Paleocurrent
Encyclopedia
A paleocurrent or paleocurrent indicator is a geological feature (typically a sedimentary structure) that helps one determine the direction of flowing water
in the geologic past. This is an invaluable tool in the reconstruction of ancient depositional environments.
There are two main types of paleocurrent indicators:
Paleocurrents are usually measured with an azimuth, or as a rake
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 rivers, the paleocurrent resulting from natural sinuosity has a natural variation of 180 degrees or more.
Below are a partial list of common paleocurrent indicators:
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
in the geologic past. This is an invaluable tool in the reconstruction of ancient depositional environments.
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, or as a rake
Rake (geology)
In structural geology rake is formally defined as "the angle between a line [or a feature] and the strike line of the plane in which it is found", measured on the plane. The three dimensional orientation of a line can be described with just a plunge and trend. The rake is a useful description of...
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 rivers, 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-beddingCross-beddingIn geology, the sedimentary structures known as cross-bedding refer to horizontal units that are internally composed of inclined layers. This is a case in geology in which the original depositional layering is tilted, and the tilting is not a result of post-depositional deformation...
- 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 marks - will have the short side of the ripple pointing down stream.
- Sole markingsSole markingsSole marks are sedimentary structures found on the bases of certain strata, that indicate small-scale grooves or irregularities. This usually occurs at the interface of two differing lithologies and/or grain sizes. They are commonly preserved as casts of these indents on the bottom of the...
/flute casts - the short, steep side will point up steam, and the long, tapered side points down stream.
- ImbricationImbrication' is the arrangement of planar bodies such that they stack in a consistent fashion - rather like a toppled run of dominoes.*In roofing, imbrication is employed in the Imbrex and tegula system.*Imbrication ....
- clasts line up in the direction of flow.
Bidirectional
- Symmetrical ripple marks - flow is perpendicular to the ridge crest.
- Tool mark - flow is along the mark.
- Parting lineationParting lineationParting lineation is a subtle sedimentary structure in which sand grains are aligned in parallel lines or grooves on the surface of a body of sand...
s - flow is along the grains.