Abstract

This paper summarizes a sedimentologic and spectral-radiometric (U, Th, and K) study of outcrops of the Cutler Formation and the transition zone between the Cutler and Honaker Trail Formations in the northeastern Paradox Basin. These Upper Pennsylvanian-Lower Permian strata represent synorogenic sediment derived from the ancestral Uncompahgre Uplift, which was cored by Precambrian metamorphic and igneous rocks. Results of this study indicate that depositional processes active during Cutler time “fractionated” the K, U, and Th, thus, allowing genetic lithofacies to be partly discriminated by their radiometric signatures. Radiometric variations are primarily due to ⁴⁰K concentrations, which are proportional to the potassium-feldspar, muscovite, and biotite contents of the rocks measured. Variations in ²³⁸U and ²³²Th do not appear to be as facies dependent.

Surface investigations were conducted at five locations in eastern Utah and four in western Colorado. A total of 2,902 total-count and 1, 812 spectral-radiometric measurements were taken from 3,644 feet (1, 111 m) of composite section. Alluvial-fan, braided-fluvial, sinuous-fluvial, flood-plain, pedogenic, playa, eolian-sandflat, eolian-dune, tidal-flat, shoreface, deltaic, and subtidal facies dominate the sections studied. Proximal alluvial-fan and braided-fluvial sandstones and their associated mudrocks have higher average equivalent potassium values (3.9-6.5 equivalent percent) than distal alluvial-fan, fluvial, eolian, and flood-plain facies (3.1-5.0 equivalent percent). Marine carbonates and playa carbonates have the lowest average equivalent potassium values (2.6-3.5 equivalent percent). Petrographic observations confirm that variations in the equivalent potassium values are due primarily to K-feldspar content.