Outcrop gamma-ray logging of siliciclastic turbidites: Separating the detrital provenance signal from facies in the foreland-basin turbidites of the Moravo-Silesian basin, Czech Republic

摘要

Standard and spectral gamma-ray (GRS) logs are widely used as indicators of facies in the subsurface. In sili-cidastics, however, the gamma-ray signal is often influenced by changes in the provenance of the K, U and Th-bearing detrital minerals. In this paper, we have compared outcrop and laboratory GRS with modal and chemical sandstone and mudstone composition and facies in an approximately 2.5 km-thick siliciclastic tur-bidite system of the Moravo-Silesian Culm Basin (Lower Carboniferous), Czech Republic. The aim was to separate the facies signal from the detrital provenance one. The silicidastics have moderately high outcrop gamma-ray values (174 API on average) and slightly lower laboratory values (127 API). Both the outcrop and laboratory data show low sensitivity to facies, which is demonstrated by the low contrast between the K, U and Th concentrations in the seven facies types ranging from proximal to distal turbidites. Markedly higher GRS variability is observed between equivalent facies at different stratigraphic levels. Major carriers of the GRS signal include K-feldspars, muscovite, sericite, biotite and albite for K, zircon, apatite, monazite and xenotime for U and monazite, thorite, REE secondary minerals, xenotime, apatite and zircon for Th. With the effect of facies filtered out, the GRS values reveal a stratigraphic variability, which coincide with the changes in the sandstone modal composition. A shift from the low-grade metamorphic and volcano-sedimentary provenance to predominantly magmatic sources with ultrapotassic plutonites in the early Late Visean is associated with a marked increase in U and Th concentrations and generally higher sandstone radioactivity compared to mudstones. Another provenance shift to high-grade metamorphic sources with granu-lites in the latest Visean is associated with a rapid decrease in Th, U and partly K concentrations and an increase in the GRS contrast between sandstone and mudstone facies. The GRS data sensitively reflect the extremely rapid exhumation of mid-crustal and deep-crustal rocks in the major source area, the Moldanubian Zone of the Bohemian Massif.