Paleocurrent

Bidirectional wave ripple, Nomgon, Mongolia

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.^[1]

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 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.^[2]

Below are a partial list of common paleocurrent indicators:

Unidirectional

Flute cast, showing paleocurrent to the upper right of pic

• Cross-bedding - 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 markings/flute casts - 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

Parting lineation, from lower right to upper left; Kayenta Formation, Canyonlands National Park.

• Symmetrical ripple marks - flow is perpendicular to the ridge crest.
• Tool mark - flow is along the mark.
• Parting lineations - flow is along the grains.

References

1. Prothero, D. R. and Schwab, F., 1996, Sedimentary Geology, pg. 48-51, ISBN 0-7167-2726-9
2. Miaoa, Xiaodong; Huayu Lua; Zhen Lid; Guangchao Caod (July 2008). "Paleocurrent and fabric analyses of the imbricated fluvial gravel deposits in Huangshui Valley, the northeastern Tibetan Plateau, China". Geomorphology (© 2007 Elsevier B.V.) 99 (1-4): 433–442. Bibcode:2008Geomo..99..433M. doi:10.1016/j.geomorph.2007.12.005.