Experimental Study of Mud-Ripple Propagation – Direct Observations of Ripple Geometry and Transport Processes

Mud deposition has long been thought to occur mainly in quiet environments that are only intermittently disturbed by weak current activity. Recent flume experiments show that muds can be transported in bedload and deposited at current velocities that would suffice to transport and deposit sand. Deposition-prone floccules form spontaneously over a wide range of experimental conditions, irrespective of the exact parameters that drive flocculation in a given experimental run. These experiments used backlighting to observe ripples, but due to the turbid nature of the flow it was not possible to observe ripple-related processes directly and in real time.

A new set of experiments provides critical first hand observations of the processes that shape and propagate mud ripples. These consist of 90 vol. % water, yet their vertical profile is indistinguishable from current ripples formed in sand (clearly developed crest and brink, lee-side slopes approximately 25-30 degrees). A combination of various imaging approaches shows that sediment is transported over the stoss side in the form of diverging boundary layer streaks, the carriers of the bulk of the bedload floccule freight. At the brink line these streaks become point sources of sediment that feed avalanches of floccule-rich sediment lobes. These propagate down the slip face like classical mudflows on a hillside. Lateral shifting of boundary layer streaks averages out sediment deposition across the slip face. Migration rates of floccule ripples vary with flow velocity (example: 25 cm/hour at V=15 cm/sec and 5 cm flow depth). The latter rate is approximately 3 to 4 times slower than comparable sand ripples because of the fundamentally cohesive nature of floccule ripples. Compaction reduces these ripples to sub-mm streaks of clay with internal laminae of a few degrees dip. Nonetheless, it is possible to recognize them in the rock record.