Paleomagnetism of Volcanic Rocks around Valsequillo Lake, Mexico, Including the Xalnene Ash

Based on geological mapping of the area around Valsequillo Lake (Mexico), volcanic and sedimentary rocks have been selected for paleomagnetic studies. In particular, the Xalnene ash layer has been studied in 10 different locations to analyze if this rock indeed recorded a reversed polarity of the geomagnetic field as proposed previously. This ash layer is of special interest because of the occurrence of human and animal footprints, suggested to have been produced shortly after the emplacement of this layer. Additionally, a sweet water limestone in gradational contact with the Xalnene ash and other volcanic rocks of the surrounding area have been studied as well. The paleomagnetic record of the Xalnene ash is complex, with strong variations of remanence intensity over short distance indicating complete remagnetization probably induced by lightning. Nevertheless, in the majority of cases a reversed polarity could be extracted using thermal and AF demagnetization. In one site of the Xalnene ash a paleointensity of 38 µT was determined. This value is close to the average paleointensity for this latitude, and not as low as expected for an excursional field intensity as observed e.g. during the Laschamp geomagnetic event ≈42 ka ago. Most other 13 sampled rock units of the region, including a lava flow of Toluquilla volcano which is supposed to be the eruption center of the Xalnene ash, also are of reversed polarity.