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Lamination (geology)
In geology, lamination is a small scale sequence of fine layers (so called laminae) that occurs in sedimentary rocks. Laminations are normally smaller and less pronounced than bedding. Lamination is often regarded as planar structures one centimetre or less in thickness, whereas bedding layers are greater than one centimetre.〔This definition can for example be found in Blatt ''et al.'' (2006), p 271〕 However, structures from several millimetres to many centimetres have been described as laminae.〔Boggs (1987), p 138〕 A single sedimentary rock can have both laminae and beds.
==Description==
Lamination consists of small differences in the type of sediment that occur throughout the rock. They are caused by cyclic changes in the supply of sediment. These changes can occur in grain size, clay percentage, microfossil content, organic material content or mineral content and often result in pronounced differences in colour between the laminae.〔Boggs (1987), p 141〕 Weathering can make the differences even more clear.
Lamination can occur as parallel structures (parallel lamination) or in different sets that make an angle with each other (cross-lamination). It can occur in many different types of sedimentary rock, from coarse sandstone to fine shales, mudstones or in evaporites.
Because lamination is a fine structure, it is easily destroyed by bioturbation (the activity of burrowing organisms) shortly after deposition. Lamination therefore survives better under anoxic circumstances, or when the sedimentation rate was high and the sediment was buried before bioturbation could occur.
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