Abstract

Spectral gamma-ray (GR) profiles were examined in well 7128/6-1, the stratigraphic reference section of the entirely subsurface Finnmark carbonate platform. Detailed bulk-chemical profiling of selected GR peaks shows that potassium (K) and thorium (Th) are mutually correlated and are a direct index of siliciclastic (aluminosilicate) content, whereas uranium (U) is uncorrelated with K, Th, and all other chemical components measured. Uranium tends to be enriched in thin shale and argillaceous carbonate layers within otherwise carbonate-dominated intervals. Uranium is thus associated with aluminosilicate minerals and is not particularly concentrated in dolomite. Two types of GR peaks are observed. Potassium-thorium-dominated peaks are suggested to indicate relatively major transgressions during which aluminosilicate detritus was derived from sources interior to the Fennoscandian shield. Uranium-dominated peaks correspond with relatively minor transgressions within intervals of cyclic shallow-water carbonate deposits. Uranium-enriched aluminosilicate detritus is suggested to be the product of extended subaerial exposure of the platform, during which U was concentrated by groundwater movement. These results can be useful as a basis for applying spectral GR signature as a tool for stratigraphic interpretation in uncored or incipiently understood carbonate sections.

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