Quaternary Stratigraphy of Valsequillo Basin, Central Mexico – Initial Results from Electrical Resistivity Soundings, Gravity and Magnetics

Initial results from electrical resistivity soundings, gravity and magnetics in the Valsequillo basin in central Mexico are presented. The basin has long been studied, in particular by H.E. Malde and co-workers who contributed to geological mapping and to establishing the stratigraphy of the basin. Here we use geophysical modeling to further investigate on the shallow stratigraphy of the volcano-sedimentary deposits, with emphasis on the northern section around Toluquilla volcano. The electrical resistivity soundings identify occurrence of 4-6 layers in the shallow 18 to 28 m, beneath the area covered by the Xalnene ash and the slopes of Toluquilla volcano. Resistivity cross-sections permit imaging of lacustrine deposits, ash layers and lavas. Gravity and magnetic models allow deeper investigation and incorporate several units mapped in past studies, particularly the lavas, Xalnene ash, upper and lower lacustrine sequences, lahars, Zacachampa/Caulapan tuffs and sediments, and the local basement formed by Balsas Group sedimentary rocks. The Toluquilla volcano is characterized by a dipolar normally polarized magnetic anomaly and a semi-circular Bouguer gravity anomaly. Results are discussed in relation to studies on the occurrence of apparent human and animal footprints on the Xalnene ash that outcrops in the Valsequillo Basin, within the context of the early human migration in the Americas. Inferences derived from paleomagnetic data for the Xalnene ash and Toluquilla volcano lavas, which have been interpreted in terms of correlation to C1r.2r chron (about 1.07 to 1.77 Ma) and Ar/Ar (~ 1.3 Ma) dating or to the Laschamp geomagnetic excursion and OSL dating (~ 40 ka), are also analyzed. These recent studies and discussions highlight the renewed interest on the stratigraphy of the Quaternary volcanic and sedimentary sequences of the Valsequillo Basin.