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Age is an interpretation

Every day geologists ask one another, ‘What age is that outcrop/formation/log pick/seismic marker?’ This deceptively simple question usually has a superficially simple answer, but an answer which masks a more complex thought process and which underlines the need for clarity of communication between stratigraphers and other geoscientists. 

To communicate our answer to the ‘What age is it?’ question, we almost always turn to the formal language of chronostratigraphy with as much precision as we dare — ‘It’s Jurassic’ or ‘It’s Bajocian’ we will reply. Few listening to such an answer will realize that it is very much an interpretation and shorthand for what should really be a much longer statement — something such as, ‘It contains an assemblage of dinoflagellate fossils indicative of the Nannoceratopsis semex biozone which in turn is currently calibrated with the Strenoceras niortense to Parkinsonia parkinsoni Tethyan ammonite biozones which currently form the standard biozones of the upper half of the Bajocian stage within the Jurassic system’. Am I being facetious? Not at all — this statement could have been much longer — for example, how sure am I that the fossil taxa present in the rocks really do indicate the Nannoceratopsis semex biozone? 

When asked what is the value of biostratigraphy or palaeontology, students and experienced professionals alike invariably reply that fossils tell you the age of rocks, as though ammonites and trilobites can speak to us like sacred oracles. We should remind them that biostratigraphy is the science of subdividing strata on the basis of palaeontological characteristics, using biozones and associated defining events — typically extinctions or inceptions of taxa. Such locally defined biozones are then calibrated, through co-occurrences with age significant fossils and often through intermediate schemes of other fossil groups, to the standard zonation schemes (ammonites, graptolites, etc) that form the basis of defining the chronostratigraphic units every geologist is familiar with (Jurassic, Oxfordian, etc). 

If that calibration process wasn’t complex enough, using jumps between biostratigraphic schema based on co-occurrences and juxtapositions, there are often further levels of uncertainty. Although biostratigraphy has its roots in the early 19th century, the process of calibration from fossils in a rock to age interpretation is still ongoing. For example, fossils and related biozones once regarded as Maastrichtian are now regarded as indicative of older Campanian ages (the zones and events remain useful for correlation — only the age labels have changed; see figure above). None of this is helped by the fact that a number of the fundamental units of chronostratigraphy — stages — which in turn are the building blocks of systems, eras, and so on, lack formal definition. At the time of writing there is no definition of what fossil event or zone constitutes the Jurassic–Cretaceous boundary. Hence one stratigrapher may calibrate an assemblage of fossils to the lowermost Cretaceous while another may use a different definition of the system boundary and calibrate the same assemblage into the uppermost Jurassic.

This can cause endless confusion for the poor end-user geologist — the oracles are giving conflicting messages or changing their mind! I have lost count of the times that I have explained that despite finding the same fossils, two stratigraphers may put different age interpretations on these occurrences for good reasons. A formation at location X is reported as being Barremian while the same formation at location Y is reported as being Aptian. Is it a diachronous formation? Not necessarily — the different ages may simply be an artefact of changing calibration. 

Let us return to that deceptively simple question, ‘What age is it?’. Rarely is a numerical age sought. Usually the question is really an oblique way of asking, ‘Does this rock correlate with this other rock?’. In many ways this is a much easier question to answer as subdivision and thence correlation of rocks on the basis of their fossil content at a local, regional, or global scale is often a more straightforward notion than the age label that usually accompanies this.

So my simple plea is to remind our fellow geologists that age is an interpretation and it is the fossil assemblage in the rock that matters. It is the zones defined by these assemblages that provide correlation, not the age labels we place upon them.

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