well as examination and analysis of samples in the laboratory.
Box 2 A story in a grain of sand
Quartz is a hard mineral that is very common in the Earth's continental crust. It is resistant to attack by chemicals and physically strong, so it tends to survive the weathering process that disaggregates and decomposes pre-existing rocks. Many sedimentary rocks contain grains of quartz. Quartz grains are recognisable by their glassy appearance (particularly on freshly broken surfaces) and lack of cleavage. Quartz is also hard enough to scratch steel.
Whether quartz grains are transported and deposited by moving air (by being blown around by the winds in a desert) or by moving water (in a river or in ocean currents) determines how rounded they become (Figure 8). The degree of rounding of quartz grains depends on the intensity and frequency with which grains collide with each other, and these factors depend on the environmental conditions. Air is less viscous than water, so windblown quartz grains collide more violently than quartz grains carried in water, which has a cushioning effect. Also, the wind speeds needed to move a sand grain of given size are higher than the speeds for flowing water. This means that collisions between grains will be much more energetic in air than in water, so the corners of windblown grains are readily knocked off, and the grains are usually very much more rounded (Figure 8b) than water-transported grains (Figure 8a).
Wind-blown sand deposited in desert environments differs from water-lain sand in another way. Quartz sand grains in a desert often have a coating of red or orange iron oxide. This red-orange coloration is typical of desert landscapes and it is derived from the insoluble ‘rusty’ residue from weathering of iron-rich minerals. Water-lain sand grains lack such an obvious coating.
The degree of sorting in a sediment is another useful method for distinguishing different types of depositional situation. Sorting is a measure of the range of grain sizes present in a sediment or sedimentary rock. A poorly-sorted sediment (Figure 9a) has a wide range of grain sizes as a result of rapid deposition, such as occurs during a storm. On the other hand, a well-sorted sediment has a narrow range of grain sizes (Figure 9c), and is the result of extensive reworking of a sediment by wind action in deserts, or wave action on beaches and in shallow shelf seas.