Paleomagnetism of the Hueyatlaco Ash at Valsequillo, Mexico

Extensive archeology excavations at Valsequillo in the State of Puebla, Mexico, by the Departamento de Prehistoria of the Instituto Nacional Antropologia e Historia (INAH) and others in the 1950s and 1960s (Irving-Williams, 1967, 1978; Steen-McIntyre et al., 1981) and more recently (Ochoa-Castillo et al., 2003) exposed the Hueyatlaco Ash. The ash is about 2 m thick and is rhyolitic. Fission-track dating of the ash reported by C. Naeser of the U.S. Geological Survey gives an age of 370,000 ± 200,000 years for zircons. Initial attempts to determine the paleomagnetic polarity of the ash at two horizons separated vertically by about 30 cm using 8-cc samples encased in plastic boxes and demagnetized by an alternating field (a.f.) to 60 milliTesla (mT) documented the following normal polarity: Horizon 1, I = 5.3, D = 0.3, a95 = 1.5, n = 6; Horizon 2, I = 30.7, D = 338.4, a95 = 8.9, n = 6 and a median destructive field of 30 mT (Liddicoat et al., 1981).

We report thermal demagnetization results for uncased samples that were collected in May 2007. Two suites of samples record the following paleomagnetic directions: Suite 1, I = 22.6, D = 342.7, a95 = 4.3, n = 6; Suite 2, I = 13.0, D = 336.8, a95 = 2.2, n = 6. The median destructive temperature is about 300C for most samples, and less than 2% of the remanence remained before the samples broke from drying at 600C. The normal polarity following thermal demagnetization is consistent with the polarity determined using a.f. demagnetization (Liddicoat et al., 1981) and with the fission-track date, placing the deposition of the Hueyatlaco Ash in the Brunhes Normal Chron (present to 0.78 my).