Detrital sedimentary rock classification and nomenclature for use in New Zealand

Abstract

Grain-size distribution and mineralogical composition are independent variables in most detrital sediments. Separate classifications are necessary to describe these sediment attributes. Essential characteristics are given when the following format is used: (sorting term) (size term): (cement) (prominent nondetrital) (detrital composition). This polynomial evokes a clear mental image of the sediment. Descriptive nomenclature and procedure are modified from earlier Folk publications. The resultant classification system is objective and practical. Gravel, sand, silt, and clay are end members of triangular diagrams that are divided into named textural categories in accord with apparent natural groupings. Modal size and sorting terms supplement the names of some categories. Rudite, arenite, and lutite are used in a compositional sense for detrital sediments whose texture is respectively indicated by the terms gravel (conglomerate or breccia), sand (sandstone), and mud (mudstone, siltstone, or claystone). Predominant mineral or rock fragment components prefix these compositional terms to provide unequivocal names (e.g., feldsarenite, schistrudite, quartz illite-lutite). Triangular diagrams are suggested for quantitative plotting of compositional data. The classification is designed for use in the field and in the laboratory. It is flexible in that the user can select the level of detailed description applicable to his needs. Adoption of this classification by New Zealand geologists would result in compilation of comparable data and would simplify communication.