Late Pleistocene Eruptive History of Nevado De Toluca Volcano, Central

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 Toluca Volcano, Central Mexico ABSTRACT Nevado de Toluca volcano, or Xinan- Younger flows of andesitic basalt, Pleis- tecatl, lies within the Mexican Volcanic Belt 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 Mexico City. 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 stratovolcano 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 lahars 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 Guerrero (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 Calimaya, 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 lahar 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/85/6/901/3443760/i0016-7606-85-6-901.pdf by guest on 02 October 2021 902 BLOOMFIELD AND VALASTRO 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/85/6/901/3443760/i0016-7606-85-6-901.pdf by guest on 02 October 2021 PLEISTOCENE ERUPTIVE HISTORY OF NEVADO DE TOLUCA VOLCANO 903 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/85/6/901/3443760/i0016-7606-85-6-901.pdf by guest on 02 October 2021 904 BLOOMFIELD AND VALASTRO TABLE 1.

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