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Jurassic to Paleogene: Part I Jurassic and Cretaceous geochronology and Jurassic to Paleogene magnetostratigraphy

Summary From the Jurassic onwards the relatively new technique of magnetostratigraphy becomes of increasing importance in geochronology and hence demands extended treatment. The original intention was to include, in one large contribution, accounts of Jurassic to Paleogene geochronology based on biostratigraphy and radiometric dating, together with separate accounts of magnetostratigraphy of deep-sea cores and land sections. Whereas, however, the Jurassic and Cretaceous chronology could be reviewed fairly concisely, this did not prove possible for the Paleogene, and a detailed discussion of biostratigraphic correlation problems and dating was necessary. Furthermore, the revised chronology is based on a new magnetostratigraphic analysis integrated with the other data. To have included the Paleogene geochronology with the rest of the material would have destroyed the overall balance and anticipated the more general and introductory accounts of magnetostratigraphy. Consequently our report is divided into two parts, the second part dealing exclusively with the new detailed analysis of the Paleogene. There is inevitably overlap with regard to the Cretaceous-Tertiary boundary but since this boundary is one of exceptional geological interest there can be little objection to thorough analysis by independent workers. In part I Jurassic and Cretaceous geochronology is reviewed successively. These sections are followed by an introductory account of magnetostratigraphy and then a general review of magnetostratigraphic investigations for the Jurassic to Paleogene interval. The concluding section deals with some geological implications of the results reported earlier. The ages discussed here in relation to the Jurassic and Cretaceous time-scale have all been recalculated to accord with the decay constants recommended during the 25th International Geological Congress (Steiger & Jäger 1977). Furthermore the discussion relies heavily on the recent compilation of time-scale data by Odin (1982); ages itemized and discussed in volume two of Odin's work are here further referred to by their item number with the prefix NDS (Numerical Dating in Stratigraphy). Author's contributions are identified by initials at the end of each section.