Sistema Regional de Información
en línea para Revistas Científicas de América Latina,
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Within the basins of Mexico and Tlaxcala in the Central Highlands of Mexico, thick pyroclastics– paleosol sequences, grouped into units T1 to T7, can be found. Unit T2 at the site Tlalpan was chosen for a mineralogical-microscopical study. The unit consists of four late Pleistocene strata of pyroclastics (toba) with associated clayey soil horizons. Apart from magmatic particles, the pyroclastics contain many opal particles, especially diatom fragments and other bio-opals. In the basal stratum of pyroclastics ‘i’, the opals are orientated within the layers and, therefore, must have been deposited together with the other components. They were not, consequently, admixed later through soil formation or bioturbation. The very well-preserved microlamination and the orientation of the individual particles render a significant loss of pore volume (compaction) after sedimentation unlikely. According to the microscopic findings, in-situ weathering is the dominating process in this stratum, as it is, probably, in the other pyroclastic strata (‘c’, ‘e’ and ‘g’) as well. The clayey soils –as the tobas– are characterized not only by weathering of volcanic glass, but also by weathering of opal particles. Colloidal silica gel is the result of weathering of opal and volcanic glass, in addition to which clay minerals are formed from the glass. Microscopic silica gel crusts and clay cutans are the characteristic forms found, the latter described more precisely as clay-mineral / silica gel compounds. Laterally adjacent to the studied profile, characteristic indurated layers, so-called ‘Tepetates’ in the pyroclastic strata ‘e’and ‘i’ can be observed. They develop out of the respective pyroclastics after their exposure following erosion. The following reasons for their formation are proposed: A comparatively dense packing of the parent material, a subsequent compaction, an intense re-location of silica gel within the matrix, its loss of water, and, probably, its partial crystallization. These results only refer to the stratigraphic unit T2 and may not be applied to other stratigraphic units.

The knowledge of past climatic and environmental conditions in central Mexico, interpreted extensively from lacustrine records, is restricted to the last ca. 50,000 years. The recent discovery of new localities of paleosol sequences in central Mexico and the reconnaissance of their usefulness as records of environmental change provides a new source of information about past climatic conditions for times prior to the last full glacial. In this paper, we analyze the mineral magnetic properties of a sequence of paleosols developed in volcaniclastic deposits and evaluate these data as paleoenvironmental proxy. The sequence consists of a modern Phaeozem soil and seven late Quaternary Luvisol paleosols outcropping in Barranca Tlalpan, Tlaxcala, Mexico. Rock magnetic analysis was used to study the concentration and type of magnetic mineralogy. The mineral magnetic data, supported by soil morphological and chemical data, are interpreted to indicate that the paleosols can be grouped in three sets, each with distinct characteristics. The Red set paleosols P7 and P6 are the most weathered, and present the highest magnetic concentration composed of Ti-magnetite and ultrafine magnetite (<30 nm). The Brown set of paleosols P5, P4 and P3, with few redoximorphic features, have coarser grains and lower concentrations of magnetic minerals. The Gray set, paleosols P2 and P1, have abundant redoximorphic features including Fe–Mn nodules, and have a multicomponent magnetic mineralogy of coarse size. Paleosols from the Red and Brown sets conserve evidence of neoformation of ultrafine magnetite grains in the Bt horizons, probably derived from pedogenic processes. However, direct observations by microscopy and rock magnetism parameters point to a later destruction of this ultrafine fraction. We consider that even if pedogenic processes resulted in the formation of ultrafine magnetite, redoximorphic conditions under humid climates are responsible of the destruction of magnetic minerals. Such conditions may have strongly prevailed in the Gray set paleosols, whereas the Red set was probably developed under contrasting seasonal precipitation climates, which favored the formation and preservation of pedogenic magnetic minerals.

Clay mineral analysis is widely used to characterize soil parent material and to relate it to the bedrock. When it is applied to a soil profile, it can give a ‘genetic signal’ of mineralogical transformations due to soil-forming processes. In this study, ten selected soil profiles in Montagnola Senese, central Italy, were analyzed for genetic signals of soil ageing, eluviation and illuviation, fragipan formation, and other processes. In previous investigations, the parent materials of benchmark soils of the area have been dated to gain an understanding of the Quaternary geomorphological evolution, which spans a time ranging from early Pleistocene to Holocene. The number of profiles selected allowed a statistical analysis of the parameters. Illite and kaolinite are the most quantitatively important phyllosilicates in the soils studied. The other clay minerals found are vermiculite, hydroxy-interlayered vermiculite (HIV), illite–HIV and illite– chlorite mixed layers, and chlorite. Soil ageing is characterized by a change in clay mineral composition that increases with age. Argilluviation is marked by a preferential accumulation of kaolinite in illuvial horizons, while formation of tongues in fragipans does not cause any significant difference in clay mineral composition between bleached and stained parts of the horizons. Furthermore, bulk density of fragipans is not related to clay mineral content. On the other hand, large bleached masses in more mature soils contain more kaolinite and vermiculite than the surrounding mass. The trends in the contents of clay minerals through the soil horizons confirm the two types of lithological discontinuities that were predicted during the field survey.

Some soils around Chennai, Tamil Nadu, are represented by ferricrete horizons, and these can be related to the underlying bedrock. In the present study, Red Soils and ferricretes have been studied from four different sites: Red Hills, Vaiyapur, Uttukkadu, and Pallavaram, which were formed from different parent rocks. Red Soils and ferricretes around Vaiyapur and Red Hills are formed from the Upper Gondwana sandstone and shale, while around Pallavaram they are formed from Precambrian charnockites. In this paper, morphology, micromorphology, and geochemistry of ferricretes and Red Soils are presented. Microfabric elements in polished thin sections provide a basis for the interpretation of processes involved in Red Soil and ferricrete formation. Micromorphology of Red Soils and ferricretes reveals high content of the clay minerals gibbsite, smectite, halloysite, and kaolinite. Fe-oxide mineralogy is represented by hematite, limonite, goethite, and magnetite. Pedogenesis has produced iron segregation that has formed, both in external and internal forms, in a great variability of colors. Cracks and fractures are filled by either kaolinite or hematite. Some of the fractures are lined with black manganese oxide representing the final depositional phase. Based on geomorphology, the soils date to the late Neogene to early Quaternary period. Climatic conditions are interpreted to have been wetter than today.  

Paleopedology is a rapidly developing branch of geoscience that investigates and interprets the carriers and elements of “soil memory” in surface and buried soils to decode the rich and variable paleopedological records of past environmental change. In this volume, Revista Mexicana de Ciencias Geológicas begins publishing papers presented at the VI International Symposium and Field Workshop on Paleopedology, held at the Colegio de Postgraduados, Montecillo, Mexico in October 2001 – an event of the International Commission on Paleopedology INQUA-IUSS, which for the first time took place in Latin America. This meeting investigated a wide variety of problems considered by paleopedologists, various case studies, and the methods used for their solution. A large part of paleosol research, whether buried or relict, is interdisciplinary in nature. Among the topics that attract major interest is the investigation of paleopedological indicators of ancient human-induced environmental change and the application of soil science methods in archaeological research. This issue contains the collection of papers presented during Session V of the Symposium: “Paleopedology and archaeology: Paleopedological evidences of ancient man-induced environmental change”, which covers various topics of soil archaeological studies. Heine summarizes decades of research on the record of human-induced soil degradation and erosion preserved in geoforms and soil sedimentary sequences in valleys, finding strong correlation with archaeological data on the settlement pattern and population density for prehispanic and colonial periods in Tlaxcala, Mexico. Bronnikova et al. and McClung et al. present models of landscape dynamics on the regional scale as background for understanding cultural evolution at the famous archaeological sites Gnezdovo (Central Russia, VIII-X century A.D.) and Teotihuacan (Mexico, II-VII centuries A.D.). In both cases, the authors correlated thoroughly the paleopedological data with the paleobotanical records, allowing the derivation of more solid and reliable conclusions about both natural and human-induced landscape change. Soil profiles under burial mounds (kurgans) of the Eurasian steppe have been fashionable objects for studies of Holocene soil and ecosystem evolution for decades. Golyeva and Khokhlova demonstrated through phytoliths that, in many cases, the soils buried under kurgans suffered strong anthropogenic disturbance before burial, that can alter or mask the signals from natural paleoenvironmental change and thus lead to misinterpretation. Alexandrovskaya and Panova studied chemical characteristics of soils and archaeological sediments in the ancient urban environment of Moscow – the case of very thick and pervasive anthropogenic soil and landscape transformation. Their data show that buried soils in the ancient city limits contain high values of various microelements derived from anthropogenic materials; the pollution with some heavy metals was higher during the Late Medieval period than at present. A specific applied aspect of paleopedology is the analysis of buried soil horizons as the source of useful materials for ancient and modern culture. Ramos et al. showed that a particular buried soil horizon has provided clay-rich material for traditional ceramic production in Tlaxcala, Mexico. The remaining papers presented in the VI International Symposium and Field Workshop on Paleopedology will be published in a special issue of Revista Mexicana de Ciencias Geológcas at the beginning of 2004. We would like to express our gratitude to the colleagues, who helped to prepare the manuscripts for publication: Elizabeth Solleiro, Ana-María Rocha, Maricela Coronado, and Hector Cabadas.

El modelo hidrogeológico de la subcuenca de Tecocomulco corresponde a una fosa tectónica de orientación NE-SW, que se amplia en extremo noreste. El relleno de esta fosa, de arriba hacia abajo, está constituido por ~600 m de sedimentos aluviales y volcanoclásticos, rocas volcánicas de diferente composición con un espesor de 500 a 1,500 m, y más abajo, aquellas de tipo calcáreo. En este trabajo se consideran éstas como unidades hidrogeológicas que tienen conexión hidráulica entre sí y forman un sistema acuífero. Hacia el centro de la planicie existe un acuitardo, que cubre parcialmente a la parte intergranular del sistema acuífero donde se comporta como semiconfinado. El modelo geológico regional permite indicar que las subcuencas de Tecocomulco, Apan y Sahagún tienen comunicación hidráulica entre sí a través de fracturas en las unidades consolidadas, así como por el material aluvial que subyace al relieve volcánico cuaternario. Considerando esto y la posición topográfica más alta de Tecocomulco, se considera que ésta es parte del área de recarga de las subcuencas de Apan y Sahagún. La configuración de datos piezométricos determina un gradiente hidráulico horizontal regional de 0.009 0/00, donde el flujo es de Tecocomulo hacia Apan y Sahagún. La interpretación de los índices geoquímicos Cl- / HCO3- y (SO42- + HCO3-) / Cl- confirma esa dirección del flujo subterráneo como E-W. El registro de conductividad eléctrica y temperatura del agua subterránea en dos piezómetros instalados en la planicie, muestra que entre 5 y 50 m de profundidad existen flujos verticales ascendentes y descendentes; de 70 a 80 m de profundidad el flujo de agua es ascendente y, a mayor profundidad, el flujo es nuevamente descendente. Esto indica la existencia de flujos subterráneos locales, intermedios y regionales. El contenido de E. colli y Enterobacter en el agua subterránea varía de un sitio a otro pero, en general, está por encima del límite permisible, definiendo un agua relacionada con sistemas de flujo muy dinámicos y poco profundos. Del balance de agua subterránea resulta un pequeño excedente de agua, sin embargo, este excedentees parte de la recarga hacia las áreas adyacentes, pero sobre todo es el que hace posible la existencia del Lago de Tecocomulo.

Along the west-central limit between the Central Mexico Mesozoic Basin and the Valles-San Luis Potosí carbonate platform exist marked variations in style and intensity of Laramide folding (Late Cretaceous-Early Tertiary). These variations are attributed to: 1) prominent inter- and intra-formational changes in lithology and bedding thickness, and 2) overall thickness contrast between the Cretaceous sedimentary sequences accumulated in the basin and the carbonate platform. Tight, NE-verging folds with fold widths in the order of meters to tens of meters formed in those sites where only thinly bedded, slightly argillaceous, basin calcareous turbidites were deposited. Folding is disharmonic in other localities where thicker beds of slide breccias, made of clasts derived from the nearby carbonate platform, are interlayered with the basin turbidites. Breccia sheets controlled the formation of mesostructures, with fold widths in the order of hundreds of meters, while second order folds, with fold widths in the order of meters, were formed in the thinner bedded argillaceous limestones. Thick strata of shallow water carbonates, deposited in and around isolated patch reefs in the platform interior, formed open (fold widths in the order of several kilometers), almost symmetrical folds with near vertical axial planes. Despite significant facies changes in the limestone, associated with the presence of patch reefs, no variations in the folding style are related with them. The occurrence of anhydrite strata (Guaxcamá Formation) under isolated portions of the carbonate platform interior also played an important role in the folding style, causing the tectonically induced accumulation of evaporite near the cores of some folds and/or formation of diapirs. This phenomenon modified the structures and caused intense fracturing in the central portion of the Sierra de Guadalcázar. In few places, anhydrite was injected along fractures in the limestone. Meteoric water infiltration and dissolution of the anhydrite developed intense karsticity and caused the formation of an extensive collapse breccia. Division of the Sierra de Guadalcázar into several domains suggests that the structural anomalies in respect to its immediate surroundings are concentrated in an area where anhydrites are exposed. Furthermore, the emplacement of the mid-Tertiary Cerro de San Cristóbal intrusive, a tin-bearing subvolcanic granite, did not caused doming by forceful injection at the core of the Sierra de Guadalcázar.

The transtensional Upper Miocene Santa Rosalía basin, located in the east-central part of the Baja California Peninsula, consists of almost 500 m of non-marine to marine sedimentary deposits, and interbedded tuffaceous beds. The Santa Rosalía basin is a NW-SE elongated fault-bounded depocenter that records the sedimentation from Upper Miocene to Pleistocene time. The sequence is divided in El Boleo, La Gloria, Infierno and Santa Rosalía Formations. The lower most stratigraphic unit is the El Boleo Formation, a 200 to 300 m thick section composed in its lower part by a 1 to 5 m thick basal limestone and gypsum bodies followed by 170 to 300 m of clastic coarsening upward fan-delta, marine and nonmarine deposits. The upper clastic part of the El Boleo Formation show intraformational unconformities, synsedimentary folds and faults, and unidirectional sedimentary structures. These occur in at least three well organized upward coarsening cycles (90-100 m thick). Each cycle represents a prograding fan-delta deposit formed probably as consequence of large and repeated vertical movements of the basin floor with respect to the source areas. This activity is related to the early stage of the opening of the Gulf of California. Each cycle started with the deposition of a unit composed by laminar fine-grained sediments accumulated in an extensive area covered by shallow standing fresh water with periodic introduction of subaqueous debris flows. Each fine unit hosts Cu-Co-Zn ore bodies in the Santa Rosalía mining district. Lateral and vertical facies changes are present in each depositional cycle, involving proximal coarse sandstone and conglomerates through fine sandstone characterized by planar and low angle cross bedding, alternating with siltstone and mudstone with ripple lamination. Early, during the formation of the Santa Rosalía basin, two ancient depocenters located north-northwest and south-southeast of the basin were developed. These depocenters were filled by sediments during the first cycle, and were separated by a ridge formed by the volcanic rocks of the Comondú Formation.

Los Azufres show a mineralogy of copper sulphides represented by idaite, digenite, covelite, bornite (which have a local distribution), and chalcopyrite.Pyrite (the most abundant species), pyrrhotite and marcasite are present in well-defined zones. The paragenetic succession of neoforming opaque minerals is greatly controlled by a reductive medium found at great depth; boiling seems to control the massive precipitation of mainly calcite and hematite. Pyrite is found in an oxidant medium, associated to zones of low permeability and acid pH. The boiling (amply testified in the beds) propitiates a massive separation of hydrogen and other volatile elements initially dissolved in the liquid phase. The ascending water is more oxidant than the one present in the deep reservoir and than the volcanic rock itself, with which it comes in contact, a phenomenon which is reflected in the evolution of the mineral paragenesis of pyrrhotite to pyrite and to marcasite and pyrite+ hematite. The thermodynamic adjustments imply an increase in the fugacity of oxigen (fO2) and a simultaneous lowering of the fugacity of hydrogen (fH2. The hydrothermal system of Los Azufres belongs to “low sulphidation” type. The isotopic composition of pyrite presents two groups: the first (from deep areas of the field, without boiling ) has values that go from FeS2 d 34S = - 1.57 %o to - 2. 970/00, and the second (from the more shallow, steam dominated zones) has values of FeS2 d 34S = - 4 %0 to - 4. 830/00. The mean value of pyrite is FeS2 d 34S = - 2.50/00, which is very similar to that of H2S in H2S d 34S = - 2.00/00. The isotopic composition of the dissolved sulphates presents a composition of SO4d 34S=+ 21.470/00, which was found to be in isotopic equilibrium with sulphides. Boiling seems to control the precipitation of sulphides as well as its isotopic composition.

A biostratigraphic zonation of the Cenomanian–Coniacian rocks of the Guerrero–Morelos basin (southern Mexico) is proposed. The stratigraphic distribution of 70 species of calcareous algae and benthic and planktonic foraminifers is used to characterize four Zones that in ascending order are: Pseudorhapydionina dubia TRZ (Total Range Zone); Whiteinella archaeocretacea IRZ (Interval Range Zone); Helvetoglobotruncana helvetica TRZ, and Marginotruncana sigali IRZ. The top of P. dubia (upper Cenomanian) is marked at the last appearance of the marker fossil, which closely corresponds to the last appearance of most miliolid benthic foraminifers. Over most of the area, the transition from shallow–marine limestones up into pelagic facies occurs within the W. archaeocretacea Zone (uppermost Cenomanian–lowermost Turonian). A characteristic of this zone is the scarcity of both benthic and planktonic foraminifers, including the zonal marker. Most large benthic foraminifers disappear in the lower part of this zone. The changes observed within the W. archaeocretacea Zone reflect the successive stages of the platform drowning. The H. helvetica (lower–middle Turonian) is characterized by the presence the nominal taxon, dicarinellids, praeglobotruncanids, whitenelids and hedbergelids. This zone is recognized in the Mexcala Formation and represents deposition in fully pelagic conditions. Toward the central and eastern part of the area in shallow–open marine facies (Cuautla Formation), this zone is represented by an assemblage characterized by hippuritids, echinoids (crinoids and roveacrinids), gymnocodiacean and udoteacean algae and scarce planktonic foraminifers. The Marginotruncana sigali (upper Turonian–Coniacian) was defined with the last appearance of H. helvetica, whilst its top was difficult to recognize. Toward the central and eastern part of the area, this zone is represented in shallow–open marine facies (Cuautla Formation) by an assemblage dominated by the hippuritid Vaccinites gosaviensis, solitary corals, gymnocodiacean algae, calcisphaerulids and very scarce planktonic foraminifers. The Cenomanian–Turonian boundary lies in the lower part of the Cuautla Formation. The appearance of hippuritid mollusks and the diversification of whiteinellids can be used to mark this boundary.