FLUME EXPERIMENTS IN MUDSTONE SEDIMENTOLOGY: FLOCCULE FORMATION IN THE DEPOSITION AND EROSION OF MUDS

A specially designed racetrack flume with a paddle belt for water propulsion avoids the destruction of clay floccules and results in more “natural” behavior of clay suspensions. Deposition experiments on pure clay suspensions (50 to 2000 mg per liter) show that floccule formation, contrary to popular notions, is a common phenomenon over a wide salinity range. At grain size distributions typical for muddy sediments (lake, tidal, marine shelf, fluvial), sand size floccules start to form and migrate as ripple-like bedforms that accrete into contiguous mud strata. The critical velocity of sedimentation is dependent upon sediment concentration. As sediment concentration increases, the flow velocity at which net deposition via floccule ripples occurs increases as well. This velocity (at 5 cm flow depth) is ~11 cm/sec for small sediment concentrations (50 mg/l) and rises to above 25 cm/sec for sediment concentrations of 1 to 2 g/l.

For extremely fine clay mixtures, flocculation occurs just as in coarser mixtures, but for a given sediment concentration the critical velocity of sedimentation is substantially lower. Rather than accreting from migrating floccule ripples, clay floccules appear to accrete analogous to adhesion ripples in eolian settings. The same type of accretion is also observed when natural muds with a biological component (finely dispersed organic matter, microbial slime and biofilms on clays) are used, although the critical velocity of sedimentation is more comparable to the pure clay experiments.

We also tested the erosion behavior of flume deposited clay beds. As expected from the cohesive nature of clays, the critical velocity of erosion substantially exceeds the critical velocity of deposition and depends on the length of the consolidation interval. Interestingly, however, once erosion commences it does not occur as a simple transfer of clays into suspension. Instead, we observe the formation of clay balls that are indistinguishable from our earlier deposited floccules and travel in bedload. As velocity is stepped up this bedload population increases, but turbidity barely rises. Only at high flow velocities (25 cm/sec and above) do we see substantial jumps in turbidity owing to partial disintegration of floccules.