Bedload Transport of Flocculated Mud and Resulting Microfabric of Deposits – Insights from Flume Studies

Abstract:

In flume experiments muds flocculate in moving suspensions. Salinity enhances flocculation efficiency, but its absence will not prevent flocculation. The exact impact of salinity is subject to further experimental work. For a given mud composition there is a critical velocity (or shear stress) of sedimentation below which floccules transfer through the basal shear layer and travel as sand-size bedload particles. These bedload floccules form migrating ripples that are in size and geometry comparable to sand ripples. For various clays (illite, kaolinite, smectite) critical velocities of sedimentation are in the 20-25 cm/sec velocity window (5 cm flow depth). At higher velocities bedload particles are dense silt size (25-60 micron) clay composites, the coarse tail of the processed clay deposit. Once the critical velocity is reached (bed shear stress ~0.2 Pa) larger floccules appear in bedload that grow to a size of several hundred microns as velocity is lowered further.

When clays are mixed with quartz silt, transport segregation occurs. Coarse silt (~15-63 microns) forms bedload ripples that travel over the flume bed at the same time as ripples that consist of flocculated clays. Fine silt (<15 microns) is integrated into clay floccules and travels separate from coarse silt. As ripples of either silt or clay migrate over the flume bed, they leave behind a thin veneer of sediment (a few particles thick), and over time a deposit results that consists of a random vertical succession of coarse silt laminae and clay rich laminae with scattered fine silt – a parallel laminated mud.

These experimental muds have direct counterparts in many ancient laminated shales from Precambrian to Tertiary. The ancient muds show the same segregation into coarse silt vs clay-rich laminae, and show scattered fine silt within clay rich layers. This strongly suggests that ancient laminated shales formed when bottom currents transported flocculated muds in bedload over the seabed.