AR-40 AR-39 AGE CONSTRAINTS FOR THE JARAMILLO NORMAL SUBCHRON AND THEMATUYAMA-BRUNHES GEOMAGNETIC BOUNDARY/

Our mid-Pleistocene Ar-40/Ar-39 age recalibration of the geomagnetic p olarity timescale is nearly in accord with the oxygen isotope, climate record calibration of the astronomical timescale proposed by Johnson (1982) and Shackleton et al. (1990). Ar-40/Ar-39 ages of a normally ma gnetized rhyolite dome in the Valles caldera, northern Mexico, yielded a weighted-mean age of 1.004 +/- 0.019 Ma. A K-Ar age of 0.909 +/- 0. 019 Ma for this rock by Doell and Dalrymple (1966) was the linchpin fo r the recognition and calibration of the Jaramillo Normal Subchron (JN S). Other Ar-40/Ar-39 ages from the Valles caldera and Ar-40/Ar-39 age s of Ivory Coast tektites indicate that the JNS began at about 1.11 Ma and ended before 0.92 Ma, probably near 0.97 Ma. The Matuyama-Brunhes boundary occurred between 0.79 Ma and 0.76 Ma on the basis of Ar-40/A r-39 sanidine ages from (1) three reversely magnetized rhyolite domes of the Valles caldera (0.793 +/- 0.018 Ma, 0.794 +/- 0.007 Ma, and 0.8 12 +/- 0.023 Ma) and pumice (0.789 +/- 0.006 Ma) from the reversely ma gnetized Oldest Toba Tuff of Sumatra and (2) pumice (0.764 +/- 0.005 M a and 0.757 +/- 0.009 Ma) from the lower and upper units of the normal ly magnetized Bishop Tuff. The age of the boundary may be close to 0.7 7 Ma as deduced from rates of sedimentation in ancient Lake Bonneville , Utah.