In today’s fast moving world of palaeontology, modern techniques are constantly being refined taking our science to new levels of data collection and analysis. We near enough take for granted that we can extrapolate maximum data from fossils and we marvel at the wonderful three dimensional restorations of skulls, such as those frequently shared by the Witmer Lab, that enable us to get inside the very head of a dinosaur.
Dating techniques of today, particularly radiometric dating, have enabled us almost unparalleled accuracy in our estimation of the ages of the various rocks and formations throughout the world, getting within around 500,000 years of their true age. No doubt this will be yet further refined and I sincerely believe that completely accurate dating will be achievable within the next ten years or so.
But this has not always been the case and it is always good to reflect how dating and correlation techniques developed in the past and why they are still of great importance and are still very much in use today.
If we were to go through a photo album that displayed various images of different time periods in our recent past, say the last 200 years, we would automatically try and work out when we believe the photo was taken. We look at the clothes that people wear, the streets, forms of transport and maybe landmarks to make a guess at the time and because of this change we can estimate the period as being the 1850’s, 1880’s and the 1900’s and so on.
Fossils do very much the same thing and enable us to help date the rocks. Evolution dictates that there is a constant turnover of life forms throughout the ages and as older taxa become extinct there is always a new one that replaces it. What this means is that each rock unit within any given formation will contain its own distinguishing set of fossils.
|William Smith 1769 - 1839|
The man who recognised this sequence in the rocks and was primarily responsible for bringing it to the scientific world’s attention was William Smith. Smith was a surveyor who, in 1793, was working on the construction of the Kennett and Avon Canal and, as the course of the canal was being cut into the terrain, keenly noticed the different types of strata that were layered on top of each other. Fascinated by what he observed, in 1794, he toured England to examine other canals which provided him with further insights.
Smith worked out that each layer of rock represented a specific moment in time and took a very long time to form and deposit layer upon layer. But, now that he had looked at the different formations throughout the country, he couldn’t work out why the same strata could be found in different sequences and thicknesses in different places. He methodically classified the strata according to its makeup and was able to build up a fine collection of fossils at the same time.
Smith made copious note of the strata that the fossils were found in and, because the fossils were so numerous, he was able to quickly work out that certain fossils were to be found in certain rocks. However, when he tried to make sense of sequencing the strata in various parts of the country he found that he could not match, what he considered to be, the matching rock types. The strata were found to be in different thicknesses and, even more apparent, were in a different order.
But then Smith realised that it was not the type of rock that mattered – rather it was the fossils they contained that were the common denominator. He observed that the same fossils could be found in different locations, in different rocks and at different depths – but they were always in the same sequence. Smith realised that this would enable him to use fossils for dating the rocks just by simply correlating the different strata by the fossils they contained and he also realised that the difference in thickness of the rocks was of no consequence since different sediments accumulated at different rates. Once he worked this out Smith was able to produce the first recognisable geological maps ever produced in the UK and, for me, was amongst the most influential and important geologists ever.
Over the years, as techniques became more refined, it became apparent that some fossils were more important for dating than others and these are known as zone fossils. These are taxa that may have only existed for a relatively short period of time but, of course, this may still cover a time period extending upwards of 10 million years or more.
The best known of the zone fossils are, of course, the ammonites and they have proved to be of considerable importance to palaeontologists worldwide. To demonstrate how zone fossils help us to date rocks, it is best to look at the diagram below.
Fossil “A” was a long lasting taxon found in all five rock units. Obviously this fossil would only allow for very basic date estimates covering a long period of time.
On the other hand, fossil “B” is only found in rock unit four and is therefore a good zone fossil. It has a limited temporal range, is only found in this rock unit and can be constrained to that time period.
You can see that fossils “C” and “D” are both reasonable zone fossils - both being fairly short lived taxa. But you can also see that they are both found in rock unit 3 and this enables to us mark that particular point in time since it is the only period where the two taxa overlap. The same can be seen for fossils “C” and “E” – this time contemporary only in rock unit 2.
Of course, zone fossils are also extremely important for correlating data for different rock formations on a world wide scale. Correlation is the term used to describe what connections exist between these variously located and different forms of strata and this is the technique that William Smith first pioneered all those years ago. Because the same zone fossils are found in rocks that were deposited in completely different environments, in different locations we know that they must have been formed during the same geological time period.
As mentioned earlier, ammonites are the most important of the zone fossils and it is easy to see why. They had a global distribution and were free-swimming organisms that, when they died, sank onto the sea bed and became fossilised in a multitude of ways in different marine environments. They were incredibly numerous and successful animals that diversified into thousands of genera and this combination of factors has enabled correlation of a wide range of sedimentary rocks. Formations, like the Oxford Clay, are separated into these zones and are often informally referred to, for example, as the Jason Zone or the Coranatum Zone – both species of ammonite.
So despite today’s advancement in modern dating techniques it is worth remembering that this very early technique is as sound now as it ever was. It may lack certain panache in the current state-of-the-art digital age but it represents a basic solid technique that is still widely referred to today.