Cross-bedding
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In geology, cross-bedding refers to inclined sedimentary structures in a horizontal unit of rock. These tilted structures are deposits from bedforms such as ripples and dunes, and they indicate that the depositional environment contained a flowing fluid (typically, water or wind). This is a case in geology when original depositional layering is tilted, and that the tilting is not a result of post-depositional deformation.
Cross-bedding structures are formed in bedforms such as ripples and dunes by the motion of sediment due to a flowing fluid. Sediment grains bounce up the windward/upstream ("stoss") side of a ripple, and then tumble down the lee side.
Cross-bedding can form in any environment in which a fluid flows over a bed with mobile material. It is most common in stream deposits (consisting of sand and gravel), tidal areas, and in aeolian dunes.
Cross-bedded sediments are recognized in the field by the many layers of "foresets", which are the series of layers that form on the lee side of the bedform (ripple or dune). These foresets are individually differentiable because of small-scale separation between layers of material of different sizes and densities.
Cross-bedding can also be recognized by truncations in sets of ripple foresets, where previously-existing stream deposits are eroded by a later flood, and new bedforms are deposited in the scoured area.
The direction of motion of the cross-beds can show ancient flow or wind directions (called paleocurrents). The foresets are deposited at the angle of repose (~34 degrees from the horizontal), so geologists are able to measure dip direction of the cross-bedded sediments and calculate the paleoflow direction. However, most cross-beds are not tabular, they are troughs. Since troughs can give a 180 degree variation of the dip of foresets, false paleocurrents can be taken by blindly measureing foresets. In this case, true paleocurrent direction is determined by the axis of the trough. Paleocurrent direction is important in reconstructing past climate and drainage patterns: sand dunes preserve the prevalent wind directions, and current ripples show the direction rivers were moving.
[edit] References
- Monroe, James S. and Wicander, Reed (1994) The Changing Earth: Exploring Geology and Evolution, 2nd ed., St. Paul, Minn. : West, ISBN 0-314-02833-1, pp. 113–114.
- Rubin, David M. and Carter, Carissa L. (2006) Bedforms and cross-bedding in animation, Society for Sedimentary Geology (SEPM), Atlas Series 2, DVD #56002, ISBN 1-56576-125-1
- Prothero, D. R. and Schwab, F., 1996, Sedimentary Geology, pg. 43-64, ISBN 0-7167-2726-9
