Boletín de la Sociedad Geológica Mexicana / 2017 / 691 Field guide to the Jurassic Otlaltepec and Tezoatlán Basins, southern Mexico: sedimentological and paleontological records of Puebla and Oaxaca Michelangelo Martini, Patricia Velasco-de León, Mildred Zepeda-Martínez, Diego E. Lozano-Carmona, Mónica Ramírez-Calderón ABSTRACT Michelangelo Martini ABSTRACT RESUMEN [email protected] Instituto de Geología, Universidad Nacio- nal Autónoma de México, CDMX, 04510, By early Mesozoic time, a major plate reorgani- A partir del inicio del Mesozoico, una reorganización de México. zation produced the breakup of Pangea, which las placas tectónicas a escala global determinó la frag- was the most recent supercontinent assembled mentación de Pangea, la cual fue el supercontinente más on Earth. Due to its paleogeographic position reciente que se ensambló en la Tierra. Durante este even- Patricia Velasco-de León along the emergent plate boundary between to tectónico global, el territorio mexicano ocupaba una Diego E. Lozano-Carmona Facultad de Estudios Superiores Zaragoza, North and South America, the early Mesozoic posición de particular interés, ya que se encontraba en el Universidad Nacional Autónoma de México, tectonic history of Mexico was dominated by límite de placa entre Norte y Sur América. Debido a CDMX, 09230, México. the development of major normal to strike-slip dicha posición paleogeográfica, la evolución geológica de faults. These faults produced a complex crustal México durante el Mesozoico temprano fue controlada configuration characterized by subsiding ba- por la actividad de fallas normales y laterales, que deter- Mildred Zepeda-Martínez sins bounded by exhuming basement highs. minaron una configuración cortical compleja y caracteri- Mónica Ramírez-Calderón Posgrado en Ciencias de la Tierra, Universi- Voluminous, continental to marine sedimenta- zada por numerosas cuencas subsidentes bordeadas por dad Nacional Autónoma de México, CDMX, ry successions were accumulated into these ba- altos de basamento en exhumación. Durante el Jurásico, 04510, México. sins during the Jurassic, representing a unique las cuencas generadas por este evento tectónico fueron los stratigraphic record related to the fragmenta- sitios de depósito de potentes sucesiones continentales a tion of the western equatorial margin of this marinas, las cuales representan un registro estratigráfico supercontinental mass. único de la fragmentación de la Pangea en su margen This field trip will provide an overview of the occidental-ecuatorial. Otlaltepec and Tezoatlán Jurassic Basins that Durante esta excursión, los participantes visitarán las are exposed in the states of Puebla and Oax- cuencas jurásicas de Otlaltepec y Tezoatlán, las cua- aca, southern Mexico. Observed outcrops are les están expuestas en los estados de Puebla y Oaxaca, exposed along two amazing canyons and con- al sur de México. Las dos barrancas que se visitarán sist of alluvial-fluvial to marine clastic deposits. contienen afloramientos bien expuestos de sucesiones Major provenance changes are recorded in clásticas aluviales-fluviales y marinas. Las sucesiones the Jurassic successions of the Otlaltepec and jurásicas de las Cuencas de Otlaltepec y Tezoatlán pre- Tezoatlán Basins and are interpreted as the servan claras evidencias de cambios en la procedencia result of exhumation of different fault-bound- de las rocas clásticas. Dichos cambios en la procedencia ed lithospheric blocks during Pangea breakup. han sido interpretados como el resultado de la exhuma- The integration of petrological and paleonto- ción de diferentes bloques litosféricos a lo largo de fallas logical data indicates that these major chang- mayores durante el progresivo rompimiento de Pangea. es in provenance of clastic rocks match with La integración de los datos petrológicos y paleontológi- changes in climatic conditions. Therefore, in cos indica que estos cambios en la procedencia coinciden this field trip, we will have the opportunity con cambios importantes en las condiciones climáticas. to explore the hypothesis that the progressive Por ende, se aprovecha la oportunidad de esta excursión exhumation of lithospheric blocks during the para explorar la hipótesis que el proceso de exhuma- fragmentation of Pangea could have produced ción de bloques litosféricos que acompañó el progresivo major topographic changes that favored local rompimiento de la Pangea pudo haber determinado una variations in climatic conditions and conse- reorganización sustancial en la topografía, favoreciendo quently, the diversification of floral assemblag- variaciones climáticas locales y, consecuentemente, la di- es in Mexico. versificación de la flora en México. Keywords: Pangea breakup, intra-con- Palabras clave: fragmentación de Pan- BOL. SOC. GEOL. MEX. 2017 tinental rift basin, southern Mexico, gea, cuenca de rift intra-continental, sur VOL. 69 NO. 3 Jurassic flora. de México, flora del Jurásico. P. 691 ‒ 709 Manuscript received: December 12, 2016. Corrected manuscript received: June 10, 2017. Manuscript accepted: June 15, 2017. Field guide to the Jurassic Otlaltepec and Tezoatlán Basins, southern Mexico 692 / Boletín de la Sociedad Geológica Mexicana / 2017 1. Introduction aim of stimulating a constructive discussion on the control that the fragmentation of Pangea exerted By the beginning of Mesozoic time, Precambrian on the climatic configuration and the paleofloristic and Paleozoic rocks that make up the backbone of evolution of Mexico and offering some perspec- INTRODUCTION / THE OTLALTEPEC BASIN present-day Mexico were located along the west- tive for future investigations. ern margin of equatorial Pangea. The progressive fragmentation of the supercontinental mass during this time produced the development of normal to 2. The Otlaltepec Basin lateral, lithospheric-scale faults that accommodat- ed the NW – SE regional extension imposed by The Otlaltepec Basin is exposed in the surround- North America-South America divergence (Pin- ings of Santo Tomás Otlaltepec (figures 1 and 2a) dell, 1985; Ross and Scotese, 1988; Dickinson and and is composed of a more than 2000 m thick Lawton, 2001; Pindell and Kennan, 2009). These clastic sedimentary succession. To the south-west, faults significantly shaped the Jurassic topography the clastic succession is faulted against the upper of Mexico, producing a complex crustal config- Paleozoic Totoltepec pluton (Kirsch et al., 2012) uration characterized by progressively subsiding, along the W-trending, sinistral normal Matanza extensional to transtensional basins bounded fault (Figure 2a). The western boundary of the Ot- by basement highs (Morán-Zenteno et al., 1993; laltepec Basin is represented by a NNE-trending Goldhammer, 1999; Campos-Madrigal et al., normal fault along which the clastic succession is 2013; Martini and Ortega-Gutiérrez, 2016). juxtaposed to Paleozoic metasedimentary rocks of Considering that the atmospheric circulation in the Acatlán Complex (Figure 2a; Ortega-Gutiér- a certain geographic area is strongly influenced rez, 1978). The northern and eastern boundaries by the topographic configuration, the progressive of the basin are not exposed. The comprehensive exhumation of fault-bounded lithospheric blocks stratigraphic framework and internal architecture during Pangea breakup may have favored chang- of the entire basin is poorly known because at es in climatic conditions in Mexico and, therefore, present only its southern part has been exhaustive- the diversification of floral assemblages. ly explored and studied. Along its southern part, This field trip will provide an overview of the Ju- the stratigraphic record of the Otlaltepec Basin rassic stratigraphic and fossiliferous records of has been subdivided into four units that, from the the Otlaltepec and Tezoatlán Basins that are lo- base to the top, are: the Tianguistengo, Piedra cated in the states of Puebla and Oaxaca, south- Hueca, Otlaltepec, and Magdalena Formations ern Mexico (Figure 1). The observed outcrops are (Figure 2b; Ortega-Guerrero, 1989; Morán-Ze- exposed along two canyons and consist of alluvi- nteno et al., 1993; Verde-Ramírez, 2016). These al-fluvial to marine clastic deposits, which show units are separated by regional-scale angular un- major changes in composition that have been in- conformities and are composed of fluvial to litto- terpreted as the result of exhumation of different ral deposits, some of which contain Jurassic flora lithospheric blocks during the fragmentation of (Ramos-Leal, 1989; Morán-Zenteno et al., 1993). Pangea (e.g. Martini et al., 2016). The documented This clastic succession is overlain by Lower Creta- compositional changes in the Otlaltepec and Tezo- ceous shallow-marine limestones of the Coyotepec atlán Basins match with major changes in climatic Formation, which were deposited during a region- conditions, as it is suggested by sandstone petro- al-scale transgression event (Figure 2b; Morán-Ze- logic data and fossil flora. We take the occasion nteno et al., 1993). of this field trip to synthesize the petrological and During this field trip we will focus on the Piedra paleontological results obtained in the last decade Hueca and Otlaltepec Formations. These units by the authors of this field guide. This is with the represent more than 80 % in volume of the Ot- Field guide to the Jurassic Otlaltepec and Tezoatlán Basins, southern Mexico Boletín de la Sociedad Geológica Mexicana / 2017 / 693 98° 00'W 97° 30'W 18 30' ° N USA MEXICO Otlaltepec Basin THE OTLALTEPEC BASIN Santo