Use of spectral gamma-ray signature to interpret stratigraphic surfaces incarbonate strata: An example from the Finnmark carbonate platform (Carboniferous-Permian), Barents Sea

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