Tephrochronology of the Toluca Basin, central Mexico

The major element glass geochemistry of 10 Late Pleistocene and Holocene te phras found in the Toluca Basin in the Trans-Mexican Volcanic Belt is repor ted for the first time, as a key step towards establishing a tephrochronolo gy for the region. The context for this tephrochronology is provided by a r eview of Late Quaternary volcanism of the basin. New tephra samples were co llected from reference profiles around the basin in order to establish majo r element glass geochemical data for tephras described in previous publicat ions, but not previously analysed. In addition, cores were taken from the b asin floor to extend the known spatial distribution of the tephra layers an d establish correlations with the lacustrine record. Improved chronological control has been provided by seven new AMS and conventional radiocarbon da tes on the major tephras. Geochemical data are presented for 10 tephras ran ging 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 bet ween sites within the Toluca Basin and between the Toluca Basin and the Bas in of Mexico. The basis for a tephrochronological framework has been establ ished. (C) 1999 Elsevier Science Ltd. All rights reserved.