The dating of the earliest sediments on earth

Published isotopic age evidence for early Precambrian sedimentary rocks containing biogenic markers is critically reviewed. The age of sedimentation can sometimes be obtained by direct analysis of the sediments themselves, but more frequently by age measurements on stratigraphically related volcanic rocks, or by interpolation between dated older basement rocks and younger intrusive rocks. Provided that stratigraphical/geochronological correlations are correct there is little doubt that sediments containing algal stromatolites were being deposited proportional3450-3300 m.y. ago, and that true microfossils occur in rocks dated at proportional3300-3200 m.y. Evidence relating to possible biogenic markers in rocks reliably dated at proportional3800-3750 m.y. is more difficult to evaluate. Much scope remains for future work. (orig.)