Paleomagnetic results for the Middle-Miocene continental Suchilquitongo Formation, Valley of Oaxaca, southeastern Mexico

Abstract

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 distrib- uted 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 suscep- tibility 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.