Mudstones, which are sedimentary rocks dominated by silt- or clay-sized particles, are very abundant, perhaps comprising up to three-quarters of the entire sedimentary record. They are usually considered to have been deposited by the very slow settling of fine dispersed grains in quiet water conditions. For this reason, the assumption has been that thick mudstones must have taken extremely long periods of time to be laid down.
However, conventional thinking about mudstones is being challenged by recent research. Silt and clay particles are susceptible to flocculation, a form of coagulation that causes fine particles to clump together into larger aggregates. This process causes silt- and clay-sized particles to settle out of suspension rapidly as ‘flocs’ or ‘flakes’ which are about the size of sand grains.
In 2007, Schieber et al described flume experiments demonstrating the formation of muddy floccules that travel in bedload in the form of ripples. This allows muds to be transported and deposited at current velocities that are sufficient to transport and deposit sand. The Indiana University team even posted an educational video on YouTube — part 1, part 2 and part 3 — showing the experiments and resulting sediments.
In the June edition of Geology, Schieber and Southard (2009) have reported additional experiments on the rapid sedimentation of muds. Observations of the sediment transport in floccule ripples is usually hampered by the turbid nature of the muddy suspensions. However, Schieber and Southard have now succeeded in imaging the sediment movement associated with these floccule ripples.
Although the ripples contain as much as 90% water (by volume), they have a shape that is very similar to the ripples formed in sandy sediments. Furthermore, even though the particles that make up the floccule ripples are fragile and much less dense than sand grains, the way in which the mud is transported across these ripples is essentially the same as in sandy ripples. The investigators say that their new experiments provide the first direct observations of the processes that shape and propagate mud ripples.
I think it’s really exciting that the deposition of mudstones, so long considered problematic for catastrophist scenarios, is beginning to be viewed in a different light. Macquaker and Bohacs (2007 p.1735) even refer to “a paradigm shift” in mudstone science. There is now a need to re-evaluate conventional interpretations of mudstone formations in the geological record and the assumed conditions under which they were deposited.
Macquaker J. H. S. and Bohacs K. M. 2007. On the accumulation of mud. Science, 318:1734-1735.
Schieber J. and Southard J B. 2009. Bedload transport of mud by floccule ripples — direct observation of ripple migration processes and their implications. Geology, 37:483-486.
Schieber J., Southard J.B. and Thaisen K. 2007. Accretion of mudstone beds from migrating floccule ripples. Science, 318:1760-1763.