Descripción: In the region of the Main Andes of San Juan, exists a high concentration of landslide deposits that can originate natural dams. The outburst of these dams generated by slide is a common process in this morphology. The analyses of satellite imagery and air photos of previous years allow reconstructing the characteristics and the origin of the slides that formed those natural dams. In this region of the Main Andes, the rapid uplift and erosion of the mountain chain has created abrupt slopes and an internal structure in the materials that determined an increase of the susceptibility to the slope collapse without triggers as the seismic activity or precipitations. The objectives of the present work are (1) to analyze the causes that formed the natural dam and produced the landslide dam and development of the Los Erizos lagoon in the middle course of the Santa Cruz river; (2) the outburst of the dam and their consequent flood; and (3) the morphologic characteristic of the study region. The temporary comparison between air photos with satellite imageries indicates that the natural dam was generated by slip of the western hillside of the Santa Cruz mountain range, located below the Cerro Estrella. However, the constant water flow to the lagoon, has increased its water level until reaching the maximum benchmark of the dam, produced the beginning of the lake drainage and weakening of the dam causing the outburst and its consequent flood. Based on satellite imagery, a temporary analysis of the lagoon is presented, indicating that monitoring is an important tool to alert in the whole San Juan river basin for possible natural dikes, preventing hazards to the populations down water. The alluvium of November 12 of 2005, discharged 32.100.000 m3 in 67 minutes and traveled 254 km in less than 12 hours.

Descripción: Structures cropping out in the Jachal Valley from the Cuesta del Viento dam to the locality of Los Tuneles are described and analyzed. These structures affect the Ordovician Yerba Loca and Los Sombreros Formations and the Paleogene-Neogene Vallecito and Rodeo Formations. The oldest units form the heights, Sierra Negra de Rodeo and Sierra de los Túneles, which are limited in their eastern flanks by the La Tranca and Los Blanquitos overthrusts, and by the angular unconformity underlying Rodeo Formation in their western ones. The regional folds show asymmetrical shapes and a wavelength comparable to the width of the Sierras, but minor structures ranging from meters to tens of meters are very common in their eastern limbs. Regional folds axis trend varies from Az 315 to Az 360 and the vergence of pre-carboniferous and andean structures point to the east, what makes very difficult to use orientation as a criterion to discriminate the differentiated deformational episodes. The comparison of sedimentological and structural features in the Rodeo area with those at the San Juan River evidences strong similarities but also important differences. Among the first ones, it can be mentioned the lithological and structural resemblance of the Ordovician units, which show NS trending axis in the eastern outcrops and point N-NW in the western ones. Main differences are the absence -in the Rodeo area- of the metapelites of the Silurian Calingasta Formation. and that of the conglomerates and rhythmites of the Devonian (?) El Planchón Formation. present in the San Juan River section.

Descripción: The advanced spaceborne thermal emission and reflection radiometer (ASTER) was used to identify different deposits of hydrothermal alteration which indicates that several important lithological groups can be mapped in areas with good exposure by using spectral-matching techniques. Different methods are tested in order to identify and map zones with hydrothermal alteration minerals using the ASTER dataset. These areas are often referred to having large quantities of clay minerals which can be detected using multispectral imagery. Several authors have developed different procedures to map these hydrothermal minerals. Among the simplest ones, band combinations and band ratios have proven to be very useful tools for identifying targets. Lithology indexes point to reinforce the spectral response of this group of minerals working with band products and ratios. So far, these techniques do not need a full image correction. Other methods here employed require further processing of the ASTER scene, especially when spectral data are used. These techniques include spectral angle mapper (SAM) classification and minimum noise fraction (MNF) transforms to segregate noise and reduce computational requirements. Spectral data used in this paper were collected from field samples using SWIR (short wave infrared) reflectance spectroscopy and derived from the scene itself. These mapping methods have been tested in areas of known hydrothermal alteration occurrences, e.g. Los Pelambres, El Pachón and Altar, and in other sector of Santa Cruz region like Carnicerias and La Coipa; all of these at the south westernmost part of San Juan province. The result of this work is here presented as a series of images showing lithology indexes and an expected mineral assembly.

Descripción: Drill samples from the Carrizal uranium ore deposit located in Western Precordillera, Province of San Juan, Argentina, were studied by petrography, ore microscopy, X-ray diffraction, energy dispersive electron microscope and microprobe methods. A mesothermal ore paragenesis composed of arsenic-nickel-cobalt-uranium-bismuth minerals was identified. The established paragenetic sequence is pyrite-bismuthinite; nickeline-gersdorffite-rammelsbergite-pechblende, in an assemblage hosted by lithic breccia. A second mineralization stage produced a porphyry copper deposit, spatially but not genetically associated with the above described assemblage. © 1997 Asociación Geológica Argentina.

Descripción: The Central Precordillera of San Juan is a thin-skinned fold and thrust belt bounded to the east by the thick-skinned structures of the Eastern Precordillera. The objective of this study is to describe and interpret the structural features found at the southern end of this fold and thrust belt. The lomas de Andapaico's folds are of asymmetric, open chevron type, and are genetically related to fault propagation. Toward the north, the folding becomes tighter and broken by faults. The regional structure consists of an imbricated fan of an array of separated to overlapping fault propagation folds, and out-of-sequence structures related to the Eastern Precordillera uplift. The major décollement thrust is located near the top of the San Juan Formation limestone. Deformation occurred in two main stages. First, an imbricate array of fault-propagation folds was developed as the thrust front migrated toward the foreland. Then, the uplift of the eastern Precordillera impeded the migration of the thrust front and may have caused the development of out-of-sequence structures in the Central Precordillera. The minimum horizontal shortening calculated is 4.8 km, and the whole structure shows a southward decreasing shortening that is shown by structures of different orders and a variety of styles of deformation.

Descripción: Ciénaga del Río Huaco Formation was deposited during the Cretaceous in the Precordillera Central (La Rioja and San Juan provinces). The unit forms part of a thick red-bed sequence including rocks of different ages (from Late Paleozoic to Tertiary) and problematic identification. In the La Troya river area, the red bed sequence is composed of the Panacán (Permian), Santo Domingo (Late Triassic- Early Jurassic), Ciénaga del Río Huaco (Cretaceous) and Puesto La Flecha Formations (Oligocene- Early Nfiocene). In the past, all these formations were included in the same stratigrapbic unit and only in recent years the different formations could be separated mainly based on their fossil contain. In the case of Ciénaga del Rio Huaco Formation, the finding of microfossils (palynomorphs and ostracods) allowed to infer a Late Cretaceous age for these rocks. However, a radiometric age of 108,1 ± 4,4 Ma presented in this paper, clearly suggest that the unit was deposited at the end of the Early Cretaceous. This age is the first Cretaceous absolute age in the Precordillera Central and enhances the time constraints of of the Cretaceous Basin.

Descripción: A palaeomagnetic study was made on the Early Permian Del Salto Formation, exposed in the Western Precordillera (31.3°S, 69.2°W). Eleven sites were sampled for siltstone and fine-grained sandstone in the middle-upper part of the succession exposed in the creek at kilometre 114, San Juan. Standard demagnetization techniques permitted the identification of a characteristic remanence that showed good within-site consistency of direction at eight sites. Application of a tectonic correction showed that the characteristic remanence is pre-tectonic. A palaeomagnetic pole for the Del Salto Formation was computed, SC: 26.8°S, 357.0°E.δp = 4.4°, δm = 6.4°, N = 8. The position of SC indicates that the sampling locality underwent a counterclockwise rotation of about 40° around a vertical axis. The age of the rotation cannot be determined from the available data but it is most likely to have happened in either Permian or Late Cenozoic times.

Descripción: Group of microstructures present in the Ordovician Yerba Loca and Los Sombreros Formations (cropping out in the vicinity of Road 150, between cuesta del Viento and Los Túneles), are described and interpreted. Cleavage, joints and fractures, shear zones, planar and sigmoid (en echelon) veins and kink bands are most common among them. Folded and domino faulted quartz veins, resulting from pressure solution contraction during formation of axial plane cleavage, are an outstanding feature and allowed calculation of contraction associated to cleavage development. The assemblage of minor structures shows coherency with the major structures hosting them and evidences a systematic displacement and stacking of rock masses from west to east. They also show the important participation of pressure solution and re-crystallization processes during deformation, along with progressive deformation due to rotation of local structures in the regional stress field.

Descripción: La Flecha Formation (Upper Cambrian) in Precordillera Central is composed of microbiolitic buildups, with a shallowing upward tendency. This paper includes the study of mega, meso and microstructure of thrombolites and stromatolites and the microfacial analysis of the muddy sequence. The following microbiolitic facies were recognized: F1: stratiform and domal stroms, with low synoptic relief, F2: stratiform thrombolites, F3: stratiform-domal thrombolites with low synoptic relief, F4: inverted cone-shaped thrombolites and F5: domal thrombolites. The associated muddy microfacies are represented by: MF6: mudstone with microbial lamination and fenestral fabric, MF7: mudstone with grumous microestructure, MF8: bioturbed mudstone and MF9: floatstone, related to storms. The depositional model of this sequence was interpreted as a carbonatic platform with a complex of non-emergent microbialitic mounds and ponds with restricted circulation.

Descripción: A sequence of plutonic mafic rocks inter-stratified with both mafic- and metasedimentary-derived migmatites is found along the San Juan valley in the center of the Sierra de Valle Fértil. This natural example shows the transition from igneous to metamorphic petrologic processes which occurred during the crystallization of mafic magmas and the subsequent partial melting of crystallized gabbroic rocks. This work studies the mineralogical changes associate to this petrologic transition. Thermobarometric estimates based on amphibole-plagioclase indicate that the mafic magmas crystallized at around 1100°C and 5 ± 0.5 kbar. The conditions under which gabbroic rocks were partially melted are estimated using two pyroxenes ther-mometry and amphibole-plagioclase thermobarometry. Similar physical conditions in the range between 740 and 840°C and 5 to 6.5 kbar are recovery from mineral assemblages in the mesosomes and leucosomes of mafic migmatites. The main mineral compositional changes that accompanied the partial process of the gabbroic rocks are: 1) depletion of aluminium content and Mg# ratio in pyroxenes; 2) depletion of anorthite mole fraction of the plagioclases; and 3) depletion of the Mg# ratio in amphiboles. These mineral compositional variations are consistent with those found by experimentally melting mafic protoliths. Experimental results showed that the temperature for promoting amphibole-dehydration melting in mafic proto-liths is of around 850°C, whereas the addition of an aqueous fluid would lower the melting temperature to around 800°C. Combining field relationships, textural features, mineral compositional variations and thermo-barometric estimates with experimental results, we demonstrated that the transition from igneous mafic rocks to mafic migmatites was due to isobaric cooling. Moreover, repetitive intrusion of mafic magmas sustained the sequence at granulite-facies conditions, so avoiding rapid cooling. The natural case from the centre of the Sierra de Valle Fértil is interpreted to expose an example of the plutonic-me-tamorphic core of the Famatinian magmatic arc, where the abnormally high metamorphic gradient provide evidence that large volume of mafic magmas reached and emplaced at paleodepths of between 16 and 20 kilometres.

Descripción: This paper reports the presence of Maastrichtian rocks in the north of the Precordillera (La Rioja province). The Cretaceous succesion (122 m thick) is a classical red bed sequence composed of sandstones, mudstones, marls and thin levels of evaporites. According to lithological features 6 facies associations have been recognized that mainly characterize a lacustrine system developed under arid to semiarid conditions. Probably as a result of an increase in aridity, eolian sediments were deposited at the top of the Cretaceous unit. At least two transgressive-regressive events have been recognized in the lacustrine sequence. The best developed is the first event formed by fossiliferous black shales corresponding to the maximum flooding stage. These rocks have yield remains of calcareous microfossils, palynomorphs and gastropods. The former comprise charophytes, with Platychara compressa (Knowlton) an Tectochara sp. A Musacchio, anda ostracods, with Ilyocypris wichmanni punctata Musacchio y Simeoni, Eucandona? sp., Cypridopsis and Ovo cytheridea? rionegrensis Musacchio.. Palinomorphs form a monospecific assemblage of algae colonies corresponding to Pediastrum. The lithological features, stratigraphic location and fossiliferous remains clearly suggest a regional correlation with the Ciénaga del Rio Huaco Formation. This correlation considerably enlarge the regional distribution of the Cretaceous sediments which were previously limited to the Precordillera of San Juan province. © 2005 Asociación Geológica Argentina.

Descripción: This paper deals with the sedimentological characteristics of piedmont deposits exposed in intramontane basins of the Precordillera. Detailed studies carried out along the upper Jáchal river and in the Gualilán area allowed identifying and characterizing six major types of piedmont accumulations: 1) talus, 2) colluvial fans dominated by gravity flows, 3) colluvial fans dominated by fluid flows, 4) collector-conoid fluvial system, 5) piedmonts dominated by channalized flows and 6) alluvial fans. The talus deposits are characterized by the dominance of clast-supported breccias (bearing minimum matrix amounts) that commonly exhibit massive bedding, planar or high angle imbricated clasts fabric. Colluvial fans dominated by gravity flows are formed by massive matrix-supported breccias with minor percentages of clast-supported breccias. On the contrary, colluvial fans dominated by fluid flows are composed of amalgamated channels infilled by both clast- and matrix-supported breccias together with intercalation of graded breccias. Accumulations of the collector-conoid fluvial systems are the most diverse in composition, including from massive matrix-supported breccias to cross-bedded conglomerates and scarce mudstones. In relation to the piedmont dominated by channalized flows, they comprise massive clast-supported breccias and less amounts of imbricated or cross-bedded ones. The alluvial fan deposits are mostly composed of cross-bedded clast-supported conglomerates, though matrix-supported conglomerates and breccias also appear as proximal deposits. Different from the previously mentioned deposits, the clast of alluvial fan accumulations shows more varied lithologic composition. The comparative analysis of the piedmonts found in the studied valleys permits to propone a conceptual model for classification of piedmont, on the basis of the extension of the basins and vertical space of accommodation.