Stylolites are serrated surfaces at which mineral material has been removed by pressure

Pressure solution

In structural geology and diagenesis, pressure solution or pressure dissolution is a deformation mechanism that involves the dissolution of minerals at grain-to-grain contacts into an aqueous pore fluid in areas of relatively high stress and either deposition in regions of relatively low stress...

 dissolution, in a process that decreases the total volume of rock. Insoluble minerals like clays

Clay minerals

Clay minerals are hydrous aluminium phyllosilicates, sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations. Clays have structures similar to the micas and therefore form flat hexagonal sheets. Clay minerals are common weathering products and low...

, pyrite, oxides remain within the stylolites and make them visible. Sometime host rocks contain no insoluble minerals, in which case stylolites can be recognized by change in texture of the rock. They occur most commonly in homogeneous rocks, carbonates

Carbonate rock

Carbonate rocks are a class of sedimentary rocks composed primarily of carbonate minerals. The two major types are limestone, which is composed of calcite or aragonite and dolostone, which is composed of the mineral dolomite .Calcite can be either dissolved by groundwater or precipitated by...

, chert

Chert

Chert is a fine-grained silica-rich microcrystalline, cryptocrystalline or microfibrous sedimentary rock that may contain small fossils. It varies greatly in color , but most often manifests as gray, brown, grayish brown and light green to rusty red; its color is an expression of trace elements...

s, sandstones, but they can be found in certain igneous rocks

Igneous rock

Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...

 and ice

Ice

Ice is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...

. Their size vary from microscopic contacts between two grains (microstylolites) to large structures up to 20 m in length and up to 10 m in amplitude in ice. Stylolites usually form parallel to bedding, because of overburden pressure, but they can be oblique or even perpendicular to bedding, as a result of tectonic activity.

Classification of stylolites

Stylolites can be classified by their geometry or their relationship to bedding .

Geometric classification

Park and Schot recognized six different geometries in stylolites:

1. Simple or primitive wave-like
2. Sutured type
3. Up-peak type (Rectangular type)
4. Down-peak type (Rectangular type)
5. Sharp-peak type (tapered and pointed)
6. Seismogram type

Relationship to bedding

1. Horizontal stylolites - This is the most commonly observed stylolite type. They occur parallel or nearly parallel to the bedding of rocks. This type is most frequently found in layered sedimentary rocks, mostly in carbonate rocks, which have not been affected by intensive tectonic structural activity or metamorphism.
2. Inclined stylolites - This type occur oblique to bedding. It appears in rock which affected and unaffected by tectonic activity, and also can found in metamorphic and layered igneous rocks.
3. Horizontal-inclined (vertical)-crosscutting stylolites - This type is a combination of horizontal and inclined types of stylolites. In this type horizontal stylolites usually have a higher amplitude then inclined stylolites. Horizontal-inclined can find in a rocks which having been affected by two successive pressure systems, first having been effected with angle to the bedding and the second, later system perpendicular to the bedding.
4. Vertical stylolites - This type of stylolites related to the bedding at right angles. It can be found in rock which have been affected and unaffected by tectonic activities, but in each case pressure acting perpendicularly to the bedding.
5. Interconnecting network stylolites - This type a network of stylolites, which related to each other with relatively small angle. This type can divided on two subtypes. Stylolites of subtype A are characterized higher amplitudes. They related to the bedding horizontally, or small angle. Stylolites of subtype B are usually appears in rocks which have been affected tectonic or metamorphic activity, or both. These stylolites have a low amplitude with undulating. Their relation to the bedding can varies from horizontal to vertical.
6. Vertical-inclined (horizontal)-crosscutting stylolites - This type is a combination of horizontal or inclined and vertical stylolite's types. In this case the inclined or horizontal stylolites were formed first and the vertical later. This type can divided on two sub type by directions of displacement of the inclined stylolites. In subtype A, the displacements could have happened during vertical stylolization, instead of in subtype B, the displacements could have happened before vertical stylolization.

Development

A stylolite is not a structural

Structural geology

Structural geology is the study of the three-dimensional distribution of rock units with respect to their deformational histories. The primary goal of structural geology is to use measurements of present-day rock geometries to uncover information about the history of deformation in the rocks, and...

 fracture, although they have been described as a form of 'anti-crack', with the sides moving together rather than apart. Proof exists in the form of eg. fossiliferous limestone

Limestone

Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....

 where fossils are crosscut by a stylolite and only one half still exists; the other half has been dissolved away. Rye & Bradbury (1988) investigated ^13/12C and ^18/16O stable isotope systematics in limestone

Limestone

Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....

on either side of a stylolite plane and found differences confirming different degrees of fluid-rock interaction.

In order for a stylolite to develop, a solution into which minerals can dissolve needs to be present, along with a pore network through which dissolved solids can advect or diffuse from the developing stylolite. Stylolite development can be improved with porosity, as it localizes stress on nonpore areas, increasing stress there. Therefore, it is suggested that bedding-parallel stylolites form in areas of high porosity, and most of the transverse stylolites form along preexisting fractures.

Significance

Stylolites are significant in several fields. In petrology, stylolites are important because they alter rock fabrics and create dissolved solids that precipitate as cement. In stratigraphy, weathering of stylolites generates apparent bedding in many stratigraphic sections and loss of material along stylolites can have a result similar to erosion, with significant stratigraphic thinning. In hydrology, stylolites prevent fluid flow and, in other settings, serve for fluid flow. Also, stylolites are indicators of compressive stress in tectonic studies, and development of transverse stylolites contributes to crustal shortening parallel to the direction of their column.