Lamination (geology)
Encyclopedia
In geology
, lamination is a small scale sequence of fine layers (so called laminae) that occurs in sedimentary rock
s. 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. However, structures from several millimetres to many centimetres have been described as laminae. A single sedimentary rock can have both laminae and beds.
percentage, microfossil content, organic material 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
to fine shale
s, mudstone
s or even in evaporite
s.
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.
(at the bottom of a lake), or mudflat
s, where the tide
creates cyclic differences in sediment supply.
Laminations formed in glaciolacustrine environments (in glacier lakes) are a special case. They are called varve
s. Quaternary
varves are used in stratigraphy
and palaeoclimatology to reconstruct climate change
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.
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...
, lamination is a small scale sequence of fine layers (so called laminae) that occurs in sedimentary rock
Sedimentary rock
Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....
s. 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. 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, clayClay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...
percentage, microfossil content, organic material content or mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
content and often result in pronounced differences in colour between the laminae. Weathering
Weathering
Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters...
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
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
to fine shale
Shale
Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...
s, mudstone
Mudstone
Mudstone is a fine grained sedimentary rock whose original constituents were clays or muds. Grain size is up to 0.0625 mm with individual grains too small to be distinguished without a microscope. With increased pressure over time the platey clay minerals may become aligned, with the...
s or even in evaporite
Evaporite
Evaporite is a name for a water-soluble mineral sediment that result from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporate deposits, marine which can also be described as ocean deposits, and non-marine which are found in standing bodies of...
s.
Because lamination is a fine structure, it is easily destroyed by bioturbation
Bioturbation
In oceanography, limnology, pedology, geology , and archaeology, bioturbation is the displacement and mixing of sediment particles and solutes by fauna or flora . The mediators of bioturbation are typically annelid worms , bivalves In oceanography, limnology, pedology, geology (especially...
(the activity of burrowing organisms) shortly after deposition. Lamination therefore survives better under anoxic
Hypoxia (environmental)
Hypoxia, or oxygen depletion, is a phenomenon that occurs in aquatic environments as dissolved oxygen becomes reduced in concentration to a point where it becomes detrimental to aquatic organisms living in the system...
circumstances, or when the sedimentation
Sedimentation
Sedimentation is the tendency for particles in suspension to settle out of the fluid in which they are entrained, and come to rest against a barrier. This is due to their motion through the fluid in response to the forces acting on them: these forces can be due to gravity, centrifugal acceleration...
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 lacustrineLake
A lake is a body of relatively still fresh or salt water of considerable size, localized in a basin, that is surrounded by land. Lakes are inland and not part of the ocean and therefore are distinct from lagoons, and are larger and deeper than ponds. Lakes can be contrasted with rivers or streams,...
(at the bottom of a lake), or mudflat
Mudflat
Mudflats or mud flats, also known as tidal flats, are coastal wetlands that form when mud is deposited by tides or rivers. They are found in sheltered areas such as bays, bayous, lagoons, and estuaries. Mudflats may be viewed geologically as exposed layers of bay mud, resulting from deposition of...
s, where the tide
Tide
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the moon and the sun and the rotation of the Earth....
creates cyclic differences in sediment supply.
Laminations formed in glaciolacustrine environments (in glacier lakes) are a special case. They are called varve
Varve
A varve is an annual layer of sediment or sedimentary rock.The word 'varve' is derived from the Swedish word varv whose meanings and connotations include 'revolution', 'in layers', and 'circle'. The term first appeared as Hvarfig lera on the first map produced by the Geological Survey of Sweden in...
s. Quaternary
Quaternary
The Quaternary Period is the most recent of the three periods of the Cenozoic Era in the geologic time scale of the ICS. It follows the Neogene Period, spanning 2.588 ± 0.005 million years ago to the present...
varves are used in stratigraphy
Stratigraphy
Stratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....
and palaeoclimatology to reconstruct climate change
Climate change
Climate change is a significant and lasting change in the statistical distribution of weather patterns over periods ranging from decades to millions of years. It may be a change in average weather conditions or the distribution of events around that average...
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.