Diagenetic processes

The physical and chemical changes that alter the characteristics of sediment after deposition are referred to as diagenesis. These processes occur at relatively low temperatures, typically below about 250 C, and at depths of up to about 5000 m. There is a continuum between diagenesis and metamorphism, the latter being considered to be those processes that occur at higher temperatures (typically above 250 C to 300 C) and pressures: metamorphism involves the destruction of the original sedimentary fabric. Sediments are generally unconsolidated material at the time of deposition and are in the form of loose sand or gravel, soft mud or accumulations of the body parts of dead organisms. Lithification is the process of transforming sediment into sedimentary rock, and involves both chemical and physical changes that take place at any time after initial deposition. Some sediments are lithified immediately, others may take millions of years: there are sediments that never become consolidated, remaining as loose material millions of years after deposition. Lithification upon deposition occurs in some limestones, evaporite deposits and volcaniclastic sediments, which may all form rocks at the time of deposition. Boundstones are formed from the frameworks of organisms that build up solid masses of calcium carbonate as bioherms, for example, coral reefs; loose material between the coral mass may be subsequently lithified but the main framework of the rock is formed in situ. Chemical precipitation out of water results in beds of solid crystalline evaporite minerals. A further example is that of pyroclastic deposits deposited from hot clouds of ash and gases: the temperatures may be high enough for the ash particles to fuse together on deposition as a welded tuff.

Burial diagenesis: compaction

The accumulation of sediment results in the earlier deposits being overlain by younger material, which exerts an overburden pressure that acts vertically on a body of sediment and increases as more sediment, and hence more mass, is added on top. Loose aggregates initially respond to overburden pressure by changing the packing of the particles; clasts move past each other into positions that take up less volume for the sediment body as a whole. This is one of the processes of compaction that increases the density of the sediment and it occurs in all loose aggregates as the clasts rearrange themselves under moderate pressure. Pore water in the voids between the grains is expelled in the process and compaction by particle repacking may reduce the volume of a body of sand by around 10%. During compaction weaker grains, such as mica flakes or mud clasts in sandstone, may be deformed plastically by the pressure from stronger grains such as quartz: fracturing, or cataclasis, of grains can also occur under pressure. When muds are deposited they may contain up to 80% of water by volume: this is reduced to around 30% under burial of a thousand metres, representing a considerable compaction of the material. Certain sediments, for example boundstones formed as a coral reef, may not compact at all under initial burial. Compaction has little effect on horizontal layers of sediment except to reduce the thickness. Internal sedimentary structures such as cross-stratification may be slightly modified by compaction and the angle of the cross-strata with respect to the horizontal may be decreased slightly.

Differential compaction

Where there is a lateral change in sediment type differential compaction occurs as one part of a sediment pile compacts more than the part adjacent to it. Possible examples are lime muds deposited around an isolated patch reef, a sand bar surrounded by mud and a submarine channel cut into muds and filled with sand. In each case the degree to which the finer material will compact under overburden pressure will be greater than the sand body or reef. A draping of the finer sediments around the isolated body will occur under compaction. This can occur on all scales from bodies a few metres across to masses hundreds of metres wide. The differential compaction effect is less marked in fluvial successions where sand-filled channel bodies are surrounded by overbank mudstones. This is because the fine sediment on the floodplain dries out between flood events and loses most of its pore waters at that stage. As a consequence the effect of overburden pressure on overbank muds and channel sands may be the same. Differential compaction effects can also be seen on the scale of millimetres and centimetres where there are contrasts in sediment type. Mud layers may become draped around lenses of sand formed by ripple and dune bedforms. Local compaction effects also occur around nodules and concretions where there is early cementation.

Pressure solution/dissolution

The burial of sediment under layers of more strata results in overburden pressures that cause more extreme physical and chemical changes. In sandstones and conglomerates the pressure is concentrated at the contacts between grains or larger clasts, creating concentrations of stress at these points. In the presence of pore waters, diffusion takes place moving some of the mineral material away from the contact and reprecipitating it on free surfaces of the mineral grains. This process is called pressure solution or pressure dissolution and it results in grains becoming interlocked, providing a rigidity to the sediment, that is, it becomes lithified. These effects may be seen at grain contacts when a rock is examined in thin-section using a petrographic microscope. Beds of limestone may show extensive effects of pressure dissolution.

Compaction effects

The degree of compaction in an aggregate can be determined by looking at the nature of the grain contacts. If the sediment has been subjected to very little overburden pressure the clasts will be in contact mainly at the point where they touch, point contacts. Reduction in porosity by changes in the packing will bring the edges of more grains together as long contacts. Pressure solution between grains results in concavo-convex contacts where one grain has dissolved at the point of contact with another. Under very high overburden pressures the boundaries between grains become complex sutured contacts, a pattern more commonly seen under the more extreme conditions of metamorphism.

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