Geochemical characterization of late Pleistocene and Holocene tephra layers from the basin of Mexico, central Mexico

In order to aid palaeoenvironmental research of Late Pleistocene and Holocene deposits of central Mexico, tephra layers collected from the sediments of the Texcoco and Chalco sub-basins, in the southern part of the Basin of Mexico, are geochemically characterized and used as stratigraphic markers. The tephra layers range in composition from basaltic andesites to rhyolites and are calcalkaline. The tephras range in age from >34,000 to ca. 2600 C-14 yr B.P. New names are used informally to designate correlated tephras. The Tlahuac tephra is present in Chalco, at a depth of 18 m; in the southeastern part of Texcoco, at a depth of around 10 m; and at the Tlapacoya archaeological site, where it had been mistakenly described as basaltic. This basalt-andesite tephra is dated to at least 34,000 C-14 yr B.P. The Tlapacoya 1 tephra is dated to between 15,020 +/- 450 and 14,430 +/- 190 yr B.P. and is present in all Chalco sections. The Tlapacoya 2 tephra corresponds to the previously described. "pomez gruesa con fragmentos de apldesita" (ca. 14,400 yr B.P.) and is present in all Chalco and Texcoco sections. The likely source of these three tephras is the volcano Popocatepetl. Tephra II at Chalco dates to 12,520 +/- 135 yr B.P. and correlates with the Upper Toluca Pumice from Nevado de Toluca volcano. These represent the first geochemical glass-shard analysis of tephras from the Basin of Mexico, and so further research is necessary before a reliable tephrochronology can be established. (C) 1998 University of Washington.