TEPHROCHRONOLOGY AND GEOMORPHOLOGY AT AN ARCHEOLOGICAL SITE IN THE MEXICAN VOLCANIC BELT

The primary purpose of this project was to gather geomorphological data and analyze geochemical data of multiple tephra layers used for stratigraphic correlations within a Mesoamerican archeological site in Michoacán, Mexico. The study site is located within the Michoacán-Guanajuato volcanic field, which is filled with numerous monogenetic cinder cone volcanoes. The archaeological site contains cliff paintings on the walls of a volcanic caldera in the vicinity of the Paricutín volcano that erupted in 1943-52. Previous dates indicate that human use of the site began prior to 5000 yr BP.

The stratigraphy of fluvial and lacustrine sediments exposed in trenches in the caldera floor indicates variations in the geomorphic environment of the caldera over the last few thousand years, which could have implications for the type of human use of the site. Stratigraphic zones of coarse, sand-sized tephra that washed into the lowest part of the caldera alternate with zones of laminated silt that settled out in quiet pools. Tephra layers were used along with radiocarbon dates to correlate and date stratigraphic sections within the site. Geochemical data of tephra layers associated with at least three separate volcanic eruptions were obtained in the field using a portable X-Ray Fluorescence (XRF) instrument. These data were evaluated to determine whether this method is effective for correlating stratigraphic sections within the caldera and nearby sites. Initial results indicate that certain tephras show distinct geochemical signatures using the portable XRF, which may make them useful as stratigraphic marker beds.