REVISIONS TO FOLK'S SAND-SILT-CLAY TERNARY DIAGRAM: A NECESSARY COMPONENT OF STANDARDIZING TEXTURE AND FACIES CODES FOR A PROCESS-BASED CLASSIFICATION OF CLASTIC SEDIMENT AND ROCK

Folk (1954, 1980) provided two ternary diagrams for classifying clastic sediment and rocks using grain size attributes of texture. The principal diagram (G:S:M) had gravel (G), sand (S) and mud (M) as end members. The secondary ternary diagram (S:Z:C) was specific for fine-grained sediments, with sand (S), silt (Z), and clay (C) as end members. It was intended for samples that lack gravel and to further differentiate the mud fraction.

Farrell and others (2012) recently revised both of Folk’s ternary diagrams to create a process-based, standardized method of classifying all clastic sediment. The revised classification is independent from composition; the terms gravel, sand, and mud are size terms for grain populations. The revised textural fields are based on a strict adherence to volumetric estimates of percentages of gravel, sand, and mud size grain populations which sum to 100%. A textural field exists for every possible combination of % G:S:M, assuming a 30% cutoff for gravel and 95% cutoffs for slightly impure to pure end members. These systematically developed textural fields form the bases for universal lithofacies codes, and they apply readily to analytical results of G:S:M that sum to 100%.

To preserve consistency with the new G:S:M diagram, Folk’s S:Z:C diagram was revised. Like Folk’s, the revised S:Z:C diagram is based on the 50% cutoff for sand and the Z:C ratio. Pure end members, however, are defined at 95% rather than Folk’s 90%. Six new textural fields (zcS, czS, zsC, szC, csZ, and scZ) are defined by segments of the alternate bisectors, S:C = 1:1 and S:Z = 1:1, that were not used by Folk.. These use percentages of all three end members (S:Z:C) and their relative order to name a textural field. The specificity in the revised S:Z:C textural fields accommodates the established nomenclature for unconsolidated and consolidated equivalents — mud/mudstone, silt/siltstone, and clay/claystone. The new ternary diagram provides bases for specifying silt or clay dominance as well as the order of abundance of the three end members S, Z, and C.

Farrell, K.M., Harris, W.B., Mallinson, D.J., Culver, S.J., Riggs, S.R., Pierson, J., Self-Trail, J.M., Lautier, J., 2012, Standardizing texture and facies codes for a process-based classification of clastic rock and sediment, Journal of Sedimentary Research, in press.