Bedforms

Bedforms are morphological patterns that form at the interface of flowing fluid and a bed of moving sediment. Typical examples include dunes and ripples. Lateral migration of bedforms can generate sedimentary structures, like ripple cross-lamination or cross-stratification. Geology students often mix up the concept of bedform and sedimentary structure: ripples are bedforms (not sedimentary structures) and ripple cross-lamination is a sedimentary structure (not a bedform). Sometimes, like in this picture of hyperpycnites from the Pennsylvanian Minturn Formation near McCoy (CO, USA), both sedimentary structure and associated bedform are preserved in a bed. The asymmetric profile of unidirectional current ripples (flow towards the left) and foreset laminae dipping to the left are clearly visible at the top of two consecutive sandstone beds, overlain by mudstone beds. Draping of bedforms by finer sediment indicates a decrease in hydrodynamic energy within a rapidly discharged, sediment-laden flow. Pencil for scale.

bedforms bedding sediment gravity flows sedimentary structures hyperpycnite ripples cross-lamination sedimentology Pennsylvanian rapid deposition sandstone