Every introductory geology course hammers home the message that the finer the grains in a sedimentary rock, the lower the energy under which it was deposited. This ‘received wisdom’ links to the ways in which grains move in moving fluids: rolling; bouncing and in suspension. A reductionist view sees this as the influence of Stokes’ Law in the boundary conditions between turbulent and laminar flow, close to the bed of flow and higher up in the fluid respectively. Stokes’ Law is invoked as that explains how spheres falling through fluids reach a steady speed related to the fluid’s viscosity. The larger the radius of the sphere, the greater that settling speed is. For the smaller size ranges settling speed is proportional to the square of the radius (laminar flow conditions), whereas for large objects it is proportional to the square root of radius (turbulent flow). This nicely explains the upward decreasing grain sizes in graded beds, formed when a mixture of grain sizes settles from moving fluids when their speed slow, as in turbidites and the on the lee sides of sand dunes. Since we often see silts and muds being deposited in low-energy lagoons and estuaries on the coast that too seems to verify the theory. However, muds that contain clay mineral particles are quite different from scaled-down spherical grains: they are platy; often have unbalanced electrical charges and are subject to Brownian motion that helps keep them in suspension. When clays suspended in fresh river water meet the sea, ions in sea water encourage the plates to clump together as aggregates or floccules that are much larger than the clay particles themselves. Another oddity is that, once deposited, clays are not as easily eroded as uncemented sands, partly due to their hosting biofilms that hold the particles together.
Despite the accepted explanation of mudstones as indicators of past low-energy conditions based on reductionist notions, suspicion of awkward complexity dates back to Henry Clifton Sorby, one of the founders of geology, who suggested that the study of mudstones and shales was a great challenge for sedimentology. In reality there are probably more than 30 parameters that govern the shifting and deposition of muds, many bound up with flocculation. Confidently discussing the true environmental conditions of mudstone deposition is often thwarted by their ease of weathering and by small animals that munch their way through muds to exploit often high contents of organic debris. Even the fissility of shales is a mystery in the field. Now and again, muds do reveal surprises, such as ripples and cross lamination, that surely reflect current action. Only experimentation can throw light on Sorby’s great challenge (Schieber, J. et al. 2007. Accretion of mudstone beds from migrating floccule ripples. Science, v. 318, p. 1760-1763). Schieber and colleagues from MIT and Indiana University used experimental flumes to investigate what happens to clay floccules, seeding the materials with fine hematite grains to show up any bedforms clearly. The muds used were from 5 to 63 mm in size, which produced floccules between 0.1 to 1.0 mm. Again and again the experiments produced migrating ripples, some like tiny barchan dunes made of clay floccules. The surprise lay in the flow speeds at which they began to form: between 10 to 30 cm s-1, much the same as those needed to produce sand ripples. Floccules were preserved in the experiments, but since they are made of clay minerals, compaction tends to destroy floccule outlines when mudstones form.
No doubt some fine-grained sedimentary rocks reflect low-energy environments, but without more careful examination of their small-scale features muds formed by energies as high as those involved in producing sandstones and many limestones will go unnoticed. Since mudstones are the most common sedimentary rocks in the geological record, some big surprises are in store.
See also: MacQuaker, J.H.S. & Bohacs, K.M. 2007. On the accumulation of mud. Science, v. 318, p. 1734-1735.