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Abstract

Changes made to the Pliocene and Pleistocene portions of the Geomagnetic Polarity Time Scale (GPTS) by matching paleoclimatic oscillations in marine sediments to variations in the earth’s orbital elements have been validated by recent 40Ar/39Ar dating. We explore here the potential for orbital chronology to improve and refine the Cretaceous and early Paleocene portion of the GPTS. Because orbital cycles mark off time in geologically short increments, their sedimentary imprint may be used to measure elapsed time between events such as magnetic reversals or biostratigraphic datums very precisely. A large number of Deep Sea Drilling Project drill sites of late Cretaceous to early Paleocene age display carbonate cycles whose mean period, estimated by paleomagnetically determined sedimentation rates, is close to 20 ky, the expected mean period of the precessional cycle. Because the cycles can be detected before and after the Cretaceous-Tertiary boundary, they offer a new tool to date the position of the extinction level within magnetochron C29R and to measure the rates of environmental and evolutionary changes across the boundary. Cycle-by-cycle correlations appear possible between South Atlantic sites; recent information from sections in Spain suggests that interhemispheric correlations can be made. We also present an astronomical time scale for the durations of the Cenomanian, Albian, and Aptian stages as defined by marine microfossil datums and the top of reversed polarity chron M-0. The durations of these stages are estimated to be 6.0 ± 0.5 my, 11.9 ± 0.2 my, and 10.6 ± 0.2 my, respectively. A cumulative cyclo-chronology from the well-dated Cenomanian-Turonian boundary places the top of polarity chron M-0, a tie point for the calibration of the M-series anomaly sequence, at 121.6 Ma.

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