stylolite

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stylolites It is a fundamental axiom of geology that bedding planes indicate changes in conditions of deposition at the sedimentary surface. However, not all the obvious ‘bedding’ surfaces are true depositional surfaces. In tectonically deformed regions the most obvious stratification may be cleavage surfaces (see slate): jointing, too, can often obscure true bedding. In undeformed regions, especially within rocks that are relatively homogeneous such as limestones, dolomites and quartzitic sandstones, a type of ‘pseudobedding’ can develop.

The process by which this forms is pressure solution, nowadays commonly referred to as pressure dissolution. Because crystalline solids under stress are more soluble than less stressed or unstressed portions, they may be dissolved at points of contact between mineral grains where there is an increased local pressure. This dissolution leads to mass transfer through an aqueous phase, either by diffusion or bulk flow, and can result in precipitation in less stressed domains.

Both tectonic stress and gravitational loading by overburden can produce pressure dissolution. Since carbonates are more soluble than quartz, pressure solution induced by gravitational loading tends to start at very shallow depths in limestone and dolomites; in quartzites it occurs at depths of 1000 m or more.

The surfaces produced by pressure dissolution may be either smooth and undulatory (dissolution seams) or serrated (stylolites), with gradations between the two. The former tends to occur in clay-rich rocks, the latter in clay-poor rocks. Although there has been a long evolution in the definition of the term stylolite, it is generally taken today to refer to a serrated interface between two rock masses that have a sutured appearance in section normal to the plane of the stylolite. These suture-like seams are marked by the presence of insoluble minerals left behind after solution of the more soluble constituents. These insoluble minerals may be clays or other silicate minerals, organic or carbonaceous material, or even iron minerals. The amplitude of the suture is much greater than the diameter of the transected grains. It cuts indiscriminately through the rock fabric, across grains, cement, and matrix; it may truncate fossils, ooliths, veins, and other stylolites.

Stylolites that are the result of overburden pressure run approximately parallel to the bedding. Those that are caused by tectonic forces may be oblique or even perpendicular to bedding. Where one cuts another the relationship can be used to unravel the diagenetic and tectonic history of the rock. Stylolites serve not only as permeability barriers but also as conduits to fluids. They thus have a considerable effect on later diagenetic processes such as dolomitization and the migration of hydrocarbons.

Harold G. Reading

stylolite An irregular, suture-like contact, produced by pressure dissolution of rock under deep burial conditions. Stylolites are most commonly found in limestones, and may be picked out by the concentration of insoluble clay residues along the stylolite surface. Up to 40% of the original thickness of a limestone sequence can be dissolved through stylolitization.