In
geology
Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other Ea ...
, lamination () is a small-scale sequence of fine layers (laminae; singular: lamina) that occurs in
sedimentary rock
Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particle ...
s. Laminae 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. However, structures from several millimetres to many centimetres have been described as laminae. 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
Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4).
Clays develop plasticity when wet, due to a molecular film of water surrounding the clay part ...
percentage,
microfossil content,
organic material
Organic matter, organic material, or natural organic matter refers to the large source of carbon-based compounds found within natural and engineered, terrestrial, and aquatic environments. It is matter composed of organic compounds that have c ...
content or
mineral content and often result in pronounced differences in colour between the laminae.
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
Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks.
Most sandstone is composed of quartz or feldspar (both silicates ...
to fine
shales,
mudstones or in
evaporite
An evaporite () is a water- soluble sedimentary mineral deposit that results from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporite deposits: marine, which can also be described as oce ...
s.
Because lamination is a small 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.
[Boggs (1987), p 142]
Laminae 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 change
In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to ...
s 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
*
Bed (geology)
*
Foliation (geology)
*
Liesegang rings (geology)
*
Speleothem
*
Stratum
References
Notes
Literature
*; 2006: ''Petrology, Igneous, Sedimentary, and Metamorphic'', W.H. Freeman & company, New York (3rd ed.), .
*; 1987: ''Principles of Sedimentology and Stratigraphy'', Merrill Publishing Company, {{ISBN, 0-675-20487-9.
Sedimentology