backbedding

Backbedding is a term used in sedimentology and stratigraphy to describe a pattern of inclined sedimentary layers that forms as beds are deposited with episodes of flow reversal or reworking. In many descriptions, backbedding is associated with backset laminae or reverse-dipping deposits that lie atop an earlier bed, indicating a change in the direction of sediment transport during successive depositional events. The phenomenon is most commonly discussed in the context of cross-bedded sandstones and related facies.

Formation and characteristics

Backbedding develops when a dune, ripple train, or channel bar migrates and then experiences a renewed surge of sedimentation in a direction that redefines the surface on which new material is laid down. The result is a succession of inclined layers that dip opposite to the primary cross-bedding or reflect abrupt changes in flow direction. It is observed in both aeolian (wind-blown) and subaqueous (waterborne) environments, including deserts with migrating dunes, river or tidal channels, and deltaic settings.

Interpretation

Geologists use backbedding as an indicator of episodic reorganization of flow and sediment supply. The presence of backset laminations can signal flow reversals, avulsion events, or rapid shifts in transport mechanisms. Care is taken to distinguish backbedding from other stratigraphic features produced by similar processes, such as laterally variable bedding or post-depositional deformation.

See also

Bedding, cross-bedding, foresets, backsets, sedimentary structures.