| Summary: | The Suchilquitongo Formation of the northeastern Valley of Oaxaca is a thick sequence of thin-bedded tuffaceous sandstones and siltstones, which locally contain Hemingfordian mammal fauna, and interbedded lacustrine limestones and rhyolitic tuffs (Etla Ignimbrite). Three new K-Ar dates of biotite and plagioclase concentrates from the Etla Ignimbrite yield an age of about 19-20 Ma. Well-defined reverse polarity paleomagnetic directions are recovered by alternating field demagnetization from 40 samples distributed in 5 sites. The overall mean direction for the Suchilquitongo Formation is B = 5, Dec = 190.9°, Inc = -37.7°, a95 = 6.0°, and k = 165, and the pole position lies at 79.0° N, 330.6° E. This direction deviates from the expected direction (Dec = 176°, Inc = -30°) for the Oaxaca Valley, by a 15° clockwise discordance in declination. A double rotation correction to compensate for structural deformation using a 20° plunge and a 10° bedding dip results in a corrected direction and pole position of Dec = 178.6°, Inc = -30.9° (88.6°N, 151.5°E), which agrees within the statistical uncertainties with the expected direction. The anisotropy of magnetic susceptibility principal axes for the Etla Ignimbrite are characterized by large angular dispersion, which does not permit to infer the flow directions and possible source location.
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