Flume studies of mud deposition: Implications for shallow marine mud deposition and the stratigraphic record (Invited)

Racetrack flumes with paddle-belt drives, designed to avoid destruction of floccules, were used to examine mud deposition from swift moving suspensions. At flow velocities that transport and deposit sand (20-35 cm/s, 5 cm flow depth) muddy suspensions produce deposition-prone floccules that form migrating floccule ripples. Mud beds that form as a consequence of floccule ripple accretion appear parallel laminated at first glance, but reveal internal low angle cross-lamination on closer inspection. In plan view, the observed pattern of ripple foresets is identical to rib and furrow structure in sandstones. In experiments at marine salinity, where clays were mixed with quartz silt, uniform as well as gradually decelerating currents produce deposits that show low relief coarser silt ripples at the base, followed by low angle inclined silty laminae and an increasing clay component upwards. Once examined with proper care, the clay-rich upper portion of the deposits shows internal laminae and cross-laminae. We also observed comparable sedimentary features in various ancient shallow marine mudstone successions. When examined at the hand specimen or drill core scale, above experimental deposits show the same features that have been widely reported from fluid-mud event layers in modern shelf settings. The latter have been attributed to gravity flows that are triggered by wave re-suspension of surficial muds. Whereas that model is consistent with the features attributed to the purported process, our experiments point to a potential alternative scenario. In addition to fluid muds, meaning high concentration sediment suspensions with mass concentrations > 10 g/l, the low concentration sediment suspensions (<10 g/l) from our experiments appear to be able to create equivalent deposits when given sufficient time for advective sediment transport. Graded muddy deposits on modern shelves and in ancient shelf successions thus may be of multiple origins. In addition to being the result of wave-triggered gravity flows, they might for example also reflect seasonal reworking of nearshore muds that are carried offshore by wind-driven circulation, storm induced downwelling of muddy suspensions, or even remobilization of bottom muds by tidal currents. Establishing the boundary conditions for these potential scenarios will require further experimentation.