Paper No. 3
Presentation Time: 8:40 AM
Age Constraints on Alleged ‘Footprints' Preserved In the Xalnene Tuff near Puebla, Mexico
Impressions in a basaltic tuff located around Valsequillo Reservoir near Puebla, Mexico have been interpreted as human and animal footprints along an ancient lakeshore and are cited as evidence of the presence of humans in North America at 40,000 yr B.P. We present new data that challenge this interpretation. Paleomagnetic analyses of the Xalnene Tuff and lavas from the volcano from which it erupted yield fully reversed magnetic polarities, indicating that the tuff was deposited prior to the last geomagnetic reversal (the Brunhes-Matuyama ~790 kyr). 40Ar/39Ar dating of Xalnene lapilli and lava from the source volcano yield indistinguishable ages of ~1.3 million years, consistent with a period of reversed magnetic polarity (C1r.2r). Additional paleomagnetic measurements of individual millimeter-size lapilli indicate that the pyroclastic grains within the Xalnene Tuff have not been disturbed or rotated since their initial deposition, thereby ruling out the possibility that the tuff was reworked by wave action along the shores of an ancient lacustrine environment. This and other evidence indicates that the marks observed in the stone quarry site are not human ichnofossils.