Abstract The Permian marine and continental sedimentary sequences in south central México and the biota contained in them are analyzed in this paper. The fossil flora and fauna are contained in Permian strata that correspond to the Guacamaya Formation in the states of Hidalgo and Veracruz; to the Patlanoaya, Los Hornos, Cuxtepeque, and Matzitzi Formations in the state of Puebla; to the Olinalá Formation in the state of Guerrero; to the Ihualtepec, Ixtaltepec, and Yododeñe Formations in the state of Oaxaca; and to the Paso Hondo Formation in the state of Chiapas. The microfaunistic association of Olinalá fusulinids can be correlated with the Guadalupe Mountains and the La Mar Formation of Texas in the U.S.A., and with the Difunta Formation of Coahuila and strata of the states of Sonora, Guerrero, and Oaxaca in Mexico. Fusulinids from the Olinalá, Ihualtepec, and Yododeñe Formations suggest that during the Late Permian (Wordian-Capitanian), these organisms were part of the same paleogeographic province that comprised Texas, Arizona, New Mexico, and California in the United States, and northern Sonora, Chihuahua, Coahuila, extending into central Mexico.

Abstract Tephrochronological studies carried out over the past decade in the area surrounding Mexico City have yielded a wealth of new radiocarbon ages from eruptions at Popocatépetl, Nevado de Toluca, and Jocotitlán stratovolcanoes and monogenetic scoria cones in the Sierra Chichinautzin Volcanic Field. These dates allow us to constrain the frequency and types of eruptions that have affected this area during the course of the past 25,000 yr. They have important implications for archaeology as well as future hazard evaluations. Late Pleistocene and Holocene volcanic activities at the stratovolcanoes are characterized by recurrent cataclysmic Plinian eruptions of considerable magnitude. They have affected vast areas, including zones that today are occupied by large population centers at Puebla, Toluca, and Mexico City. During Holocene time, Nevado de Toluca and Jocotitlán have each experienced only one Plinian eruption, ca. 10,500 yr B.P. and 9700 yr B.P. respectively. During the same period of time, Popocatépetl had at least four such eruptions, ca. 8000, 5000, 2100, and 1100 yr B.P. Therefore, the recurrence interval for Plinian eruptions is less than 2000 yr in this region. The last two Plinian eruptions at Popocatépetl are of particular interest because they destroyed several human settlements in the Basin of Puebla. Evidence for these disasters stems from pottery shards and other artifacts covered by Plinian pumice falls, ash-flow deposits, and lahars on the plains to the east and northeast of the volcanic edifice. Several monogenetic scoria cones located within the Sierra Chichinautzin Volcanic Field at the southern margin of Mexico City were also dated by the radiocarbon method in recent years. Most previous research in this area was concentrated on Xitle scoria cone, whose lavas destroyed and buried the pre-Hispanic town of Cuicuilco ca. 1665 ± 35 yr B.P. The new dates indicate that the recurrence interval for monogenetic eruptions in the close vicinity of Mexico City is also <2000 yr. The longest lava flow associated with a scoria cone was erupted by Guespalapa and reached 24 km from its source; total areas covered by lava flows from each monogenetic eruption typically range between 30 and 80 km2, and total erupted volumes range between 0.5 and 2 km3/cone. An average eruption rate for the entire Chichinautzin was estimated at ~0.5 km3/1000 yr. These findings are of great importance for archaeological as well as volcanic hazard studies in this heavily populated region.