Tephrochronology of the Toluca Basin, central Mexico

The major element glass geochemistry of 10 Late Pleistocene and Holocene tephras found in the Toluca Basin in the Trans-Mexican Volcanic Belt is reported for the first time, as a key step towards establishing a tephrochronology for the region. The context for this tephrochronology is provided by a review of Late Quaternary volcanism of the basin. New tephra samples were collected from reference profiles around the basin in order to establish major element glass geochemical data for tephras described in previous publications, but not previously analysed. In addition, cores were taken from the basin floor to extend the known spatial distribution of the tephra layers and establish correlations with the lacustrine record. Improved chronological control has been provided by seven new AMS and conventional radiocarbon dates on the major tephras. Geochemical data are presented for 10 tephras ranging in age from ca. 25,000 BP (Lower Toluca Pumice) to about 8500 BP (Tres Cruces Tephra). Based on geochemical data, it is possible to correlate between sites within the Toluca Basin and between the Toluca Basin and the Basin of Mexico. The basis for a tephrochronological framework has been established. (C) 1999 Elsevier Science Ltd. All rights reserved.