J. W. Geissman C.E. Chapin Michael J. Kunk Christopher D. Henry W. C. McIntosh 1992 <p><span>A discontinuous record of late Eocene-Oligocene geomagnetic polarity has been determined using high-precision (±&lt;0.15 m.y.)&nbsp;</span>⁴⁰<span>Ar/</span>³⁹<span>Ar sanidine dating and a paleomagnetic study of 37-27 Ma ignimbrites in New Mexico, Colorado, and Texas. This record provides age control for several geomagnetic polarity reversals that occurred during three periods of intense ignimbrite volcanism: 36.8-33.5 Ma, 32.7-31.4 Ma, 29.1-26.9 Ma. The relative timing of these polarity reversals permits four possible correlations with the geomagnetic polarity time scale (GPTS). The preferred correlation yields calibration ages for Chron C10R (28.0-29.0 Ma) and Chron C13R (34.4-33.1 Ma) that indicate an Eocene-Oligocene boundary age near 33.4 Ma, some 0.3-0.6 m.y. younger than boundary ages indicated by other recently proposed GPTS calibrations based on terrestrial and marine sedimentary sequences.</span></p> application/pdf 10.1130/0091-7613(1992)020<0459:COTLEO>2.3.CO;2 en GSA Calibration of the latest Eocene-Oligocene geomagnetic polarity time scale using 40 Ar/ 39 Ar dated ignimbrites article