SWISHER, CARL C., III, Geochronology Center, Institute of Human Origins, Berkeley, CA

ABSTRACT: The Impact of {40}Ar/{39}Ar Dating on the Calibration of the Paleogene Time-Scale

Geochronology, over the past five years, has undergone a dramatic transformation culminating in the virtual replacement of conventional {40}K-{40}Ar with {40}Ar/{39}Ar dating as the primary method used for the calibration of the geologic time-scale. This transformation has been a direct consequence of the coupling of fully automated high resolution mass spectrometers, micro-volume extraction lines and laser heating with {40}Ar/{39}Ar dating methodology.

The application of this new technology to the calibration of the Paleogene time-scale is radically changing long held views as to the age and correlation of global marine and nonmarine events. For example, recent {40}K-{40}Ar and {40}Ar/{39}M ages have shifted the age of the Eocene/Oligocene boundary from approximately 37-36 Ma to slightly younger than 34 Ma. Middle Eocene events occurring within Chron 21N, previously dated at 49-50 Ma, are now calibrated by sanidine ages to be 45-46 Ma. In the North Sea, boreholes and on-land sections in Denmark, sanidine ages from No.60/+19 ash and No.70/-17 ash that occur slightly above the NP9/NP10 boundary, have indicated an age of approximately 55 Ma for the Paleocene/Eocene NP9/NP10 boundary. This age is significantly younger than previous age stimates of 57.8 Ma to 56.5 Ma for the Paleocene/Eocene boundary as proposed by most recent time scales. Controversy over the age of the Cretaceous-Tertiary boundary remain as new {40}M/{39}Ar ages, although analytically precise, range in age from 66.4 to 65.0 Ma.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.