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Stylolite

A stylolite is an irregular discontinuity or non-structural fracture in limestone and other sedimentary rocks. Stylolites result from compaction and pressure solution during diagenesis and may be enlarged by subsequent groundwater flow. Stylolites appear as jagged discontinuities in outcrops and hand samples and are often lined with insoluble clays, opaques (such as iron oxide), or dark organic matter.

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The term stylolite is derived from the Greek for pillar, stylo. Compare the French 'stylo' for pencil, and 'stylite' for 'pillar saint', an ascetic deliberately living standing on top of a pillar.

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^[1]. Proof exists in the form of eg. fossiliferous limestones where fossils are crosscut by a stylolite and only one half still exists; the other half has been dissolved away. Rye & Bradbury (1988) ^[2] 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.

References

^ Fletcher, C.C. and Pollard, D.D. 1981 Anticrack model for pressure solution surfaces. Geology, 9, 419-24.
^ Rye, DM, and Bradbury, HJ (1988): Fluid flow in the crust: an example from a Pyrenean thrust ramp. American Journal of Science (288): 197-235.