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

Knowledge about the rates of pedogenic processes is essential to understand landscape development and history. It can be attained by the quantitative investigation of soil chronosequences. In this work, the development of Chromic Luvisols in the Mediterranean is addressed. The soils investigated are located on five uplifted Pleistocene marine terraces in south-western Sicily, and have developed on calcareous marine and fluvial sediments. The soil on the highest terrace is a Ferri-Profondic Luvisol, while soils on the lower terraces represent a Chromi-Profondic Luvisol (Bathifragic), a Chromi-Profondic Luvisol and two Bathicalci-Chromic Luvisols. The soils on the 1st (lowest) and 2nd terrace are 100 cm thick, the one on the 3rd terrace, 160 cm and those on the 4th and 6th (highest) terrace more than 200 cm. All soils are decalcified, while greater CaCO3 contents in the Ap horizons of the soils on the three lower terraces are due to deposition of younger calcareous fluvial sediments. Soil pH (water) drops from pH 7.7–8.9 in the soils on the three lower terraces to values below pH 7 in the soils on the two higher terraces. In general, the chemical alteration and the leaching of silicic acid increase with elevation, and are most pronounced on the 4th and 6th terrace. Clay illuviation and associated translocation of Fe, Al and K is pronounced in all soils. The clay minerals include kaolinite, illite and expandable three-layer silicates. Irregularly interstratified clay minerals are composed of illite, vermiculite, smectite and chlorite, and usually constitute 20–40 % of all clay minerals, indicating proceeding soil development. The comparatively more advanced weathering and the high kaolinite content on the most elevated terrace point to soil development under warmer and wetter climate, that is early Pleistocene or earlier. The soil on the 4th terrace is more developed than the soils on the less elevated terraces. The 3rd terrace was exposed to local rejuvenation by sedimentation in a lagoon/lacustrine environment. Above the lacustrine layer, the Chromi-Profondic Luvisol on the 3rd terrace shows a similar development as the soils on the two lower terraces. Therefore, we assume that the marine terraces around Menfi in south-western Sicily emerged at least at three different time steps; the 6th terrace and the 4th terrace developed at different times, whereas the lower three terraces formed at similar times. The latter may have formed also at the same time, and later dislocated to different elevations by tectonics.

In order to recognize the soil and geomorphologic characteristics of the northeastern part of Valle de Guadalupe, Baja California, and its implications on the runoff volumes in the region, 59 soil samples were collected from the first 5 cm of the soil layer, taking into account the surface relief, soil classification and vegetation cover of each sampling point. Soil moisture content and textural analyses were carried out in the laboratory for each sample. Three raster images were used to complement field data: an Ikonos image, an Aster Image and the Instituto Nacional de Estadística Geografía e Informática digital geologic map I11D82. The Ikonos image was used to calculate the normalized differential vegetation index, the Aster image to evaluate the field slope, and the digital map to obtain geologic information for the study area. Following the runoff curve number method, a computational algorithm was developed (for Ermapper 6.4 software), by using the three raster images and the field data. The algorithm was employed to study the surface runoff potential in the Serranía Matcuatai and to evaluate its runoff volume contributions to Valle de Guadalupe. The mean annual runoff volume contribution was estimated in 258,000 ± 123,000 m3. It was also found that 88% of the study area surface has a runoff potential and only 12% is favorable to infiltration.

To investigate the effects of hydrothermal alteration on the chemistry of volcanic rocks, we analyzed the whole rock chemical composition (major and trace elements, including rare-earth elements – REE) of two distinct portions of a drill well core sample from the Los Azufres geothermal field, Mexico. This highly hydrothermally-altered sample allowed us to study, for the first time, the mineralogical and chemical effects imposed by hydrothermal alteration on the mm scale in this important geothermal system. Mineralogically, hydrothermal alteration in the sample is mainly represented by chloritization of primary olivine and pyroxene, argillitization of primary plagioclase, as well as banded chlorite and quartz, with significant amounts of epidote and hematite. The “altered” part of the sample contains intensely altered remnants of the fresh rock, which show intense silicification, hematization, and dissolution boundaries. Most major and trace elements were mobilized from the original rock. Major element composition reflects the silicification, chloritization, and epidotization processes taking place in the geothermal system. The rare-earth elements La and Ce, as well as Yb and Lu were probably partially re-deposited during alteration. The positive anomaly of Eu may suggest that Eu is being concentrated in hydrothermal epidote after its release from plagioclase to the geothermal fluid. The high-field strength elements such as Zr, Ti, and P, show as well significant hydrothermal alteration-related decrease in the highly-altered rock. The geothermal fluid responsible for this hydrothermal alteration was probably oxidizing, of high temperature (>250º C), and enriched in REE and other trace elements.

Rates of soil-forming processes were evaluated for the three major models of pedogenesis: (1) normal “top-down” soil development (pedogenesis transforms the parent material under a geomorphically stable surface), (2) soil development accompanied by pedoturbation (turbational model), and (3) soil development accompanied by or alternating with sediment deposition on the soil surface (sedimentational model). According to the normal model, the rates of pedogenic processes exponentially decrease with time, so that in 2–3 ka after the beginning of pedogenesis, temporal changes in the soil properties become negligibly small, which is typical of soils in the steady state. Under humid climate conditions, the rates of many processes (accumulation and mineralization of humus, leaching of bases, migration of sesquioxides, textural differentiation, and so forth) are much higher than under arid climate conditions. For example, the rate of textural differentiation in soils of humid climates is one to two orders of magnitude higher than that in soils of arid climates, and the rate of humus accumulation and rejuvenation is several times higher. The rate of the zoogenic turbation of soil material with the input of fine earth onto the soil surface and gradual burial of archaeological artifacts into the soil is maximal in Chernozems, where it is two times higher than in Kastanozems and gray forest soils (Humic Luvisols) and five times higher than that in Albeluvisols and Podzols. For soils developing according to the sedimentation model, the degree of development of the soil profile depends on the rate of sedimentation on the soil surface. Thus, with high rates of alluviation (>2.5 mm/yr), pedogenetic processes cannot significantly transform the alluvium. At the moderate rate of alluviation (0.3–1.0 mm/yr), typical aggradational soils –Fluvisols– are formed; the alluvial stratification is well seen in their profiles. At the low rate of alluviation (<0.1 mm/yr) and a considerable duration (>1000 yr) of pedogenesis, floodplain soils have relatively similar properties as the corresponding zonal soils (Luvisols, Chernozems).

Hainan Island is located in the South China Sea. Basalts of different geological ages are widely distributed in the north of the island, and soils developed from them form a well dated soil chronosequence. The absolute ages of basalt from which soils have developed varied from 1.0×104 a to 1.81 Ma. A few indicative soil features, especially typical geochemical features, were studied and related to the stages of soil development. Relative depletion/enrichment ratios of macroelements and microelements, as well as of rare earth elements (REE), were calculated and several indices were proposed to illustrate the development of the soil chronosequence. The study found that Fe and Al were relatively enriched, while Ca, Mg, K and Na as well as Si were strongly lost during rock weathering and soil formation. Indices based on soil macroelements, such as the chemical index of alteration (CIA), the chemical index of weathering (CIW), the silicon-aluminum ratio (Sa), and the Silicon-aluminum-iron ratio (Saf) could not adequately express the sequential weathering degree of the chronosequence. The weathering index (WI) gave a good relationship with soil age and can be taken as an index of soil development. Ba/Nb correlated significantly with soil age and can be used as an indicator of soil development. REE content increased linearly with soil age so it can express the degree of soil development. The relative depletion rates of major elements showed that Si was lost up to 80% of the original content before 1 Ma and remained in constant concentrations afterwards. Easily mobilized elements were lost quickly during the initial stage of weathering and more than 90% of them were depleted within the first 1.0×104 a.

In this paper we report the oldest geologic world record of Ptychodus mortoni, from the Vallecillo Member (Agua Nueva Formation), at Vallecillo, Nuevo León, northeastern Mexico. The presence of the ammonite Watinoceras coloradoense, allows the placement of the sediments of the Vallecillo member within the lower Turonian (Watinoceras coloradoense zone). With reference to this new data, the presence of Ptychodus mortoni in the Vallecillo member extend the temporal distribution of this species into the earliest Turonian. Also, this study suggests that the utility of this shark species as biostratigraphic control in late Cretaceous sediments should be revised in greater detail.

The Zapalco-Amacuzac area in the Morelos and Guerrero states, southern Mexico, constitutes one of the classic sectors where Paleogene continental beds of the Balsas Group were originally described. The exposed succession has an apparent thickness of ~2,500 m and it is constituted by two distinct sedimentary units separated by a slightly angular unconformity and by two intercalated ignimbrites (35.1 ± 0.4 and 38.5 ± 0.7 Ma, sanidine, K-Ar). The basal beds of the lower sedimentary unit (Tepetlapa Formation) are constituted by matrix-supported conglomerates that grade upwards to sandstone with a poorly defined planar parallel stratification and some intercalated andesitic and mafic lava flows. One of the andesites yielded a whole rock K-Ar date of 44.5 ± 0.7 Ma. Intercalations of conglomerate beds increase again toward the top of this unit. The upper sedimentary unit (Huajintlán Formation) is constituted by a ~300 m thick succession of matrix-supported conglomerate with some lenses of conglomeratic sandstone. This unit unconformably underlies the Tilzapotla ignimbrite whose best age estimate is 34.3 Ma (sanidine Ar-Ar). The beds of the two sedimentary formations dip east to northeast with higher dips for the lower unit (50–30º). Recognized facies in both units are indicative of an alluvial fan environment with more proximal facies for the Huajintlán Formation. The studied succession is located immediately northwest of a large elliptical (50×30 km) dome structure that resulted from the tumescence stage, in the early evolution of the Tilzapotla caldera, previous to the collapse. The northwestern termination of the dome merges and interferes with the NNE-trending Laramide fold and thrust front of the Iguala Cacahuamilpa highs, near Taxco. In the study area, the dome contours display a local inflection defined by a change in the dip direction of the Morelos and basal beds of the Tepetlapa Formation from north to east. The slight angular unconformity between the Tepetlapa Formation and the overlaying units indicates a first stage of eastward tilting related to an uplift episode in the Taxco area, probably originated by the emplacement of silicic magmas (ca. 38 Ma). The proximal facies of the overlaying Huajintlán Formation and the age (35.1 ± 0.4 Ma) of the Piedras Altas Tuff , indicate that this formation resulted from the rejuvenation of the source area triggered by the tumescence and the initiation of the volcanic activity in the nearby Tilzapotla caldera area. This is confirmed by the higher dips (~50º) of the Tepetlapa Formation near the dome structure and the decreasing dips toward the northeast. The age of one andesite lava flow (44.5 Ma) intercalated in the lower unit is indicative of an Eocene age for the beginning of the Balsas Group accumulation in this area.

The geometry of the aquifer of the Villa de Reyes graben (San Luis Potosí, México) was inferred from the study of the underground electric properties in the La Pila – Jesús María region. The results of 104 vertical electrical soundings (VES) were used to establish two georesistivity trends with N-S and NE-SW directions. The N-S trend is related to the San Luis Potosí graben and the NE-SW trend to the Villa de Reyes graben. The basement of the continental basins in these tectonic depressions is formed by volcanic rocks with resistivities (ρ) between 67 and >500 Ωm. The lower values are interpreted as fractured rocks containing water, and the higher values as dry, unfractured basement. The San Luis Potosí and Villa de Reyes grabens are partially filled by upper Cenozoic continental sediments. Coarse-grained sediments, such as gravel and sand deposits, predominate in the southern portion of the studied area. These sediments have ρ values in the order of 21 to 35 Ωm. In the northern part of the area, the coarsest deposits are restricted to the borders of the basin and the fine-grained sediments –such as silt and clay, with ρ values of 10 to21 Ωm– predominate toward the central part. Three geoelectric pseudosections were prepared using apparent resistivity (ρa) values derived from the VES. On the basis of these pseudosections, a buried horst, probably related to the Villa de Reyes fault system, is inferred near the western border of the valley. The data derived from the pseudosections are also used to speculate about the tectonic events that led to the formation of the valley. It is proposed that the N-S trending San Luis Potosí graben is older and was partially overprinted in the area by the NE-SW trending faults of the Villa de Reyes graben.

A peculiar volcanic episode represented by peralkaline ignimbrites and rhyolitic domes, has been recognized among the Tertiary volcanic sequences of central Sonora. Field studies, 40Ar/39Ar geochronology, mineral chemistry and geochemistry (major and trace elements), contribute to establishing the volcanic stratigraphy and the characteristics of these lavas. 40Ar/39Ar dates give very similar ages of ~12 Ma, showing that all the peralkaline rocks from central Sonora belong to the same volcanic episode. Peralkaline rocks have a typical anhydrous mineral association (sanidine+fayalite+Fe-rich, green clinopyroxene), very distinctive from that of the calc-alkaline silicic rocks in the region. These high-silica magmas have relatively high iron, and low alumina contents that classify them as comendites. The REE patterns are characterized by a slight enrichment in LREE, a strong negative anomaly in Eu, and flat HREE pattern. The low concentrations in Ti, P, Sr and Ba are in accordance with the high degree of differentiation of these liquids. The existence of such a middle Miocene peralkaline episode, with a wide geographic extension, constitutes a clear geological marker in the region. The intimate association of these lavas with clastic sedimentary deposits and the eruption, shortly after (~11 Ma), of differentiated tholeiitic magmas (islandites), show that the middle Miocene volcanism was clearly related to extensional tectonism and lithospheric thinning processes which precede the opening of the Gulf of California.

Recent research in some basins of the Sonora State, including the Moctezuma basin, has permitted the collection of equids and bison fossils which have been located within continental sediments that ovelie the Báucarit Formation. Studies on the stratigraphy of the late Tertiary sedimentary basins of central Mexico have demonstrated the diversity of mammals they contain. It has been possible to define the stratigraphic indexes and the correlation of the faunas of this age in Mexico with those of the Great Plains of North America. However, in the northern states of Durango, Coahuila, Chihuahua and Sonora, our knowledge about the stratigraphy of the sedimentary basins containing late Cenozoic vertebrate fossils is restricted only to some isolated and incomplete reports. This fact contrasts with the abundance of the vertebrate fossils described in the faunas of the Grate Plains in the states of Arizona, Texas, and New Mexico, and the State of Florida. In this paper we present the preliminary results of the studies in the Moctezuma basin, in particular at the locality of San Clemente de Térapa, where the association of  Equus excelsus, Equus conversidens and Bison sp. was found.