CrossBedding

Crossbedding refers to inclined internal laminae or sets within a bed that dip at an angle relative to the overall stratification. These inclined layers form when sediments are deposited on moving surfaces such as migrating dunes, ripple fields, or foresets within channels, producing a series of inclined beds called cross-bedding. The phenomenon is most common in sandstones formed in aeolian (wind-driven) or fluvial (water-driven) settings, but can also occur in deltaic and tidal environments.

In aeolian dunes, grains are deposited on the windward side and accumulate as inclined foresets as a dune migrates. The result is trough or planar cross-bedding with foresets dipping downwind. In fluvial settings, migrating ripples or dunes in a braided or meandering stream create similar structures, sometimes called trough cross-bedding; cross-beds can climb as the depositional surface rises.

Terminus: The features include foresets, bounding surfaces, and cross-bed sets; climbing cross-stratification occurs when successive cross-beds accumulate with increasing bed thickness. The direction of the dip of cross-beds records the paleocurrent direction during deposition.

Significance: Crossbedding is a diagnostic indicator of depositional processes and paleoenvironments and is widely used to infer paleo-flow directions. It also influences reservoir quality in hydrocarbon geology by creating permeability anisotropy within sandstones.

Occurrence: Cross-bedding is widespread in the rock record; notable examples include large-scale eolian cross-bedding in the Navajo Sandstone of the American Southwest and other desert dune deposits.

In interpretation, multiple cross-bedding directions within a succession can reflect changing flow directions or reworking; careful analysis is required.