The Camp et al. discussion of Milliken’s (2014) compositional classification of fine-grained sediments and rocks is a welcome opportunity to draw further attention and deliberation to an important topic. Camp et al. do not, however, address the conceptual model that underlies the classification: that the composition of the primary grain assemblage is a key predictor of the pathways of chemical and mechanical modification that control the evolution of bulk rock properties in the subsurface. If one accepts this fundamental linkage between grain composition and diagenesis, objections raised by Camp et al., while interesting and worthy of discussion, should not be used to discourage a thorough testing of the proposed classification as the alternatives do not address these linkages. The classification has potential value for prediction of bulk rock properties relevant to exploitation of shales as reservoirs and seals, for oil, gas, and CO2, and supports a broader understanding of the role of fine-grained sediments in fluid flow and elemental cycling in the sedimentary portions of the crust.

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