I've come across this question in a Petrophysics exercise regarding the Gamma Ray well logging. And I don't know how to solve it.
This is the "believed" AFAIK session
Natural radioactivity on a sedimentary environment is mainly due to a Potassium isotope (K40), responsible for around 20% of total radiation, with a lower although reasonable contribution from Thorium (Th232) and a variable Uranium (U235) amount. Shalestones are rich in K40 and also has metallic ions as Th232 and U235.
For a sandstone, however, radioactivity can arise either from mineral content on the matrix or, the fluid content belonging to the formation or mainly from shale contamination.
End of the AFAIK session
Someone suggested that younger the rocks, lesser the diagenesis and thus more presence of K40 bearing elements, like feldspar and mica. This results in a radioactivity not originated from the shale, so tertiary rocks would show more radioactivity. Someone else pointed out that older doesn't mean more under diagenesis, which opposes to the first reasoning.