A paleocurrent or paleocurrent indicator is a
geological
Geology (). is a branch of natural science concerned with the Earth and other astronomical objects, the rocks of which they are composed, and the processes by which they change over time. Modern geology significantly overlaps all other Earth s ...
feature (typically a
sedimentary structure
Sedimentary structures include all kinds of features in sediments and sedimentary rocks, formed at the time of deposition.
Sediments and sedimentary rocks are characterized by bedding, which occurs when layers of sediment, with different parti ...
) that helps one determine the direction of flowing
water
Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...
in the geologic past. This is an invaluable tool in the reconstruction of ancient
depositional environment
In geology, depositional environment or sedimentary environment describes the combination of physical, chemical, and biological processes associated with the deposition of a particular type of sediment and, therefore, the rock types that will b ...
s.
There are two main types of paleocurrent indicators:
*''Unidirectional'', which give a clear, single direction of flow
*''Bidirectional'', which give a good linear direction, but it is unclear which direction along the linear trend the water flowed.
Paleocurrents are usually measured with an
azimuth
An azimuth (; from ) is the horizontal angle from a cardinal direction, most commonly north, in a local or observer-centric spherical coordinate system.
Mathematically, the relative position vector from an observer ( origin) to a point ...
, or as a
rake
Rake may refer to:
Common meanings
* Rake (tool), a horticultural implement, a long-handled tool with tines
* Rake (stock character), a man habituated to immoral conduct
* Rake (poker), the commission taken by the house when hosting a poker game
...
on a bedding plane, and displayed with a
Rose Diagram to show the dominant direction(s) of flow. This is needed because in some depositional environments, like
meandering river
A meander is one of a series of regular sinuous curves in the channel of a river or other watercourse. It is produced as a watercourse erodes the sediments of an outer, concave bank ( cut bank or river cliff) and deposits sediments on an inn ...
s, the paleocurrent resulting from natural sinuosity has a natural variation of 180 degrees or more.
Below are a partial list of common paleocurrent indicators:
Unidirectional
*
Cross-bedding
In geology, cross-bedding, also known as cross-stratification, is layering within a stratum and at an angle to the main bedding plane. The sedimentary structures which result are roughly horizontal units composed of inclined layers. The origina ...
– the axis of a trough cross bed or the down-dip direction of a tabular cross bed point the direction of paleo flow.
*Current
ripple mark
In geology, ripple marks are sedimentary structures (i.e., bedforms of the lower flow regime) and indicate agitation by water (Ocean current, current or wind wave, waves) or directly by wind.
Defining ripple cross-laminae and asymmetric ripples ...
s – will have the short side of the ripple pointing down stream.
*
Sole markings/
flute cast
Sole marks are sedimentary structures found on the bases of certain strata, that indicate small-scale (usually on the order of centimetres) grooves or irregularities. This usually occurs at the interface of two differing Lithology, lithologies ...
s – the short, steep side will point up stream, and the long, tapered side points down stream.
*
Imbrication – clasts line up in the direction of flow.
Bidirectional
*Symmetrical
ripple mark
In geology, ripple marks are sedimentary structures (i.e., bedforms of the lower flow regime) and indicate agitation by water (Ocean current, current or wind wave, waves) or directly by wind.
Defining ripple cross-laminae and asymmetric ripples ...
s – flow is perpendicular to the ridge crest.
*
Tool mark – flow is along the mark.
*
Parting lineations – flow is along the grains.
References
Sedimentology
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