Mudrocks: Siltstones, Mudstones, Claystones & Shales
2015. "Mudrocks: Siltstones, Mudstones, Claystones & Shales", A Color Guide to the Petrography of Sandstones, Siltstones, Shales and Associated Rocks, Dana S. Ulmer-Scholle, Peter A. Scholle, Juergen Schieber, Robert J. Raine
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Shale and mudstone are both widely used terms for fine-grained terrigenous clastic rocks (although some use fissility as a requirement for the use of the term “shale”), but there is at present no broadly agreed upon terminology for naming and classifying these rocks (see discussions in Schieber et al., 1998, and Potter et al., 2005). Because in past stratigraphic and sedimentologic studies, the great majority of fine-grained rocks have been designated as shales (such as, for example, the Cretaceous Eagle Ford Shale that is, in many places, a marl or even a limestone) or the Monterey Shale (a diatomite or chert depending on diagenesis), we will use the term shale in this chapter, with the understanding that it includes what some prefer to identify as mudstone. Also, because the most basic definition of shales, that they be dominated by particles smaller than 62.5 μm (e.g., Blatt et al., 1980), implies that shales span the clay-silt boundary, a good many rocks that are identified as siltstones in the literature also qualify as shales (and vice versa). The naming of any rock must on one hand convey a maximum amount of information about the rock, yet at the same time accept incompleteness for the sake of brevity. What would the key properties of a shale be with that purpose in mind? Grain size, for example, while a useful property of sandstones, lacks utility for shale because it is too small to be readily discerned. Mineral composition, though unquestionably useful, is again stymied by small grain size. XRD or whole rock chemistry data would be needed to make it workable. In fact, without advanced instrumentation, the most accessible properties of a shale are probably its color, its relative softness (lithification state), its reaction with hydrochloric acid (is it calcareous?) and textural features such as lamination, bioturbation, etc. Whereas it is not uncommon to see geologists use color charts to describe rock color, simpler qualifiers (gray, red, green, greenish, etc.) are in many instances sufficient. Relative softness can be tested be scratching the sample with a nail, and most geologists also would have some hydrochloric acid handy. So, aside from color, most of what we are able to say about a shale at first encounter is decidedly of qualitative nature, and that circumstance makes textural features very valuable when we set out to describe and categorize shales.