Sistema Regional de Información
en línea para Revistas Científicas de América Latina,
el Caribe, España y Portugal

The characterization by means of spectroscopic techniques of a natural montmorillonite collected at Cuencamé, Durango is described. The composition of the tetrahedral and octahedral layers was analyzed by magic angle spinning Nuclear Magnetic Resonance (NMR) for the nuclei of 29Si and 27Al. The superficial properties of the clay were quantified using the technique of adsorption-desorption of nitrogen as well as the Brunahuer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods. The results indicate that the analyzed clay is a Na-montmorillonite with a molecular layer of water adsorbed between their laminae, where the Si atoms in the phyllosilicate Q3 structure display different composition of Si-nAl (n = 0, 1 and 2). About 3.3% of the Al atoms in the structure are replacing Si atoms in the layer. The presence of the paramagnetic Fe+3 and Mn +2  ions in the structure was determined by electronic paramagnetic resonance after the analysis of the acidified clay treated with HCl and CF3SO3H. Additionally, the results acquired by atomic absorption, scanning electron microscopy, infrared and gravimetric thermal analysis are discussed.

Detailed paleontological studies of the basal strata of the La Peña Formation from a stratigraphic section of the State of Nuevo León, northeastern Mexico, reveal a rich ammonite assemblage representative of the local biozone indicative of the uppermost level of the early Aptian corresponding to the Dufrenoyia justinae Taxon Range Zone. The Lower Aptian assignment of this Mexican biozone is corroborated by its distinctive ammonite assemblage characteristic of both uppermost early Aptian standard ammonite zones of the stage; namely the Dufrenoyia furcata Zone, of the tripartite Mediterranean standard scheme, and its equivalent Tropaeum bowerbankii Zone of the northeastern Europe bipartite scheme. Thus, in Mexico, the shift of deposition from a shallow carbonate platform locally represented by facies of the Cupido Formation, to the deeper facies of the La Peña Formation took place sometime during the late early Aptian and not in the late Aptian as previously assigned. To avoid diachronic interpretations of the Cupido-La Peña formational contact, the ammonites analyzed in this study for biochronostratigraphic purposes, were compared to other coeval assemblages reported from the basal strata of the La Peña Formation on a regional scale.

The Sierra de Las Cruces volcanic range is located in the eastern portion of the Trans-Mexican Volcanic Belt. This range constitutes a morphological boundary between the basins of Mexico (2,240 m a.s.l.) and Toluca (2400 m a.s.l.). The range is 110 km long and has a variable width of 47 km to the north and 27 km to the south. The Sierra de Las Cruces consists of eight overlapped stratovolcanoes, named from south to north: Zempoala, La Corona, San Miguel, Salazar, Chimalpa, Iturbide, La Bufa, La Catedral, and other minor structures as Ajusco volcano. These volcanoes underwent alternated episodes of effusive and explosive activity, from the Pliocene to the Pleistocene, during which they were affected by faulting. This study presents a morphostructural model of the Sierra de Las Cruces through the analysis of aerial photographs, digital processing of satellite imagery, shadow relief models, thematic maps, and field work. The results indicate that Sierra de Las Cruces is made of three blocks: north, central and south, bounded by E-W faults. Each block is characterized by specific altitudes, slopes, density and trends of morpholineaments, drainage patterns, relief energy, dissection depth and orientation of drainage. The landforms and the structural analysis of the rocks defined three main faults system: N-S, NE-SW and E-W. These systems have interacted as normal faults from Pleistocene to Recent times. Reactivation of these faults has played an important role in definig the morphostructural arrangement of Sierra de Las Cruces and the configuration of the Mexico basin basement.

The epiclastic deposits related to volcanic slope instability and erosion include matrix facies of debris avalanche, cohesive debris flow, non-cohesive debris flow and tills. Because of their similarity in textural features, a granulometric approach is here presented in attempt to compare these four types of epiclastic deposits and discuss they similarities or difference on the basis of their origin, transport and sedimentary processes. We compared the granulometric distribution of 337 samples on the basis of density diagrams, where mean and standard deviation are compared. Main achievements are: best sorted deposits are non-cohesive debris flows, whereas the poorest sorted are cohesive debris flows; till deposits are the coarsest, whereas non-cohesive debris flows are the finest. By looking only at the granulometric distribution of the matrix (<-1f), better sorting corresponds to higher content of sand respect to silt and clay fraction. These results show that particles transported by debris avalanches, debris flows and glaciers do no suffer fragmentation or significant grain segregation during transport. In contrast, non-cohesive debris flow particles are easily segregated and yield better sorting. Finally, the granulometric distribution of these four types of epiclastic deposits associated to volcanic aprons (cones, edifices) depends more on the granulometry of the source material than on transport and depositional processes. This work shows that the granulometric characteristics of epiclastic deposits should be supported by additional textural information to better constrain their classification.

Microfossils from an outcrop of the coral reef and rudist-bearing calcareous upper member of the Mal Paso Formation just north of Chumbítaro, State of Michoacán, Mexico, indicate a deepening trend and transition from nearshore through outer shelf depositional environments upward through the sampled stratigraphic interval. The microbiota is mostly composed of species of calcareous algae and foraminifera. The identified calcareous algae are: Pseudolithothamnium album Pfender, 1936; Cayeuxia kurdistanensis Elliott, 1957; Acicularia americana Konishi and Epis, 1962; and Dissocladella sp. cf.D. savitriae Rama Rao and Pia, 1936. The species of foraminifera are: Nezzazata sp. cf. N. isabellae Arnaud-Vanneau and Sliter, 1995; Buccicrenata subgoodlandensis (Vanderpool, 1933); Cuneolina parva Henson, 1948; Pseudolituonella sp.; Praechrysalidina sp.; and Rotalipora appenninica (Renz, 1936). In addition, a species of stromatoporoid is illustrated and an indeterminate tube-shaped calcitic microorganism is described as an incertae sedis. From the base of the section upward, four biofacies are defined by the co-occurrences of these taxa: a benthic foraminiferal assemblage, a coral assemblage, a caprinid - dasycladacean assemblage, and a coral - miliolid assemblage. This report documents the first detailed examination of the microbiota of the calcareous upper member of the Mal Paso Formation. Data from this analysis of the microbiota supplement earlier paleoenvironmental interpretations based on studies of macrofossils, mainly scleractinian corals, rudists, and other mollusks, and carbonate facies relationships.The combined stratigraphic ranges of the microfossil species identified from this measured section of the Mal Paso Formation support an age determination of late Albian. The occurrence of Rotalipora appenninica (Renz, 1936), a planktic foraminiferan, in the uppermost portion of the exposed stratigraphic section is especially significant because its presence indicates a marked deepening of the depositional environment which can be correlated with the onset of the global late Albian marine transgression and drowning of Tethyan carbonate platforms that is known as the R. appenninica - event. A late Albian age was also suggested by previous studies of other taxonomic groups that have been discovered in the same stratigraphic section, particularly the species of the rudist bivalve genus Mexicaprina Coogan, 1973.

An integrated study of planktic foraminiferal tests (Orbulina universa and Globigerinoides trilobus) imaging techniques and chemical/isotopic analyses has been carried out at the Miocene section of the Pelotas Basin (South Brazil) with the purpose of record and evaluate the effects of the diagenesis in its wall texture and isotopic composition. The characterization of the foraminiferal tests preservation prior performing isotopic analysis for paleoceanographic studies is essential to ensure the choice of suitable material for obtaining reliable data. Scanning electron microscopy, backscatter scanning electron microscopy, energy-dispersive X-ray spectrometry and stable isotope measurements were used to evaluate the post-depositional effects on the tests. It was possible to identify features of dissolution, neomorphism (recrystallization) and coating of autigenic minerals. The stable isotopic data define two compositional groups. The heavier and less scattered values characterize a preserved paleoenvironmental signal as the lighter and scattered ones indicate a signal derived from post-depositional alterations. Additionally, the characterization of the distinct types of diagenetic changes and their textural products provide a guideline for the evaluation of the diagenetic effects of deeply buried fossil foraminifera.

A study of fossils (plant, bivalves and vertebrates) and sedimentary deposits was undertaken in a section of the Palo Pintado Formation (Payogastilla Group, Upper Miocene), southern Salta Province, Argentina. The section consists of 1,387 m of continental fluvial deposits. On the basis of fluvial architecture analysis, the sedimentary paleoenvironment was reinterpreted and the results were related to the paleocommunities and the paleoclimate conditions in which they may have developed. On the basis of stratigraphic and sedimentological features, we introduce new paleoenvironmental interpretations: the strata are related to an intermediate class between low and high sinuosity rivers forming a wandering sand-gravel fluvial system with small lagoons. Channel and overbank deposits can be distinguished within this river system: Accumulations within channels are composed of gravels bars and sandy bedforms, whereas overbank deposits are represented by crevasse channels, small crevasse channels, and floodplain deposits. In the floodplains, lagoons and marshes can be recognized in which aquatic and marsh communities developed, whereas during the dry season, or during longer dry intervals, suggested by desiccation cracks and thin gypsum beds, savannas and grasslands would had developed. The coasts of the lakes and marshes, which intermingled with fluvial riverbeds, were inhabited by hygrophilous communities and woodland. The aquatic and marsh paleocommunities and the hygrophilic forests were stable and developed under a hot and humid, tropical to subtropical climate, which is also indicated by the development of clay mineral assemblages of illite, smectite and kaolinite. These paleoclimatic conditions occurred at a time when global climatic changes led to drier conditions and to the establishment of xeric vegetation. The paleocommunities defined for the Palo Pintado Formation contain members of various current phytogeographic provinces from northern Argentina and other South American regions, although the modern communities had not been established at that time, and the fossil ones can be considered as their predecessors.

A juvenile of Pterocnemia pennata (MACN 14780) from the Bonaerian (early Late Pleistocene) locality of Salto (Salto district), in the north of Buenos Aires Province, Argentina is presented. This is the northernmost record for the species. The specimen consists of the hind limbs and fragments of fore limbs, phalanxes and vertebrae. The aim of this work is to describe and analyze those features of the hind limb that could indicate the postnatal ontogenetic stage (juvenile, subadult and adult) of the fossil. Characteristics related to the osseous maturation of the epiphyses as well as proximal and distal regions of the bones and to the periosteum texture were analyzed. The specimen hind limbs showed features which could be attributed to an immature age: the femur lacks the intercondylar region and the proximal epiphyses of the tibiotarsus and tarsometatarsus are absent. In addition, the proximal region of the tarsometatarsus is expanded and bulky and the metatarsals are not completely fussed among each other. The texture of the periosteum is characterized by numerous porous in the femur and striations in the tibiotarsus and tarsometatarsus, which are indicative of immatureness. The assignment to the postnatal ontogenetic stage could be misinterpreted if the fragmentary remain belongs to those parts that do not show immaturity features in a juvenile (e.g. proximal region of femur, distal region of tibiotarsus and/or tarsometatarsus). The presence of a new specimen of Pterocnemia pennata in a territory outside its current geographical distribution is another evidence that reinforces the hypothesis of arid conditions for this region during the early Late Pleistocene.

In the present contribution, the presence and distribution of siliceous microremains in characteristic soil types of the southwestern Entre Ríos Province (La Curtiembre soil series) are analyzed. The phytolith extraction was made following a methodology that comprises the physical and chemical processing of the samples. The results show that the phytolith assemblages are dominated by graminoid phytoliths, associated with arecoid ones. The graminoid types are characterized by the presence of prismatic elongated, fan-shaped and polyhedrical phytoliths among the largest components. Among the smallest phytoliths, bilobate and saddle types are abundant, especially in the samples from the upper and lower levels, whereas in the middle levels, roundel or truncated cones are more abundant. Echinate globular phytoliths are present with scarce variations across the profiles. The abundance analysis allows interpreting the presence of three phytolith sources in soil profiles. The first one is the parental material, the Tezanos Pinto Formation, principally abundant in the lower levels of the profiles. The second one originated in a past community that was present in early stages of the soils development, represented in the middle level samples of the profiles that clearly show evidence of material mobility along each profile. The third one is the phytolith assemblage of the upper levels, with a composition mainly linked to the current vegetation. The analyses demonstrated that these soils developed in a mixed community in association to a grassland dominated by panicoid and danthoniod affinities components, jointly with palms.

In the Toluca valley, important surface displacements have occurred in response to the activation of recently generated fractures, which locally condition the infiltration of contaminated surface water. This is the case of a fractured zone in the community Santiago Tianguistenco (Mexico State), to which wastewater, mainly domestic, from about 14,000 habitants is discharged through a canal. The objective of this paper is to quantify the infiltration process in the unsaturated zone of the mentioned fractured zone by combining technological development and numerical modeling, both supported by intense field work. In the course of the study, vertical electrical soundings were carried out and a monitoring well was drilled, which provided information to define the lithological features of the materials involved. These consist of fractured basalt, covered by a silt-clay horizon of variable thickness. Also, precipitation, evaporation, soil water content and water table variations were quantified by automatic devices, and the resulting values were used to calibrate and set the initial and boundary conditions in a numerical model of flow and transport. In this model, the studied zone is conceptualized as consisting of several layers, similar to a continuum porous media. The results obtained in the different simulations show that the nature of the first layer is determinant for the infiltration rate. In the last two months of observation, the fractures were partially sealed as a result of settling of solids suspended in the wastewater, a process that contributed to retard the infiltration process. The simulations for extreme conditions show that a conservative contaminant (without reactions) would go through the vadose zone in a time range of two to four years for the worst and best case, respectively.