C. Israelson et al., U-PB DATING OF CALCITE CONCRETIONS FROM CAMBRIAN BLACK SHALES AND THEPHANEROZOIC TIME-SCALE, Earth and planetary science letters, 141(1-4), 1996, pp. 153-159
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.