Black fissile shale in Pennsylvanian cyclothems in the western part of the Illinois Basin shows low-level radioactivity as measured by alpha-particle emission. Preliminary analysis showed a ten-fold variability in radioactivity among closely spaced samples, with no apparent concentration in particular zones of the thin shale units. These relations favored design of a sampling experiment for evaluation of average radioactivity of the shale and for detection of regional radioactivity gradients.
Interlocked regional and local sampling designs were set up and provided 176 samples for a regional study and 32 samples for a local study, including evaluation of the experimental error. The interlocked designs furnished data for estimating stratigraphic variability of samples spaced from about 1 foot to 18 miles apart.
Strip mines, mine dumps, and some well cores provided regional samples of the 3-foot black fissile shale bed overlying Coal No. 6 in an area about 200 miles long and 20 miles wide. The local samples were collected from four localities within a single mine. In both sampling designs the maximum variability occurred at the lowest sampling level, indicative of a “spotty” distribution of radioactivity in the black shale. Neither the geological evidence nor the statistical analysis suggested any strong regional gradient in radioactivity over the area studied. Both the geological and statistical data support the inference that the radioactive material was associated with the depositional environment, rather than introduced by wholly diagenetic processes.