Wacke, Graywacke and Matrix--What Approach to Immature Sandstone Classification?

ABSTRACT Huckenholz (1963) showed that the original arkose and graywacke overlap completely in both texture and composition; perhaps suppression of one or both terms in indicated. Graywacke has the longer, if not more significant (albeit confused), history. For 100 years in Germany and 50 in Britain, before development by Sorby of thin section petrography, graywacke was used as a purely hand specimen term based chiefly upon texture. It seems pointless to change this emphasis for an important, large, texturally-similar yet compositionally-varied rock clan. Modal analysis and interpretation of immature sandstones presents many problems, especially that of matrix which is increasingly attacked as a classificatory parameter, chiefly of maturity. But it matters not how or when conspicuous matrix of graywackes formed for it to be useful for descriptive "coarse" separation of pure (arenite) and impure (wacke) sandstones; genesis of fine material must be interpreted in any sand, recent or ancient. Finer subdivision can be made on a compositional basis following flexible schemes such as Gilbert's. Sedimentary structures, tectonics, provenance, depositional process and environment should be avoided in petrographic classification. If textural and compositional maturity be accepted as the prime guides in classification, then purely quantita ive placing of a rock within each maturity spectrum could replace all troublesome varietal names.