Wednesday, April 22, 2015

On Rock Classification

Two interesting articles:

1) In the Journal of Sedimentary Research (behind paywall) Kitty Milliken proposes a tripartite classification of fine grained sedimentary rocks, those with grain assemblages with greater than 50% of particles by weight or volume less than 62.5 µm (4 Phi). There are a number of names for these types of rock; mudstone, claystone, pelite, argillite to name a few. This classification categorizes the rocks according to the composition, thereby indicating the source of the grains. Composition in turn controls to a large measure bulk rock properties upon burial and interaction with fluids, thus enabling general predictions about their economic and engineering qualities.


A tripartite compositional classification is proposed for sediments and sedimentary rocks that have grain assemblages with greater than 50 percent of a weight or volume of particles smaller than 62.5 µm (4 Phi). Tarl (terrigenous–argillaceous) contains a grain assemblage dominated by more than 75 percent of particles of extrabasinal derivation, including grains derived from continental weathering and also volcanogenic debris. Carl (calcareous–argillaceous) contains less than 75 percent of particles of extrabasinal derivation debris and among its intrabasinal grains contains a preponderance of biogenic carbonate particles including carbonate aggregates. Sarl (siliceous–argillaceous) contains less than 75 percent of particles of extrabasinal derivation and contains a preponderance of biogenic siliceous particles over carbonate grains.

These three classes of fine-grained particulate sediments and rocks effectively separate materials that have distinct depositional settings and systematic contrasts in organic-matter content and minor grain types. In the subsurface the grain assemblages that define these classes follow contrasting and predictable diagenetic pathways that have significant implications for the evolution of bulk rock properties, and thus, assigning a fine-grained rock to one of these classes is an important first step for predicting its economic and engineering qualities. For purposes of description these three class names can be joined to modifier terms denoting rock texture, more precise compositional divisions, specific grain types of notable importance, and diagenetic features. 

2) In Earth Magazine, a delightful article (open access) titled Geologic Column: The Rumpelstiltskin Factor by Ward Chesworth, professor emeritus at the University of Guelph, Canada.

Chesworth muses on the importance of naming objects and whether it is better to be a "lumper" or a "splitter" i.e. whether it is better to organize variation in to as few groups as possible or whether it is better to draw finer and finer distinctions and place a smaller range of variation into its own distinct cubicle.

An excerpt:

Excessive splitting can lead to problems, though. If an overly meticulous taxonomist kept on splitting hairs ad absurdum, we would wind up with a classification resembling an advanced case of logorrhea, the kind of thing guaranteed to drive working geologists to the brink. C.B. Hunt staged his own rebellion against this tendency when he considered the plethora of names invented for minor igneous intrusions. He expressed his displeasure by sarcastically concocting one more, cactolith, which he described as “a quasi-​horizontal chronolith composed of anastomosing ductoliths whose distal ends curl like a harpolith, thin like a sphenolith, or bulge discordantly like an atmolith or ethmolith.” He insinuated it into his 1953 U.S. Geological Survey professional paper on the Henry Mountains of Utah, and from there it crept under the radar into the first edition of AGI’s very own “Glossary of Geology.” Unfortunately, some humorless jobsworth banned it from all subsequent editions.

Sprinkled with more anecdotes, this is a fun read.


  1. Thanks for the tip! Will check this out. Kitty sits in the adjacent wing of our office building...

  2. great... you seem to be surrounded by a lot of interesting people! :)