diagenesis

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diagenesis The term ‘diagenesis’ is attributed to von Gumbel who coined it at the end of the nineteenth century. It includes all processes which affect sediments after their deposition and during burial, excluding the effects of tectonism and metamorphism. Diagenesis usually, but not always, results in the production of an indurated or lithified (i.e. hard) rock. Diagenesis grades imperceptibly into metamorphism: the division between them is taken at the point where the first metamorphic index mineral appears.

Diagenesis is controlled by several factors: the composition and grain size of the original sediment; the rate of deposition and environment of deposition; the rate of burial and phys-ical and chemical changes during burial. It is also influenced by the character of adjacent sediments. Diagenetic changes are both physical and chemical. Water is expelled as the component grains are packed together by overburden pressure on burial. Expelled water escapes into more porous adjacent sediments where it may cause solution or precipitate mineral cements according to the chemical conditions.

Compaction is the main physical process. Organic sediments such as peats, which originally contain more water than sediment, are reduced to a fraction of their original thickness. Fine-grained siliciclastic muds which may contain up to 80 per cent water when originally deposited, retain only about 10 per cent of it after burial. During compaction of siliciclastic muds the flaky clay particles are realigned to be parallel to each other and impart a fissility (a direction of easy splitting) to the rock, which is commonly called shale. Sands and coarser sediments, which can contain up to 50 per cent water on deposition, undergo minor compaction. Individual grains are rotated slightly and may slip and even sometimes fracture as they are forced into a tighter packing. However, this produces only minor changes in porosity. Miocene sands in the Gulf Coast of the USA retain porosities of around 30 per cent at depths of 3226 m. Where sands contain interstial mud or grains of mud or fine-grained rocks they may undergo greater changes of porosity. The softer grains are smeared between more resistant grains to produce a matrix and hence reduce the pore space.

The character of the chemical changes undergone by sediments during diagenesis is controlled by the activity of ions in the pore-water solution between the grains, its Eh (redox potential) and pH as well as the temperature and pressure. These changes can take place in several different environments. For example, they may occur in the continental realm above the water table—the vadose zone; or in the permanently shallow saturated zone—the phreatic zone, in either freshwater or marine conditions. They may also occur in the zone of deep burial.

Although the climate of the environment of deposition controls the pathways of early chemical diagenesis in terrestrial and coastal situations, the effect of surface water lessens during burial. The pore waters become more reactive as the solubility of various mineral phases increases with increasing temperature and pressure.

Organic matter in sediments is very reactive to diagenetic change. The fairly large polymers of primary organic matter are degraded by bacteria during burial and converted to monomers to form the complex organic compound kerogen as well as releasing biogenic gas (methane and carbon dioxide). As temperature increases during burial, kerogen breaks down to yield oil and wet gas—the process known as catagenesis—and ultimately it passes into the zone of metagenesis, where only dry gas is released.

The humic organic matter of peat is also altered on burial, water and volatiles being lost during the process of coalification. The carbon content of the organic matter increases, that is, it is said to increase in rank. The parent material, peat, is converted to soft brown coal and then to hard brown coal and ultimately via bituminous coal to anthracite under the influence of increased pressure and temperature, methane gas being produced during the process.

Evaporites undergo changes on burial owing to reactions between interstitial brines and previously deposited salts to produce new suites of minerals. Fine-grained siliceous deposits originally composed dominantly of opal lose water and recrystallize as chalcedony, and then eventually quartz. Fine-grained carbonates are replaced so that they consist entirely of calcite and can suffer further diagenesis and may be replaced by dolomite.

Fine-grained siliclastic deposits are also prone to diagenetic change. The clay mineral kaolinite can develop early in acidic conditions but on burial changes to other polymorphs which are, however, destroyed at higher temperatures or can be converted to other clay minerals as the water chemistry changes. Smectites and vermiculites develop in alkaline waters during early diagenesis, as well as minerals such as attapulgite (authigenic minerals). They may be converted to kaolinite if they are brought in contact with acidic pore waters. However, usually during burial they become dehydrated and transformed to mixed-layer clays, and then to illite or chlorite according to the pore-water chemistry. Illite and chlorites form more stable polymorphs during diagenesis in the presence of alkaline pore waters, and the amount of the two minerals increases with time. It is thus difficult to ascertain the nature of the original or early diagenetic clay in Palaeozoic rocks, but this becomes increasingly easier in Mesozoic and Cenozoic rocks.

The coarser grains of siliciclastic sediments usually survive chemical diagenetic changes with only minor modification, although some minerals such as feldspars are sometimes broken down to clay and carbonates. In carbonate deposits individual grains are either recrystallized or replaced by calcite when they were originally composed of aragonite or high-magnesium calcite. Grains become coated with a precipitated mineral cement consisting mainly of quartz, carbonates, iron oxides, and clay. Some of this cement is of local origin but most of it originates from water expelled from adjacent mudstones. In some instances complex sequences of cements are present which were deposited at various times during diagenesis and have different origins. Studies using staining techniques, cathode-luminescence, ion-probes, and isotopic analyses have helped greatly in unravelling the details of cementation histories.

In carbonates (mainly limestones), cementation may occur very early while the sediment is exposed on the sea floor, but continues during burial, often with several episodes of cementation. Carbonates are very easily altered. Both coarser and fine-grained carbonates may be replaced by silica (chert), phosphate and dolomite at various times during their diagenetic history.

Deep burial sometimes leads to pressure solution at the points of contact of grains in siliciclastic and carbonate sediments. This produces sutured interpenetrative contacts between grains, and these may form thin extensive sheets of concentrations of insoluble material called stylolite seams. Whereas most diagenetic changes produce a denser indurated deposit, sometimes, solutions driven out of adjacent sediments can lead to solution of earlier cements and the production of high-porosity zones in the subsurface.

The cumulative affect of diagenesis is to modify the porosity and permeability of sediments and thus their economic potential.

G. Evans

Bibliography

Burley, S. D.,, Kantarowicz, S. D.,, and and Waugh, B. (1985) Clastic diagenesis. In Brenchley, P. J. and Williams, B. P. J. (eds) Sedimentology: recent developments and applied aspects, pp. 189–228. Blackwell Scientific Publications, Oxford.
Dickson, J. A. D. (1985) Diagenesis of shallow-marine carbonates. In Brenchley, P. J. and Williams, B. P. J. (eds) Sedimentology: recent developments and applied aspects, pp.173–188. Blackwell Scientific Publications, Oxford.
Leeder, M. R. (1982) Sedimentology, process and product. George Allen and Unwin, London.

diagenesis All the changes that take place in a sediment at low temperature and pressure after deposition. With increasing temperature and pressure, diagenesis grades into metamorphism. Diagenetic processes such as compaction, dissolution (see PRESSURE DISSOLUTION), cementation, replacement, and recrystallization are the means by which an unconsolidated, loose sediment is turned into a sedimentary rock, e.g. sand into a sandstone, or peat into coal. See also CARBON ISOTOPES; and OXYGEN ISOTOPES.

diagenesis All the changes that take place in a sediment at low temperature and pressure following deposition. With increasing temperature and pressure, diagenesis grades into metamorphism. Diagenetic processes such as compaction, dissolution, cementation, replacement, and recrystallization are the means by which an unconsolidated, loose sediment is turned into a sedimentary rock (e.g. sand into sandstone, or peat into coal).

diagenesis Physical and chemical processes whereby sediments are transformed into solid rock, usually at low pressure and temperature. Pressure results in compaction, forcing grains together and eliminating air and water.

diagenesis See taphonomy.