Descripción: In 1833 during his journey across the Buenos Aires Pampas, Charles Darwin made observations that reflected his thoughts on two major landscape units, Pampa interserrana and Pampa deprimida, later identified by other authors. Darwin grouped the Pampean sediments into a single unit, the Pampean Formation, based upon the lithological homogeneity and the large extension of the deposits; the unit was thought to be of estuarine-marine origin and attributed to the Recent Epoch considering the paleontological content (vertebrates and mollusks). At present, the Pampean sedimentary succession, which accumulated approximately during the last 11-12 Ma, is interpreted as a pedosedimentary sequence due to the ubiquity of pedogenetic features throughout the deposits. Four main subcycles of sedimentation are identified related to reactivations of the Pampean landscape. At a regional scale, the outcrop distribution of Pampean sediments of different ages suggests the dominance of more stable conditions since the late Miocene-Pliocene in a vast area of Pampa interserrana, documented by the formation of calcretes. However, sedimentation during the late Pliocene-Pleistocene was active within the domain of the Salado tectonic basin and Sierras de Tandil. The regional disparity shown by the Pampean stratigraphic record reveals the major morphostructural differences of its basement.

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: The geomorphologic analysis of the study area led to distinguish some formations not considered in previous geological research, as well as to explain the characteristics of the present relief. Thirteen principal geomorphic units produced by different geomorphic processes are recognized. Their variety and incidence through the time are expressed in the glacial morphogenesis concentrated in the western part of the Cordillera del Viento, in the volcanic landscape that predominates in the northern and eastern sectors (calderas, volcanoes, lava flows), in the structural control that defines the morphology of the fluvial process in the central zone and a firstly recognized important landslide to the north of Tricao Malal. As a result of the methodology employed two geological units of different age in the western part of the Tromen-Waile volcanic field were distinguished, which were initially interpreted as a single formation. A limited but consistent structural analysis of the region is also 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: Although the depositational environment assigned by Darwin to the large erratic blocks and gravels in the Río Santa Cruz valley has been reinterpreted, his geomorphological and stratigraphic observations are still in force. The large erratic blocks he described as crowning the Condor Cliff terrace and spread at the bottom of the valley just east of this locality (Sites 2 and 3), are now interpreted as indicators of the maximum glacial expansion in Patagonia. Similar blocks, though of a different lithology, accumulated over a lower terrace located up-valley (Site 4), are now linked to moraines and glacifluvial terraces of the Penultimate Glaciation. Finally, in addition to the erratic block discovered by Darwin in the lower Río Santa Cruz valley (Site 1), there are others - recently discovered - which probably account for a catastrophic event ascribed to a big glacier-lake outburst during the last interglacial.

Descripción: Considering the high concentration of rock avalanches in the northern part of the Neuquén province of Argentina (36°-38°S and 70°-71°W) and their association with lacustrine basins forming natural dams, their catastrophic collapse seems not to be such a common process as could be expected for these cases. The present work depicts the area immediately affected by the outburst flood corresponding to the Navarrete dam catastrophic collapse (175 × 106 m3), through a sedimentological analysis, generated since the arroyo Colorado was blocked due to a rock avalanche deposit during not well constrained postglacial times. The outburst flood dispersion is initially inferred based on morphological criteria and then determined from a sedimentological analysis of its matrix. Furthermore, the matrix is compared to that of the fluvial terraces, which have been sampled along the arroyo Colorado. This comparison shows that even though differences among them are not so significant through most of the study area, matrix frequency starts to be distinctive from the most distant point of sampling.

Descripción: The coastline position in Claromecó, Buenos Aires province was determined, featuring character elements identified in relation to the position of the shoreline. In comparison on the determination of coastline and shoreline, it appears that the first results of the geomorphologic interpretation, while the position of the shoreline is subject to a number of uncertainties if there are no long-term measurements. The nomenclature for the coastal areas is not always clear, hampering the interpretation of the processes occurring in the coast and shore. The correct identification is important technical, scientific and legal. Current legislation states that the boundary between public and private territory corresponds to the shoreline. The use of plant species in the determination of the coastline and shoreline, by itself is not determinative if not accompanied by the geomorphologic reconnaissance of the area. The exact determination of shoreline and coastline is essential in the implementation of geological surveys of major importance for the proper planning and development of the coastal area. In the task of delimitation of the coastal area beach ridges were found; one located about 5 m a.s.l has an age of 4,370 years BP.

Descripción: Large landslides of different types not previously recorded are located in the left bank of the Río Curri Leuvú valley, north of the Tricao Malal village. There are situated among the Cerro Palao and the Palao pond and Riscos Negros site to the north. The largest landslides (Malal Cura and Arroyo Mogolillo landslides) correspond to the multiple rotational progressive type. They are developed on Mesozoic sedimentary rocks, covered by Cenozoic basalt flows. The debris and earth flows are developed in areas covered by the volcanic rocks of the Charilehue Formation (Middle Miocene) and continue north of study reaching the latitudes of the Grande and Medio streams and also the Colimamil stream valley southeast of Cerro Domuyo. The impact of these phenomena in the fluvial system was important and produced ephemeral and permanent blocking as seen in the La Turbia stream with coeval ponding. It is hypothesized that the landslide occurrence is related to excess or long-lasting periods of rain in postglacial times. It is also indicated the presence of rock glaciers in the Cerro Palao.

Descripción: The main environmental problems existing in Buenos Aires metropolitan area were studied in an area located on the Pampean plains (Llanura Pampeana) at about 32°S and 66°W. Since its foundation in 1580, the city of Buenos Aires has experienced a striking population growth, to its present level of 13,000,000 inhabitants. The main geoenvironmental problems that pose a threat to Buenos Aires population are: flooding, regional rise in the water table, disposal of domestic and industrial waste in land-fill sites, mining dust, expansive soils and contamination of soils, water and air. This paper focuses on the geological, geomorphological and edaphic aspects, which may have controlled the occurrence of these processes. Thematic mapping of the main physical environmental features would appear to be one of the main tools for solving these problems. © 2004 Asociación Geológica Argentina.

Descripción: Soil-landscape relationships were studied in the central region of Santa Cruz, southern Argentina, between 49-51°S on the Atlantic coast westward to the international border with Chile. Two different sectors were distinguished. The western area, including the Patagonian Andean Cordillera and piedmont is characterised by strong landscape variations, a udic-xeric humidity regime and Nothofagus woodlands; the eastern area has an arid regime, grassland-shrub steppe and large flat structural plain landscape. As a result of the different landscape-relief features and parent materials, the two areas have different soils associations. The western area is characterised by an Andisol-Mollisol-Entisol sequence, whereas the eastern area has an Ardidisol-Entisol trend. The principal characteristics and properties of the soils are analysed and discussed in relation to variations in the conditions of formation.

Descripción: Landforms and soils in the Traful River Valley, Neuquén Province. The Andes Patagónicos Region is characterised by the presence of strong west-east bioclimatic, geomorphological and lithostratigraphical gradients, that controlled the genesis and geographical distribution of soils. In this contribution soil-landscape relationships in a sector of a transitional zone in the northern Andes Patagónicos are studied. The Traful River valley is located in Neuquén Province aproximately at 40°40′S and 71°15′W. Diverse geomorphological units were distinguished which were formed through glacial and fluvioglacial action, and partially modified in recent times by fluvial and eolian processes and by mass wasting. Holocen ashes and lapilli were deposited in different volcanic events and constitute the soils'main parent material. We recognize soils belong to three orders: entisols, mollisols and andisols. A strong gradient of variations in soils properties was observed. The Andisolization process decrease with distance from the Cordillera zone. © 1999 Asociación Geológica Argentina.

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: Deception Island (South Shetlands, Antarctica) is one of the most active volcanoes in Antarctica, having erupted recently in 1967, 1969 and 1970, damaging scientific stations on the island. It is also seismically very active. The island has attracted the attention of many researchers as it constitutes an undisturbed natural laboratory to study seismo-volcanic events and how they affect landscape modelling and evolution. One of the most remarkable geological and geomorphological features on Deception Island is the linearity of its easternmost coastal landform, the origin of which remains unknown. Some answers, based on presence of strike-slip fault or on the ice cap and beach geomorphological dynamics, have been reported in the literature. Our new work provides several indications of the existence of a dip-slip submarine fault, parallel to the coast (NNW-SSE), which suggests a tectonic origin for this morphological feature. Uplifted marine terraces, incision of a fluvial network over the ice cap, normal faulting parallel to the coast in the north and south rock heads bounding the beach and sharp shelf-break with rather constant slope, constitute some of this evidence. Terrace uplift and fluvial channel incision decreasing southward from Macaroni Point, indicates possible tilt movement across this inferred fault plane. © Antarctic Science Ltd.

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 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: 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: This thematic map describes the principal features of the landscape, a relative sequence of its geomorphic evolution, and arrives to important conclusions about its regional stratigrapby. The regional landscape is composed and polycyclic. The geoforms are related to different exogenous processes (glacial, fluvial and mass-wasting). An endogenous process (volcanism) formed the more distinctive features; a great tectonic-volcanic caldera and the preglacial Copahue stratovolcano. Some minor and youngest volcanic forms were formed during postglacial times. A previous fluvial cycle was replaced by glacial morphogenesis (valley glaciation) during the Quaternary, due a climatic intervention. A huge and very thick (500-800 m) ice-blister was lodged into the caldera. The ice-blister exceded the caldera's edges forming several glacial diffluences and giving an important and additional supply to neighbouring glacial valleys (Hualcupén and Trocoman), out of to the depression. The Pleistocene glaciation shows two local cycles of volcanic eruptions: pre- and post-glacial. The origin of many local lakes was related to glacial exharation. Others lakes have different genesis: by blocking slide, crater-lake and glacial exharation associated with important structural weakness (Caviahue Lake related to the Trapa-Trapa lineament or Caviahue graben). During postglacial times important rock-avalanches occurred in the Hualcupén valley: the Cajón Chico and Compul rock-avalanches were related to seismic shocks. Previously, their accumulations were interpreted as terminal moraines of a second glaciation. A fluvial rejuvenation phase was the consequence of a blocking of Agrio River by the Agrio volcanic flows, during the Holocene. After that, the river eroded this obstacle restablishing its previous base level. The Agrio cascade represents the upstream present position of the consequent rejuvenation wave. © 2005 Asociación Geológica Argentina.

Descripción: Variation in genome size of 24 populations belonging to 11 NW Patagonian species of Berberis was analysed as a function of the environment and geographical location. The variation showed three levels of discontinuity, two of which corresponded to diploid species (2n = 28) while the third corresponded to polyploid species (2n = 56). Diploids with DNA content ranging from 1.463 pg to 1.857 pg included Berberis cabrerae, B. chillanensis, B. montana, B. serrato-dentata and B. bidentata. Diploids with DNA content ranging from 2.875 pg to 3.806 pg included B. linearifolia, B. darwinii, B. parodii and B. empetrifolia. The genome size of the polyploid species B. buxifolia and B. heterophylla ranged from 5.809 pg to 6.844 pg. Principal component analysis (PCA) was applied to represent the variability of environmental conditions. The eigenvectors of the principal component axes showed that PCl discriminates the populations according to rainfall, types of vegetation and geomorphology; altitude and latitude, on the other hand, contribute to PC2 and PC3, respectively. From these results it is concluded: (1) that diploids with lower DNA content grow in high-elevation sites having greater rainfall but lower water availability; (2) diploids with higher DNA content are associated with half-elevation forests where the vegetative period is longer, the water availability is greater and the temperatures are higher; and (3) the distribution pattern of polyploids is considerably wider than that of diploids, which are geographically and ecologically restricted to forest areas. These results suggest that the C-value plays an important role in the ability of the species to adapt to different growing conditions. (C) 2000 Annals of Botany Company.