ACCRETION OF MUDSTONE BEDS FROM MIGRATING FLOC-RIPPLES

That muds are mainly deposited in quiet environments that are only intermittently disturbed by weak currents is a widely held assumption among sedimentologists. For the past 18 months we have conducted flume studies to closely observe mud deposition and erosion under controlled conditions. In the process we determine thresholds of deposition and erosion for various clay types and a range of depositional parameters (salinity, sediment concentration, flow velocity, flow depth, etc.). These experiments were inspired by observational evidence (field studies and petrography) that suggests abundant current activity during mudstone deposition, and further point towards mud transport and deposition at current velocities that would also transport and deposit sand.

Our experiments so far have shown that floccule formation is a common phenomenon, even in distilled water and at low sediment concentrations (some 10 mg/l). Flocs congregate into migrating patches that appear to be the equivalent of ripples in coarser sediments. As more floccules accumulate at the flume bottom, floc-ripples grow larger and downstream dipping foresets can be visualized with marker sediments. With further clay addition and/or lowering of flow velocity, floc ripples eventually become stationary, spread out, and coalesce to form a contiguous mud bed. With further sediment addition floc ripples continue to migrate over this initial bed and build up a layer of mud. Magnetic fabric studies of mudstones in the rock record indicate preferred alignment of clay particles. Such alignment could be the result of mud bed accretion from migrating floc-ripples, and suggests that the observed mode of mud bed accretion may be a common process in nature.