Lamination (geology)

In geology, lamination is a small scale sequence of fine layers (so called laminae) that occurs in sedimentary rocks. Lamination is normally smaller and less pronounced than bedding. Lamination is often regarded as every planar structure smaller than 1 cm, and bedding as every planar structure larger than that.[1] However, structures from several millimetres to many centimetres have been described as laminae.[2] A single sedimentary rock can have both laminae and beds.

Contents

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.[3] 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 even 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.

Origin

Lamination develops in fine grained sediment when fine grained particles settle, which can only happen in quiet water. Examples of sedimentary environments are deep marine (at the seafloor) or lacustrine (at the bottom of a lake), or mudflats, where the tide creates cyclic differences in sediment supply.[4]

Laminations formed in glaciolacustrine environments (in glacier lakes) are a special case. They are called varves. Quaternary varves are used in stratigraphy and palaeoclimatology to reconstruct climate changes during the last few hundred thousand years.

Lamination in sandstone is often formed in a coastal environment, where wave energy causes a separation between grains of different sizes.

See also

References

Notes

  1. ^ This definition can for example be found in Blatt et al. (2006), p 271
  2. ^ Boggs (1987), p 138
  3. ^ Boggs (1987), p 141
  4. ^ Boggs (1987), p 142

Literature