How importent are dating methods
A clastic sedimentary rock is made up of older rock and mineral fragments, and when the rock forms it is almost certain that all of the fragments already have daughter isotopes in them.Furthermore, in almost all cases, the fragments have come from a range of source rocks that all formed at different times.Over time, the Ar remain embedded within the crystal, unless the rock is subjected to high temperatures after it forms.The sample must be analyzed using a very sensitive mass-spectrometer, which can detect the differences between the masses of atoms, and can therefore distinguish between Ar in the case of the K-Ar method).If we dated a number of individual grains in the sedimentary rock, we would likely get a range of different dates, all older than the age of the rock.It might be possible to date some chemical sedimentary rocks isotopically, but there are no useful isotopes that can be used on old chemical sedimentary rocks.
One of the isotope pairs widely used in geology is the decay of K is a radioactive isotope of potassium that is present in very small amounts in all minerals that have potassium in them.
An example is shown in Figure 8.16; radiocarbon dates from wood fragments in glacial sediments have been used to estimate the time of the last glacial advance along the Strait of Georgia.
Figure 8.16 Radiocarbon dates on wood fragments in glacial sediments in the Strait of Georgia [SE after Clague, J, 1976, Quadra Sand and its relation to late Wisconsin glaciation of southeast British Columbia, Can.
The need for an accurate chronological framework is particularly important for the early phases of the Upper Paleolithic, which correspond to the first works of art attributed to Aurignacian groups.
Carbon-14 is the only method used for the direct dating of organic pigments, but indirect methods are used to date subsequent deposits on rock art (thermoluminescence, OSL, Uranium/thorium, etc.).