![]() Furthermore, by selecting an event associated with the correlative conformity location a link with the original description of a stage is maintained by relating the GSSP to the event that drove the original definition. We would not argue that it is entirely erroneous to ignore the sequence stratigraphic setting of a stage boundary, but we would suggest that considering sequence stratigraphy will provide a guide to the best palaeontological (or other) guide event. Even if this were correct, one would be tempted to ask which biozones and how are they in turn defined? Hancock (1977) has also expressed the view that stages are nothing more than groupings of biozones. Thus in the Jurassic, at least conceptually, biostratigraphy rules absolutely and often the only question is to find a GSSP locality with a good ammonite succession, other events in the rock record being considered of secondary or no importance. This is not surprising in that for the Jurassic Period at least, stages have long been regarded as groupings of ammonite zones ( Arkell, 1956). Here, as stated by the authors, the GSSP lies within a TST (there is a prominent correlative conformity to a sequence boundary further down in the section). For example, the GSSP for the base Pliensbachian has recently been defined ( Meister et al., 2006) at the base of the jamesoni Zone in Yorkshire. The Jurassic Period contains good examples of such GSSPs (see Morton, 2006 for a review of Jurassic GSSPs). THE SEQUENCE STRATIGRAPHIC SETTING OF STAGE BOUNDARIES This matches the limits of biostratigraphic resolution so sea-level and climate change events can often be regarded as “geologically instantaneous”, although clearly not instantaneous in the strict sense of this word. There is increasing evidence that sea-level change and climate change can be very rapid – measured in thousands rather than millions of years (e.g. ![]() We are conscious that some geologists will question what the duration of events such as those listed above is. Events like these often have important historical significance for recognition of stage boundaries. Such natural events include unconformities resulting from sea-level changes and changes in palaeoceanography or palaeoclimate. This is exactly why he observed faunal turnover at their boundaries, and is what sequence stratigraphy would predict ( Holland, 1995).Ī more philosophical, but nonetheless important, argument in GSSP selection has been relating the selection of a GSSP to what has been called a (synchronous) “natural event” in the rock record. Because d’Orbigny was working mainly on outcrops in platform locations (“up-systems tract” in a sequence stratigraphic sense), many of his stages are bounded by unconformities (i.e. Despite much debate in the geological literature over the last 150 years as to the definition of stages, it seems that we may be coming full circle and that sequence stratigraphy provides the vehicle to express d’Orbigny’s original views within a modern geoscience framework. Stages were described as “the expression of the boundaries which Nature has drawn with bold strokes across the whole globe” (d’Orbigny, 1842 as quoted in English translation by Rioult, 1969). In keeping with the prevailing view in the mid-19 th Century that “cataclysmic events” controlled Earth history, d’Orbigny believed that each of his stages resulted from faunal turnovers in response to sudden events in Earth history (see reviews of Monty, 1968 Rioult, 1969 Torrens, 2002).
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