Some of the relative and absolute methods of dating rocks near Whanganui are outlined by Dr Alan Beu of GNS Science. Fossil correlation is important as is the counting of climate cycles represented in the rocks. Fission track dating and paleomagnetism both provide absolute dates to tie the relative dating to.
DR ALAN BEU
The relative and absolute ages of the rocks are worked out by all sorts of different techniques. I mean there are probably something like four that are mostly commonly used at Whanganui, and the result is what we call integrated stratigraphy that uses the results of all of those fitted together.
The first is the fact that some of the fossils are not living now, and we know that some of them go out consistently at a particular age throughout New Zealand and you can use those to date rocks, and that’s purely relative dating – you know that, relative to that fossil, that other one went out earlier and so on.
One of the most important techniques for dating rocks is the changes in the Earth’s magnetic polarity. The geomagnetic pole has swapped from its current north direction to the opposite way around, quite a few times through geological history. The most recent one is 780,000 years ago, and it’s possible to measure the magnetic directions in rocks, so you can measure what the field was in the past, and you can take these samples through the cliffs at Whanganui and recognise the point where the 780,000 year reversal occurs. So that’s the first tie point for an absolute age of the rocks.
The other important technique for telling the absolute age of the rocks is fission track dating of tephra – volcanic ash deposited in the sediment. Fission track dating is done on grains of the mineral zircon, which is a very, very hard and resistant mineral. Some of the chemicals within the tephras break down by normal radioactive decay and the particles that they give off leave little tracks – blow little holes essentially through all the minerals and glass shards and so on in the rock. If you look down a very high-powered microscope, you can see the really minute holes made in them by radioactive breakdown, and the density of the track is what tells you the age. So that can be used over a very long period of time – they’re used you know for 500 million year old samples as well – but they’re particularly useful and accurate for these volcanic ashes because in many other cases there’s nothing else to use.
The fourth method is counting the glacial-interglacial cycles that you basically you start from the top and assume that the youngest terraces over the top of everything are the last interglacial before present, and you can count gradually back through them all using your dates on tephras and the last geomagnetic reversal as tie points to be sure that you’ve got your count correct. Once you have that count correct, you know that each packet of sediment with unconformity planes separating the bodies of sediment represents a particular glacial-interglacial cycle.
Tim Naish, GNS Science
GeoTrack International Ltd, Melbourne
Dr Phil Shane, University of Auckland