Early Cambrian sediments on the East European platform in North Estonia, represented mostly by clays and silty clays, were deposited under normal marine conditions. The sediments were never affected by significant tectonic or thermal events after sedimentation 530 Ma ago, and the clays still have high water contents. The clay fraction, divided into four sub-fractions, was studied using X-ray methods and Rb/Sr dating. Decomposition of the XRD curves was used to quantify the amounts of illitic minerals in the sub-fractions. Dating by Rb/Sr showed that the finest fraction (<0.06 mu m) was formed 50-150 Ma after sedimentation. The coarser fractions also contain considerable amounts of diagenetically formed minerals. This shows that neoformation of illitic minerals in marine sediments with high water/sediment ratios is a very important process even at temperatures <35 degrees C.