UPb dating of calcite concretions from Cambrian black shales and the Phanerozoic time scale

The UPb dating of carbonate offers considerable potential for accurate and direct dating of sedimentary rocks. Here we show that carbonate concretions from the Upper Cambrian black shales in southern Scandinavia have remarkably high U to Pb ratios (238U204Pb up to 288,000) permitting precise geochronology. A fine-crystalline early diagenetic concretion yields an age of 509.8 ± 5.1 Ma, providing an accurate age for the late Cambrian, whereas a coarse ‘cone-in-cone’ concretion has yielded an age of 478.2 ± 4.9 Ma, defining the final stage of sediment compaction and indicating a protracted dewatering and diagenetic history. These results demonstrate the power of this method for directly calibrating the geological time scale and constraining diagenetic histories.