Back to the Field: The Limits of Standard Scales in High-Resolution Stratigraphy. Examples from Northwest Africa and the Middle East

The standard references regarding the geological timescale present charts where biostratigraphic scales and defining events are integrated and placed against the standard chronostratigraphic units (stages, periods, etc.). These charts may give the impression that these are known facts rather than an interpretation that may shift as more information becomes known. The importance here is that geologists who are not biostratigraphers assume that the purpose of biostratigraphy is to provide ‘age dating’ (i.e., ‘these fossils indicate an Aptian age’). They are typically unaware of what a complex and uncertain process arriving at such a statement can be. These uncertainties were brought to us in the course of studies of the Cretaceous of SW Iran and Morocco. In both areas, we were able to recognize a number of ammonite, planktonic foraminifera and calcareous nannofossils key species, and hence the standard biochronozones. However, what surprised us, based on the ammonite successions, was that a completely different calibration of the planktonic foraminifera and nannofossil zones to the ammonite standard, and hence standard chronostratigraphy, was suggested than might be assumed by reference to the standard charts. This highlighted the role of endemism in biostratigraphy and the need for research into biostratigraphic/age calibration in order to provide reliable ‘age dating’.