Revista de la Asociación Geológica Argentina

Analysis of spectral signatures obtained from Sentinel-2 images and their relationship with sediment transfer in the Lower Valley of the Chubut River, Patagonia, Argentina

Nicolas Scivetti — 2023-12-04

This article analyzes the characteristics of the surface cover and the lithostratigraphic units present in a particular sector of the Valle inferior del Rio Chubut (VIRCH) in Argentina, Patagonia based on reflectance values obtained from Sentinel-2 multispectral images. The aim of the study is to identify the main lithostratigraphic units that contribute sediments to tributaries rivers of the Rio Chubut through the analysis of the spectral signatures of a fluvial–alluvial fan. In turn, the accuracy of the used supervised classification algorithms for the study area is discussed. The obtained results suggest that the fluvial – alluvial fan would be composed almost entirely of sediments from Sarmiento and Río Chico groups. Therefore, these units would be, at least in the study area, the main sources of sediments.

Geological conditioning in the occurrence and evolution of mass movements in the Fitz Roy River watershed, Santa Cruz province

Daniela Schmidt — 2023-07-21

In the Torre glacier watershed and along the Fitz Roy River valley, Southern Patagonian Andes, there are several mass movements as a consequence of the continuous loss of glacial conditions. From geomorphological detail analyses, we identified the recent and past mass movements, interpreted their origin and estimated the age related to the retreat of the Torre glacier for, finally, proposed what conditioning and triggering factors caused these movements. Among the movements identified stands out a megalandslide, with approximately 115 million of m³, developed from the south slope of the valley. From the analysis, we interpreted a lithological-structural control as the main conditioning factor, attributing a probable seismic event, which occurred in the Pleistocene-Holocene, as the main triggering event. In addition, from the study of the identified mass movements, we evidenced the close relationship between the glacial retreat and its thinning, and activity of gravitational processes as a result of the relaxation of slopes. According to this, we revealed climate and geomorphological control as the main triggering factor in the development and initiation of these movements. Finally, understanding geomorphological evolution in this area is important for the future evaluation of the existing geological risks.

Spatial distribution and evaluation flow occurrence, based on inventory and morphometric analysis of the San Juan River Basin, San Juan Province, Argentina

Natalia Noemí Rios — 2023-09-26

This work aims to determine the flows susceptibility occurrences areas, spatial distribution and characteristics. This achieved through a regional morphometric analysis, thematic maps evaluation, derived from a Digital Elevation Model, flash flood active deposits inventory, and the Kernel density calculation. The San Juan river basin has 39604.86 km², encompassing almost all human activities in the province. Due to the regional nature of the study, it divided into four sub-basins to obtain detail. Based on the results, it is deduced that the basin has a torrential nature, and the terrain conditions would contribute to increasing the flow of the main rivers during intense precipitation events. Flash flood deposits are mainly established in relatively smooth terrains with slopes of up to 16°, mostly distributed in mid-slope and east-facing piedmont areas, as well as low-lying floodplain areas or flat areas. These areas are more prone to experiencing these events.

Geohydrological assessment of a coastal sector located between Bahía Bustamante and Camarones, Chubut, Argentina

Maria del Pilar Alvarez — 2023-12-04

In arid coastal areas, groundwater is usually brackish and coastal ridges are often sites where freshwater lenses are generated. The purpose of the study was to explore hydrogeologically and geoelectrically the area between Bahía Bustamante and Camarones in order to evaluate the feasibility of the sequence of coastal ridges in that area to constitute freshwater lens reservoirs. A regional water source survey was carried out, where the electrical conductivity of the water and the phreatic level were measured. On the Pleistocene and Holocene littoral ridges, vertical electrical soundings and soil description and infiltration tests were carried out. The studies carried out indicate that the Holocene ridges present more favorable conditions for rainwater infiltration and its storage in the form of freshwater lenses than the Pleistocene ones. However, the presence of freshwater lenses has not been identified in any of the coastal ridges explored. The development of soils with argillic and calcic horizons is probably the main reason why the Pleistocene cords do not constitute preferential recharge zones. Although the Holocene cords present favorable characteristics to host freshwater lenses, the reduced areal development of these geoforms could be a conditioning factor for their development. The results obtained show the need to carry out more detailed hydrogeological studies to verify the geohydrological functioning of the area.

Micromorphology, mineralogical and microbiological characterization of silica speleothems in Cerro Colorado Breccia, Sierras Australes (Buenos Aires, Argentina)

Silvia Grill — 2023-12-04

The micromorphological, mineralogical, and microbiological analysis of silica speleothems developed in the BCC (Northwest of the Sierras Australes) allowed for a new contribution on the environmental conditions and genesis of these deposits. The origin of the 11 types (15 subtypes) of recorded speleothems was related to general and specific mechanisms that control water circulation (gravity, capillarity, surface tension), microenvironmental conditions (relative humidity (RH), condensation, temperature, evaporation, and insolation), and biological activity. The speleothems present inside the tafoni showed markedly humid conditions (RH > 60%, condensation, slow water circulation), which would have particularly favored the formation of flowstones and stalactites/stalagmites with thicknesses less than 20 mm, compact textures, and gypsum/calcite trichites at their free ends. On the other hand, those found in fractures and outside the tafoni (drier environments) were associated with more porous microforms (gours, boxwork, and peloidales) and smaller dimensions (< 5 mm thickness). Opal-A was associated with the dissolution/precipitation process of quartz (carried out by water and biological activity). Clays would be the product of weathering of silicates; muscovite, chlorite, and plagioclases that are part of the BCC. The presence of microorganisms (palynomorphs, diatoms, and phytoliths) allowed for inferring biological involvement in the development of the speleothems. The thin thickness of the speleothems, the presence of opal-A, and the recorded pollen associations limited their age to recent times (Late Holocene?).

Geomorphological analysis and identification of threats associated with climate change in the Torrecillas Glacier basin, Los Alerces National Park, Patagonia Argentina

Agustina Reato — 2023-12-04

In the Torrecillas Glacier watershed, located in the North Patagonian Andes, in the Parque Nacional Los Alerces, paraglacial processes that are introducing essential changes in the area have been identified. The glaciology and geomorphology of this sector are complex, characterized by the coexistence of a free glacier at the headwaters and a debris-covered glacier downstream within a watershed exhibiting significant geomorphological activity. The past and present morphodynamics of this area have been studied to identify possible future behaviors, especially those related to Glacial Lake Outburst Flow events. Field control and the analysis and comparison of temporal satellite images, aerial photographs and Digital Elevation Model allowed the recognition of the rapid retreat of the covered glacier in the last 14 years and the development of a greater intensity of paraglacial phenomena. Besides, in recent years a Glacial Lake Outburst Flow type event was caused by the sudden entry of more than 12.000 m³of debris, snow, and ice into the proglacial lake. Precipitation and temperatures above the average are being proposed as the triggering factor. Additionally, the detailed study of the Glaciar Torrecillas watershed allowed the recognition of threatening areas. Due to rising temperatures and decreasing precipitation in the last decades, and the short-term climate variability registered in the area, it is necessary to study and monitor unstable sites to detect gravitational phenomena.

Late Pleistocene-Holocene structures in the Central Precordillera: Their potential as seismogenic sources

Federico Miguel Haro Sanchez — 2023-12-04

Between 29° and 32° S, along the eastern border of the Andean orogen, numerous earthquakes have been recorded that have caused changes in the landscape throughout geological history. In the southwestern flank of the Matagusanos depression, located in the eastern foothills of the Precordillera Central geological province, a detailed study was conducted on structures exhibiting Quaternary tectonic activity. Four distinct geomorphological units were identified: mountainous, hilly, transition, and depressed, all of which are closely linked to the tectonic evolution of the Precordillera. The transition unit revealed various indicators of Quaternary tectonic activity, including anomalies in the drainage pattern, fault scarps, alignment of springs, and displaced alluvial terraces. These characteristics suggest Quaternary structures that have been grouped and defined in this study as part of the El Cajón Fault System. Within this context, four parallel fault scarps oriented in a north-south direction were identified, characterized by a west-facing slope and reaching maximum heights of 4 m. Associated with one of these scarps, a high-angle thrust fault with a westward inclination (345°, 81° NE) was observed in a natural outcrop. This fault juxtaposed the Mogna Formation (upper Pliocene) over Quaternary (Q2) alluvial deposits. An Optically Stimulated Luminescence (OSL) dating of the Quaternary sediments affected by the fault yielded an age of 8880 +/- 2000 years, with no evidence of deformation in the more recent levels (Q1). This suggest that the fault did not have activity following the early Holocene. From the morphotectonic analysis (planar and linear markers) of the El Cajón Fault System, it is suggested that this structure is a potential seismogenic source located very close to the city of San Juan and three hydroelectric dams along the San Juan River. This could generate a seismic event ranging between Mw=5.7 and Mw=6.3.

Geomorphology of the piedmont of the Serranías Occidentales: Influence of a folding morphostructure in the Sierras Pampeanas Australes, San Luis, Argentina

Guillermo Ojeda — 2023-12-04

The geomorphology of the western piedmont of the Serranías Occidentales, within the southern Sierras Pampeanas, is presented. The hierarchical analysis of the relief allowed us to distinguish 4 Geomorphological Regions, 2 Geomorphological Associations and 11 Geomorphological Units. Within the Geomorphological Association of the Piedemonte Occidental de las Serranías Occidentales, we recognize erosional and aggradational geoforms. The latter are grouped into 4 main types: A) large sandy-muddy alluvial fans, B) sandy-muddy terminal fans, C) small gravelly-sandy alluvial fans, and D) piedmont with channelled flows. The characteristics of alluvial fans are closely related to their watersheds; the relationship between the fan area shows a positive trend for all cases; similarly, the textural characteristics of the deposits are directly related to the source zone. However, the sandy-muddy fans present sizes and slopes that differentiate them from the alluvial deposits defined for other Pampean piedmont of the southern sector; their dimensions greatly exceed the frequent sizes, while the slope values are lower. The deposits that make up the fans are linked to sedimentary sequences with Pleistocene to upper Holocene ages and the current wedging of its main channel is interpreted to be associated with a climate variation and changes in their local base levels, which could be linked to the decrease in the flow of the Desaguadero River and neotectonic activity.

Geomorphology and superficial parameters: the interaction for soil development in the Precordillera Oriental of San Juan, Argentina

Raúl Emmanuel Ocaña — 2023-12-04

The study area is located 25 km northwest of the San Juan city, in a sector of the Precordillera Oriental. The main objective is to evaluate whether the geomorphometric properties and surperficial characteristics of alluvial geomorphic surfaces condition the development and spatial distribution of the soil and vesicular horizons. A digital elevation model was used for geomorphometric analysis and maps derived from it. During the field work, data were collected regarding surface and soil properties on three alluvial units: one of the Pleistocene (alluvial level 3) and two of the Holocene (alluvial level 2 and alluvial level 1). Then a correlation of variables was carried out with the objective of developing a model based on the general relationship between geomorphometric variables (slope, vertical distance to the channel network, wind effect, topographic roughness index), vesicular horizon properties (thickness and texture) and superficial properties (rock and sediments covers). The soils analyzed show the presence of vesicular horizons in ten of the eleven soil profiles. The greatest development of the vesicular horizon requires low values of slope and roughness and its greatest development occurs in the oldest levels linked to high values of rock cover, wind effect and vertical distance to the channel network.

Climate and environmental changes recorded in Quaternary sediments of the Rolling Pampas, Buenos Aires province, Argentina

Yamile Rico — 2023-12-04

Loessic sediments of the Rolling Pampa, assigned to the Pampeano and La Postrera Formations, are presented. Paleoclimatic and paleoenvironmental interpretations are made, the magnetostratigraphy of the profile analyzed is defined, and a tentative stratigraphic correlation with oxygen isotope stages is proposed.

It is a pedo-sedimentary succession composed of silty, silty-sandy and muddy sediments of aeolian and fluvial-eolian origin modified by pedogenesis to different degrees; paleosols are classified as gleysols, argilisols, and protosols. The modal mineralogical components come mainly from the Andean sector. The most notable variation is recorded in the proportion of vitroclasts, whose increase (≥ 75%) evidences the occurrence of 2 events of greater pyroclastic activity, one prior to the Matuyama/Brunhes change in magnetic polarity and another contemporary with the deposition of the La Formation.

According to the determined magnetostratigraphy, the sediments of the profile have an age of less than 1.78 Ma (Early Pleistocene (Calabriense) – Holocene). The silicophytolith association shows that in this time interval, the landscape of the Rolling Pampas was similar to the current one, with open to semi-open environments, covered by mesothermal grasslands that favored the development of soil profiles. In this context, climatic changes, variations in the sedimentation rate/pedogenesis relationship, topography and position of the water table, interpreted through the behavior of magnetic parameters and mineralogical and micromorphological analyses, were decisive in its degree of evolution, giving rise to soil profiles with distinctive characteristics.

Preface: Quaternary and geomorphology

María Romina Onorato — 2024-01-21

This special issue contains ten selected works corresponding to the VIII Argentine Congress of Quaternary and Geomorphology, organized in the province of San Juan from September 19 to 23, 2022. The works describe detailed research on exhibitions that were carried out during the development of the congress and cover geomorphological and quaternary studies and analysis. Specifically, this special edition of RAGA includes works related to fluvial geomorphology based on spectral signatures and the study of geological conditions in the occurrence of mass removal processes in the Argentine Patagonia region. A morphometric analysis of basins and evaluation of the occurrence of alluvium in the province of San Juan is also presented. Likewise, this edition contains geohydrological evaluation studies of the coastal sector of Chubut, characterization and analysis of siliceous speleothems of the Sierras Australes.
On the other hand, two contributions are presented referring to climate change, the first referring to the threats in the Torrecillas Glacier basin and the second to climatic and environmental changes recorded in Quaternary sediments of the Pampa Ondulada of Buenos Aires. Finally, this special issue includes a neotectonic analysis in the Central Precordillera sector, studies of the influence of a morphostructure in the Southern Pampean Mountains and a final contribution that points out the interaction between geomorphology and surface parameters for soil development.
The editorial committee thanks the colleagues who collaborated as referees of the works published here. Likewise, we appreciate the effort made by the authors to adapt their contributions to the referees' suggestions and editorial standards.