FLUME STUDIES OF GRAIN SIZE DISTRIBUTION IN MOVING CLAY SUSPENSIONS: RELATIONSHIP TO FLOCCULE COMPOSITION, FLOW VELOCITY, BOTTOM SHEAR STRESS, AND ACCRETIONARY BEDFORMS

Flume experiments have shown that clays accrete into clay beds from moving turbulent suspensions via formation of floccules that congregate into floccule ripples that pile up on the flume bottom. In order to examine whether the nature of these floccules is independent of flow velocity, grain size distribution in moving kaolinite suspensions (grain size 01 to 60 microns) was monitored as experiments were conducted at successively lower flow velocities.

In these experiments, the main body of the suspension was sampled at every flow velocity, and the grain size distribution of the samples was analyzed with a Micromeritics Sedigraph. A systematic shift of grain size distribution towards increasingly finer grained suspended load was observed as velocity was lowered. This implies that the bedload floccules are initially constructed of only the coarsest clay particles at high velocities, and that finer clay particles become incorporated into floccules as velocity is lowered. In kaolinite, the finest particles are thin platelets and the coarser particles are stacks of multiple platelets. Thus, the edges of coarse particles carry a higher charge and floccules formed from them are stronger and more shear resistant than floccules constructed only from single platelets. As a result, at high flow velocities only coarse particle flocs can survive the shear forces imposed at the passage into the viscous sublayer of the flow. At low velocities, floccules constructed from a mixture of coarse and fine particles, though of lower strength, are able to pass into the viscous sublayer as well. Thus, as velocity is lowered the grain size distribution in the remaining suspension is increasingly shifted towards the finer end of the grain size spectrum.

These observations have the following implications for the rock record: (1) clay beds deposited from decelerating flows should show subtle internal grading of coarser clay particles; (2) clay beds deposited from continuous fast flows should show a uniform distribution of coarse clays; and (3) still settled clays should produce uniform beds dominated by fine clays.