Radioactive elements tend to become concentrated in the residual melt that forms during the crystallization of igneous rocks. The radioactive minerals in sedimentary rocks are derived from the weathering of igneous rocks . If the sedimentary rock were dated, the age date would be the time of cooling of the magma that formed the igneous rock. The date would not tell anything about when the sedimentary rock formed. Glauconite is a good example.
Glauconite contains potassium, so it can be dated using the potassium-argon technique.
Cosmic rays from the sun strike Nitrogen 14 atoms in the atmosphere and cause them to turn into radioactive Carbon 14, which combines with oxygen to form radioactive carbon dioxide. Living things are in equilibrium with the atmosphere, and the radioactive carbon dioxide is absorbed and used by plants. The radioactive carbon dioxide gets into the food chain and the carbon cycle. All living things contain a constant ratio of Carbon 14 to Carbon 12. At death, Carbon 14 exchange ceases and any Carbon 14 in the tissues of the organism begins to decay to Nitrogen 14, and is not replenished by new C-14.