Geochemical response to Upper Precambrian rift basin sedimentation and Lower Palaeozoic epicontinental sedimentation in south Norway

Abstract

Geochemical analyses of 138 samples from the Upper Precambrian and Lower Palaeozoic are presented in diagrams showing the stratigraphical distribution of each element. The analyses include the major elements and the following trace elements: Sc, U, B, Ba, Co, Cr, Cu, Ni and Ga. The Upper Precambrian sedimentary rocks were deposited in an Upper Precambrian rift valley basin on the Baltic Shield and are mineralogically immature and controlled by the composition of the local basement. The Middle Cambrian to Upper Ordovician Alum Shale was deposited in a stagnant epicontinental sea covering most of the Baltic Shield. These shales are very fine-grained illite shales derived from distant sources and probably deposited to a large extent as eolian dust at 1 mm per thousand years. Ordovician and Silurian shales, however, have a high content of chlorite and are enriched in Mg, Fe, Cr, Ni and Co, and indicate derivation from Lower Ordovician igneous rocks, probably ophiolites, which were exposed following abduction along the continental margin. While the Upper Precambrian sparagmites, which have local sources, show rapid lateral variation in mineralogy and geochemistry, the Lower Palaeozoic sediments show a laterally very persistent but vertically more abrupt geochemical pattern which now has been confirmed in different parts of the Oslo region. The Lower Palaeozoic carbonate rocks in the Oslo region are mostly calcite limestone. It is concluded that epicontinental sea sediments probably had a very low mobility of their pore water due to low pressure gradients (low relief) and sealing shales. This diagenesis has been essentially isochemical.