Acta Geológica Lilloana

Petrography and geochemistry of the Vicin granitoid, Macara, Ecuador

Christian Wladimir Romero Cóndor — 2025-03-14

In Ecuador, the magmatic history of the Alamor–Lancones basin remains enigmatic due to the lack of specific studies characterizing igneous bodies. This research focuses on the petrographic and geochemical analysis of the Vicin intrusive body in the Vicin Viejo section southwest of Macará, Ecuador. The Vicin granitoid intrudes volcaniclastic sequences of the Ciano Formation and the Potrerillos intrusive body, manifesting as sills, laccoliths, dikes, and domes, aligned with the Cerro Viejo reverse fault. Petrographically, the Vicin intrusive is a quartz-rich granitoid, with mineralogy dominated by quartz, potassium feldspar, plagioclase, actinolite, muscovite, and garnet. It exhibits phaneritic, inequigranular, and holocrystalline to hypocrystalline textures, medium to coarse grain size, and dioritic and gabbroic xenoliths. Geochemical data indicate that the Vicin intrusive corresponds to calc-alkaline affinity granites, saturated in silica and exhibiting metaluminous to peraluminous characteristics. The concentrations of trace elements and rare earth elements suggest that the magmatism is associated with a volcanic arc environment, characteristic of an active continental margin in a compressive orogenic domain

Morphometry applied to flood hazards in the Los Sarmientos river basin, La Rioja, Argentina

Oscar Juarez — 2025-03-21

The Los Sarmientos River basin has characteristics in which torrential floods are generated in summer times, with a history of extraordinary events and discontinuous hydrometeorological data, that affected the population. The objective of this work is to contribute to determine the possible dangers of growth in Los Sarmientos' river basin, located in Chilecito (La Rioja, Argentina), by analysing the dynamics of the basin through the parameters and indices morphometrics. In the environment of a geographic information system (QGIS), and in a quantitative database, parameters and indices related to: the basin’s shape, relief, drainage network and intensity of concentrated drainage, were calculated. The results indicate that the basin analyzed is elongated in shape and has a large altitudinal unevenness, associated with the exhumation that occurs in the mountain region, possibly a high erosive power characterizes the main course, in search of its base level. The hydrological behaviour of the basin under study has morphological, structural, lithological, and topographical dependence; runoff concentrations occur suddenly in the upper basin and become attenuated as the topography/unevenness decreases, thereby reducing the possibilities of sudden concentrations. In turn, it depends on other factors such as the intensity and duration of the storm, likewise, the existence of previous rains reduces infiltration and favouring rapid drainage and making it potentially dangerous in the possible events of extraordinary flood.

Sedimentary dynamics of the shallow water facies of Puncoviscana basin in the Neoproterozoic – Early Cambrian transition, NW Argentina

Vanina L. López de Azarevich — 2025-05-19

The Neoproterozoic-Early Cambrian deposits from NW Argentina conform the infilling of the Puncoviscana Basin. It represents an elongate, northward opened, foreland marine basin, developed between Amazonia, Antofalla and Pampia Terranes derived from the fragmentation of Rodinia. This basin registers sedimentation of basal diamictites and platform to deep marine deposits to the west, followed by cap carbonates that transitionally pass to shallow clastic sequences with few interlayered pyroclastites (Puncoviscana Formation). Shallow marine Puncoviscana Formation contains diagnostic sedimentary structures belonging to intertidal and shallow subtidal environments. Tidal processes are evident from lenticular to flaser stratification, soft-sediment deformation, tidal rhythmites, herringbones and double-crested ripples. Facies associations resemble shoreface environments along the west margin, which evolved to tidal flats laterally and upper sequence. The tides flooded over a mesotidal coast, while palaeocurrents’ analysis display bimodal bipolar and polymodal patters due to a reversal N-S flood and ebb circulation, and littoral drift. From published zircon ages’ population sourced to the Puncoviscana Formation we interpret that subsidence of the basin was episodic. The initial episode of active faulting and subsidence (~570-545 Ma, deposition of the lower Puncoviscana Formation) was followed by a ~10 Ma-episode of sea level rise (middle Puncoviscana Formation) and a final ~20 Ma-episode that included tectomagmatic activity that progressed to uplift and closure.

Regional geology of Formosa province, Argentina

Hugo G. Marengo † — 2025-05-19

The province of Formosa, in the northern sector of the Chaco-Paranaense basin, extends from the Eastern foothills of the Subandean System to its base level in the alluvial plains of the Paraguay and Paraná rivers. The sedimentary section of Formosa contains Paleozoic sediments of marine, continental and glaciogenic provenance, originated in the Andean foothills, distributed in three main basins or “depocenters”, bounded by "tectonic thresholds”, between the Sierras Subandinas and the Caburé High (including Zapla, Copo, Caburé and Rincón formations), Charata
(or Las Breñas) and the Asunción High in Paraguay. The Mariano Boedo exploration well was the only one, in Formosa, that reached a Cambrian granite at -1864m., below the sedimentary pile. Post-Cretaceous sequences in Formosa are filled of Tertiary and Pleistocene sediments,
among which the sediments of the "Paranaense ingression" stand out, culminated during the Upper Miocene. Before this marine ingression occurred that one of the Laguna Paiva Formation (Upper Oligocene – “Chattiano?”). The Chaco Formation is interspersed between both ingressions. The modern sedimentary sequence of the geology of Formosa, between Pleistocene to recent, extended throughout the Gran Chaco, in its phytogeographical significance, including sediments of large alluvial fans of fluvial origin, with aeolian and lacustrine intercalations, originated in the eastern Andean sector. The red sediments of the fans, containing Pleistocene fossils, are actually found at the base of the ravines of the Pilcomayo and Bermejo rivers, and are interpreted as corresponding to the upper section of the Old Pilcomayo and Bermejo Fans.