Hierarchical subdivision of the Cenozoic Era: A venerable solution, and a critique of current proposals

Abstract If Tertiary and Quaternary are to be formally defined and ranked, then only one topology and two ranking options for the major subdivisions of the Cenozoic Era are consistent with the principles of hierarchical classification. These are: 1) for the Tertiary and Quaternary to be ranked as Suberas (with the Paleogene and Neogene ranked as Periods of the Tertiary), or 2) for the Tertiary and Quaternary to be ranked as Periods (with the Paleogene and Neogene ranked as Subperiods of the Tertiary). The first option would leave the Period rank empty for Quaternary time, however, so the second option is preferred here. In neither case is an extension of the Neogene to the present permitted. If desired, however, “Early Cenozoic” and “Late Cenozoic” could be defined as formal unranked chrons, with the latter term providing an unambiguous expression for the same concept as the extended Neogene. Attempts to justify an extended Neogene by reference to a statement arising from the 1948 International Geological Congress fail because they are based on a misinterpretation of that statement. Likewise, an appeal to the alleged original meaning of the Neogene fails because it is factually superficial and would also have unacceptable consequences if applied as a general principle to the rest of the time scale. Four arguments against a ranked Tertiary and/or Quaternary involve the status of these names as parts of an obsolete classification; the disparity in their durations; an analogy between them and the unranked Precambrian; and their supposed ambiguity. These arguments are readily refuted. Arguments in favor of a ranked Tertiary and Quaternary include the continuing widespread use of these names, their practical relevance for geologic maps, and their role in honoring the pioneers of stratigraphy. A recent recommendation by the International Commission on Stratigraphy (ICS) would decouple the beginning of the Quaternary from the beginning of the Pleistocene, have the Neogene and Pliocene overlap the Tertiary/Quaternary boundary, and have the Gelasian Age belong to both the Pliocene and the Quaternary. These results violate the existing Guidelines of the ICS. This same proposal would also regard the Quaternary as a Subera within the Neogene Period (the latter being extended to the present). However, this arrangement violates the most fundamental rule of hierarchical classification in that a unit of superior rank would be included entirely within a unit of inferior rank. Given these problems, the International Union of Geological Sciences (IUGS) must reject the ICS recommendation. An improved version of the ICS proposal (with the Quaternary ranked as a Period and the Paleogene and extended Neogene ranked as Suberas) would still prohibit the Tertiary from being ranked and would either violate “Simpson's rule” or else require a new name for the traditional concept of the Neogene. Because the classification of the Cenozoic advocated here is strictly hierarchical, violates none of the existing Guidelines of the ICS, obeys Simpson's rule, requires no new names, allows the Tertiary, Quaternary, Paleogene, and Neogene to be formally ranked, and has been in general use for more than a century, it is to be preferred. Only two current options for defining the Quaternary and Pleistocene are valid, namely, to leave the beginnings of these units where they are now (Vrica GSSP at 1.8 Ma), or to move both to the Piacenzian/Gelasian boundary (Monte San Nicola GSSP at 2.6 Ma). The latter option is not prohibited by Lyell's “types” of the Older Pliocene nor (after 2008) by the existing Guidelines of the ICS, but if enacted could jeopardize the stability of the rest of the standard global time scale.