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Palynological Evidence for Middle Miocene Vegetation in the Tehuacán Formation of Puebla, Mexico
Abstract and Figures
Palynological assemblages from the Tehuacán Formation (TF), geochronologically dated as Middle Miocene (15.6 ± 0.4 Ma), provide evidence of a highly diverse flora that, at the generic level, is similar to the extant flora in the Tehuacán Valley. We propose that, during Miocene times, plant communities may have been formed of similar botanical elements to those seen today in the region, with some taxa adapted to semiarid conditions. While major temperate floristic elements of Pinus, Quercus, Juniperus, cloud forest and mexical vegetation can be recognized, components of tropical deciduous forests, such as Burseraceae, Leguminosae and Cactaceae, are also present, indicating semiarid conditions. Semiarid local conditions are also inferred from the geological record, consisting of lacustrine and alluvial fan deposits, which contain abundant evaporites beds. This lithology was formed under high evaporation and moderate precipitation conditions, as usually occurs in small basins fed by a seasonal input of water in semiarid environments. Important differences in the vegetation from the TF palynoflora as compared to older associations from south-central Mexico can be inferred, such as an increase in the diversity of semiarid taxa, belonging to Leguminosae and Burseraceae, and the oldest abundant occurrence of the Cactaceae.
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... The current intricate physiography and diverse climatic conditions of Mexico have favoured the existence of exceptional botanical richness with high degrees of endemism (Rzedowski, 1978;Ferrusquía-Villafranca, 1993). For the past six decades, Cenozoic studies in this country have documented palynomorph assemblages throughout the Palaeogene (Martínez-Hernández et al., 1980;Tomasini-Ortíz and Martínez-Hernández, 1984;Rosales-Lomelí et al., 1992;Martínez-Hernández and Ramírez-Arriaga, 1999;Ramírez-Arriaga and Martínez-Hernández, 2003;Ramírez-Arriaga et al., 2006, 2014bRamírez-Arriaga and Reyes-Salas, 2014;Carrasco-Velázquez et al., 2009;Corona-Esquivel et al., 2010), during the Palaeogene/Neogene transition (Langenheim et al., 1967;Biaggi, 1978;Galván-Escobedo et al., 2017) and in the Neogene (Carrasco-Velázquez et al., 2008;Martínez-Hernández, 1992;Martínez-Hernández and Tomasini, 1986;Ramírez-Arriaga et al., 2014a,b, 2015Ramírez-Arriaga and Reyes-Salas, 2014;Ferrusquía-Villafranca et al., 2016;Rosales-Torres et al., 2017). Knowledge regarding Mexican Cenozoic vegetation history, mainly supported by palynological findings, helps us to understand how floristic richness increased and evolved into several plant communities over time, until the establishment of the present-day phytogeographic distribution. ...
... 4.8 capturing software were used for palynomorph photography. Terrestrial and marine palynomorph identification was done by comparison using specialised literature (Tschudy and Van Loenen, 1970;Elsik, 1974;Graham and Jarzen, 1969;Graham, 1987;Frederiksen, 1988;Ramírez-Arriaga et al., 2006, 2014a, 2014b. In order to standardise the counting of palynomorphs, one slide per sample was scanned and all the taxa present were recorded. ...
... It is a relictual flora that grows in disjunct areas, characterised by summer-dry as well as sub-humid areas with summer rainfall (Axelrod, 1975). Chaparral has just been reported from the upper Eocene-lower Oligocene Cuayuca Formation and the Miocene Tehuacán Formation (Ramírez- Arriaga et al., , 2014a. ...
Terrestrial and marine palynomorph assemblages from a total of 42 productive samples from San Gregorio Formation core LB1 were analysed. Marine palynomorphs, such as dinocysts, acritarchs, copepod eggs, among others, dominated the associations. With regard to terrestrial palynomorphs, dicotyledonae (e.g. Anacardiaceae type, Chenopodipollis spp., Brossipollis spp., Euphorbiaceae type, Fabaceae type, Quercoidites sp., Polygonaceae type and Sterculiaceae type) were more abundant than monocotyledonae (e.g. Liliacidites spp. and Graminidites sp.). The recovered palynoflora gave evidence of two temperate highland communities: Pinus forest and cloud forest. Furthermore, representatives of the local semiarid vegetation (Brossipollis, Chenopodipollis, Ephedripites and Graminidites), growing throughout a palaeoaltitudinal gradient from the uplands down to the shoreline, such as chaparral, tropical deciduous forest, coastal grassland and coastal dune also occurred. Terrestrial taxa richness varied between 5 and 57, the diversity index ranged between 1.2 and 3, and evenness oscillated between 0.4 and 1. As for marine palynomorphs, the dominant dinoflagellate cysts were Cleistosphaeridium sp., Cordosphaeridium sp., Chiropteridium lobospinosum, Homotryblium sp., Hystrichokolpoma rigaudiae, Lingulodinium machaerophorum, Operculodinium centrocarpum, Polysphaeridium sp. and Spiniferites spp., suggesting that San Gregorio Formation core LB1 was deposited in a neritic marine environment. The dinocysts Chiropteridium lobospinosum and Tuberculodinium vancampoae support a late Oligocene age for the San Gregorio Formation at LB1. Marine taxa richness oscillated between 5 and 18, the diversity index ranged from 0.2 to 2.4 and evenness fluctuated between 0.1 and 0.9. CONISS statistical analysis of the terrestrial and marine palynomorphs allowed us to group the SGF assemblages into four palynozones.
... The current intricate physiography and diverse climatic conditions of Mexico have favoured the existence of exceptional botanical richness with high degrees of endemism (Rzedowski, 1978;Ferrusquía-Villafranca, 1993). For the past six decades, Cenozoic studies in this country have documented palynomorph assemblages throughout the Palaeogene (Martínez-Hernández et al., 1980;Tomasini-Ortíz and Martínez-Hernández, 1984;Rosales-Lomelí et al., 1992;Martínez-Hernández and Ramírez-Arriaga, 1999;Ramírez-Arriaga and Martínez-Hernández, 2003;Ramírez-Arriaga et al., 2006, 2014bRamírez-Arriaga and Reyes-Salas, 2014;Carrasco-Velázquez et al., 2009;Corona-Esquivel et al., 2010), during the Palaeogene/Neogene transition (Langenheim et al., 1967;Biaggi, 1978;Galván-Escobedo et al., 2017) and in the Neogene (Carrasco-Velázquez et al., 2008;Martínez-Hernández, 1992;Martínez-Hernández and Tomasini, 1986;Ramírez-Arriaga et al., 2014a,b, 2015Ramírez-Arriaga and Reyes-Salas, 2014;Ferrusquía-Villafranca et al., 2016;Rosales-Torres et al., 2017). Knowledge regarding Mexican Cenozoic vegetation history, mainly supported by palynological findings, helps us to understand how floristic richness increased and evolved into several plant communities over time, until the establishment of the present-day phytogeographic distribution. ...
... 4.8 capturing software were used for palynomorph photography. Terrestrial and marine palynomorph identification was done by comparison using specialised literature (Tschudy and Van Loenen, 1970;Elsik, 1974;Graham and Jarzen, 1969;Graham, 1987;Frederiksen, 1988;Ramírez-Arriaga et al., 2006, 2014a, 2014b. In order to standardise the counting of palynomorphs, one slide per sample was scanned and all the taxa present were recorded. ...
... It is a relictual flora that grows in disjunct areas, characterised by summer-dry as well as sub-humid areas with summer rainfall (Axelrod, 1975). Chaparral has just been reported from the upper Eocene-lower Oligocene Cuayuca Formation and the Miocene Tehuacán Formation (Ramírez- Arriaga et al., , 2014a. ...
Dinoflagellate cysts from the LB-5 core in the San Gregorio Formation (SGF) from the La Purísima area, Baja California Sur, Mexico, limit the studied successions age to Oligocene-Miocene (28-20 Ma). Our results allow correlation of this core with the type locality and another well-dated section of the SGF in the region, namely the nearby La Purisima section, with an unequivocal Oligocene-Miocene boundary (O/M - 23 Ma). Lithology and dinoflagellate cyst data indicate mainly marine sedimentary environments, with a transgression from continental at the base to upper bathyal (200-500 m) in the O/M level (∼ 243 m), and a regression towards the top of the unit. Dinoflagellate cysts are virtually absent in some intervals, probably due to oxidation during diagenesis. The abundance of heterotrophic taxa during intervals with high cyst concentrations indicates that palynological preservation was adequate for quantitative analyses in the rest of the samples. Samples with dinoflagellate cyst concentrations >2,000 cysts/gram of sediment (c/g sed), the abundance of heterotrophic taxa, plus the presence of phosphoritic layers and diatomites, indicate high biogenic productivity during the deposition of the SGF. Intense upwelling conditions in the area are probably associated with the cold events Mi-1 (∼23 Ma) near the O/M and the Mi-1a (∼21.7 Ma) at ca. 170 m depth. Quantitative dinoflagellate cyst data in the LB-5 core suggest lower productivity (mean =203 c/g sed) in the Oligocene than during the Miocene (mean =848 c/g sed) interval. The absolute abundances of the Miocene intervals indicate similar dinoflagellate cyst concentrations in the LB-5 core and modern samples from the Pescadero Basin in the southern Gulf of California.
... Although paleoclimatic and paleobotanical studies of the Oligocene in Mexico are few, they suggest that in the north and center of the country the predominant climatic conditions were subhumid tropical with some degree of seasonality, and that the vegetation was similar to dry tropical forests, chaparrals, or savannas (Cevallos-Ferriz and Calvillo-Canadell, 2012;Pérez-García et al., 2012;Ramírez-Arriaga et al., 2014). The climatic conditions that prevailed in Mexico during the Oligocene have been explained as the result of a long process in which seasonal climates were slowly replacing more stable and humid tropical ones (Graham, 1999;Ferrari et al., 2000;Gómez-Tuena et al., 2007;Cevallos-Ferriz and Calvillo-Canadell, 2012). ...
... The basal portion of the sedimentary sequence is composed of calcareous rocks, sandstones and siltstones. Towards the top are shales and gypsum predominate with interspersed tuffs (Dávalos-Alvarez et al., 2007;Ramírez-Arriaga et al., 2014). According to lithological data, the sedimentary environment is interpreted as a continental sequence with lake facies that were deposited in endorheic basins. ...
... The geological record of the Tehuacán Fm. gives indications about the seasonality that existed in the region. The lithology suggests the existence of seasons of floods and droughts, being marked in the upper part of the formation and moderate towards the base (Dávalos-Alvarez, et al., 2007;Sainz-Reséndiz, 2011;Ramírez-Arriaga et al., 2014). This seasonality has been suggested by the gypsum and evaporite beds intercalated with siltstone that have been interpreted that occurred in the Neogene lakes of central Mexico (Dávalos-Álvarez et al., 2007;Sainz-Reséndiz, 2011;Ramírez-Arriaga et al., 2014). ...
Climatic conditions that prevailed during the deposition time of the Tehuacán Formation, Mexico (upper Oligocene) are proposed based on a previous model using wood anatomical characters. Different paleoclimatic variables were calculated, and the anatomical characteristics of the Tehuacán Formation paleoflora were compared with the characteristics of other extant and fossil plant communities to infer water conductive capacities of the Tehuacán Fm. plants. The paleoclimatic inference suggests that the environmental conditions under which the fossilized plants of the Tehuacán Fm. grew were seasonally warm and humid. Statistical analysis and the presence of growth rings in some woods suggest that the paleoflora of the Tehuacán Fm. were similar to extant tropical communities such as tropical semi-deciduous forests and the tropical deciduous forests, most probably representing a transitional community: containing plants that were efficient in transporting water like those in the tropical wet forests, but with phenological adaptations typical of drier tropical forests.
... The Balsas Depression biotic province, a wide, and disjunct area of SDTDF occupying the Río Balsas Depression (Rzedowski 1978, Morrone 2014, Pavón-Vázquez et al. 2017, is one of the most important hotspots for endemism (Sousa and Delgado-Salinas 1993, García-Trejo and Navarro-Sigüenza 2004, Becerra and Venable 2008 in which species represent old or recent elements of the Mexican flora (Sousa and Soto 1987, Sotuyo et al. 2007, Ramírez-Arriaga et al. 2014. The Río Balsas Depression is an east-west oriented basin that forms the lowlands of the Balsas River drainage basin surrounded by mountains, some of which exceed 2,000 m above see level and have a horizon that extends towards the east of the Tehuacán-Cuicatlán Valley and Valles Centrales in Puebla and Oaxaca states. ...
... Dusky Hummingbird occupies arid montane scrub, seasonally dry tropical deciduous forest and open-to-semiopen areas with scattered trees (gallery forests) from 900 to 2,200 m above sea level (Hutto 1992, Howell andWebb 1995; Figure 1). The current geographic distribution of P. sordida clearly matches the regional rain shadows (the Balsas river basin and the Tehuacán-Cuicatlán Valley and Valles Centrales) associated with the Miocene aridification (Ramírez- Arriaga et al. 2014, Medina-Sánchez et al. 2020). The divergence time estimated (11.2-7.6 Myr) for the split between P. sordida and the remaining lineages in the emeralds subclade (Hernández-Baños et al. 2020) is consistent with the Late Miocene average age of Bursera species and its strong conservatism to SDTDF (Becerra 2005. ...
The Dusky Hummingbird Phaeoptila sordida (= Cynanthus sordidus), occurs in the Balsas Basin, a region with a complex biogeographical history, and in the Tehuacán-Cuicatlán Valley and Valles Centrales in Puebla and Oaxaca, Mexico. However, the biogeographical and evolutionary history of these two regions of Mexico is poorly understood. We aimed to understand the genetic structure and phylogeographic history of P. sordida, a range-restricted hummingbird to these two regions, as a proxy to study the evolutionary history of the Balsas Basin, by using mitochondrial DNA (mtDNA) sequences and nuclear microsatellites. Geographic structure was evident for both markers; however, some discordance was observed between the mitochondrial and nuclear markers. Based on mtDNA, samples from the Balsas Basin form one haplogroup, well separated from the Tehuacán-Cuicatlán Valley and Valles Centrales samples. In contrast to this, nuclear microsatellites uncovered two slightly different genetic clusters restricted to different habitats: samples from the Western Balsas restricted to the seasonally deciduous tropical dry forest, and samples from the Eastern Balsas-Tehuacán/Cuicatlán-Tehuantepec area in shrub and dry forested habitats. As expected by the interglacial refugia hypothesis, ENM predicted that the distribution of P. sordida was more contracted and fragmented during the Last Inter Glacial and more expanded at the Last Glacial Maximum. Consistent with that observed for other range-restricted hummingbird species, ENM predictions and a strong signal of population expansion indicate that the geographical range and population size are unstable over time, as compared to widespread hummingbird species, and that ecological and climatic factors possibly impacted its diversification. This study contributes to the debate that disputes the integrity of the Balsas Basin as a biogeographical unit and urges for the conservation of endemic species in the Balsas region and interior Oaxaca.
... Palynomorph studies in Mexico mentioned fluctuating climate that became increasingly temperate with semiarid regions intermixed with subhumid regions, and increased climatic seasonality were responsible for the expansion and divergence of tropical deciduous forests [52][53][54]. Further evidence drawn from lithological studies indicated that the mountainous regions became more arid while the low-lying areas received maximum precipitation [55]. This evidence supports the dispersal time and genetic divergence of a closely related taxa of Ceratozamia into present-day Honduras and bordering eastern Guatemala and western Belize. ...
Ceratozamia Brongn. is one of the species-rich genera of Cycadales comprising 38 species that
are mainly distributed in Mexico, with a few species reported from neighboring regions. Phylogenetic
relationships within the genus need detailed investigation based on extensive datasets and reliable
systematic approaches. Therefore, we used 30 of the known 38 species to reconstruct the phylogeny
based on transcriptome data of 3954 single-copy nuclear genes (SCGs) via coalescent and concatenated
approaches and three comparative datasets (nt/nt12/aa). Based on all these methods, Ceratozamia is
divided into six phylogenetic subclades within three major clades. There were a few discrepancies
regarding phylogenetic position of some species within these subclades. Using these phylogenetic
trees, biogeographic history and morphological diversity of the genus are explored. Ceratozamia
originated from ancestors in southern Mexico since the mid-Miocene. There is a distinct distribution
pattern of species through the Trans-Mexican Volcanic Belt (TMVB), that act as a barrier for the
species dispersal at TMVB and its southern and northern part. Limited dispersal events occurred
during the late Miocene, and maximum diversification happened during the Pliocene epoch. Our
study provides a new insight into phylogenetic relationships, the origin and dispersal routes, and
morphological diversity of the genus Ceratozamia. We also explain how past climatic changes affected
the diversification of this Mesoamerica-native genus.
... In Mexico, the analysis of the Miocene biome has been carried out based on fossil fish from the center (Guzmán, 2015) and south of Mexico (Cantalice and Alvarado-Ortega, 2019), and mammals from the Baja California Peninsula, the Sierra Madre Occidental, the Altiplano, and the south of Mexico (Ferrusquía-villafranca, 2003;Ferrusquía-Villafranca et al., 2014). In the case of vegetation, notable studies are those using woods (Martínez-Cabrera, 2004;Castañeda-Posadas et al., 2009;Martínez-Cabrera and Cevallos-Ferriz, 2008), fruits (Estrada-Ruiz and Cevallos-Ferriz, 2007), leaves (Hernández-Villalva et al., 2013;Zayas-Ocelotl et al., 2014), and pollen (Lenhardt et al., 2006(Lenhardt et al., , 2013Ramírez-Arriaga et al., 2014). The prime example of a Miocene fossil locality in Mexico is Simojovel, Chiapas because of the extraordinary preservation of organisms contained in amber, with ages between 23 and 13 My (Riquelme et al., 2014). ...
Concern about the course of the current environmental problems has raised interest in investigating the different scenarios that have taken place in our planet throughout time. To that end, different methodologies have been employed in order to determine the different variables that compose the environment. In paleoecology, these variables are used for the reconstruction of paleoenvironments. Therefore, the objective of the present study was to reconstruct the environment of the early Miocene of Simojovel, Chiapas, using the taxonomic affinity of flower structures preserved in amber as a proxy. We used the method of the nearest living relative (NLR) for an estimation of the paleovegetation, which resulted in a high evergreen forest with the presence of mangrove elements and flooding zones. The paleoclimatic reconstruction was obtained using the Mutual Ecogeographic Range (MER) method, which resulted in a mean annual temperature of 24.19 ± 1.79 °C and a mean precipitation of 1745 ± 336 mm per year. These results indicate that the environmental conditions of the Miocene of Chiapas exhibited a tropical climate warmer (+1.99 °C) and drier (−337.7 mm) than the Present. Thus, the use of methods alternative to conventional ones used in plant macrofossils, together with a good proxy, such as flowers in amber as in our study, can help to infer the conditions of a fossil locality using quantitative and qualitative parameters.
Biogeographic disjunction patterns, where multiple taxonomic groups are shared between isolated geographic areas, represent excellent systems for investigating the historical assembly of modern biotas as well as fundamental biological processes such as speciation, diversification, niche evolution, and evolutionary responses to climate change. Studies on plant genera disjunct across the Northern Hemisphere (NH), particularly between eastern North America (ENA) and eastern Asia (EAS), have yielded tremendous insight on the geologic history and evolutionary assembly of rich temperate floras. However, one of the most prevalent disjunction patterns involving ENA forests has been largely overlooked: that of taxa disjunct between the eastern US (EUS) and cloud forests of Mesoamerica (MAM), with examples including sugar maple (Acer saccharum), dogwood (Cornus florida), sweetgum (Liquidambar styraciflua), redbud (Cercis Canadensis), hop hornbeam (Ostrya virginiana), and beechdrops (Epifagus virginiana). Despite the remarkable nature of this disjunction pattern, which has been recognized for over 70 years, there have been surprisingly few recent efforts to empirically examine its evolutionary and ecological origins. Here I synthesize previous systematic, paleobotanical, and phylogenetic and phylogeographic studies to establish what is known, and unknown, about this disjunction pattern to provide a roadmap for future research. I argue that this disjunction pattern, and the evolutionary history and fossil record of the Mexican flora more broadly, represents a key missing piece in the broader puzzle of Northern Hemisphere biogeography. I also suggest that the EUS-MAM disjunction pattern represents an excellent system for examining fundamental questions about how plant traits and life history strategies mediate plant evolutionary responses to climate change, and for predicting how broadleaf temperate forests will respond to the ongoing climatic pressures of the Anthropocene.
El total de 37 taxones de anfibios, reptiles (incluyendo dinosaurios) y aves, encontrados en sitios paleontológicos y arqueológicos, representan casi la mitad de los 72 taxones encontrados para mamíferos fósiles en Oaxaca. Esto indica la falta de estudios
hacia estos grupos de vertebrados, tal como sucede en el resto del país. Temporalmente, se tiene representados al Jurásico Medio, Jurásico Superior y Cretácico Inferior en el Mesozoico de Oaxaca; mientras que para el Cenozoico se tiene al Oligoceno, Pleistoceno tardío y Holoceno, quedando un hiatus temporal de 85 ma., entre el Cretácico Inferior y el Oligoceno, así como otro hiatus de 28 ma., entre el Oligoceno y el Pleistoceno. La falta de fósiles de anfibios y reptiles en estas edades se puede deber a la falta de colecta de material o a que restos de estos se encuentran guardados aún en gabinetes. En contraste, se han encontrado restos fósiles de 28 taxones mamíferos en cinco localidades del Paleógeno y Neógeno. Espacialmente, la presencia de fósiles de anfibios, reptiles y aves de Oaxaca en las provincias biogeográficas muestra una alta representatividad en la Sierra Madre del Sur y un sitio en la Costa del Pacífico. En comparación con el registro de mamíferos fósiles en el estado de Oaxaca, se encontró que existe una alta representatividad en la Sierra Madre del Sur, tendencia que se observa a nivel nacional hablando de mamíferos fósiles. También se tienen registros en las provincias de Oaxaca y Golfo de México, además de un único registro en la Costa del Pacífico. Esto sugiere que deben existir sitios fósilíferos con restos de anfibios, reptiles y aves en las regiones de Oaxaca y el Golfo de México, donde se han encontrado restos de mamíferos.
The morphological evolution of the basins in the Sierra Madre del Sur (SMS), southern México is poorly understood. This work explains for the first time the geomorphological development of the tectonic, fluvially-interconnected SMS basins named San Juan Raya (SJRb) and Zapotitlán (ZAPb). The evolution of the SJRb and ZAPb are analysed within the context of the transformations of the well-studied Tehuacán basin (TEHb). A new interpretation of a series of tectonic features of the TEHb valley area is also presented. Published geological data and extensive field work provided the basis for our geomorphological and evolutionary interpretation of basin evolution of this part of Mesoamerica during the late Cenozoic. Stratigraphic and sedimentary records suggest that after the late Cretaceous-early Cenozoic orogeny the TEHb and ZAPb were closed basins, and that the TEHb graben system was activated during the Paleogene as a response to the dominant regional NW-SE trending faults. We propose that the ZAPb and SJRb formed sequentially during the Neogene as a result of new E-W, N-S and NE-SW faults. The continuation of the TEHb extension during the Oligocene widened its lowland area and allowed the formation of an extensive lake. No alluvial or fluvial records of this interval are found in the ZAPb and SJRb. No sedimentation rather than formation and subsequent erosion of such sediments is supported by the basin morphology and by the absence of re-worked alluvial deposits at the outlet area where both connect to the TEHb. By middle to late Miocene the TEHb lost its endorheic configuration, ending the lake-type deposition while new faults initiated the opening of the ZAPb. Intensive tectonics, alluvial deposition and the confinement of the Tehuacán lake to the north sector of this basin characterised the Pliocene. During the late Pliocene to the early Pleistocene the formation of the SJRb was initiated. Quaternary faulting related to basin extension along the north watershed of the SJRb and ZAPb is supported by independent data on the biogeography of the cactus Mammillaria pectinifera. We introduce the idea that the departure from the regional NW-SE fault alignment that formed the major Miocene basins to a more local E-W trend that formed Neogene-Quaternary basins was probably a response to the latest post-orogenic relaxation of the crust in the Mixteca terrane.
p>Se describe un bosque mesófilo de montalia dominado por Engelhardtia mexianna del norte del Estado de Oaxaca (México). Esta comunidad prospera entre 1400 y 1800 m de altitud, sobre laderas de fuerte pendiente, expuestas a un régimen de muy frecuentes neblinas, con precipitación media anual de 5000 a 6000 mm y sin que en ningún mes llueva en promedio menos de 200 mm. Es un bosque denso y perennifolio, de 30 a 40 m
de alto, rico en epifitas, con predominancia absoluta de Engelhardtia en el estrato arbóreo superior. Una gramínea bambúsea (Yushania sp.) el helecho arbustivo Alsophla salvinii así como varias Melastomaceae (Miconia) y Palmae (Chamaedorea) destacan cuantitativamente a niveles inferiores. Con respecto a las relaciones geográficas de la flora, predominan elementos neotropicales, les siguen en import ancia los pantropicales y escasean los holárticos.
Al realizar el anális palinológico de los sedimentos del Mioceno Inferior del NW del Estado de Chiapas (México), se encontraron tres muestras con franca predominancia de polen de Engelhardtia. Se presentan los espectros de estas muestras y al compararlos con gráficas y listas de la lluvia de polen actual del bosque de Engelhardtia del N de Oaxaca, se encuentra un buengrado de similitud. Las diferencias mas notables entre ambas lluvias
de polen consisten en: 1a porcentajes ligeramente mayores de polen de Engelhardtia en el bosque actual, 2a porcentajes superiores de esporas de pteridofitas (sobre todo de Polypodiaceae) en los hosques miocénicos, 3a falta en los espectros actuales del polen de Tricolpopollenites y Picea, de los cuales el primero parece haberse extinguido por completo y el segundo dejó de existir en el centro y sur de México desde el Pleistoceno Superior.
El género Engelhardtia tuvo una amplia distribución en el hemisferio boreal duralite el Terciario, pero en el presente parece hallarse restringid o a pequenas áreas en: a) el sureste de México y Centroamérica, y b) el sureste de Asia.</p
Paleopalynology, second edition, provides profusely illustrated treatment of fossil palynomorphs, including spores, pollen, dinoflagellate cysts, acritarchs, chitinozoans, scolecodonts, and various microscopic fungal and algal dispersal bodies. The book serves both as a student text and general reference work. Palynomorphs yield information about age, geological and biological environment, climate during deposition, and other significant factors about the enclosing rocks.
Extant spores and pollen are treated first, preparing the student for more difficult work with fossil sporomorphs and other kinds of palynomorphs. Recognizing that palynomorphs occur together in rocks because of chemical robustness and stratigraphic distribution, not biological relationship, the central sections are organized stratigraphically. Among many other topics presented are the sedimentation and geothermal alteration of palynomorphs, and palynofacies analysis. An appendix describes laboratory methods. The glossary, bibliographies and index are useful tools for study of the literature.
Based on the lithology of the Izucar de Matamoros (IzM) sections, and the occurrence of index taxa such as Armería, Cicatricosisporites dorogensis, Corsinipollenites, Eucommia, Momipites coryloides, Momipites tenuipolus, Mutisiapollis, and Ranuculacidites operculatus, a correlation between the IzM sections and the Cuayuca Formation stratotypes is proposed. The IzM sections are Late Eocene‐Early Oligocene, and they are part of the evaporitic member (Mcy) of the Cuayuca Formation. These new data are consistent with the paleoenvironmental interpretations proposed for the formation. It was probably deposited under local xeric conditions in a semiarid climate that allowed the development of grassland (Gramineae with Amaran thaceae‐Chenopodiaceae and Ephedra) and a thorn shrub community with Acacia, other Leguminosae, Linutn, and Plumbaginaceae. The neighboring communities were probably tropical deciduous forests, low tropical deciduous forest, thorn forest, and chaparral. There were also regional temperate vegetations such as a Picea‐Pinus forest, and a cloud forest community. Using parsimony analysis of endemicity, a biogeographic method, a palynofloristic relationship between the Cuayuca Formation and the Pie de Vaca Formation (Late Eocene‐Early Oligocene) is proposed. Both formations are within the Balsas Group, and are correlated with San Gregorio Formation (Oligocene) of southern Baja California.
A sequence of amphibolite facies ortho- and paragneisses is exposed along the coast of southern Oaxaca state, between Puerto Angel and Santiago Astata, Mexico. The lithologies present include garnet+biotite+/-sillimanite schists, garnet+diopside marbles, hornblende+cummingtonite amphibolites, and quartz+feldspar+biotite+/-hornblende orthogneisses. These rocks show evidence of at least two superposed folding events: the first is suggested by folded foliations in restites within migmatized portions of the gneisses, while the second is indicated by asymmetric, north-verging folds affecting the migmatitic foliation. Associated with these folds are mylonitic shear zones tens of meters wide with shallow to moderate southerly dips and top-to-north kinematic indicators.
Biotite+/-hornblende bearing granites and granodiorites of Oligocene age intrude the metamorphic rocks. One of these bodies, the 29±0.2 Ma Huatulco granodiorite, is truncated along its northern edge by an E-W trending, subvertical mylonitic shearzone up to two km wide with subhorizontal stretching lineations and left-lateral kinematic indicators. The westward extension of this shearzone is the tectonic contact between the Xolapa complex migmatites and the granulite facies, grenville age Oaxaca complex gneisses to the north. The mylonites are cut by granodioritic porphyritic rocks that yielded a K/Ar biotite age of 23.7+/-1.2Ma, indicating that the mylonitic deformation occurred between 29 and 24 Ma. The mylonitic rocks of the shearzone are in turn affected by phyllonites, pseudotachylites, cataclasites, and active faults, which document further exhumation and left-lateral activity of the shearzone.
Strain analyses of these rocks were carried out using two different methods. The first was developed during this project and consists of an extension of the Fry method in which the density of the cloud of Fry dots is quantified to aid the recognition of the central vacancy characteristic of the Fry diagram. The second is the scanner method, which consists of retrodeformation of a digitized photograph of the strained rock until an isotropic fabric is obtained. Strain analyses on rocks from the Chacalapa Fault mylonite zone result in strongly prolate ellipsoids with subhorizontal maximum extensions which suggest a strongly transtensional deformation.
The Al in hornblende geobarometer was used to calculate the depth of crystallization of some of the Oligocene plutons. The results indicate that they crystallized at depths between 20 and 10 km, which, in conjunction with the available isotopic age data allow the calculation of cooling rates of up to 119 (+55/-21) ºC/Ma. The amphibole-plagioclase geothermometer was used to estimate temperature conditions during crystallization of amphibolites within the Xolapa Complex. The results indicate that the minimum for the thermal peak must lie above 650ºC.
The effect of left-lateral strike-slip faulting on river channels in the study area was assessed. The apparent offset of stream channels reaches a maximum of 3.8 km and a cumulative total of 11.4 km distributed over 5 faults with a spacing of 5 km between faults. Moment tensor analysis of 52 earthquakes shows earthquakes with inverse, normal and oblique kinematics with a notable absence of strike-slip events. The oblique events are deep (between 100 and 150 km) and concentrated in the area beneath the Tehuantepec Isthmus. These oblique events are associated with a lithosperic-scale fold in the subducted plate caused by a combination of different ages of subducted crust and the curved geometry of the Middle America Trench at this point.
The exhumation and past and present left-lateral kinematics along the Pacific coast of Mexico are associated with the SE passage of the Chortis block toward its present position and the present-day NE-directed oblique subduction of the Cocos plate beneath North America.