Sedimentary processes

Sediments are formed by the breakdown (both physical and chemical) of pre-existing rocks, which may be of igneous, metamorphic or sedimentary origin.

The main factors that control the breakdown of rocks into sediments are:

  • climate
  • topography
  • vegetation
  • properties (physical and chemical) of the rock.

The particles that are broken down are called sediments. Sediments are classified according to their size, ranging from silts and muds up to gravels and boulders.

Sediments can then be transported from their source, often to great distances. The main factors that control the transportation of sediments are:

  • water
  • wind (particularly in arid regions)
  • gravity (with all sediments flowing downhill regardless of the slope)

Studying sediments

Sedimentary geologists (known as sedimentologists) tend to study both present-day sediments and older sedimentary rock sequences.

Principle of uniformitarianism

The principle of uniformitarianism is that processes which operate on the Earth's surface today are similar to those that operated in the past. It is a fundamental principle in sedimentary geology and was first proposed by Charles Lyell in 1830. Using this principle when studying present-day sedimentary environments (e.g. coral reef systems, delta systems, river systems), we can determine fundamental principles such as:

  • rates of sedimentation
  • geometry of sediment sequences
  • rates of compaction
  • amount of water present in the sediments

which can then be applied to much older sedimentary rock sequences.

Stratification

Horizontal layering in sedimentary rocks is called bedding or stratification. It forms by the settling of particles from either water or air (the word sediment comes from the Latin sedimentum, meaning settled). Layer boundaries are natural planes of weakness along which the rocks can break and fluids can flow. As long as the sequence of layers has not been deformed or overturned, the youngest layers are at the top and the oldest are at the bottom. This sequence of stratification is the basis for the stratigraphic time scale. These observations were first made by a Danish physician, Nicolaus Steno, who in 1669 formulated the principles of horizontality, superposition (younger layers on top of older ones) and original continuity (sedimentary layers represent former continuous sheets).

Sedimentary layers - some terms

  • Laminations are thin discrete layers of rock.
  • Formations are groups of sedimentary rocks which have formed at the same time and contain similar sedimentary rocks. They are mappable units that formed under distinctive environmental conditions.
  • Unconformities are major time-gaps between layers.

Preservation of sedimentary sequences

Most sedimentary sequences that are preserved in the rock record are formed from catastrophic deposition such as floods, mud flows, rock slides and melting of glaciers. For a sediment sequence to be preserved and lithified (turned into rock), it must be covered over by younger sediments soon after it is deposited and water within the sequence must be expelled (this usually achieved through compaction by the weight of overlying layers).

Weathering and erosion

Weathering is the process where rocks break down under the effects of water and air. It consists of two processes which always act together:

  • fragmentation (known as mechanical or physical weathering)
  • decay (known as chemical weathering)

Erosion is the process of the movement of weathering products, by water and air.

The smaller the pieces (or fragments), the greater the surface area available for chemical attack and the faster the pieces decay. Important agents in weathering are rainfall, wind, ice, snow, rivers, seawater, vegetation and living organisms.

Soil is both a factor in weathering and the result of it. Once soil starts to form, rock weathers more rapidly and more soil is formed. Rainwater, which is mainly H2O, also contains small amounts of dissolved CO2 and H2S which react strongly with many rock materials.

Weathering and feldspars

Feldspar is the most common mineral in many igneous and metamorphic rocks, particularly in granites. In temperate climates, granites are quite resistant to erosion whereas in humid to tropical regions, granites decay easily. This is because in areas of greater rainfall, the feldspars decay into clay minerals that are then easily released from the rock. Feldspars generally only remain untouched in very arid climates.

The most common clay mineral formed is kaolinite. In the weathering of feldspars to kaolinite, potassium, sodium, calcium and silicon are released from the feldspar structure and go into solution. Since the speed of chemical reactions increases with increasing temperature, weathering occurs at a much faster rate in tropical areas than in more temperate ones. Under extreme weathering conditions in tropical climates, the clays themselves decay further into the mineral gibbsite (Al (OH)3) the main ore of aluminium. The resulting rock is a bauxite or laterite.

Weathering and carbonates

Whereas feldspar minerals decay into clays, carbonate minerals can completely dissolve. Caves are a characteristic product of the weathering of limestone (rocks made up predominantly of the carbonate minerals calcite and dolomite) in humid climates.

The main agent of weathering in this case is groundwater containing dissolved CO2 (carbon dioxide). This dissolution of limestone results in the enrichment of the groundwater in Ca (calcium) ions, leading to what is known as 'hard water'. CO2 is extracted from the atmosphere and from organic material in the soil and dissolves in the groundwaters. The greater the weathering of limestone, the more CO2 is removed from the atmosphere. As limestone dissolves faster than silicate rocks, the chemical weathering of limestone accounts for more of the total chemical erosion of the land surface than any other rocks, even though much larger areas of the Earth consist of silicate rocks.

Resistance of minerals to weathering

Measurements of weathering in the field can be combined with experiments in the laboratory to determine the relative resistance of minerals to weathering. The products of weathering are generally more resistant to further weathering than other minerals, particularly iron oxide and clay minerals. As quartz is generally insoluble and chemically stable, it tends not to weather readily under most conditions. The order of mineral stability under weathering conditions is related to the stabilities of chemical bonds and crystal structures under different temperatures and pressures.

Weathering in different rock types

Layered rocks break into slabs or plates along bedding planes whereas massive rocks break along regularly spaced planar cracks, called joints. In some igneous rocks, the joints take the form of sheets - sets of parallel closely-spaced planar surfaces. In areas of large daily temperature gradients, thermal expansion often accompanies frost action and chemical weathering.

Exfoliation is the peeling-off of large curved sheets or slabs of rock from the weathering surface of an outcrop. Spheroidal weathering is a similar phenomenon in which rounded boulders split off layers or shells from the weathered surface.

Shapes of weathered fragments

The shapes of fragments caused by weathering and erosion are largely inherited from the patterns of joints, bedding and other structures in the parent rock rather than being produced by transport. The size of fragments is a good clue to the intensity of mechanical erosion. In general, the higher or steeper the landscape, the larger the fragments. Once fragments have been moved from their source rock, they enter new environments of weathering and erosion and undergo further breakdown. Once boulders fall into streams they break and abrade quickly, and the size of pebbles downstream decreases rapidly with distance from the source. The different sizes and shapes of eroded particles (from huge boulders to clay-sized particles) can be attributed to the characteristics of their source rocks and their distance from the source.

Rates of erosion

Rates of erosion can be averaged over regions and are called rates of denudation (measured in mm per thousand years). These rates are greatest in valley glaciers and badlands (deeply eroded areas) and lowest in areas of low relief and temperate and rainforest regions. The rate of denudation is primarily controlled by topography and climate. Human influences accelerate the rate of denudation by three to ten times, with the highest figures being recorded in areas of intensive land use.

One of the highest rates of denudation measured is in the Tamur Basin of the Himalayas. The combination of steep slopes, unconsolidated material (sediment), a glaciated terrain and human modification has resulted in a rate of 4700 mm per 1000 years.

Organic matter and sedimentary processes

The biosphere (all biological activity such as plants, animals, and their remains) also plays a vital role in sedimentary processes. All organic matter eventually decomposes, releasing vital nutrients (such as N, Ca, C) into the soil and sea. Both coal and oil are formed by the interaction of buried organic matter with sedimentary processes.

Diagenesis

Diagenesis is the alteration of the mineralogy and/or texture of sediments at low temperatures and pressures. It affects sediments close to the Earth's surface. There are two main processes operating:

  • compaction: by overlying sediments, involving the close-packing of the individual grains by eliminating the pore space and expulsion of entrapped water
  • cementation: development of secondary material in the former pore spaces which then binds the sedimentary particles together. This material may be introduced from the passage of groundwater or derived from solution.


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