Oceanic magnetic lineaments and the calibration of the late Mesozoic — Cenozoic time-scale

Summary The pattern of magnetic reversals from mid-Cretaceous times onwards is now well-known from the study of ocean-floor spreading systems. Integration of the pattern of polarity reversals with the standard biostratigraphical time-scale is now well-advanced and, as a result, a series of recently proposed time-scales use the detailed pattern of magnetic lineaments, calibrated radio-metrically at a few points only, as their major control. The anomaly sequence generally accepted as standard is based on a single track across the South Atlantic and its representativeness of at least the greater part of the Cenozoic has never been seriously challenged. Assumption of a constant spreading rate with time has enabled detailed and apparently very accurate time-scales to be generated. However, spacings within anomaly sequences in the same oceanic area vary notably from track to track and oceanic areas themselves may exhibit characteristic patterns. Clearly, in detail, the rate of ocean-floor spreading with time is not constant, and the role of patterns of magnetic reversals in the construction of time-scales needs reevaluation in the light of this fact.