BEDLOAD TRANSPORT OF COARSE, MEDIUM AND FINE MUD – FLUME STUDIES WITH GRADED QUARTZ POWDERS

In addition to clays, natural muds invariably contain a substantial quantity of detrital quartz in the 62 – 1 micron grain size range. For a better understanding of potential transport and depositional processes, a series of flume experiments were conducted with commercial quartz powders that had maximum grain sizes of 50 microns, 40 microns, 30 microns and 5 microns. The coarsest grain size in each sample constituted less than 10% of the total sample, and for the 50, 40, and 30 micron grades at least 50 % of the sample was finer than 10 micron. The “fine fraction” of each sediment batch was removed in an initial experiment step, and then the coarse fraction was tested for critical velocity of sedimentation.

These quartz powders show critical velocities of sedimentation of 45 cm/s; 40 cm/s, and 20 cm/s (5 microns); and ripple migration stopped at 20 cm/s (50 and 40 microns), 25 cm/s (30 microns), and 10 cm/s (5 microns). There is visual evidence (SEM observations) for floccule formation in the 30-5 micron size range, suggesting that cohesion between grains becomes important at finer grain sizes and smaller flow velocities. Whereas the coarser size grades (50, 40, 30 microns) form migrating barchan ripples, the finest size grade (5 microns) forms migrating transverse ripples once velocity drops below critical. This effect probably reflects the fact that at 5 microns we have abundant sediment in suspension, and that sediment is continually transferred to bedload over a longer time period.

Based on these observations one might assume that in the rock record, the average grain size of pure silt ripples may allow a more refined assessment of paleoflow velocities. However, in natural systems organic matter and microbial biofilms can complicate matters. In our experiments, the flume had been treated with sodium hypochlorite for disinfection, however in experiments that ran longer than 1 week biofilm induced floccule formation was observed. If biofilm interference occurs it is likely that under natural conditions, when biofilms and organic matter exists, fine grained silica will inevitably co-flocculate with organic matter and form mixed mineral-organic aggregates.