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

摘要

Spectral gamma-ray (GR) profiles were examined in wel17128/6-1, the stratigraphic reference section of the entirely subsurface Finn- mark carbonate platform. Detailed bulk-chemical profiling of se- lected GR peaks shows that potassium (K) and thorium [Th) are mutually correlated and are a direct index of siliciclastic ( alumi- nosilicate) 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 associ- ated with aluminosilicate minerals and is not particularly concen- trated 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 sub aerial exposure of the platform, during which U was concentrated by groundwater movement. These re- suIts can be useful as a basis for applying spectral GR signature as a tool for stratigraphic interpretation in uncored or incipiently un- derstood carbonate sections.