Significance of K-Ar ages of authigenic illitic clay minerals in sandstones and shales from the North Sea

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Abstract
Petrographic, X-ray diffraction, and microprobe analyses have been used to assess the significance of illite K-Ar ages from sandstones of two North Sea wells. Three closely spaced samples in one well from the upper Statfjord Formation yield similar ages (69-79 Ma) although the illites formed from different precursor minerals. Pore-filling illite in the upper Brent and the Upper Skagerrak Formations from a second well formed by replacing groundmass clays and other detrital minerals. The average layer charge and K+ content increase slightly with depth (0.69-0.80 K+) due to minor reaction and crystal growth during burial diagenesis. These K-Ar ages increase from 15 to 33 Ma within a 500 m depth interval. The K-Ar age vs. depth relationship for these samples corresponds to the burial rate during the middle Tertiary. In examples of extensive illitization of pore-filling clays in sandstones with little subsequent evolution of the clay minerals, the K-Ar ages indicate the age of diagenetic events.In contrast, illitic minerals in shales from the Skagerrak Formation in the second well yield an age (108 Ma) that is much older than the clays in the sandstones, but is still younger than stratigraphic age. The K-Ar ages from illitic clay in shales reported in the literature can get younger, older, or remain essentially unchanged with increasing depth. These age vs. depth trends reflect the complex interplay of crystal growth and dissolution during diagenesis, as well as probable contamination by non-recrystallized detrital illites. The K-Ar ages of illitic clays, therefore, evolve in a different manner in shales than in sandstones.