Because they succumb to erosion easily, mudrocks do not outcrop well, except on the coast or in arid lands. Often they show little if any stratification that field workers can distinguish from the partings imparted by compaction and dewatering, which make shales from them. Yet they are repositories of a great deal of information (see Earth Pages archives – Methane hydrate – more evidence for the ‘greenhouse’ time bomb). In hand specimen their two main components, silt-sized quartz grains (<62 microns) and clay minerals (>4 microns) only become distinguishable by chewing! They are irresolvable using optical microscopes, and detailed work needs scanning electron microscopy.
Silt to clay proportions in mudrocks are variable. The first is generally taken as an indicator of suspended debris from land masses and its proximity to where the mud accumulated. The more clay, the further muds were from exposed continents, or so sedimentologists used to assume. That approach has taken a hard knock from some recent detailed work on Devonian mudrocks (Schieber, J. et al., 2000. Diagenetic origin of quartz silt in mudstones and implications for silica cycling. Nature, v. 406 31 August 2000, p 981-985).
Jurgen Schrieber of the University of Texas (Arlington), Dave Krinsley of the University of Oregon and Lee Riciputi of the Oak Ridge National Laboratory in Tennessee used scanning electron microscopy, cathodoluminescence and ion-probe techniques to discriminate between detrital quartz grains and those formed by precipitation of silica from pore water in the original muds. Those grains that do not luminesce probably formed by silica solution and reprecipitation, and the Devonian mudrocks contain mainly non-luminescent quartz grains. Oxygen isotope ratios from individual grains confirm this in situ origin. The researchers had no reason to suspect that their Devonian samples would give such results, and assumptions based on silt to clay ratios from any mudrock are now in doubt.
Worse still, silt in ocean-floor muds, cores of which form the linchpin for Pleistocene climate studies, has been a rough and ready way of estimating wind speeds as climate shifted from glacial to interglacial conditions. These silts could be precipitates too, and the variations in their proportions may stem from changes in the delivery of dissolved silica from land to the oceans.
See also: Kemp, A. 2000. Probing the memory of mud. Nature, v. 406 31 August 2000, p 951-953