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 H olocene deposits of central Mexico, tephra layers collected from the s ediments of the Texcoco and Chalco sub-basins, in the southern part of the Basin of Mexico, are geochemically characterized and used as stra tigraphic markers. The tephra layers range in composition from basalti c andesites to rhyolites and are calcalkaline. The tephras range in ag e 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 Chal co, at a depth of 18 m; in the southeastern part of Texcoco, at a dept h of around 10 m; and at the Tlapacoya archaeological site, where it h ad 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 dat ed 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 pre viously described. ''pomez gruesa con fragmentos de apldesita'' (ca. 1 4,400 yr B.P.) and is present in all Chalco and Texcoco sections. The likely source of these three tephras is the volcano Popocatepetl. Teph ra II at Chalco dates to 12,520 +/- 135 yr B.P. and correlates with th e Upper Toluca Pumice from Nevado de Toluca volcano. These represent t he first geochemical glass-shard analysis of tephras from the Basin of Mexico, and so further research is necessary before a reliable tephro chronology can be established. (C) 1998 University of Washington.