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Pleistocene Glaciation of Volcano Ajusco, Central Mexico, and Comparison with the Standard Mexican Glacial Sequence" by Sidney E

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Pleistocene Glaciation of Volcano Ajusco, Central Mexico, and Comparison with the Standard Mexican Glacial Sequence * QUATERNARY RESEARCH 22, 242-246 (1984) LETTERS TO THE EDITOR Comment on "Pleistocene Glaciation of Volcano Ajusco, Central Mexico, and Comparison with the Standard Mexican Glacial Sequence" by Sidney E. White and Salvatore Valastro, Jr. The observations of a Pleistocene glacial determining the stratigraphic succession of sequence for volcano Ajusco that includes tephra, both field characteristics and labo- deposits of three glaciations, two Neogla- ratory examinations were carried out. Ra• cial advances, and associated periglacial diocarbon dating of tephra layers has been activity by White and Valastro (1984) is no regtricted to charcoal logs and branches doubt an important addition to the "Stan• imbedded in the volcanic deposits. In ad• dard Mexican glacial sequence." dition to these data paleosols, wood frag- The purpose of this letter is to call atten• ments from gravels, peat, and calcrete (ca- tion to some reconstructions of the late liche) deposits were dated by radiocarbon. Quaternary glaciations of the highest vol- Other age determinations of the late Qua• canoes of the Cordillera Neovolcänica de ternary deposits are being attempted Mexico. Apart from the Standard Mexican through studies in archaeology and prehis- glacial sequence published by White (1962 tory. Pollen studies in cores of Sediments in a, b) two decades ago, a chronostratigraphy small volcanic craters, maars, and lakes based on comprehensive research on La (Ohngemach and Straka, 1983) cover the Malinche Volcano, Pico de Orizaba, Iztac- time span of approximately 35,000 to 0 yr cfhuatl, Popocatepetl, and Nevado de To- BP. The results are compared with the chro- luca volcano has been elaborated (Heine, nology of the glacial deposits. Furthermore, 1971, 1973a, b, c, 1975, 1976a, b, 1978, relative age-dating methods have been used 1980, 1983a, b, 1984; Heine and Ohnge- to demonstrate age differences in the tili se• mach, 1976). Different late Quaternary stra• quence; such relative dating methods in- tigraphic successions for each volcano can clude topographic position, morphologic be recognized within the area (Fig. 1). The shape of the moraines, rock-weathering Pa• slopes of the volcanoes are dissected by rameters, thickness of eolian "toba" Sedi• barrancas (erosion gullies), radiating from ments, soil properties (Miehlich, 1974), and the upper parts of the forest belt and de- Vegetation cover (including liehen data of scending to the basins. Thus the strati- Holocene glacial and periglacial deposits). graphic successions can easily be traced The chronology of the late Quaternary from one barranca to another by different glacial deposits is summarized in Figure 2. layers of glacial and periglacial deposits, Tills with poorly preserved moraine forms paleosols, debris, fluvial gravels and sands, are those of the M I glacier advance be- and loess-like so-called "toba" Sediments tween 36,000 and >32,000 yr B.P. and of which are interbedded with tephra (the tills deposited during the M II glaciation term tephra pertains to all pyroclastic frag- about 12,000 yr B.P. Lateral and end mo• ments, such as fine and coarse ash, lapilli, raines deposited during the M III glaciation volcanic bombs, and blocks), lava flows, between 10,000 and 8500 yr B.P. show well- and ignimbrite deposits. Different tephra preserved morainal forms, as do the Holo• layers as well as some paleosols are of great cene Neoglacial deposits which are divis- use as stratigraphic markers within the late ible into two advances (M IV; 3000-2000 Quaternary deposits of the volcanoes. In yr B.P.; M V, Little Ice Age). In the cor- 0033-5894/84 $3.00 Copyright © 1984 by the Univcrsity of Washington. All rights of reproduetion in any form reserved. FIG. 1. Index map of the Cordillera Neovolcänica, Mexico. Circles indicate the location of the volcanoes: 1, Nevado de Colima; 2 Nevado de Toluca; 3, Ajusco; 4, Tläloc; 5, Telapön; 6, Iztaccihuatl; 7, Popocatepetl; 8, La Malinche; 9, Cofre de Perote; 10, Pico de Orizaba; 11, Cerro Pena Nevada; 12, Durango mountaneous area; 13, Tacanä; 14, Tajumulco. relation diagram (Fig. 2) most geologic-cli- equals the M III glaciation, which means matic unit boundaries are based either on that the Albergue glaciation can be dated maximum or minimum radiocarbon dates between 10,000 and 8500 yr B.P. These data or on relative-age criteria. In most cases are in good agreement with the Observation radiometric controls are very reliable and by White and Valastro (1984). The two the presented boundaries will likely not be Neoglaeial advances on Ajusco are shifted on the time bar. matched with the MIV glaciation of around Comparisons of the Standard Mexican 3000 to 2000 yr B.P. according to the height glacial sequence (White, 1962a, b) and the of the moraines above sea level (Heine, Pleistocene glacial sequence for volcano 1976b). Ajusco (White and Valastro, 1984) with our In an effort to determine radiocarbon above-mentioned late Quaternary chron- ages of the glaciations on Ajusco volcano, ostratigraphy suggest that certain glacia• organic matter in buried soils was obtained tions on Ajusco compare with those on from different exposures by White and Va• other volcanoes of the Cordillera Neovol• lastro (1984). According to them the radio• cänica de Mexico. The Marques glaciation carbon ages appear to provide minimum probably is equivalent to an early M I gla- ages for the Marques glaciation and for an cier advance of our stratigraphy and thus advance and retreat of the Santo Tomas gla- may be dated to about 36,000 to >32,000 yr ciers because of possible unrecognized con- B.P. The Santo Tomas glaciation on Ajusco, tamination (p. 28). According to our expe- too, appears to be equivalent to the M I rience with buried soils, the reliability of glaciation, because in the Santo Tomas tili, radiocarbon ages is relatively good. So, if well-developed soil profiles have formed; we put the stratigraphic units with the given this soil formation is similar to our fBol radiocarbon ages of Ajusco volcano into paleosol with an age of 26,000 to about our chronostratigraphy (Fig. 3), the radio• 16,000 yr B.P. The Albergue glaciation carbon ages fit very well, apart from the 1 ' Radiocarbon dates MV J $ Valsequitlo area + RbFrioPass | x Malinche volcano j O Iztacdhuatl.Popocatepetl MIV YH • Nevado de Toluca volcano X • O XX I A El Seco/Oriental area • Tlaxcala area fBo3 p* ®x$ 1 • Dating by archaeology.historical X | sources 3 l MIII2 1 1 I fBo2 1 h ] X M II LI j A fBol X X 1 X A M I j 1 •I 1 + 1 2 3 4 5 6 7 8 9 14 15 20 30 x103yrBP Pinus/Mixed 2 c 2* ä-ä forest Abies Pinus/Mixed forest CL<CL <£S 1 ,i, i VI (V) iIVHll Zones' 2200-2OM maltitude I Pinus Pinus.poor in Quencus Pinus with Quercus Pinus.rich in witnout Quercus HUI Quercusl I £ 2 2 I HOL 0 C E N E j -t- fTTTTTTI cold more humid »': M more arid rrm cool H-s-H alternating more humid - more arid FIG. 2. Correlation diagram. Geologie-climatic unit boundaries are based on radiocarbon ages, tephrochronologic correlations, soil development, pollen analyses, sedimentologic criteria, and to- pographic position. The pollen zones refer to Ohngemach and Straka (1983). Till and/or moraines of the glaciations are numbered: M I, M II, M III (1, 2, 3), M IV, and M V. The paleosols are fBol, fBo2, and fBo3. section D (volcanic and colluvial deposits canoes in the east than on Ajusco is under- and Marques tili in Valle de Doberman). If standable in terms of the difference not only we assume that in section D the exposed in size and height (White and Valastro, tili does not belong to the Marques glacia• 1984) but also in distance from the Gulf of tion, the radiocarbon age of about 27,000 yr Mexico. Our M II glacier advance occurred B.P. is equivalent to our fBol paleosol and around 12,100 yr B.P., but only on the high must not be considered a minimum age be• volcanoes east of the basin of Mexico cause of contamination. (Heine, 1983a, 1984); the M II glacial epi- The difference between the greater sode is characterized by a short duration of number of glaciations on the higher vol• presumably less than 200 yr and was caused Ajusco glaciation (WNte and Valastro 1984) landes während des Spätglazials (Wisconsin) und Holozäns. Erdkunde 27, 161-180. Heine, K. (1973c). Studies of glacial morphology and Ii!* tephrochronology on the volcanoes of the Central Mexican Highland. In "IX. INQUA-Cong. Ab- stracts", pp. 144-145. INQUA, Christchurch. Heine, K. (1975). "Estudios de la Geomorfologia Gla• cial del Cuaternario Superior de Volcanes Mexi- canos, con una Resena de Desarrollo del Clima." El Proyecto Mexico de la Fundaciön Alemana para la Investigaciön Cientifica, vol. VII. Steiner, Wies• baden. Heine, K. (1976a). Blockgletscher- und Blockzungen- Generationen am Nevado de Toluca, Mexiko. Die Erde 107, 330-352. Heine, K. (1976b). Schneegrenzdepressionen, Kli• maentwicklung, Bodenerosion und Mensch im zen• • K Glacier advance Glacier advance C age of fossil soil (Heine) (White ard Valastro 1984) (Whitean ard Valastro 1984) tralmexikanischen Hochland im jüngeren Pleistozän und Holozän. Zeitschrift für Geomorphologie FIG. 3. Preliminary comparison of the Ajusco glacial Suppl. 24, 160-176. sequence (White and Valastro, 1984) with that of the Heine, K. (1978). Neue Beobachtungen zur Chro- Cordillera Neovolcänica. nostratigraphie der mktelwisconsinzeitlichen Ver• gletscherungen und Böden mexikanischer Vulkane. Eiszeitalter und Gegenwart 28, 139-147. by intense preeipitation rather than by a Heine, K. (1980). Quartäre Pluvialzeiten und klima• temperature decrease. The nonexistence of morphologischer Formenwandel in den Randtropen the M II glaciation on Ajusco confirms our (Mexiko, Kalahari).
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