What the heck do I call her? This question is not only asked by new parents or quarreling lovers or David Allen Coe, and it includes queries raised by geoscientists studying fine-grained sedimentary rocks—shale, claystone, mudstone, mudrock, lutite, pelite, and argillite? The recent explosion of efforts to understand fine-grained rocks (motivated by recent realization of their economic significance) generates a compelling need to bring order to the discussions. To do so, this paper by Milliken proposes a tripartate compositional classification for fine-grained sedimentary rocks, those with greater than 50% particles less than 62.5 µm. The classification scheme is a function of the abundance of particles of extrabasinal origin and the preponderance of carbonate versus biogenic siliceous particles. This scheme, although simple, is highly functional in that it also separates rocks with distinct depositional settings, organic matter content, and diagenetic pathways—and thus economic and engineering attributes.
A compositional classification for grain assemblages in fine-grained sediments and sedimentary rocks by Kitty Milliken