Geologic setting of the Peña de Bernal Natural Monument, Querétaro, México: An endogenous volcanic dome Gerardo J. Aguirre-Díaz1,*, Alfredo Aguillón-Robles2,*, Margarito Tristán-González2, Guillermo Labarthe-Hernández2, Margarita López-Martínez3,*, Hervé Bellon4,*, and Jorge Nieto-Obregón5,* 1Centro de Geociencias, Universidad Nacional Autónoma de México, campus Juriquilla, Boulevard Juriquilla 3001, Juriquilla, Querétaro, C.P. 76230, México 2Instituto de Geología, Universidad Autónoma de San Luis Potosí, San Luis Potosí, México 3División de Ciencias de la Tierra, Centro de Investigación Científi ca y de Educación Superior de Ensenada, Carretera Ensenada- Tijuana No 3918, Zona Playitas, C.P. 22860, Ensenada, México 4European University of Brittany, 6538, Domaines Océaniques, Institut Universitaire Européen de la Mer (IUEM), Université de Bretagne Occidentale, 29280 Plouzané, France 5Facultad de Ingeniería, Universidad Nacional Autónoma de México, Ciudad Universitaria, Delegación Coyoacán, 04510, México, D.F., México ABSTRACT INTRODUCTION above sea level [masl]). Either elevation chosen from these new values shows that the Peña de Peña de Bernal is a natural monument Peña de Bernal, at 20°45′N, 99°57′W Bernal is apparently the highest monolith of located near the town of Bernal, in Queré- (Fig. 1), is a prominent peak that was declared a the world. taro State, central Mexico. It is one of natural reserve area by the State Government of Previous regional geologic mapping has con- the tallest monoliths of the world, with a Querétaro in 2007 (Fig. 2). Since 30 September sidered Peña de Bernal as a Paleocene monzo- maximum height of 433 m. Peña de Bernal 2009, it has been included in UNESCO’s Intan- granodioritic pluton (Segerstrom, 1961) or a was recently declared Intangible Cultural gible Cultural Heritage sites (UNESCO, 2009) subvolcanic Miocene felsic body (Carrillo- Heritage of Humanity Patrimony by United in order to preserve the traditions of Otomí and Martínez, 1998). However, the mechanism of Nations Educational, Scientifi c, and Cultural Chichimeca indigenous peoples as well as the emplacement has remained unknown. Before Organization (UNESCO). In spite of being Peña de Bernal itself. Furthermore, Peña de Ber- this work, there were no scientifi c studies spe- both a natural and cultural monument, nal is known as the main entrance to the Sierra cifi cally on Peña de Bernal’s geologic nature little is known about its origin, physical Gorda mountains area, a range that was declared and origin. The purpose of this study is therefore characteristics, and chemical composition. in 2001 as a Biosphere Reserve by UNESCO to provide detailed information on the geologic It is a leucocratic-igneous rock intruding (2001). In a few publications and in some web setting, including the stratigraphy, structure, marine Mesozoic sedimentary rocks and has sites, including the offi cial municipal (county) geochronology, and geochemistry, to answer been misinterpreted as a pluton of Eocene web page of the Bernal area and that of the fundamental questions of when and how Peña or older age. However, this study shows National Institute of History and Anthropology de Bernal was formed and why it is topographi- that Peña de Bernal is a dacitic dome with (INAH) of Mexico, Peña de Bernal is identifi ed cally prominent with respect to its surroundings. SiO2 = 67 wt% and an age of 8.7 ± 0.2 Ma. as one of the highest monoliths in the world, The complete Peña de Bernal body includes with monolith defi ned as a prominent and large GEOLOGY three plugs that crop out in an ~3.5 × 1.5 km monolithologic rock (Fig. 2). The exact height area elongated N40°E. Texture of the rock is of Peña de Bernal has not been well constrained, Peña de Bernal is located at the intersection porphyritic, nearly holocrystalline (80 vol% with reported heights ranging between 288 and of three major geologic provinces: the Miocene– crystals and 20 vol% glass), with a mineral 360 m (e.g., Coenraads and Koivula, 2007; Quaternary Mexican Volcanic Belt (Demant , assemblage of pyroxene, hornblende, bio- INAH, 2009; Municipio Ezequiel Montes, 1978; Aguirre-Díaz et al., 1998; Siebe et al., tite, plagioclase, and quartz, plus accessory 2009). From the topographic data of the offi - 2006), the mid-Tertiary Sierra Madre Occiden- apatite and zircon. Peña de Bernal dacite cial Mexican agency for National Geographic tal ignimbrite-dominated sequence (McDowell is a spine-type endogenous dome that was Studies and Information (Instituto Nacional de and Clabaugh, 1979; Aguirre-Díaz et al., 2008), forcefully intruded through the Mesozoic Geografía Estadística e Informática [INEGI], and the Mesozoic Sierra Madre Oriental fold- sequence practically as a solid plug. 2002) and from altimeter measurements carried thrust belt (Suter, 1987; Bartolini et al., 1999), out by us at the base and at the top of Peña de which in this area corresponds to the Sierra *Emails: Aguirre-Díaz: [email protected] Bernal, we have calculated a height of 433 m on Gorda sector (Fig. 1). A simplifi ed geologic map .mx; Aguillón-Robles: [email protected]; López- Martínez: [email protected]; Bellon: herve.bellon the northern side (Fig. 3). Height is 405 m on the of the Peña de Bernal area is shown in Figure 4. @univ-brest.fr; Nieto-Obregón: nieto@servidor southern side of Peña de Bernal with respect to The stratigraphy is briefl y described below, from .unam.mx. the plain where Bernal town lies (2049 meters the oldest to the youngest unit (Fig. 5). Geosphere; June 2013; v. 9; no. 3; p. 557–571; doi:10.1130/GES00843.1; 16 fi gures; 6 tables. Received 5 July 2012 ♦ Revision received 14 January 2013 ♦ Accepted 20 February 2013 ♦ Published online 4 April 2013 For permission to copy, contact [email protected] 557 © 2013 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/9/3/557/3344305/557.pdf by guest on 26 September 2021 Aguirre-Díaz et al. 100°30'W 100°00'W MEX N S I 57 E R R A G EXPLANATION O San José R Iturbide 21°00'N D Toll highway El Zamorano A Highway Volcano San Miguel Tolimán State line Tolimán Town Zamorano Study area City Colón San Martín Volcano Santa Rosa Cadereyta de Jauregui Ajuchitlán Bernal MEX 57 Ezequiel Villa Montes Progreso Apaseo El Grande Tequisquiapan QUERÉTARO 20°30'N San Pedro Escobedo 32ºN Apaseo El Alto MEX S I E R R A M A D R E O R I E N T 57 S I E R R A M A D R E O C C I D E N T A L 0510 20 U. S. A. San Juan del Río Kilometers 24ºN MESA CE Gulf Sierra NTRAMa of L AL Occidendre Mexico tal Pacific Mexican Volcanic Belt Ocean Mexico City 0 200 400 16ºN Km Guatemala 112ºW 104ºW 96ºW 88ºW Figure 1. Index map showing the position of Peña de Bernal with respect to some of the geological provinces of Mexico and the geography of the local area; the study area is highlighted in the inset. Las Trancas Formation (Jt) fi ssile shales, calcareous limonites, and dirty with minor amounts of graywackes. Bedding limestones with pyrite and minor amounts of is generally undulated, with thicknesses in the Las Trancas Formation was defined by graywackes and fl int (Segerstrom, 1961, 1962; order of tens of meters. The shales and lime- Segerstrom (1961) as a Jurassic–Early Cre- Chauve et al., 1985; Carrillo-Martínez, 1990). stones of Las Trancas Formation were inten- taceous (Kimmeridgian–Barremian) marine In the Bernal area, this unit occurs as a thinly sively deformed during the Cretaceous–Tertiary sedimentary sequence. It consists of dark-gray bedded sequence of fi ssile shales and limestones (K-T) Laramide orogeny (Chauve et al., 1985; 558 Geosphere, June 2013 Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/9/3/557/3344305/557.pdf by guest on 26 September 2021 Geologic setting of Peña de Bernal dome, Querétaro, México at the locality of Dedhó, and Texamites sp. in the Tolimán canyon, confi rming the Late Cretaceous epoch of this unit. Soyatal Formation is thinly to 500 km medium bedded, showing boudinage primary MEXICO bedding. It is dark gray when fresh, and light gray Peña de to white when weathered. As in Las Trancas, this N Bernal unit was folded and northeastwardly tectonically transported during the K-T Laramide orogeny Mexican (Chauve et al., 1985). Las Trancas Formation Volcanic Belt was thrusted on Soyatal Formation during this orogeny (Suter, 1987; Carrillo-Martínez, 1998), forming klippes at top of younger Soyatal. In the Bernal area, the top of Soyatal Formation is A eroded, and thus, a complete thickness estimate was not possible, but a minimum of 300 m is 0 0.5 ~1 Km NE Peña de Bernal peak Bernal town considered our best approach. San Antonio Tortuga Main plug de la Cal plug Lower Tertiary Continental Clastic Sediments (Ts) Continental detrital deposits crop out to the NW of the mapped area and nearby the town of San Martín (Fig. 1). Within the mapped area these deposits occur as small patches in the SW lower fl ank of Peña de Bernal too small to be shown in Figure 4, and to the southwest of Cerro B Azul, in the northeastern portion of the area. Thickness ranges from 0 to 20 m. Near Bernal Figure 2. (A) Panoramic view toward the north of Peña de Bernal monolith and cor- this unit consists mainly of conglomerates with responding regional index map (inset). (B) Panoramic view toward the east of Peña de clast sizes of 2–30 cm in diameter, mainly from Bernal from summit of San Martín volcano. Note the elongated shape of the main plug of shales of Las Trancas Formation and black fl int the Peña de Bernal dacitic dome and the rounded plug of Tortuga.