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

The Rabanpalli Formation exhibits two types of shales, viz. grey and calcareous shales. These shales (grey and calcareous) have been analysed for major, trace, and rare earth elements to find out their source rocks characteristics and paleo-oxygenation conditions. The grey shales have higher concentration of SiO2, Al2O3, Fe2O3, K2O, Zr, Th, U, V, Cr, La, Ce, and Y than calcareous shales, whereas calcareous shales are enriched in CaO, Mn, Sr, Ba, Cu, and Zn, which indicate that the carbonate phase minerals are higher in calcareous shales. The positive correlation of K2O with other elements, and abundance of Al2O3, Ba, Th, and Rb suggest that these elements are primarily controlled by the dominant clay minerals. La/Sc, Th/Sc, Th/Co, Th/Cr, and Cr/Th ratios of shales were compared with those of sediments derived from felsic and basic rocks (fine fraction), upper continental crust (UCC) and post-Archean Australian average shale (PAAS) ratios, which reveal that these ratios are within the range of felsic rocks. The La/Sc vs. Th/Co plot also suggests the felsic nature of the source rocks. The shales show slightly light rare earth element (LREE) enriched and flat heavy rare earth element (HREE) patterns with negative Eu anomaly, and are similar to the granitoids from Dharwar Craton, which suggest that the Archean Dharwar Craton contributed the sediments to the Bhima basin. The geochemical parameters such as U, authigenic U, U/Th, V/Cr, Ni/Co, and Cu/Zn ratios indicate that these shales were deposited under oxic environment.

In this work we reconstructed the thermal history of some chondrules of Nuevo Mercurio H5 ordinary chondrite. We selected pyroxene-rich chondrules of different textural groups (RP, C and PP) showing diopsidic rims and exsolution textures and obtained 149 WDS microprobe chemical analyses. In order to calculate the temperature, we used the QUILF program for two-pyroxene thermometry, which yielded minimal values of crystallization because Ca-poor pyroxene formed prior to Ca-rich pyroxene. Presence of contraction cracks, clinoenstatite Raman spectra and enclosed olivine in enstatite phenocrysts suggest formation temperatures much greater than 1200 ºC. The highest pyroxene temperatures were obtained for augite blebs and grains (1065–1232 ºC), pigeonite lamellae (1037–1103 ºC) and enstatite cores (1277–1284 ºC). On the other hand, calculated temperatures for augite lamellae and diopsidic rims are 933–817 ºC, whilst enstatite reached temperatures from 915–988 ºC. We found in the five chondrules of Nuevo Mercurio that diopside rims and Ca-rich augite lamellae reached a minimum temperature range between 817 ºC and 933 ºC, and interpreted this temperature range as the peak of thermal metamorphism. We conclude that pyroxene temperatures from Nuevo Mercurio chondrules had two origins: the highest, albeit minimum temperatures (1283–915 ºC) preserved from chondrule formation, and the lowest temperatures (933–817 ºC) reflecting prograde thermal metamorphism well inside the chondritic parental body, as suggested by the low cooling rate reported by other authors of 15 ºC/Ma. We performed mineragraphic analysis of troilite + Fe-Ni alloys in order to verify that the metamorphic peak did not reach the eutectic temperature of this opaque minerals association at 988 ºC. X-ray diffraction data detected sodic plagioclase, a secondary recrystallization product confirming the thermal metamorphism grade 5.

The Oaxaca fault is a Cenozoic structure located in southern Mexico. The extensional deformation related to the northern sector of this fault system formed a half-graben and a topographic depression known as the Tehuacán valley. The Cenozoic strata deposited in the valley recorded a progressive deformation phase with four pulses: p1, p2, p3, and p4. Between the Upper Cretaceous and the first Cenozoic strata exists an angular unconformity, which was associated to the Laramide orogeny. After this orogeny, the tectonic regime in the Tehuacán valley changed from shortening to extension. The extension produced brittle normal faults which were the first structures in the northern sector of the Oaxaca fault defining p1. The minimum age of p1 is constrained by the older Cenozoic strata of the valley that range in age from the early to middle Eocene. The pulse p2 occurred between the late Eocene and the early Oligocene and was produced by the propagation of faults within the system; this pulse was recorded in the Calipan ramp. A change in the regional base level is marked by a disconformity, which was associated to a pulse p3 that occurred between the late Eocene to the base of late Oligocene; during this pulse the strata of the Eocene and early Oligocene were strongly tilted. In the late Oligocene, the base level returned to the valley and the Tehuacán Formation (late Oligocene – middle Miocene) began to be deposited; this indicated the end of p3. The progressive deformation continued throughout the Miocene (p4) with the development of the youngest ramp within the fault system and the deposit of the San Isidro conglomerate (middle to late Miocene). The northern sector of the Oaxaca fault is constituted by four en échelon normal faults with a small lateral-slip component forming a left-stepped arrangement. Considering the characteristics of the litostratigraphic units, the en échelon array of the faults and the identified Cenozoic pulses of deformation, we concluded that the northern sector of the Oaxaca fault grew through relay ramps with a migration and propagation from south to the northwest.

A structural model of the Chicxulub crater is derived from aeromagnetic anomaly modeling, borehole information and magnetic mineral data. Magnetic susceptibility measurements from borehole cores and samples in the crater show that suevite-like breccias have a variable strong magnetic signature, which is related to basement and melt clasts. The crystalline component estimated from clast analyses in the suevite-like breccias has on average higher magnetic susceptibilities (up to 1200×10-5 SI) than that of impact melt (~500×10-5 SI) and crystalline basement (400×10-5 SI). Reduction to the pole and downward analytical continuations show the discrete composite character of the anomaly, with inverse dipolar anomalies. The second-derivative of magnetic anomaly depicts five concentric rings, with the external ring correlating with the cenote ring and marking the surface expression of crater rim. The analytical signal and the radially averaged spectrum yield an estimate of the averaged depth to the magnetic sources, ranging from 1000 to 6000 m. There are three major magnetic sources within the Chicxulub crater: 1) the melt unit, 2) the suevite-like breccia, and 3) the central uplift. Using all these data, including new 2-D magnetic models, a new structural model is proposed. It reveals a system of regional vertical faults that explain the magnetic signal over the southern sector of the crater, whereas a 2.5 km deep central uplift and highly magnetized breccia sequences and melt sheet might be the sources of the main magnetic anomalies.

Large complex impact craters form by collapse from initial excavation stage of a deep narrow bowl-shaped transient cavity. Fracturing and shattering of solids with finite tensile shear limits occur related to shock-induced damage of target material, with fracturing and fragmentation occurring during transient cavity crater collapse processes. Geophysical studies of subsurface crater structure may assist in studying shock-induced effects of deformation and fracturing of target rocks. Here we present initial results of a study of subsurface fracturing/deformation in the Chicxulub crater from seismic reflection data. The analysis is based on the instantaneous seismic attributes envelope amplitude, instantaneous frequency and Q factor, at selected sectors of the crater by looking at petrophysical properties and seismic attenuation. Shock effects with shattering and fracturing of Mesozoic target rocks show a trend to decrease away from the rim zone. Cretaceous carbonates show less attenuation inside the crater than in exterior sectors. The relative attenuation quality factor Q is lower in sections outside the crater rim as compared with higher Q values inside the rim, and particularly at depth within the Cretaceous sequence. Carbonates in the western sector are characterized by slightly larger attenuation than in the eastern sector, suggesting radial asymmetries in fracturing/deformation within Chicxulub.

Results of micromagnetic and microstructural studies of individual chondrules from the Allende carbonaceous meteorite are presented. Allende is a member of the CV3 carbonaceous chondrites, and part of the oxidized meteorites with iron in silicates and oxides. Magnetic hysteresis data in terms of plots of parameter ratios give relationships with chondrule size and shape, in particular with magnetization ratio (Mr/Ms) and coercivity (Hc). Morphology, internal structure and elemental composition are investigated by scanning electron microscopy and spectrometric analyses. Chondrules show low ranges of magnetization ratios (Mr/Ms from 0 to 0.22) and coercivity (Hc from 3 to 24 mT). Low values suggest that chondrules are susceptible to alteration and re-magnetization, which affects paleointensity determinations for the early planetary magnetic fields A linear relation of Mr/Ms as a function of Hc is found up to values of 0.17 and 17 mT, respectively. This relation correlates with internal microstructure and composition, with compound chondrules showing higher hysteresis ratio and parameter values. Chondrules with hysteresis parameters falling outside the major trend show internal structures, composition and textures indicative of compound chondrules, and fragmentation and alteration processes. Microprobe analyses show distinct mineralogical assemblages with spatial compositional variation related to chondrule size, shape and microstructure.

The Escalón meteorite, a crusted mass weighing 54.3 g, was recovered near Zona del Silencio in Escalón, state of Chihuahua, México. The stone is an ordinary chondrite belonging to the high iron group H, type 4. Electron microprobe analyses of olivine (Fa18.1) and pyroxene (Fs16.5), phosphate, plagioclase, opaque phases, matrix and chondrule glasses are presented. The metal phases present are kamacite (6.08 % Ni), taenite (31.66 % Ni), high nickel taenite (50.01 % Ni) and traces of native Cu. The chondrules average apparent diameter measures 0.62 mm. X-ray diffraction pattern shows olivine, pyroxene and kamacite. Alkaline-type glass is found mainly in chondrules. This meteorite is a stage S3, shock-blackened chondrite with weathering grade W0.

Pinos volcanic complex is an uplifted area that exposes Mesozoic strata and mid-Tertiary volcanic and sedimentary rocks. Its stratigraphy, deformation style, and volcanism are characteristic of the Mesa Central region of central Mexico and the southeastern segment of the Sierra Madre Occidental (SMO) volcanic province. The oldest rocks in Pinos are marine carbonate sedimentary and siliciclastic rocks that underlie a red bed sequence (Pinos red beds) interlayered with felsic volcanic rocks, in turn partially covered by a voluminous lava dome complex. The Pinos red bed sequence is at least 900 m thick and it is formed by well-lithified conglomeratic sandstone and matrix-supported, generally fine-grained to medium-grained, polymictic conglomerate. Clasts in the Pinos red beds were derived from the Mesozoic basement, subaerial felsic volcanic rocks of unknown provenance, and tourmaline-bearing muscovite granite. Interlayered volcanic rocks include ash-fall tuffs, a densely welded ash-flow tuff, and water-laid or reworked pyroclastic material. The main components of the dome complex are a dark-red, porphyritic potassium-rich trachyte, and a buff-colored, porphyritic rhyolite, for which we report lava mingling (the first one in the SMO volcanic province). Field relations at the Pinos volcanic complex demonstrate a close temporal relationship between felsic volcanism and extension. Faulting in Pinos is complex as it includes arrays of Cenozoic normal faults with NS, NW, and NE trends, for which cross-cutting relations are ambiguous. A combination of mapping, K-Ar geochronology, petrographic work and interpretation of the magnetic polarity of the volcanic units allow us to establish that repeated pulses of synextensional volcanism occurred during the period between ~32 and 27 Ma. These data demonstrate that extension in the Mesa Central is older than 29–27 Ma, the oldest previously recognized episode of extension. The earliest (≥ 32 Ma) pulse of extension may be related to a regional (~250 km long) NW-trending fault system that divides the Mesa Central into two domains with contrasting stratigraphy and different geomorphic aspect. The Pinos red bed sequence includes clasts of the Peñón Blanco granite (40Ar/39Ar = 50.94 ± 0.47 Ma), that in addition to providing a minimum age for the end of the Laramide orogeny in the Mesa Central, offers evidence of uplift and denudation of the granite before or synchronous with red bed deposition.

We investigated the development of Albeluvisols and Podzols with time in southern Norway. The Vestfold region at the western shore of the Oslofjord was chosen because it is characterized by continuous glacio-isostatic uplift for the last 12,000 years. Due to the permanent elevation process, no distinct marine terraces have been built, and the age of the sediments continuously increases with distance from the modern coastline. Albeluvisol development was assessed in a soil chronosequence on loamy marine sediments with ages ranging from approximately 1,800 to 10,200 years. The most obvious change during soil development was that after 4,500 – 5,000 years light tongues intruded from the E horizon into the B horizon, and became more pronounced with time. The combined thickness of the A and E-horizons was constant at 40 ± 3 cm in 9 of the 12 profiles and did not change with age. The organic matter content of the A-horizons, the fine silt to coarse silt ratio of the Btg horizons and the Feo/Fed ratio all decreased with soil age, whereas the thickness of the organic surface horizon and B horizon, as well as the Fed/Fet ratio all increased. Podzol development was investigated in a chronosequence on sandy beach sediments, the ages of the soils ranging from 2,400 to 8,500 years. All soil properties investigated –the organic matter content of the B horizons, clay content, Feo, Alo, Sio, Feo/Fed and Fed/Fet – tend to increase with advancing podzolization, and are strongly correlated with soil age. Topsoil pH values decrease with age. The characteristic Bh and Bs horizons had developed after approximately 4,000 years.  

An extensive chronosequence of Chernozem-type soils developed on loamy sands was studied in the pre-Ural steppe, Russia. Paleosols were buried under kurgans that belonged to three main periods of construction: the Early Bronze Age (forth to third millennium BC), the Early Iron Age (eighth century BC to forth century AD) and the Middle Ages, the time of the Mongols and the Golden Horde (thirteenth to fourteenth centuries AD). Several paleosol profiles of the same archaeological chronointerval were studied as a soil chronosequence. This study restricted soil chronosequences not only to the duration of one archaeological culture but to the separated chronological phases of it. This allowed the documentation of soil property changes in time with the maximal possible resolution, and to understand the time scale of those changes. In order to take into account the changes of sets of soil properties in time, all soil features under study were attributed to either “arid” or “humid” environmental conditions, and numerical grades were assigned to each of them. For each soil of the chronosequence, the sum of numerical grades for the “arid” and “humid” properties were calculated and plotted against time. Two important conclusions were made after analyzing the curves: (1) it is necessary to distinguish the direction of change of the properties under study, and (2) the short time scale (sub-centennial or centennial) over which changes of soil properties occur. The morphological and analytical properties that change in sub-centennial or centennial time scales include the character of the lowest boundary of the humus horizon, the degree of biological activity (coprolites, humus-enriched root and mesofauna channels), the morphological patterns of carbonate accumulation, and the percentages of humus, carbonate and exchangeable sodium down through the profiles.