Adams and Waever reported in 1958 that because of analytical difficulties, few data are available on the thorium and uranium contents of sedimentary rocks. More than 200 new thorium and uranium determinations have been made by a gamma-ray spectral technique and by an alpha activity-fluorometric uranium technique. Together these two independent techniques can be used as an experimental test of secular radioactive equilibrium. Only rarely in this study have fresh samples of ancient sedimentary rocks been found out of radioactive equilibrium. The accuracy of the thorium-to-uranium ratio determinations is more than
sufficient for many geologic studies.
The thorium-to-uranium ratios in sedimentary rocks range from less than 0.02 to more than 21. Ratios in many oxidized continental deposits are above 7, whereas most marine deposits have ratios much below 7. Thus, the thorium-to-uranium ratio varies with sedimentary processing and depositional environment. A cyclothem and several other sedimentary sequences illustrate the use of this ratio to distinguish environments and processes. The thorium content of shales varies much less than the uranium content. By mineral and trace-element analysis an attempt has been made to evaluate the resistate, hydrolyzate, clay, and precipitate (evaporite) contributions to the thorium and uranium contents of sedimentary rocks. These data also provide some insight into the details of the mobilization, transportation, and fixation of thorium and uranium in the sedimentary cycle.
Field tests indicate that quantitative potassium, uranium, and thorium determinations can be made with a spectral gamma-ray logging instrument. Logs obtained with such instruments may provide an important additional means for subsurface interpretations. The authors also concluded that:
- The distribution of thorium and uranium in sedimentary rocks is largely determined by:
(a) the oxidation and leaching of uranium during weathering, with the immobile thorium left behind in resistates or adsorbed on clays and hydrolyzates, and
(b) the distribution of resistate minerals containing a high concentration of thorium and uranium. - Products of the high Th/U-ratio (more than about 7) environment are materials that are thoroughly weathered, oxidized, and leached, e.g., certain continental shales and bauxites.
- Products of the low Th/U-ratio (less than about 2) environment have resulted where uranium was removed from sea or other surface waters and fixed, e.g., marine black shales (Chattanooga shale), limestones, and Florida phosphate rock.
- Products of the intermediate Th/U-ratio (about 2 to about 7) are incompletely weathered and leached of uranium or are mixtures of materials from low and high-ratio environments.
- The general distribution of thorium and uranium in sedimentary rocks is summarized in Figure 18, where primary igneous rocks with a Th/U ratio of 3 to 4 and thorium contents of 10 to 15 ppm are fractionated upon complete weathering in three directions:
(a) to sea water with a very low Th/U ratio (0.0002),
(b) to hydrolyzates and oxidized clays with high Th/U ratios and high
thorium contents, and
(c) to resistate minerals with high Th/U ratios and high thorium contents. - Th/U-ratio logs offer numerous possibilities as aids to subsurface interpretations, particularly in regard to the location of unconformities under which there are well weathered fossil soils, in distinguishing between marine and continental shales, possibly in recognizing porous zones, dolomitized zones, and high-residue zones in limestones, and in characterizing sedimentary sections for correlation.
- The data and conclusions presented have been and should continue to be of great assistance in interpreting total gamma-ray logs and in interpreting gamma- ray logs that separate the thorium, uranium, and potassium contributions to the gamma-ray flux.
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