Stylolite

Stylolites or styolite (Greek: stylos, pillar; lithos, stone) are serrated surfaces within a rock mass at which mineral material has been removed by pressure dissolution, in a process that decreases the total volume of rock. Insoluble minerals, such as clays, pyrite and oxides, remain within the stylolites and make them visible. Sometimes 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, cherts, sandstones, but they can be found in certain igneous rocks and ice. 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.

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

Park and Schot recognized six different geometries in stylolites:

A stylolite is not a structural 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 fossiliferous limestone 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 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.

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