Cross-bedding - Significance

Significance

Cross beds can tell geologists much about what an area was like in ancient times. The direction the beds are dipping indicates paleocurrent. The type and condition of sediments can tell geologists the type of environment (rounding, sorting, composition…). Studying modern analogs allows geologists to draw conclusions about ancient environments. Paleocurrent- The rough direction of sediment transport can be determined by seeing a cross section of a set of cross beds. However, to get a true reading, the axis of the beds must be visible. It is also difficult to distinguish between the cross beds of a dune and the cross beds of an antidune. This could lead to misinterpretation since dunes dip downstream while antidunes dip upstream.

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.