A COMPOSITIONAL CLASSIFICATION FOR GRAIN ASSEMBLAGES IN FINE-GRAINED SEDIMENTS AND SEDIMENTARY ROCKS

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 mm (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.