DEPTH-DEPENDENT TRANSFORMATION OF KAOLINITE TO DICKITE IN SANDSTONES OF THE NORWEGIAN CONTINENTAL-SHELF

Replacement of kaolinite by dickite has been observed to occur with in creasing depth of burial in sandstones from three different basins on the Norwegian continental shelf. In the Garn Formation (Middle Jurassi c) of Haltenbanken, samples from 1.4-2.7 km below the sea floor (110 d egrees) contain kaolinite, whereas deeper than 3.2 km (130 degrees C), mainly dickite is present. In the Statfjord Formation (Late Triassic- Early Jurassic) from Gullfaks and Gullfaks Sor Fields, transformation of kaolinite to dickite occurs at similar to 3.1 km below the sea floo r (120 degrees C). From the Sto and Nordmela Formations (Lower to Midd le Jurassic) to the Troms Area, kaolin polytypes have been identified in only two shallow and two deep samples, but the results are consiste nt with the transformation depth determined in two other areas studied . These occurrences are significant because they allow the temperature of the kaolinite/dickite transformation to be established with greate r confidence than had been possible previously. Also the observation o f this transformation in all three areas so far examined indicates tha t it may be a general and predictable feature of kaolin-bearing sandst ones worldwide and therefore a potentially reliable paleogeothermomete r. In most cases, the kaolinite occurs as relatively large vermicular crystals, whereas dickite forms more euhedral, blockier crystals. This morphological difference, together with the nature of the structural difference in octahedral occupancy between the kaolinite and dickite, suggests that the transformation occurs by dissolution and reprecipita tion, rather then in the solid state.