Snow avalanches, landslides, and debris flows are the primary mechanisms delivering wood to streams in steep, forested headwater catchments. Tree mortality and bank erosion, in contrast, play a relatively major role in recruiting large wood (LW) in larger watersheds. The 2008-2009 explosive eruption of the Chaitén volcano in southern Chile caused widespread forest destruction, depositing several meters of volcanic sediments and dead wood in the Blanco River valley. Tephra fall, pyroclastic density currents, and dome collapses filled the valley, creating thick deposits of lithic-rich gravelly sand and buried wood. Post-eruption fluvial processes and channel adjustments have since eroded these deposits, mobilizing buried dead wood into the channel and creating an additional, previously understudied LW recruitment process. The exhumation of buried wood pieces could increase downstream risks during floods, emphasizing the importance of understanding this process. Given that buried wood cannot be visually identified, the prospecting noninvasive geophysics technique based on Ground Penetrating Radar (GPR) was used in this study to perform continuous scanning of the lithic-rich volcanic deposits along the Blanco River. Three key research questions were addressed: 1) Can the internal morphology and structure of volcanic deposits be described using GPR?; 2) Can GPR detect the signature of buried wood within these deposits?; and 3) Is it possible to determine the spatial distribution of buried LW using high-resolution 3D subsurface mapping of GPR reflections? Following initial calibration, GPR scanning produced radargram profiles that were post-processed into 2D and 3D representations of the deposits, successfully identifying buried LW, whose spatial distribution was subsequently mapped using 3D GPR analysis. This study demonstrates that GPR is a rapid, non-invasive, and precise tool for characterizing the morphology and internal structure of river sediments affected by volcanic activity, facilitating the identification of buried LW.

The Milna deposit (54°37’ S, 67°33’ W) is situated near Lago Fagnano, in the south-central part of Isla Grande de Tierra del Fuego. The presence in the deposit of laminations, dropstones, and convoluted structures, as well as a low organic matter content, suggest a lacustrine origin under relatively cold conditions, likely in direct contact with glacial ice. Radiocarbon ages obtained in this study define a depositional period from ca. 31.5 to ca. 11.5 ka cal BP, or even older. This new evidence points towards a glacial environment for the study area well prior to the Last Glacial Maximum; that is, older than ca. 25 ka cal BP.

In mature oil-producing basins, managing water resources presents challenges due to the potential of enhanced oil recovery (EOR) techniques to counteract reservoir depletion. The water sources in production systems, whether from naturally infiltrated meteoric water, connate formation water, or fluids derived from EOR, add complexities to the isotopic and geochemical signatures of groundwater in reservoir fluids. The present study reports new data on the isotopic and chemical composition of groundwater associated with Oligocene Mugrosa Formation reservoirs in the Middle Magdalena Basin (MMB), Colombia. This research integrates conventional hydrochemical parameters (pH, electrical conductivity, and major ion composition), stable isotope data (δ18O and δ2H), and radiogenic isotope ratios (87Sr/86Sr). The water samples were collected from production wells with selective completions in operational levels (depths ranging from ~800 to 1,950 m) of the Mugrosa Formation across fields P1-P6, ensuring samples representative of the targeted reservoir intervals. These water samples are unaffected by EOR operations, thereby providing data that reflect the natural, undisturbed conditions of the reservoirs. Ionic and isotopic analyses indicate interaction between meteoric and connate waters within the MMB. Meteoric waters, which are chemically less evolved and isotopically depleted in δ18O and δ2H, exhibit short relative residence times in the system and are primarily present along the eastern part of the basin, particularly in the eastern flank of the Nuevo Mundo Syncline. In contrast, connate waters are characterized by longer relative residence times, more evolved chemical compositions, and isotopic enrichment in δ18O and δ2H. These latter samples are located on the western flank of the Peña de Oro Syncline and in proximity to the Arrugas and Casabe thrust faults. This study suggests that groundwater in the MMB originates from two primary sources: 1) infiltration of meteoric water, and 2) upward migration of deep saline waters derived from Cretaceous formations. Meteoric water infiltrates along the eastern basin margin and migrates through the Mugrosa Formation, where water-rock interactions progressively alter their chemical composition, most notably increasing sodium, calcium, and potassium concentrations. Connate waters from underlying Cretaceous formations ascend along reverse fault planes, mixing with meteoric waters and modifying their chemical and isotopic signatures. This mixing results in elevated chloride concentrations and an enrichment in δ18O relative to local meteoric water values.

In Memoriam

A small body of orbicular granodiorite crops out on the Recoba Hill, immediately east of the town of Chaitén, in the mainland area called Chiloé continental, in southern Chile. The rock comprises cm-sized igneous cores with a single shell of fine-grained plagioclase-quartz-K-feldspar assemblages. It is hosted in a Miocene granodiorite, and it is crosscut by aplite and mafic dikes. No other mention of orbicular rocks has to date been reported for the >1,000 km long North Patagonian Batholith, suggesting that the conditions necessary for their formation were infrequent. Thermobarometric determinations indicate pressures lower than 2 kbar (less than 6 km depth) for its formation, a level much shallower than the estimated source depth of the older rocks of the batholith.

The wetlands along the Leyes-Tulumaya stream in central-western Argentina (32° S) are part of an old, inactive paleo-stream bed of the Mendoza River. These wetlands have been drastically impacted during the last 20 years, reducing their water areas or even completely drying out. The causes and driving factors of this major environmental impact have yet to be explored. Understanding the interaction of the different natural and/or anthropic factors influencing these wetland area changes in the recent past is imperative to apply proper management and conservation plans. In this contribution, the water areas of three lakes along the Leyes-Tulumaya stream are mapped using a GIS environment from satellite imagery to track variations in the last two decades. Assuming that natural variables have been forcing these environmental changes, annual precipitation, soil moisture, evaporation indexes, and the stream flow of the Mendoza River, are analyzed. Changes in the lake water areas along the Leyes-Tulumaya stream are, however, hardly explained due to these natural variables, so anthropogenic factors might have been key and thus need to be further explored.

During 23 days in February 1984, the team made up by Wes Hildreth, Bob Drake, and Judy Fierstein explored and sampled much of the western half of the Tupungato volcano, a Chile-Argentina natural landmark at ~33.3° S, including its summit area. The project remained unfinished, and this impressive, ~6,550 m-high volcanic edifice was no longer the focus of significant studies. Forty years later, Wes Hildreth has given up and graciously handed over to us all his data, so it can now be shared with the geological community. Bob Drake dated six samples of Tupungato through the K-Ar technique. In a dacite dome from the southern summit, interpreted as the youngest Tupungato activity, a hornblende age of 831±116 ka is reported. A long-standing controversy is thus resolved: Tupungato has no Holocene activity. 1.26±0.6 Ma to 932±90 ka plagioclase ages are reported from three andesite lavas from the lower to middle flank of the volcano, constraining the age of the edifice to the Early Pleistocene. Finally, hornblendes from two pre-Tupungato dacitic deposits at the volcano’s western foothill were dated at 11.4±0.5 Ma and 9.45±0.6 Ma. These much older deposits may represent pyroclastic rich facies similar to the ones described in the upper sections of the Tunuyán Conglomerates, in Argentina.

The eastern flank of the Argentine Frontal Cordillera at ~30°30’ S is a remote area with rugged topography and high elevations that hinder geological field mapping. This task is also challenging because several outcrops are dominated by Permian-Triassic and Cenozoic volcanic rocks of intermediate to acid compositions with scarce geochronological age constraints. In this work, we use ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) data to clarify the structural and stratigraphic framework of the region. Visible and near infrared (VNIR) and shortwave infrared (SWIR) band combinations, integrated with field observations, are effective in identifying first-order geological features. In addition, the Semi-Automatic Classification Plugin (SCP; a free open-source plugin for QGIS) is capable of discriminating lithological units and is especially useful to visualize their distribution. Results presented here allow us to characterize Permian-Triassic and Cenozoic units, propose tentative correlations between the Permian-Triassic rocks, and outline major tectonic structures in the study area. This work demonstrates the capability of ASTER data to improve geological mapping, especially in those places where field work is difficult.

Because of their social, ecological, and economic impacts, landslides are an issue of global concern. Episodes occurring worldwide have been associated with natural and human causes, although the latter have been poorly explored. Despite recent scientific insight into intensive irrigation as a cause, research remains scarce. This paper focuses on determining the anthropogenic influence on the triggering of earth flows by long-term intensive irrigation, based on temporal and geospatial information. This analysis covers a sector of Northern Patagonia in Argentina, an area of transhumant pastoralism. Our results show that excessive water input creates favorable conditions for the triggering and evolution of earth flows in the region, even under water deficit conditions. As a result, on 22 September 2020, a collapse was recorded which mobilized ~19,000 m³ of earth debris along 360 m and produced economic losses estimated at 168,000 USD. Other landslides are expected to occur in the area if irrigation amounts remain at the same level.