Mammoth bones embedded in a late Pleistocene lahar from Popocatepetl volcano, near Tocuila, central Mexico

The accidental discovery in July 1996 of the Tocuila mammoth site in the ea stern Valley of Mexico was followed by intensive scientific escavation, whi ch yielded remains of at least seven individuals of Mammuthus columbi in an area measuring only 28 m(2). The high density of mostly disarticulated mam moth bones identified 3 m below the surface in a massive, 130 cm-thick laha r deposit of sandy ash and rounded pumice fragments is unusual and suggests a high-energy depositional environment, Samples of charcoal found within t his deposit yielded radiocarbon ages ranging between 10 220 +/- 75 and 12 6 15 +/- 95 yr B.P. The Lahar was studied in greater detail in order to estab lish its provenance. Determination of grain-size distribution and magnetic properties of the deposit, as well as petrographical. mineralogical, and ge ochemical analyses of pumice clasts, all indicate that the lahar was derive d from volcanic fallout deposits that originated at Popocatepetl volcano,lo cated 50 km south-southeast of the site, Popocatepetl underwent a major phr eato-Plinian eruption, dated ca, 14 000 yr B.P., at the end of the last maj or glaciation, Dispersal of the fallout was mostly toward the northwest and north; deposits were found on the slopes of Tlaloc mountain (4150 m), 20 k m southeast of Tocuila.At the beginning of deglaciation and the onset of mo re humid conditions similar to 3000 yr later, lahars derived from the fallo ut deposits picked up the bones of already dead animals and then deposited them on the shores of Lake Texcoco and/or simply covered bones without majo r displacement.