Origin of limestone nodules in the lower Palaeozoic of the Oslo region

Nodular limestones occur in several stratigraphic horizons in the Lower Palaeozoic of the Oslo Region. In the Middle and Upper Cambrian and the Lower Ordovician the nodules occur isolated in the black alum shale. In the Middle and Upper Ordovician and the Silurian, nodules are more closely spaced in a grey matrix. All transitional stages between continuous limestone beds and nodular limestones can be observed. Erosional features and burrows suggest that the nodules formed prior to any deep burial. The author considers the nodules to be the remains of continuous carbonate layers that were later dissolved due to prolonged exposure to sea water unsaturated with respect to carbonate in an environment of very slow sedimentation.