How can shale have more radioactive radiation than sandstone?

Sandstone is mostly quartz, silicon dioxide, and neither silicon nor oxygen are naturally radioactive. Shale is mostly clay, which contains significant potassium, which is very slightly radioactive. read more

matter and clays, they are generally more radioactive than other shales or sedimentary rocks.[63] This radioactivity can be measured with sensitive equipment at black shale outcrops or equipment can be lowered into an uncased well, with radioactivity readings used to identify, map, and determine the thickness of the black shale facies. read more