Kongsberg-Bamble formasjonen

Abstract

A prominent friction breccia running from Modum in north to Kristiansund in south divides the pre-Cambrian of Southern Norway into two parts that differ markedly in their geology, petrography, metamorphism and ore deposits. The westernmost unit comprises rocks of unquestionable sedimentary origin - sandstone with ripple marks, conglomerates, etc. - alternating with effusive lava flows. Folded layers, conglomerates pulled out to long lenses, and, in places, a high degree of metamorphism, are indicative of the effect of an old mountain chain superimposed on the regional metamorphism. The easternmost unit (the Kongsberg-Bamble Formation) comprises no rocks of obvious sedimentary origin, nor can any unquestionable traces of volcanic activity be seen. The whole unit exhibits a parallel schistosity of crystallization without any crosscutting folding or faulting. A general pneumatolytic and hydrothermal activity starting at high and ending at low temperature is much in evidence along a certain zone from north to south. The presence of low temperature minerals in this formation only a few kilometers east of typical high temperature mineral assemblage west of the breccia, is of more than ordinary interest and forms the basis of certain conclusions regarding the origin and the geological history of these rocks.