U-PB DATING OF CALCITE CONCRETIONS FROM CAMBRIAN BLACK SHALES AND THEPHANEROZOIC TIME-SCALE

The U-Pb dating of carbonate offers considerable potential for accurat e and direct dating of sedimentary rocks. Here we show that carbonate concretions from the Upper Cambrian black shales in southern Scandinav ia have remarkably high U to Pb ratios (U-238/Pb-206 up to 288,000) pe rmitting precise geochronology. A fine-crystalline early diagenetic co ncretion 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 sedime nt compaction and indicating a protracted dewatering and diagenetic hi story. These results demonstrate the power of this method for directly calibrating the geological time scale and constraining diagenetic his tories.