Potassium-argon dates and the Tertiary floras of North America

Authors:
  • JEJ. F. Evernden
    J. F. Evernden
  • GJG. T. James
    G. T. James
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Abstract
Thirty-nine Tertiary floras principally from Oregon, California, and Nevada were dated by the potassium-argon method on feldspars in basalt. 22 dates are in agreement with ages determined from plants and/(or) mammals; 10 dates disagree by as much as 5 m.y.; and 7 dates disagree by 5 to 14 m.y. The disagreements suggest that paleobotanical age assignments based on distributional changes of regional floras or based on the floral succession method are not applicable at Stage-Age level of refinement, probably not at a refinement of less than 5 m.y. and on occasion not even at the epoch level. Investigation of the relation between chemical composition and argon retention in volcanic feldspars shows that feldspars of low and high potassium content retain argon whereas those of intermediate potassium content lose argon.