Age and archaeological implications of Xitle volcano, southwestern Basin of Mexico-City

The Pedregal lavas are fresh, well-exposed basaltic flows erupted from the Xitle scoria-and-cinder cone in the southwestern part of the Basin of Mexic o. These lavas cover an area of 70 km(2) and were emplaced over pyramids an d other buildings (e.g. Cuicuilco and Copilco archaeological sites). Today, a part of Mexico-City (including the National University) is built on the flows. Initial strombolian activity produced an ash fallout layer, which was immed iately followed by effusive emplacement of lava flows. The Xitle cone grew on the north-facing slope of Ajusco volcano, and lava Bowed down to the N-N E until it reached the basin floor. More than 30 radiocarbon dates have been obtained by several workers on cha rcoal samples from beneath the lava, and several ages for the eruption have been proposed from these dates. Most dated samples were not directly produ ced by Xitle's eruption but instead are artifacts of human activity that pr edates the eruption. Thus, these ages (mostly about 2000 BP) are older than the eruption. A new age of 1670 +/- 35 years BP (AD 245-315) obtained on c harcoal samples collected just beneath the lavas is favored for the Xitle e ruption. These samples originated by ignition of vegetation during the empl acement of hot scoriaceous tephra. The new age is within the Classic period of Mesoamerican archaeology, whereas the earlier reported ages are at the end of the Preclassic. The new age carries important implications for the t iming of population shifts within the Basin of Mexico. (C) 2000 Elsevier Sc ience B.V. All rights reserved.