OBSERVATION OF CURRENT RIPPLES COMPOSED OF MUD IN MODERN TERRESTRIAL FLOOD DEPOSITS

Current ripples composed of mud have been reported from flume experiments and the rock record. Flume experiments indicate that at flow velocities that transport and deposit sand (20-35 cm/s, 5 cm flow depth) clay suspensions produce deposition-prone floccules that form migrating floccule ripples. Although the resulting clay beds appear parallel laminated, they contain certain diagnostic textures that identify lateral accretion, such as low-angle downlap of laminae and updip lamina truncations. The latter have also been identified in several examples from the rock record, but until now there has been no documented case of mud ripples occurring in modern natural settings. Mud ripples were observed in flashflood deposits in Indiana. These ripples formed under a thin sheet of water (5-10 cm deep) at flow velocities estimated at 20-50 cm/s. Ripple geometries are transverse to lunate with wave lengths from 4 to 20 cm. Ripple crests range from 4 to 15 mm in height. They show sediment avalanching on foresets and are internally cross-laminated. Although the ripples have the same general geometry and dimensions as the much more familiar sand ripples, they contain between 70-80 vol. % water. When still submerged they offer little resistance to touch and experience fabric collapse when disturbed. Grain size analysis of samples from these ripples indicate that ~50% of the particles are finer than 10 microns, and that the modal grain size is 20 microns. Although this is somewhat coarser than mud ripples produced in flume experiments, these muds readily form floccules in freshwater. It is therefore most likely that they formed as migrating ripples of flocculated mud, directly analogous to flocculated clay ripples observed in flume experiments. Mud ripples as described here are probably widespread in a range of depositional environments characterized by muddy suspensions. Given that muddy suspensions are a common feature of terrestrial floods, it is quite likely that many of the laminated muds in ancient fluvial mud successions accreted via migrating ripples of flocculated mud.