The ICS International Chronostratigraphic Chart

Abstract

IUGS) uses the units and formally defined boundaries as international standards: they are included in the library of GeoSciML, an application for globally accessing standards-based geoscience data and information. Furthermore, the GSSPs (Global Boundary Stratotype Section and Point) that define the boundaries are recognized as international geostandards, at which ICS encourages the placement of markers, educational exhibits, and even 'golden spikes' in wellattended dedication ceremonies (Schmitz et al., 2011: their Fig. 4; Morton, 2012). Here we present the most recently updated version of the ICS Chart dated January 2013 (Fig. 1). From earlier editions, we have continued the organisation of the chart in four columns. Three columns present the Phanerozoic, showing four systems in each column. At the lowest level they display 34 + 34 + 32 stages. The stage names are derived from stratotype areas, and the stages are defined primarily on marine faces. The fourth column shows the Precambrian, subdivided to system level. An innovation to the layout is that the stages of greater temporal duration are shown with thicker intervals in the columns. The numerical ages make clear that the chronostratigraphic units are not of equal temporal duration and that the geological time scale is not a linear one. The three Phanerozoic columns span 145, 214 and 182 Ma respectively, but have equal heights in the chart. Within these columns, each stage is given a fixed proportion of the column height (e.g., 2%). The remaining height (100-(34 x 2) = 32% in the example) is then distributed proportionally to the stages that cover more than the fixed proportion on a linear timescale for that column. With the design we have tried to communicate visually, in improved style, that the division of Phanerozoic geological time is irregular and governed by the stratigraphical successions on which the units were originally defined, to which many subsequent revisions have been made. The chart design is intended to be advantageous in the daily use for both professional geologists and other users alike. Global divisions and 'Golden Spikes' Units of all ranks are in the process of being defined by GSSPs for their lower boundaries, including those of the Archean and Proterozoic, the latter long defined by Global Standard Stratigraphic Ages (GSSA). The status of each GSSP is displayed in the chart by small golden-spike icons at the base of the divisions that they define. The status of each GSSA is indicated similarly with clock icons. For boundaries in the Phanerozoic for which no GSSP is currently ratified, or which lack constraining numerical ages, an approximate numerical age (~) is provided. Note that numerical ages do not define units in the Phanerozoic and the Ediacaran, they are only defined by GSSPs. The International Commission on Stratigraphy (ICS) has a long tradition of producing international charts that communicate higher-order divisions of geological time and actual knowledge on the absolute numerical ages of their boundaries. The primary ob...

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