Scott Buchanan said this (in the context of igneous rocks):
The fossil record allows us to trace back some lineages for millions of years. Fossils are found in sedimentary rock layers, which record a succession of biological forms. Radioactive dating of igneous rocks above and below these sedimentary layers provide absolute dating of the sedimentary layers and thus of the fossils within them.
But Shaun Doyle said this (in the context of tuff deposits):
One of the most (in)famous examples is the rather convoluted story of the hominid fossil KNM-ER 1470.5
After the discovery of KNM-ER 1470, the tuff deposit associated with the fossil was first ‘dated’ by the K-Ar [Potassium-Argon] method at 212–230 million years (Ma), but since hominid fossils are ‘clearly’ not that old, the ‘date’ was rejected.
Analysis of selected samples gave an ‘age’ of 2.9 Ma, which was considered acceptable. This was ‘corroborated’ with numerous other methods, and was widely accepted. That is, until another palaeontologist, Basil Cooke, said those dates were 800,000 years too old based on pig fossils.
The pig fossils won the argument—over the five different dating techniques that were all consistent with each other in giving a ‘date’ around 2.7–3.0 Ma. Therefore, the presence of certain types of fossils and evolutionary assumptions provide the primary dating information, and are often used to override other dating methods, even when those other dating methods are all consistent with each other!
- Buchanan, S. (2012). STAN 4. Letters to Creationists. Available https://letterstocreationists.wordpress.com/stan-4/. Last accessed 30th Aug 2015.
- Doyle, S. (2014). ‘Precambrian rabbits—death knell for evolution?‘ in Journal of Creation. Volume 28(1) 2014, p. 10.