Descripción: An environmental magnetic study of mainly fluvial "postpampean sediments" assigned to the Lujan Formation (Luján, Buenos Aires Province, Argentina) was carried out. The results obtained suggest a depletion of detrital ferromagnetic minerals during the pedogenetic processes. A characteristic magnetic signal can be distinguished in Holocene paleosols and soils of both eolian and fluvial "post-Pampean sediments" (Fm. Luján and Fm. La Postrera . This signal is analogous to that determined in loessic Pampean sediments assigned to the Buenos Aires Formation. It is inferred that the depletion of detrital ferromagnetic minerals is a general process in the Chacopampean area. This process may be associated with the genesis of a superparamagnetic fraction (SP), which could indicate dry periods during the climatic event. The depletion could also be associated with the genesis of a high coercitivy fraction, analogous with those determined in paleosols in the Pampean loess.

Descripción: The use of the magnetic signal of soils and paleosoils as a climatic function. The correlation between an index of potential water storage (PWS) with magnetic signal in soils and paleosoils, developed in loessic sediments has been proposed. The analyzed data suggest the existence of climatic thresholds that affect the genesis, preservation or depletion of ferrimagnetic minerals. Soils and paleosoils characterized by a positive PWS have an appropriate environment that favors the depletion of ferromagnetic minerals due to mainly reductive loss. Such soils are characterized by a depletion of detrital ferrimagnetic minerals, as in northeastern Pampean plain soils and paleosoils of Argentina and SE of China. A negative PWS prevents highly reducing conditions in the soil, and the detrital ferrimagnetic minerals are preserved. On the other hand, the environmental conditions of these soils allow the formation of new nannoparticle minerals. These conditions produce a net magnetic enhancement of the soil, as observed in European soils (Russia, Czech Republic, Rumania), Asian (paleosoils from the loess plateau of China) and Northern African ones (Tunisia). The influence of parent material in the magnetic signal is also discussed.

Descripción: In this communication, the genesis of great anomalous seismic activity registered during austral summer 2003/2004 on Deception Island (Antarctica) is discussed, and related with changes on the chemical composition of fumarolic gases and vent deposits. Here we present a comparative analysis performed to explain this activity. We propose a possible parcial seal of the microfracture system. This effect allows an increase on pressure follow by reopening of microfractures, resulting on a sequence of seismic events and changes on the chemical composition of fumarolic emissions.

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: El Infiernillo Mining District is located in the San Rafael Block (34°38'20";68°47'37"), province of Mendoza, Argentina. It is a porphyry Cu (Mo) type deposit which is hosted by a volcaniclastic sequence. It develops an alteration halo with a concentric pattern consisting of a central quartz neck surrounded by a potassic alteration zone and outwards a phyllic halo and small polymetallic veins. The potassic halo has a paragenesis of K-feldspar-quartz-(biotite). Three silicification phases are related to this alteration stage, one of them (with temperature between 550° and 390°C and salinity between 35 and 47 %) genetically linked to the copper mineralization. The phyllic alteration consists of quartz-illite-(rutile) with minor albite. The albitization process points out to an early phase of this phyllic stage. The fluid inclusions together with the textures of the quartz neck suggest the shallowing of the active hydrothermal system. The disseminated ore paragenesis consists of bornite, chalcopyrite-molibdenite, pyrite, (galena-sphalerite). The presence of molibdenite-quartz veins indicate the transition from the potassic to the phyllic stage while the pyrite-quartz ones point out to a transition to a fragile deformation regime during which the polymetallic veins, with a paragenesis of chalcopyrite- sphalerite, pyrite, (Ag) tetrahedrite, galena were formed. The petrologic characteristics of the volcaniclastic host rocks reveal that they correspond to the lower section of Choiyoi magmatic cycle. The magmatic and structural controls constrain the age of this ore deposit to the Early-Middle to Late Permian.

Descripción: The relationships between landscapes features evolution and soil genesis and distribution are studied. The area is located in southern Entre Rios province (33o00'-33o45'S and 58o30'-59o17'W). The Parana River Delta, has a complex genetic evolution, due to river processes, marine ingressions and aeolian actions. These processes resulted in an unstable landscape, that imprinted its particular features in soils. Nine geomorphic units were recognized: 1) Gualeguay river alluvial plain and terraces, 2) Arroyo Ñancay aluvial plain and terrazes, 3) Minor streams aluvial plains, 4) Gualeguay river delta, 5) Deltaic plain, 6) Beach ridges, 7) Old lagoon and tidal plain, 8) Loessic plain and 9) Dunes. Soils are mainly Entisols, Inceptisols and Mollisols, and a spatial trend is recognized in the Delta Region. Soil evolution is controlled by a) an active morphodynamic, b) soil landscape features that resulted in a dominant aquic regime of soils and c) Late Quaternary climatic fluctuations that resulted in sea level variations. Finally, soil development, soil features and distribution and soil drainage are studied in relation with landscape main features and evolution. © 2004 Asociación Geológica Argentina.

Descripción: The main mineralizations of the Sierra Pintada uranium district, San Rafael Massif, Mendoza, are associated with the Choiyoi volcanic province. In the lower section of this magmatic cycle uranium deposits hosted by epiclastic sandstones are predominant. In the upper section, small vein-type deposits of low economic significance are found instead. During the emplacement of these Permian volcanic and sedimentary sequences two different stress regimes, which conditioned the mineralizing systems, were dominant: transpressional (San Rafael orogenic phase) and transtensional (post-orogenic stage). A strain fabric analyses was performed in an attempt to evaluate the control exerted by the structures in the distribution of the uranium mineralizations during these two stages of deformation. So that, a contribution to a better understanding of the genesis of these deposits will aid to the generation of new exploration guides for uranium in the Choiyoi province. In the case of the deposits related to the lower Choiyoi, three orders of magnitude could be defined for the structural control. The fabric of the structures determining the distribution of these mineralizations reflects the structural grain of the San Rafael orogeny. The deposition of uranium hosted by the upper Choiyoi sequences was directly or indirectly influenced by the structures generated under the post-sanrafaelic stress field.

Descripción: Factors of soils formation, parent material, and relief in western part of Santa Cruz province (southermost Patagonia) were studied in rio de las Vueltas valley beetween Laguna del Desierto and Lago Viedma (49oS-49o30′S y a 73o W). There is great soil variability related to a strong north-west-south-east bioclimatic and geomorphic gradient. Soils were studied in a NW-SE transect, where three different sectors could be distinguished. The western area, including the Patagonian Andean Cordillera and piedmont is characterised by strong landscape variations due to glacial processes, colluvial, glacial, cineritc and glacifluvial parent materials, a udic-xeric regime and Nothofagus woodlands. The landscape of the eastern area is mainly of glacial-fluvioglacial, fluvial origin, with aeolian landforms, an and regime, grassland-shrub steppe and large flat structural plain landscape. Because of their different landscape-relief features and parent materials, both areas have different soils associations. The western area is characterized by a distrudepts, udivitrands/hapludands and haploxerolls sequence, meanwhile the eastern by an haplargides, torriortents and haplocalcides trends. In the western area, podzolisation features were recognized. In the north-western sector, Bs and E horizons were recognized, and low pH base saturation values, due to climatic conditions, indicate ongoing podzolization. Elsewhere in the south-eastern sector calcium carbonate from the surface, basic pH and total saturation are soils main features. © 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: 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.