Descripción: In this work, the modeling of plate tectonic processes associated with crustal deformation through a numerical framework for non-collisional Andean type orogens is presented. The main objectives of the numerical framework are to develop a numerical model in order to represent large-scale deformation in Andean type subduction zones, to include appropriate physical and mathematical laws that describe the behavior of the lithosphere, to model surface erosion taking into account dominant winds direction and the synorogenic topography of the orogen as the crustal erosion caused by friction between the subducting and the overriding plate and to include isostatic compensation behavior taking into account flexural rigidity. © 2005 Asociación Geológica Argentina.

Descripción: A process of stream piracy caused by the active retrograde erosion of Limay Chico stream - a tributary of Limay river- is described. It is located in the southern part of headwaters of the Alicurá drainage basin, tributary of Collon Cura river. It is supposed that the increment of the headward erosion is related to the postglacial climatic conditions and a coetaneous deepening of the Limay river channel, a process expressed by modern and lower erosion terraces. Moreover the retrograde erosion was enhanced by the special location of the Limay Chico valley along an important line of structural weakness, the Limay Chico fault. The fluvial capture and the consequent diversion of the Alicurá basin upper part showed previous favorable conditions: 1) the topographic lower disposition of Limay Chico stream and their base level; 2) its steeper slope and the location of Limay Chico valley along a zone of structural and litologicalk weakness; 3) transverse arrangement of the Alicurá river's headwaters; 4) its gentle slope and location of their basin and base level at a topographic higher level. Several wind gaps were recognized and were interpreted two probable elbows of capture and barbed pattern. Also the increment of discharge of Limay Chico stream caused in their basin local phenomena of static rejuvenation. It is suppose a close genetic relation between the landslides observed along the steep fault scarp promoted by the Limay Chico Fault and the postglacial climatic change and the local erosional deepening of Limay Chico valley. The slumps characterized by the coalescence of their scan, generated the irregular and ruged landscape of the western side of Limay Chico valley. On the eastern valley slope the slumps reach minor expression. Longitudinal profils and slopes of the analized streams are included.

Descripción: The purpose of the paper is to analyze the geomorphology and hydrodynamic of the coastal area, to model the evolution of the coastal cohesive cliffs and the handing dune barrier system along southern Buenos Aires coast. The morphology and evolution of coastal transverse profile are used as geoindicator of erosional susceptibility for planning and manage the coastal communities' development. The vulnerability to erosion increases from dune coasts, to stabilized dunes, cliffs covered with ramps, cliffs with ramps, cliffs with mixed ramps, inactive cliffs and active cliffs. Human intervention such as afforestation, urbanization, mining and interruption of the littoral drift by Quequen harbor groins, have altered the eolian and marine sand supply in the area, modifying the coastal profile configuration, trending to the disappearance of sand ramps.

Descripción: Three of the biggest barrier spits of Argentina are studied in order to determine the relationship between the coastal planform and littoral dynamics. The studied landforms are the spit of Cabo San Antonio of an age of the 5800 years BP, the north spit of Caleta Valdés with a maximum age of approximately 5700 years BP and El Páramo spit of San Sebastián bay with an inferred age of approximately 6000 years BP. The orientation regarding the coastal planform of the three spits, allows to establish the relationship between landforms and coastal dynamics, being obtained the predominant waves that gave origin to them. Also the possible consequences for the modification or erosion of these landforms, that they would take to the disappearance of coastal environments of significant ecological value and the submergence of some adjacent areas; just as it would happen in Caleta Valdés.

Descripción: The coast of Lobería and Necochea is characterized by cliffsand narrow beaches. The inlet of the Quequén Grande river used to alter the coastline. The construction of jetties related to the Quequén Harbour induced huge accumulations of sand towards Necochea, and a critic erosive process of cliff and beach to the east. Beach width reduces from 300 m at the west to 40-60 m to the east. Considering regional sand scarcity, the eastern beaches are characterized by more sand variability. Greater variations in the sand balance were assigned to human activity. Waves coming from the E, SE and S deliver more energy along this eastern sector. On the other hand, submerged zones of these beaches have less availability of sand. Greater availability of fine sand was recognized towards Necochea. The Quequén Grande river has much effect on the estuarine dynamics during ENSO-triggered floods, although sedimentation is dominated by silt supply.

Descripción: According to the global predictions by the International Panel of Climate Change (IPCC), the sea-level rise due to ocean thermal expansion and glacier mass loss will average 0.09 - 0.88 m between 1990 and 2100, with a trend toward an increasing rise rate. Potential impacts of this increasing rise rate relate to changes in the coastal area, including floods, increased coastal retreat, and saline intrusion in the coastal acquiferous and estuaries. In order to achieve an appropriate coastal management scheme in the Province of Río Negro, the area was divided into zones to show information of the level of hazard on a simple map. To identify areas of geological hazard related to permanent and/ or episodic floods, a database was prepared. The threatened or in-danger areas were identified by means of coastal vulnerability indices, also called susceptibility indices, which were calculated by means of an algorithm that relates different variables obtained from coastal geology, and oceanographic and climatological information. This index is defined by seven variables and can be used to identify areas in risk of erosion or permanent/temporary flood. The most vulnerable areas to sea level rise are Balneario El Salado (Playas Doradas), San Antonio Oeste - San Antonio Este, Caleta de los Loros, and the Río Negro mouth. By contrast, vulnerability of the coastal area located to the south of Punta Pórfido is low. © 2004 Asociación Geológica Argentina.

Descripción: The region is situated in the Neuquén Cordillera at 38° 30'S latitud close to the international border. Geomorphic and structural features are presented which indicated a glacial morphogenesis for the depression that contains the Moquehue- Aluminé lakes. Previous works have identified a large volcanic complex related to Plio-Quaternary volcanic activity located at the east of the present volcanic arc. In this sector various caldera depressions and associated domic bodies have been identified. From the nine proposed calderas only the Pino Hachado caldera has been studied in detail. The Meseta del Arco and Nacimientos del Aluminé are disregarded as calderas. The erosional genesis of the steep scarps previously interpreted as caldera margins is also identified with glacial erosion. Facies and types of the local Pleistocene glaciation, its extension, their flow direction and its quite probable diffluence (Paso del Arco) through coeval glaciers situated north of the international border are also analyzed. Local characteristics of volcanic and eolian morphogenic and the mass-wasting process are described. Brief comments about pre-glacial drainage and the previous proposal of Bío Bío-Aluminé trough on the region are added. Transversal topographic-geological profiles discard west of the Bío Bío-Aluminé Fault a structural unit related to tectonic depression of Bío Bío-Aluminé trough and the Nacimientos del Aluminé and Moquehue calderas.

Descripción: A rock-avalanche (named Las Cañas) was distinguished near the Potrero de Leyes rock-avalanche previously recognized in the southern Sierras Pampeanas. It is located on the western slope of the Sierra Grande de San Luis (San Luis Province), which is regarded as the range uplift front during the Andean tectonism. On account of its greater degradation than the Potrero de Leyes rock-avalanche, the deposits described here are considered to be older in age. Their morphometric attributes could be only roughly calculated, because fluvial erosion has removed a large portion of its initial shape and volume. The avalanche-related deposits form round-shaped hills, which can be distinguished from alluvial fan deposits by their higher altitude, photogeological signature and degree of degradation. Two fault sets have also modified its original surface configuration. It is considered that this phenomenon was also triggered by an earthquake. Hence, the recognition and analysis of these geomorphological processes provides a useful addition to palaeosismological analysis and is significant for an adequate evaluation of the the seismic hazard of this region. © 1998 Asociación Geológica Argentina.

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: The previous analysis of the prehistoric slide phenomena in the main Andes of Neuquén Province in Argentina (36o-38oS), triggered the study of those located south of this la-titude. Therefore a new set of mass wasting deposits has been identified, whose trigger factors and corresponding tectonic framework, where they are located, differ from the northern slides. Their study was performed in three specific areas: Aluminé, Chapelco and Arroyo Limay Chico. Earth flows predominate in relation to slumps. The only exception is the Chapelco rock avalanche. Their occurrence seems to be related to the presence of Limay Chico member of the Caleufú Formation, whose lithology (expanding clays), particular rheology before water saturation, and particular disposition underlying basaltic lava and conglomeradic plains were conditioning factors. The trigger factor of the Present and prehistoric earth flows has been intense orographic precipitations. Water saturation reduced friction and cohesion in materials, incrementing water pore pressure and producing slope instabilities. Rotational slides are related to the slope basal erosion produced by lateral river migration and additionally to precipitations. Slope orientation has been important in determining location of main slides. Lateral slope release through the flanks of glacial valleys due to debutressing because of the glacial retire in the area, has produced instability and rock fracturation that led to slide phenomena. The Chapelco rock avalanche is particularly related to copious precipitations during postglacial times. Thus sliding main trigger factors south of 38oS differ from those pro-posed for the northern area where earthquakes dominated. Their specific age is still unknown, although their geomorphologic analysis allow to constraint a postglacial age for all these occurrences. However, temporal relationships are lacking for the specific case of the arroyo Limay Chico, beyond the glaciated area.

Descripción: An ancient alluvial fan and its subsequent evolution are described. Ancient alluvial fan accumulations have been interpreted as the northern continuation of the rodados patagónicos of the Montemayor Plateau, a proposal that is contested. The methodological recognition of the ancient alluvial fan and its genetic and temporary distinction, were made from geomorphologic analysis and altimetric evaluation. Time assignment is a problem not yet solved. The methodology led to three different geomorphic systems or levels related to successive episodes of aggradation and erosion, with increasing lower positions, which coincide with a complex paleodrainage ancient alluvial fan. its successive developments are linked to proposed proto-rivers Chubut 1, 2, 3 and to a Simpson paleo-valley. Despite the mentioned limitations, we propose that the evolutionary model of ancient alluvial fan responds to a structure of cut and fill which is supported by the poly-formational composition of an ancient alluvial fan, and a gradual southward migration of the paleo-drainage. Based on geomorphologic analysis and altimetric distinction an upper level, composed of the remains of three plains (Pa, Pm and Pb), a middle level with two terraces (T1 and T2) and a complex lower level and five terraces (T3, T4, T5 T6 and T7) were recognized. Relict evidences of paleo-drainages are proposed based on the Estancia Paz and Don Morgan-Saraza paleo-valleys in the upper level. The age of the ancient alluvial fan based on the geomorphic evolution of the study area is tentatively assigned to an uncertain period between the late Pleistocene and late Sangamon interglacial at the end of the last glaciation.

Descripción: Phosphate prospecting in marine sedimentary rocks of the Marginal and Austral basins of the Isla Grande de Tierra del Fuego resulted in the first discovery of phosphatic beds in the Paleogene Rio Claro Formation. Phosphatic anomalies of up to 20.1% P2U5 in concretions and up to 2% PaOs in bulk rock offer promising opportunities for systematic prospection on the Isla Grande de Tierra del Fuego. The phosphatic beds are conglomerates, coquinas and conglomeratic sandstones interbedded with thick mudstone packets. They belong to the condensed-type phosphatic beds, which are the product of erosion and reworking of concretions developed in fine sediments. Phosphate concretions containing glauconite, together with abundant glauconite in the same and closely associated beds, indicate a common syn-sedimentary to earlydiagenetic origin for both minerals, via recycling of sedimentary iron. This process occurred on a shelf with low sedimentation rates, pointing to high sea-level stands and marked interruptions in the sedimentation of the thick Paleogene sedimentary sequence of the Austral Basin . The deposit point to the existence and extension of this early Cenozoic phosphogenic episode to high latitudes in the Southern Hemisphere. © 1998 Asociacidn Geoldgica Argentina.

Descripción: The rise in the world demand for food poses a challenge to our ability to sustain soil fertility and sustainability. The increasing use of no-till agriculture, adopted in many areas of the world as an alternative to conventional farming, may contribute to reduce the erosion of soils and the increase in the soil carbon pool. However, the advantages of no-till agriculture are jeopardized when its use is linked to the expansion of crop monoculture. The aim of this study was to survey bacterial communities to find indicators of soil quality related to contrasting agriculture management in soils under no-till farming. Four sites in production agriculture, with different soil properties, situated across a west-east transect in the most productive region in the Argentinean pampas, were taken as the basis for replication. Working definitions of Good no-till Agricultural Practices (GAP) and Poor no-till Agricultural Practices (PAP) were adopted for two distinct scenarios in terms of crop rotation, fertilization, agrochemicals use and pest control. Non-cultivated soils nearby the agricultural sites were taken as additional control treatments. Tag-encoded pyrosequencing was used to deeply sample the 16S rRNA gene from bacteria residing in soils corresponding to the three treatments at the four locations. Although bacterial communities as a whole appeared to be structured chiefly by a marked biogeographic provincialism, the distribution of a few taxa was shaped as well by environmental conditions related to agricultural management practices. A statistically supported approach was used to define candidates for management-indicator organisms, subsequently validated using quantitative PCR. We suggest that the ratio between the normalized abundance of a selected group of bacteria within the GP1 group of the phylum Acidobacteria and the genus Rubellimicrobium of the Alphaproteobacteria may serve as a potential management-indicator to discriminate between sustainable vs. non-sustainable agricultural practices in the Pampa region. © 2012 Figuerola et al.