1985 Articles

A Cretaceous and Jurassic geochronology

Kent, Dennis V.; Gradstein, Felix M.

An integrated geomagnetic polarity and geologic time-scale for the Jurassic and Cretaceous periods is presented, based on various methods according to the availability of definitive isotopic ages. An age-calibrated sea-floor-spreading model is used to interpolate the ages of the Kimmeridgian to Barremian, and the Campanian to Maestrichtian stages. Numerical age estimates for the Aptian to Santonian stage boundaries follow published isotopic age determinations. The hypothesis of equal duration of ammonite zones is employed as a vernier to apportion time for the Hettangian to Oxfordian stages. The new scale results in ages of 208 Ma for the base of the Jurassic, 144 Ma for the Jurassic/Cretaceous boundary and 66.5 Ma for the top of the Cretaceous. The integrated biostratigraphic, magnetostratigraphic, and geochronometric record serves as a working hypothesis for geologic correlation of Jurassic and Cretaceous strata.