Cenozoic chronostratigraphic terminology: In defense of formal subseries
Authors: Aubry Marie-Pierre 
Published: January 2024

Ever since the series and corresponding epochs of the Cenozoic began to be defined over 180 years ago, the Earth science community has recognized bi- and tripartite lower/early, middle, and upper/late divisions of these units. As chronostratigraphy became more precise, these divisions assumed an essential role in the integration of the continuous deep-sea successions where the tools for worldwide correlation were developed, and the historic but disjunct sequences on land in which the stages/ages of the time scale were defined. Rather than being discarded as too vague, the essential value of these subdivisions has been tacitly recognized by describing them in terms of the newly recognized global stages, allowing their boundaries to be identified by the GSSPs (Global Stratotype Section and Point) of the lowest component stage. In this way, and without noticeable controversy, the modern Cenozoic literature treats the lower, middle and upper divisions of its series as elements within the chronostratigraphic hierarchy, i.e., as de facto subseries. Their status in the hierarchy has nonetheless been questioned recently by several members of the ICS (International Commission of Stratigraphy) Bureau on the basis that subseries, as such, have not been explicitly defined by ratified GSSPs. Accordingly, this rank has been omitted from versions of the ICC (International Chronostratigraphic Chart), a product of the ICS. Such omission fails to consider that subseries (and by inference subepochs) are valuable in circumstances where individual stages are inappropriate or often not applicable, e.g., in such disciplines as seismic stratigraphy and climatostratigraphy. The status of subseries in the Cenozoic Erathem is presently under discussion, and there exists within the Subcommissions on Paleogene and Neogene Stratigraphy of the ICS the view that such divisions should have informal status (i.e., non IUGS-ratified subseries/subepochs). This would seemingly contradict the primary goal of the ICS, which since 1986 has focused on establishing a functional common language for all whoworkwith geological time, by making the widely used concepts of subseries and subepochs open to misunderstanding. It also begs the question, as to why should something that consists of GSSP-defined units, and which is in turn a component of higher units in the chronostratigraphic hierarchy, not be considered a formal chronostratigraphic unit in its own right? The reality is that whether formal or informal, the subseries/subepochs of the Cenozoic will continue to be broadly used. Therefore, the interest of the ICS and the Earth science community is best served by formally recognizing them in the ICC hierarchy. This will clarify the problem of inconsistent capitalization of the terms lower, early, middle, upper and late where these are in fact part of a formal unit name and not just a modifier for an indefinite interval, and it will satisfy the broad consensus of the profession, as evidenced during a recent open meeting of the ICS during the 2nd International Congress on Stratigraphy in Graz in 2015.