DISTRIBUTION OF RADIOACTIVITY AND RADIOGENIC HEAT PRODUCTION ACROSS THE SEDIMENTARY BASIN IN NEBRASKA, CENTRAL UNITED STATES

Sedimentary basins across North America have recently started to be analyzed in relation to the thermal property characterization of radioactive heat production. ²³⁵U, ²³²Th, and ⁴⁰K are the primary heat producing radioisotopes, however, most sedimentary rocks only have minor abundances of these radioisotopes. The overall radiogenic heat contribution to the total heat generation in sedimentary basins is largely unknown, so the potential influence of any variability in distribution and abundance per depth of these isotopes may be substantial.

Drill cuttings were collected from 18 heat flow holes in Nebraska in the 1980's for thermal conductivity measurements. The locations were largely of the Northern, Northwestern and Eastern edges of Nebraska. Recently, these cuttings were combined stratigraphically, based on their radioactivity and depth. The cuttings were poured into pint sized containers, averaging 8-9 600 to 700 g containers per test hole. A total of 173 containers were created, all undergoing a gamma ray spectrometry analysis using a Ge-Li GRS. The eU and eTh were determined in ppm and the K in pct. This data was then used to calculate the heat production and heat flow present. Stratigraphic columns were created for each test hole, comparing the radioactive concentrations, heat production, as well as heat flow per depth, in order to show the variation and distribution across the state of Nebraska.

A total of 5 containers were of Paleozoic carbonate origin, averaging a heat production of 1.0 mW m^-2. 35 were of a siliciclastic Mesozoic origin, yielding an average of 1.4 mW m^-2. Lastly, 28 were of a Cenozoic siliciclastic origin, with an average heat production of 1.4 mW m^-2, respectively. The abundance and variation was largely influenced by the age and sequence of deposition, however, they do not correlate to any geophysical anomalies in Nebraska. This variability shows that further research is needed in accordance to radioactivity in sedimentary basins.