Faculté des sciences

Chronostratigraphy and geochronology: a proposed realignment

Zalasiewicz, Jan ; Cita, Maria Bianca ; Hilgen, Frits ; Pratt, Brian R. ; Strasser, André ; Thierry, Jacques ; Weissert, Helmut

In: GSA Today, 2013, vol. 23, no. 3, p. 4-8

We propose a realignment of the terms geochronology and chronostratigraphy that brings them broadly into line with current use, while simultaneously resolving the debate over whether the Geological Time Scale should have a “single” or “dual” hierarchy of units: Both parallel sets of units are retained, although there remains the option to adopt either a single (i.e., geochronological) or... Plus

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    Summary
    We propose a realignment of the terms geochronology and chronostratigraphy that brings them broadly into line with current use, while simultaneously resolving the debate over whether the Geological Time Scale should have a “single” or “dual” hierarchy of units: Both parallel sets of units are retained, although there remains the option to adopt either a single (i.e., geochronological) or a dual hierarchy in particular studies, as considered appropri-ate. Thus, geochronology expresses the timing or age of events (depositional, diagenetic, biotic, climatic, tectonic, magmatic) in Earth’s history (e.g., Hirnantian glaciation, Famennian-Frasnian mass extinction). Geochronology can also qualify rock bodies, stratified or unstratified, with respect to the time interval(s) in which they formed (e.g., Early Ordovician Ibex Group). In addition, geochronology refers to all methods of numerical dating. Chronostratigraphy would include all methods (e.g., biostrati-graphy, magnetostratigraphy, chemostratigraphy, cyclostrati-graphy, sequence stratigraphy) for (1) establishing the relative time relationships of stratigraphic successions regionally and worldwide; and (2) formally naming bodies of stratified rock that were deposited contemporaneously with units formally defined at their base, ideally by a GSSP (Global Boundary Stratotype Section and Point = “golden spike”) that represents a specific point in time. Geochronologic units may be defined and applied generally by either GSSPs or—as currently in most of the Precambrian—by Global Standard Stratigraphic Ages (GSSAs). Geochronologic units would continue as the time units eons/eras/periods/epochs/ages, and chronostratigraphic units as the time-rock units eonothems/erathems/systems/series/stages. Both hierarchies would remain available for use, as recommended by a formal vote of the International Commission on Stratigraphy in 2010. Geological context helps determine the appropriate usage of the component units.