K. BLOOMFIELD Institute of Geological Sciences, Overseas Division, 5 Princes Gate, London SW7 1QN, England S. VALASTRO, JR. Radiocarbon Laboratory, Balcones Research Center, University of Texas at Austin, Austin, Texas 78757

Late Pleistocene Eruptive

History of Nevado de

Volcano, Central

ABSTRACT Nevado de Toluca , or Xinan- Younger flows of andesitic basalt, Pleis- tecatl, lies within the Mexican tocene to Holocene in age, flank the moun- Fieldwork and radiocarbon dating pro- (Mooser, 1969) some 80 km west- tain to the east, and there is a marked vide evidence of two major explosive erup- southwest of . It rises to 4,565 geological resemblance between the valleys tions and an intervening minor eruption. m, the fourth highest of the major vol- of Toluca (the Lerma basin) and Mexico The Nevado, a central built canoes of central Mexico. Morphologically, (Ordonez, 1902). up in late Pleistocene time, was eviscerated it is a volcanic ruin (Waitz, 1915), because The early workers concluded that the by a violent Vulcanian-type eruption about it has the form of a dissected cone with Nevado de Toluca was formed at the be- 25,000 yr B.P., which deposited thick, cold, moderate slopes, markedly elongated east ginning of the Pliocene Period (Ordonez, lithic around its flanks. After a and west, with a roughly elliptical crater at 1902), but Mooser's maps (1969) show it period of widespread soil formation, dated the summit about 0.5 km by 1.5 km in size. as a Quaternary feature. In common with at 24,500 yr B.P., a relatively minor Intense mechanical erosion has profoundly the other major volcanoes of central Mex- Vesuvian-type eruption produced small modified the shape of the summit crater, ico, the Nevado lies at the intersection of amounts of pumice that fell only on the but a noticeable break in the wall on its strong north-trending and east-trending eastern and northeastern slopes of the vol- eastern side is probably an original feature. fracture systems (Mooser, 1969). cano. A major Plinian-type eruption took An almost perfect small dome rises some place at 11,600 yr B.P., directed mainly 100 m from the center of the irregular cra- TEPHRA SEQUENCE east-northeast, which spread thick dacitic ter floor. The tephra deposits have been studied in 2 pumice over an area of at least 1,700 km . Little work has been carried out on the some detail, and there are numerous excel- This eruption probably produced the Nevado de Toluca since the days of lent continuous sections exposed in the ac- "tripartite" pumice, which forms a marker Ordonez (1902), Flores (1906), and Waitz tively eroding barrancas both east and for the stratigraphy of early man in the ad- (1909), who gave generalized accounts of north of the volcano. To the west, the joining Valley of Mexico. The vent orifice the geology. Mooser and (1961, steeper slopes are covered by pine forest, was subsequently plugged by a dome of ve- unpub. data) have produced several photo- and the rocks are exposed only in artificial sicular dacite. Key words: stratigraphy, geologic maps that include the area of the cuts. The best exposures are in the vicinity Quaternary, volcanology. volcano. of the small towns of , Zaragoza, The Nevado de Toluca is an eroded, and San Lorenzo Cuautenco, but it is possi- INTRODUCTION polygenetic, central stratovolcano made up ble to trace the deposits and overlying This paper gives the first results of a largely of dacitic and andesitic lava flows pumice beds along the northern flank of the study of the development of the Nevado de (Fig. 1). Its piedmont slopes are mantled by volcano and westward as far as the Toluca volcano, which is being carried out thick lahar and fluvial deposits, which are Toluca-Temascaltepec road. Lateral by the University of Mexico. The tephro- capped by widespread pumice beds. The changes are gradual, but some distinct dif- chronology, petrology, and geochemistry of dome within the crater, known as El Omb- ferences exist between the beds exposed on the volcano are being considered, but only ligo, is a dacitic plug that has blocked a cen- the east and west flanks of the volcano. the significant preliminary conclusions of a tral vent. On a regional scale, the volcano The generalized tephra sequence east of radiocarbon dating program are described was formed on a series of older Tertiary the Nevado is shown in Figure 2 and is well here. volcanic rocks that rest on Cretaceous beds. exposed in the Barranca de Jaral, 3.5 km

Geological Society of America Bulletin, v. 85, p. 901-906, 3 figs., lune 1974

901

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west-northwest of Calimaya. At the base, a 3b). Locally, the upper part of the lahar has lain by finely current-bedded, gray-buff silt sequence of completely unsorted, blue-gray been reworked and comprises fluvial sand or fluvial sand but this i.and invariably andesitic breccia beds (fresh and completely and gravel, but the solum is always de- grades up into a pale-buff, leached solum unwelded, with an estimated maximum veloped. Immediately above the humic with root impressions. The organic horizon thickness of at least 100 m) rests on an paleosol there is a characteristic sequence of of this paleosol is absent or weakly de- eroded surface of Tertiary volcanic rocks. pale-yellow pumice lapilli with thin ash veloped, and locally the B horizon has been These beds, thought to represent cold beds, the lower Toluca pumice (Fig. 3b). removed; thus, a strong break separates it lahars, grade upward into stony, pale- This is partly air-fall and shower-bedded fron the overlying upper Toluca pumice as brown paleosol with weak columnar struc- pumice, but more commonly, it shows the shewn by Figure 3b. This upper pumice ture and some root impressions, which in- effects of reworking, because the lapilli are unit is well developed on the northern and variably carries a strong, clayey, humic subrounded, well sorted, and bedded. In eastern slopes of the volcano and has a paleosol about 25-cm thick (Figs. 3a and most places, the lower pumice unit is over- maximum observed thickness of 5 m. It is

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re-worked pumice upper Toluca pumice lithic sand 11 581 170 air-fall pumice re-worked charcoal sand

unconformity

valley-fill lahar

lower Toluca pumice

24780 • i 250

basal sand 24 400 •ûû.-'OU ±350

'm coarse iahar CJDVA •ïêls m Qî •dìD'à: m êQ?û f Tertiary volcanic rocks volcanic rocks

Figure 2. Tephra sequences on east and west flanks of Nevado de Toluca volcano showing mean I4C ages in yr B.P. Note that, in places, the upper Toluca pumice rests directly on coarse lahar or Tertiary volcanic rocks. The third column is a x6 enlargement of the base of the upper pumice unit.

divisible into two distinct subunits sepa- To the west of the Nevado, the tephra se- rated by a narrow band of coarse ash quence is somewhat different from that to loosely cemented with pinkish-brown clay, the east; nevertheless, correlations between which forms a characteristic ledge on the two may be made (Fig. 2). Two distinct weathered surfaces. In the Calimaya area, types of Toluca lahars have been recog- most of the upper pumice is composed of Figure 3. The upper and lower Toluca pumice units, nized: an older one that is widespread and Barranca de Jaral. a. Type section showing lower shower-bedded pumice blocks and lapilli, made up of coarse, bouldery material, and a pumice (L.P.) resting on strong paleosol developed on but there is a distinctive layer of medium- to younger, more sandy type that fills pre- lahar and separated from thick upper pumice (U.P.) by fine-grained lapilli at the base of the upper existing valleys. The base of the older lahar an unconformity, b. Close-up of the 24,500 yr B.P. paleosol below the lower pumice and the base of the subunit. The top of the upper pumice sequence is not exposed and, to the south of upper pumice. grades into modern soil and is present just the mountain, the lahars appear to grade below the surface over a wide area. The into widespread clastic sediments derived break between the upper and lower Toluca from Tertiary volcanic rocks. The younger pumice units is a strong, low-angle uncon- deposits, which are made up of typical lithic sand fills slight irregularities in the formity because, in places, and particukrly blue-gray, andesite-derived material, rest underlying paleosol surface. The upper 1 m in valley bottoms, the former unit oversteps on a strong paleosol developed on the older or more of the younger lahars invariably onto the lower blue-gray lahars. In addi- lahar and also on both weathered Tertiary comprises well-bedded fluvial sand and tion, a valley-fill lahar occurs locally be- and Nevado volcanic rocks. At the base of gravel. Above this is a sequence of yellow- tween the two pumice units on the east side the lahar, a thin layer of yellow feldspathic orange pumice lapilli, for the most part of the Nevado. sand overlain by a bed of coarse, blue-gray, subrounded, well stratified and graded, and

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TABLE 1. '"C AGES OF PALE0S0LS AND CHARCOAL FROM THE NEVADO DE TOLUCA AREA

Sample Laboratory Age 'n yr B.P. Locality Material dated no. no.

Group A KBC-2 Tx-1518 24,410 l 590 Stream bank 1 km due south of San Pedro Humic paleosol developed on typical blue-gray Toluca lahar and T1amisco: 19°04' N.; 99°39' W. overlain by pumicu lapilli of the lower Toluca pumice KBC-3 Tx—1519 25,(¡20 ± 680 Side of path 2 km southwest of San Juan Do. Tilapa: 19°12' N.; 99°41' W. KBC-4 Tx—1520 24,930 i 670 Stream bank to west of road, 0.5 km of do. Calimaya: 19°09' N.; 99°37' W. KBC-5 Tx—1521 25,020 ± 590 Erosion gully 1 km northwest of -aliitaya: do. 19°09.5' N.; 99°37.5' W. KBC-6 Tx-1522 25,¿50 ± 760 Northern tributary stream of Bar-anca do. de Jaral, 3 km west-northwest of Calimaya: 19°09.5' N.; 99°39' W. KBC-7 Tx-1523 23,540 ± 600 Gully 1.5 km west-northwest of Nativitas: do. 19°12.5' N.; 99°39' W. KBC-8 Tx—1524 24,260 ± 670 Gully on south side of Barranca de Jjral, do. 3 km west-northwest of Calimay: 19°13' N.; 99°39' W. Group B KBC-17 Tx-1596 24,£90 ± 280 Toluca- road, 2 km south of Humic paleosol developed on weathered Tertiary volcanic rocks and Cajones: 19°03' N.; 99°52.5' W. overlain by youngur valley-fill fine-grained lahars KBC-18 Tx—1597 24,440 ± 550 Toluca-Sultepec road, 1 km south of do. Cajones: 19°03.5' N.; 99°52.5' W. KBC-19 Tx—1598 21,170 ± 170 Toluca-Sultepec road, 4 km southwest of do. El Capulin: 19°04' N.; 99°52' W. Group C KBC-21 Tx—1600 11,550 ± 100 Toluca-Sultepec road, 0.5 km north of Charcoal from 1- ;o 2-cm-thick layer between upper Toluca pumice and Agua Blanca: 19°06' N-; 99°50.5' W. lahar KBC-22 Tx-1601 11,850 ± 220 Toluca-Sultepec road, southwest flan< of Charcoal from thin layer between upper Toluca pumice and pale brown Cerro Gordo: 19°07' N.; 99°50' U. paleosol developed on Cerro Gordo volcanics KBC-23 Tx-1602 11,470 ± 90 Toluca-Sultepec road, 2.5 km northeast of Charcoal from thi t layer resting on fluvial lahar-derived sand and Cerro Gordo: 19°08' N.; 99°49' W. overlain by upper Toluca pumice KBC-24 Tx-1603 11,050 ± 130 Toluca-Sultepec road, 1.5 km south of Charcoal-rich soi' from thin layer resting on weathered Tertiary turnoff to Nevado: 19°09' N.; 99°49' W. andesite and covered by upper Toluca pumice KBC-15 Tx-1594 24,400 ± 430 Borrow-pit on Mexico-Tenango road about Humic paleosol horizon from complex sequence of at least eight 1.5 km east-southeast of San Pedro paleosols overlaiii by Holocene basaltic ash Atlapulco: 19°14' N.; 99°23' W.

Age calculations based on '"c half life of 5,568 yr and modern reference standard of 95% of National Bureau of Standards oxalic acid. Ages give.i to one standard deviation.

obviously redeposited. However, the base sible for transport immediately after depo- convert the acetylene to benzene, which of the pumice is made up of a layer of air- sition. was assayed for 14C, using a Packard fall lapilli up to 10 cm thicx, which is over- The upper pumice unit has been recog- TriCarb Model 3002 liquid scintillation lain by a persistent thin bed of mantle- nized over a wide area to the east of the counter. Chemical yields averaged 88 per- bedded, gray, lithic sand. Charcoal frag- Nevado de Toluca. It is present on the cent. Measurements of 12C/13C were not ments occur along the con:act between the weathered Tertiary volcanic rocks making made, and the results were not corrected for pumice and the lahar and, in places, be- up the Sierra de las Graces, and locally, it is 13C fractionation. tween the pumice and weathered volcanic overlain by basaltic ash from Holocene It is apparent from Table 1 that the 14C rocks. monogenetic cinder cones. ages fall into three spatia and (or) temporal The lower Toluca pumice unit has a lim- groups: A, B, and C. The ages of all the ited distribution and is interpreted as a RADIOCARBON DATING samples in group A agree to within 1

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pies is 11,580 ± 70 yr B.P. Here there may lowed by a quiescent period of widespread Kamchatka peninsula (Gorshkov, 1959). be two subgroups with mean ages of soil formation at about 24,500 yr B.P. Soils Preliminary estimates of the volume of 11,900 ± 120 yr B.P. (Tx-1600 and of this same age have been found on the lithic tephra produced during the two erup- Tx-1601) and 11,260 ± 70 yr B.P. western slopes of the Sierra de las Cruces tions (>0.25 km3) suggest that the Nevado (Tx-1602 and Tx-1603), which might be (see Tx-1594, Table 1), and on de Toluca was originally at least as large as explained by the different sources for the volcano some 200 km to the east of Toluca present-day , and possibly wood that produced the charcoal. (Heinke and Heide-Weise, 1972); they larger. Within the three groups, the I4C results could be the equivalent of the soils formed The upper Toluca pumice extends east- show excellent agreement. The consistent in the Rocky Mountains during the Bull ward into the Valley of Mexico as much as ages of group A vindicate the applicability Lake—Pinedale interglaciation (Richmond, 85 km from the Nevado. A distinctive band of the radiocarbon method to older 1965). of "pomez tripartita de grano fino," also paleosols. Although the analyses give A relatively minor Vesuvian-type eruption known as pumice X, is found in many shal- strictly the "mean residence time" of the followed the period of soil formation to pro- low excavations in the Mexico City area; it soil carbon, plus the time that has elapsed duce the lower pumice that apparently fell rests locally on peat beds dated at 12,900 yr since then (Scharpenseel, 1971), the ages only on the eastern and northeastern slopes B.P. and is an important stratigraphic obtained for this relatively old material of the volcano. This was succeeded by the marker for early man (Mooser, 1967; must approximate closely the date of soil production of relatively fine-grained lahars Cornwall, 1971). This was thought to have formation. that flowed down and infilled valleys, mainly been derived from Iztacihuatl volcano, but, The ages in groups A and B are so similar on the western side of the mountain. Distinc- in an earlier work (Mooser and that they point to a widespread phase of tive beds of air-fall lithic ash and lapilli may González-Rui, 1961), the same pumice was soil formation at about 24,500 yr B.P., also be about this same age. deduced to have come from either which postdated the formation of the older A long period of dormancy and erosion Popocatépetl or the Nevado de Toluca and Nevado de Toluca lahar but preceded the followed the first explosive eruptions, pro- to mark a late Pleistocene volcanic erup- eruption of the lower pumice. Similar but foundly modifying the shape of the crater and tion. Preliminary mineralogical studies in- younger soils developed on the lahar;, of re-excavating prelahar valleys in the pied- dicate that the "pomez tripartita de grano , California, derived from mont slopes. During this time, fluvial sand fino" bears a strong similarity to the upper coniferous vegetation, are known to have and gravel, derived from the lahars and from Toluca pumice, and this should be substan- taken only 205 yr to attain a maximum or- the underlying weathered Tertiary volcanic tiated when further radiocarbon dating and ganic carbon content (Dickson and rocks, were deposited, particularly on the chemical and mineralogical work have been Crocker, 1953). western slopes of the volcano. To the east, completed. The four 14C dates of group C are of somewhat drier conditions resulted in the samples obtained from a thin layer of char- formation of loesslike deposits on the lower ACKNOWLEDGMENTS coal that lies on an undulating weathered pumice. Alejandra G. Varela and Teresia Lopez- surface immediately beneath the upper At 11,600 yr B.P. a second major violent Cepero assisted with the radiocarbon dating, pumice (Fig. 2). Since the basal pumice in- eruption took place, emanating from the and G. Sánchez-Rubio helped in the field. D. variably comprises a thin layer of air-fall center of the eroded crater. This was a S. Barker, S. E. Clabaugh, D. A. Cordoba, E. lapilli, the charcoal probably represents Plinian-type eruption, directed east- M. Davis, and P. W. Lambert made construc- vegetation that was killed off and burned northeast, and it produced vast amounts of tive comments on the manuscript. Financial by the first hot cloud of tephra. This, there- dacitic pumice (the upper pumice unit), assistance was received from the University fore, gives an excellent date for the onset of which fell on the eroded surfaces of lahars, of Texas at Austin, the Instituto de Geología, the eruption that produced the main upper volcanic rocks, and soil. The eruption took Universidad Nacional Autónoma de Toluca pumice. place in two main phases separated by a short México, and the British government's Over- period of quiescence, and its products were seas Development Administration. ERUPTIVE HISTORY OF NEVADO entirely air-fall. Local pink pumiceous DE TOLUCA VOLCANO lahars, which postdate the upper pumice REFERENCES CITED From the evidence of the new 14C age de- unit, appear to be mudflows that infilled ex- terminations, we can now reconstruct the isting valleys. Cornwall, I. W., 1971, Geology and early man in late Quaternary history of the Nevado de Finally, a plug of vesicular dacite was in- central México: Geol. Assoc. London Proc., Toluca volcano in broad outline. truded into the vent orifice, appearing at the v. 82, p. 379-392. The age of formation of the original lava surface as a dome, and this was the end of Dickson, B. A., and Crocker, R. L., 1953, A chronosequence of soils and vegetation near stratovolcano is not known. Attempts at eruptive activity. However, small vents of Mount Shasta, California. II. The develop- dating the mafic minerals in andesite by the stained and coated volcanic agglomerate ment of the forest floors and the carbon and conventional K-Ar method have not been around the edge of the crater suggest that nitrogen profiles of the soils: Jour. Soil Sci., successful, due to the presence of low fumarolic activity continued for some time. v. 4, p. 142-154. amounts of potassium and radiogenic Lenticles of native sulfur occur in excava- Fitch, F. J., 1972, Selection of suitable material argon as well as excess argon discrepancies tions near the central plug. for dating and the assessment of geological caused by late hydrothermal and fumarolic error in potassium-argon age determina- tion, in Bishop, W. W., and Miller, J. A., activity (Fitch, 1972; Fitch and Miller, CONCLUSIONS eds., Calibration of hominoid evolution: 1971). All we can deduce at present is that Two major late Pleistocene explosive Edinburgh, Scottish Academic Press, p. the volcano was built up in late Pleistocene eruptions occurred; one a Vulcanian-type 77-91. time, more than 25,000 yr ago. some 25,000 yr B.P., and the other a Fitch, F. J., and Miller, J. A., 1971, Atmospheric A violent Vulcanian-type eruption took Plinian-type, 11,500 yr B.P. The older erup- argon correction in the K-Ar dating of place about 25,000 yr ago, eviscerating the tion seems to have been similar in type and young volcanic rocks: Geol. Soc. London original volcano and depositing thick, cold magnitude to the cataclysmic 1956 erup- Jour., v. 127, p. 277-280. lahars around its flanks. This phase was fol- tion of the volcano Bezymianny on the Flores, T., 1906, Le Xinantecatl ou Volcan

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Nevado de Toluca: Internar. Geol. Cong., American sympoMum on the Upper Mantle, Yaalon, D. H., ed., Paleopedology: Origin, 10th, México [D. F.J 1906, Excursion México, 196Í!, Group If, Upper Mantle, nature, and dating or paleosols: Internat. Guide 9. petrology and .ectonics: Inst. Geofis., Soc. Soil Sei. and Israel Univ. Press, p. Gorshkov, G. S., 1959, Gigantic eruption of the U.N.A.M., Mexico, v. p. 15-22. 77-88. volcano Bezymianny: Bull. Volcanol., v. 20, Mooser, F., and Ganzález-Rul, F., 1961, Erup- Waitz, P., 1909, Excursion Geolögica al Nevado p. 77-120. ciones volcánicas y el hombre primitivo en de Toluca: Soc. Geol. Mexicana Bol., v. 6, Heinke, K., and Heide-Weise, H., 1972, la Cuenca de México, in Homenaje a Pablo p. 113-117. Estratigrafía del Pleistoceno reciente y del Martínez del Río en el XXV aniversario de 1915, Der gegenwärt ge Stand der mex- Holoceno en el Volcan de la Malinche y la edición de Los Orígenes Americanos: ikanischen Vulkane ur.d die letzte Eruption region circunvecina: Fundación Alemana México, p. 137-141. des Vulkans von Colima (1913): Zietschr. para la Investigación Científica, México, Ordóñez, E., 1902 Le Xinantecatl ou Volcan Vulkanol., v. l,p. 247-274. Proyecto Puebla Tlaxcala, Comunic. 5, Nevado de Toluca: Soc. Cieut. "Antonio 1972, p. 3-8. Alzate" México Mem., v. 18, p. 83-112. Mooser, F., 1967, Tefracronologia de la Cuenca Richmond, G. M., 1965, Glaciation of the Rocky de México para los úkimos treinta mil Mountains, in Wright, H. E., Jr., and Frey años: Inst. Nacional de Antropología e His- D. G., eds., The Quaternary of the United toria México Bol. no. 30, p. 12-15. States: Princeton, New Jersey, Princeton MANUSCRIPT RECEIVED BY THE SOCIETY JUNE 1969, The Mexican Volcanic Belt, structure Univ. Press, p. 217-230. 15,1973 and development. Formation of fractures Scharpenseel, H. W., 1971, Radiocarbon dating REVISED MANUSCRIPT RECEIVED NOVEMBER 30, by differential crusta heating: Pan- of soils — Problems, troubles, hopes, in 1973

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