Why is it almost impossible to date sedimentary rocks using radiometric dating techniques?
In addition to the above answers, radiometric dating techniques (eg, Pb-Pb, U-Pb, K-Ar, Rb-Sr etc) are based on the amount of the parent radioisotope locked in a mineral when crystallization took place. When molten igneous material cools and hardens, the atoms of the parent radioisotope are no longer free to move about the igneous body and the “clock” is set. As a true sedimentary rock does not have a history of minerals crystallizing from an igneous melt, it is not possible to provide a radiometric date of sedimentary rocks. The exceptions are volcanic ash beds in a conformable sedimentary sequence and volcanic dikes cutting sedimentary strata can be used to provide “older than” and younger than” absolute dates.
