Geomorphology of the Copahue Volcano region and vicinities (Central-western Neuquén)

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

Geomorphologic analysis of the lower and middle section of the drainage basin of the Curri Leuvúriver, Neuquén

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.Fil:Folguera, A. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

The "Ñireco Caldera" in the central-western Neuquén Andes (38°50'S - 70°50'O)

In the southern segment of the Copahue- Pino Hachado block, next to the Moquehue-Alumine lacustrine basin, an anomalous concentration of calderas, resulting from a Pliocene-Quaternary volcanic activity was recognized. The Ñireco caldera, which was proposed as part of this caldera complex, is the reason of the present work. The authors object the interpretation of such a morphostructural feature on the basis of a detailed geomorphologic, structural and stratigraphic analyses of the area. The limits of the proposed caldera coincide mostly with fluvial valleys that seem to be controlled by previous structures like faults and regional lineaments. There are no evidences that could suggest the presence of a volcanic complex that theoretically contains such a depression, or for its external and internal flanks. The central sector of the proposed caldera presents the highest altitude values of the region (± 2,900 m a.s.l.), which highly surpass those of the Principal Cordillera at these latitudes. The local stratigraphy of the area, is dominated by the oldest rocks of the region (Upper Paleozoic granitoids and Triassic volcanic and volcaniclastic sequences), and shows not temporal coincidence with the Pliocene-Quaternary volcanic activity, well represented immediately to the north. The study area falls on the contrary into a thick-skinned west-verging fold-and-thrust-belt, where tectonic inversion played a central role.Fil:García Morabito, E. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Geomorphological evolution and relative chronology of terrace levels adjacent to the Chubut river mouth to the Atlantic (Chubut Province)

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.Fil:di Tommaso, I. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Geomorphology of the Moquehue and Aluminé Lakes: Considerations about the Meseta del Arco and Nacimientos del Aluminé calderas

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.Fil:di Tommaso, I. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

The rock avalanche of Cerro Los Cardos (37° 10′S, 70° 53′ W) in the northern region of the province of Neuquén

A new and important rock avalanche not previously recognized is described, which is located 25 km west of Andacollo town. It shows two scars (northern and southern ones) where the southernmost one is interpreted as the most important, developed in the western flank of a reduced basaltic plain, where at its northern part of Cerro Los Cardos is located. The movement is complex showing several breakaway zones developed into the landslide deposit, considered as secondary scars, all of them, main and secondary ones, have a NW trending. The movement mechanics seems to have started as a planar landslide considering the local almost vertical bedding of the Lileo Formation. However, the concave nature of the breakaway zones allows to infer a rotational movement after the initial stage of sliding. The movement corresponding to the southern breakaway zone was projected as a flux to the south of the Lileo valley, presumably in successive pulses as indicated by the multiple secondary scars. The movement corresponding to the northern break away zone moved to the west and after-wards seems to be controlled by the slope of the valley, moving through to the Palao valley. The western slope of the Palao valley was covered by avalanche deposits corresponding to a run-up. Los Rojos lake is located in a depression presumably generated by the landslide and consequent fracturing of a rock block detached from a Los Cardos's lavic structural plain. Several factors, particularly the structure, favoured the development of the landslide. We propose a seismic trigger for the Los Cardos's avalanche, having into account the regional seismotectonic characteristics (neotectonic front) and the local structure (lineaments, faults). Other landslides, some of them with volume similar to an avalanche deposit, have been included in a regional map. Moreover, modifications to the regional stratigraphy, particularly referred to the volcanic units, are proposed based on morphological criteria. Other geoforms related to other geomorphological processes, fluvial, volcanic and glacial are identified. © 2005 Asociación Geológica Argentina.Fil:Folguera, A. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Las Cañas rock-avalanche: Another manifestation of these phenomena on the western slope of the Sierra Grande de San Luis

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

Mass wasting in the neuquén cordillera south of 38°: Its trigger factors

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.Fil:Folguera, A. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Ultraestructura de la Babesia bigemina.

Se determina la morfología y mecanismos de reproducción y alimentación de B. bigemina, tal como se revela en el microscópio electrónico. Se reportan estudios preliminares en otros tipos de parásitos hemáticos, incluyendo estudios en varias especies de Babesia como B. canis, B. caballi y B. rodhaini. La cepa de B. bigemina fue obtenida de animales infectados naturalmente en el Valle del Cauca en Colombia y se purificó en animales esplenectomizados hasta cuando la parasitemia fue alta. La B. bigemina revela tener varias formas, encontrándose la forma de pera como la más común. El tamaño promedio es de 4.5 micras de largo por 2.3 micras de ancho. El hemoparásito muestra tener una membrana citoplasmática densa y simple, y reticulo endoplasmático de aspecto vesicular compuesto de gránulos de diferentes densidades. El citoplasma aparece como una masa homogénea en la cual se aprecian otras estructuras como vacuolas. La reproducción de la B. bigemina parece realizarse en 2 formas, siendo la más común por gemación, al final de la cual se forman 2 o más gemas. Otra forma de reproducción es por división binaria. El mecanismo de alimentación no ha sido revelado en forma muy clara. Los cuerpos polares, componentes citoplasmáticos pueden asumir la función de reservorio de alimentación del parásito. No se encontraron residuos de digestión de hemoglobina en el parásito, lo que indica que la digestión es más completa en la Babesia que en otros parásitos hemático