143-11 A Test of Spectral Gamma-Ray Response in Lithologic, Depositional Environment, and Provenance Discrimination in the Cutler Group, Eastern Utah

Abstract:

Spectral gamma-ray measurements are increasingly used to characterize sedimentary rocks because variations in potassium (K), uranium (U), and thorium (Th) reflect mineral composition, provenance, and clay content. This study evaluates whether spectral gamma-ray signatures can be useful in distinguishing lithology and clastic facies associations representing eolian, fluvial, shoreline bar-channel system, and shallow marine depositional environments within the Late Pennsylvanian to Permian Cutler Group near Moab, Utah.  

One hundred sixty-five measurements were collected with a handheld gamma-ray scintillometer at one-foot intervals through a composite measured stratigraphic section. At each location, detailed sedimentologic observations were recorded.  

In analysis of the lithologies, limestone and sandstone produced similar calculated gamma-ray API values; however, sandstone contained higher potassium concentrations, whereas limestone contained higher uranium concentrations. Thorium values were similar between lithologies. Elevated potassium in sandstone likely reflects arkosic sediment derived from the ancestral Uncompahgre highlands, whereas elevated uranium in limestone may reflect post-depositional secondary uranium enrichment.  

Sandstone data were further examined by facies association (FA) to evaluate relationships between spectral gamma-ray measurements and depositional environments. Clay-rich sandstones were excluded from analysis to isolate non-clay mineralogical effects. The eolian facies (FA1) are characterized by very fine- to fine-grained trough cross-bedded sandstone with flat contacts and local convolute bedding. Shoreline facies are represented by two subdivisions in FA2. The shallow marine facies (FA2a) consist of intensely bioturbated very fine- to fine-grained sandstone interbedded with silty sandstone and mudstone and contains the highest sandstone uranium concentrations (average 7.8 ppm), potentially related to mud-lined burrows and spreiten. The bar and channel facies (FA2b) consist of fine- to medium-grained sandstones with shell lags, limited bioturbation, and sigmoidal bedding.  

The fluvial facies (FA3) consist of medium- to coarse-grained trough cross-bedded sandstone with concave up channel bases and contains the highest potassium concentrations (4.3-6.8%), consistent with less mature, western-driven sediment. FA1 and FA2 (a, b) generally exhibit lower potassium concentrations than FA3. Thorium values overlap in all facies associations.  

In conclusion, spectral gamma-ray data clearly distinguish sandstone and limestone lithologies and differentiate fluvial deposits (and related eastern provenance) from other depositional environments through potassium variations, whereas uranium and thorium values overlap amongst most facies and provide limited depositional discrimination with the potential exception of the burrowed shoreface facies.

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