ABSTRACT: 19th and 20th century stratigraphy often concerned itself primarily with classification and nomenclature, during what can be termed the heroic and codex ages of stratigraphy. In contrast, 21st century stratigraphy will fall within the post-modern age. In possession of agreed classification schemes, future stratigraphers will concentrate on (i) the reconstruction of earth environments and processes (including evolution) through time, (ii) the eficient location and recovery of useful earth resources, and (iii) the study of those geological hazards that can be understood within a stratigraphic context. The first objective - reconstructing environments through time - requires the use of a conceptual framework similar to the one that we term the geological time scale (GTS). The 21st century GTS will be based on GSSP designations at the base of all geological Periods and, ultimately, Ages, i.e. it will comprise an internationally agreed chronologic hierarchy. Recognition of local chronologic schemes (as distinct from biostratigraphies based on Oppelzones) will thereafter serve no useful purpose and local “Agesâ€ will become redundant. Globally, recognition of a separate but completely parallel chronostratigraphic classification will also serve no useful purpose, and this hierarchy too will be abandoned. Correlation of events into the GTS will be undertaken using a wide variety of methods, including numeric dating, fossil occurrence, physical and chemical properties, tephrochronology and astrochronologic retrodictions. Biostratigraphy, though remaining a vital tool, especially for Phanerozoic strata, will carry no necessary correlation primacy. Meeting the second and third objectives - locating and recovering earth resources, and studying hazards - requires first and foremost the creation of detailed geological maps and stratigraphic columns. The lithostratigraphic hierarchy of Bed-Member-Formation-Group-Supergroup is an efficient and mostly objective classification whereby useful maps and columns are created. Because geological mapping is concerned with local stratigraphic detail and complexity, it cannot, like chronology, be organized within a global nomenclature. Over different large areas, different major, genetically-related packages of sediments correspond to the form ation, filling and sometimes destruction of sedimentary basins - as driven by regional tectonic events, and as influenced by regional climatic and oceanographic histories. At the supra-Group or supra-Supergroup level, major sediment assemblages of this type are separated by regional unconformities, as recognized by the creation of a category of Unconformity-bounded Units (UBU) in the 1994 2nd edition of the International Stratigraphic Guide. Whether or not UBU are continued with as a formal unit of classification, the strong need will persist for the type of regional, unconformity-bounded units that have successively been termed Sequence and Synthem, for use as the highest level within the lithostratigraphic hierarchy.
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