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The 87Srf6Sr isotope ratio is homogeneous in sea water at anyone time, but has fluctuated during geological time along the path which can be reconstructed by the Sr-isotopic analysis of marine precipitates (e.g. carbonate or phosphate fossils) of known age. The resultant 'seawater curve' can be used as a relative dating tool by analysing carbonate/ phosphate fossils from a marine sediment, locating its position on the curve and reading off the corresponding age. Such ages are independent of biofacies or faunal provincialism. The hypothetical chronological resolution, which depends upon analytical reproducibility and the gradient of the seawater curve during the period of interest, lies between 0.4 and 1.0 Ma for much of the Cenozoic and Mesozoic. However, the practical resolution is limited in sediments of certain ages by uncertainties in the definition of the seawater curve; this may be improved with further work.A study of Cenozoic sediments from the northern North Sea produced ages on submilligram samples of bioclastic carbonate with precisions of ±0.5-3.0 Ma. When integrated with seismic studies, the results placed important sequence boundaries at approximately 35 Ma, 34-30 Ma and 22 Ma. These boundaries represent hiatuses of various durations. The data illustrate the important potential of Sr-isotope stratigraphy for the production of precise burial histories, which possess the distinct advantage of being based upon dates calculated by an objective numerical method with quantifiable uncertainties.New data presented here indicate that the seawater curve could be defined very precisely for the Jurassic by the analysis of fossils which resist diagenetic modification. Belemnites are ideal: 0 18 0 and 013C values show no trace of diagenetic overprinting, and 87Sr/86Sr values show a regular variation with time. The gradient of the seawater curve in the Jurassic is such that dating of Jurassic sediments with a precision of ± 1 Ma could be realistic, as the seawater curve becomes more closely bracketed. Even with present knowledge, correlation of Jurassic sediments between wells or between reservoirs with a resolution of about ± 0.5 Ma is possible, based solely on comparison of 87Sr;S6Sr ratios.The high quality and resolution of dating and correlation using Sr-isotope stratigraphy, combined with the simplicity and cost-effectiveness of the method, and the increasing availability of access to the analytical instrumentation, suggest that Sr-isotope stratigraphy is set to become a routine technique for basin analysis and correlation in reservoirs .

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