Sistema Regional de Información
en línea para Revistas Científicas de América Latina,
el Caribe, España y Portugal

Several Upper Cretaceous plesiosaur specimens recovered from southernmost Chile are described here. These were collected from upper levels of the Dorotea Formation exposed on three different localities (Sierra Baguales, Cerro Castillo, and Dumestre). The new material includes the first record of Aristonectes (Plesiosauria, Elasmosauridae), previously recorded from Argentina, central Chile, and Antarctica. Additional specimens include associated postcranial skeletons as well as isolated elements. Among these, we recognize the presence of aristonectines in the three studied localities, while non-aristonectine elasmosaurids were only collected from Cerro Castillo. The specimen from Dumestre is remarkable by being a small-sized adult, indeterminate aristonectine, and could be related to known representatives from Antarctica. These new finds prove the abundance of aristonectines as well as intermediate elasmosaurids along the Magallanes Basin during the uppermost Cretaceous, while extreme long-necked elasmosaurids as well as polycotylids seems to be completely absent during this time span. This key record from southernmost Chile and its strategic placement in the middle part of the Weddellian Province gives the chance for complementing the paleobiogeography of Upper Cretaceous plesiosaurs from the Southern Hemisphere. As a first result, a faunal turnover is observed during the early Maastrichtian, when extreme (very-long necked) elasmosaurids and polycotylids disappeared from the austral record. Since the early Maastrichtian and towards the late Maastrichtian, aristonectines became differentially abundant along the southeastern Pacific and Antarctica, but moderately represented in the southwestern Atlantic. On contrary, intermediate elasmosaurids were scarce in the Antarctic-Pacific realm, but abundant in the Atlantic. The updated record of austral plesiosaurs suggest a first stage of interchange from the Northern into the Southern Hemisphere, and through the Atlantic seaway, at least since the Coniacian to the late Campanian-early Maastrichtian. During the early Maastrichtian, aristonectines were relatively frequent in the New Zealand-Antarctica archipelago, becoming abundant along southern South America during the late Maastrichtian.

A recording of 1,115 gravimetric stations, the review of 368 wells, and the petrophysics measurements of 106 samples from representative outcrops have been used for a comprehensive geological/geophysical study of Santiago Basin. 2.5D and 3D gravimetric modeling, constrained by regional geology, soil and bedrock densities, edge-basin outcrops, depth (minimum) to basement from wells, and detailed modeling of heterogeneous bedrock and mid-crustal blocks, provided a well-constrained depth to basement model. Model results indicate the presence of a relatively shallow basin with an average of 250 m depth, and three sub basins with depth in excess of 500 m, but comprising less than 30% of the basin surface. From erosion rates in central Chile we estimate a basin infill lasting between 10 to 20 Ma. Basement topography/geomorphology, undercover a structural pattern dominated by NE and NW-trending structures that can be traced out of the basin, westwards in the Coastal Cordillera and eastwards in the Main Cordillera, with second order relevance of NS structures in the eastern border of the basin. This observation, further supported by natural crustal seismicity and basement-derived-magnetic signatures, suggests that the basin origin is mainly controlled by inherited old structures oblique to the margin. Active seismicity along these traverse NE and NW structures suggest that permanent deformation, and associated seismic hazard in the basin is mostly concentrated along these structures. The dynamic response of soils, in terms of the natural resonance frequency, shows that the basement-to-sedimentary/infilling-impedance-ratio is proportional to the amplitude of the resonance peak. On the other hand, the expected correlation between fundamental frequency and depth to basement is only partially supported by the empirical evidence. The difference between a greater gravimetric depth-to-basement compared to lesser seismic depth-to-basement, is attributed to changes in mechanical stiffness with depth compaction with minor effects in bulk density. Finally low enthalpy geothermal resources of the Santiago Basin is analyzed considering depth to bedrock, water table estimates and simple Darcy’s-temperature coupled flow modeling. Results show that high groundwater temperature is restricted to deeper parts of southern sub-basin, which improves direct uses of geothermal energy for heating purposes.

Planchón-Peteroa volcano started a renewed eruptive period between January 2010 and July 2011. This eruptive period was characterized by the occurrence of 4 explosive eruptive phases, dominated by low-intensity phreatic activity, which produced almost permanent gas/steam columns (200-800 m height over the active crater). Those columns presented frequently scarce ash, and were interrupted by phreatic explosions that produced ash columns 1,000-3,000 m height in the more intense periods. Eruptive plumes were transported in several directions (NW, N, NE, E and SE), but more than half of the time the plume axis was 130-150° E, and reached a distance up to 638 km from the active crater. Tephra fall deposits identified in the NW, N, NE, E and SE flanks covered an area of 1,265 km2, thickness variable from 4 m (SE border of active crater) to ~0.5 cm 36.8 km SE and ~8 km NW from active crater, respectively, corresponding to a minimum volume of 0.0088 km3. Tephra fall deposit is exclusively constituted of no juvenile fragments including: lithics fragments as main component, quartz and plagioclase crystals, some oxidized lithics, and occasional presence of Fe oxide, and less frequently Cu minerals, as single fragments. We present new field-based measurements data of the geochemistry of gas/water from fumaroles and acid crater lakes, and fall deposit analysis, that integrated with the eruptive record and GOES satellite data, suggests that the eruptive period 2010-2011 has been related to an increasing of heat and mass transfer from hydrothermal-magmatic reservoirs, which would have been favoured by the formation and/or reactivation of cracks after 8.8 Mw Maule earthquake in February 2010. This process also allowed the ascent of fluids from a shallow hydrothermal source, dominated by reduced species as H2S and CH4, during the entire eruptive period, and the release of more oxidizing fluids from a deep magmatic reservoir, dominated by acid species as SO2, HCl and HF, increasing strongly after the end of the eruptive period, probably since October 2011. The eruptive period was scored with a magnitude of 3.36, corresponding to a VEI 1-2.

Ammonites of the genus Santafecites Etayo-Serna and subgenus Olcostephanus (Viluceras) Aguirre-Urreta and Rawson are described for the first time from Chile. The succession of Olcostephaninae from the Chañarcillo Basin of northern Chile is described in the light of new collections and revision of historical material. The occurrence of mixed Andean and Mediterranean faunas supports the correlations proposed with the ammonite scales of the Neuquén Basin and the Mediterranean Province.

Based on their petrography and chemistry, 18 tephra analyzed from two lake and bog cores and one outcrop in the upper Río Cisnes valley are believed to have been derived from nine different eruptions of the Mentolat volcano, four of the Melimoyu volcano, and one from the Hudson volcano. Some of these tephra correlate chronologically and petrochemically with previously documented large eruptions of these volcanoes, including the Late-Glacial Ho eruption of Hudson (17,340 cal yrs BP), the mid-Holocene MEN1 eruption of Mentolat (7,710 cal yrs BP), and the Late-Holocene MEL2 eruption of Melimoyu (1,680 cal yrs BP). A Melimoyu-derived tephra from the outcrop occurs in glacial-lacustrine sediments and is considered to pre-date the Last Glacial Maximum (>19,670 cal yrs BP). The data suggest that none of the tephra were produced by explosive eruptions of the Maca, Cay and Yanteles volcanoes.

U-Pb detrital-zircon geochronology of two discrete outcrops of mica schists of the western border of the Domeyko Cordillera in the Region of Atacama, northern Chile, indicates that the maximum age of sedimentation of their protolith corresponds to the Late Carboniferous to Early Permian. The Early Permian granitoids of the Sierra Castillo Batholith that intruded the metamorphic rocks show ductile deformation and were emplaced within hot crust not much later than the greenschist-facies metamorphism peak that affected their host rocks. Therefore, the metamorphism of the Quebrada del Carrizo Metamorphic Complex and El Jardín Schists is constrained temporarily between the maximum age of sedimentation of detrital zircons (314±11 to 291±5 Ma) and the crystallization of Early Permian intrusions (292.2±6.6 to 278.3±5.8 Ma), thus pointing to Early Permian metamorphic peak. Concentration of U-Pb ages between 400 and 600 Ma indicate eastern detrital input sources, such as the Pampean and Brasiliano orogenies and the Ordovician-Silurian Famatinian magmatic arc of northwestern Argentina. Other concentration of detrital-zircon U-Pb ages between 900 to 1,200 Ma reflect contributions of magmatic rocks of age of the Proterozoic Sunsas orogeny (Grenville). Whereas, only few grains of zircon with U-Pb ages older than 1,200 Ma occur and these may correspond to a minor contribution zircon from South American cratonic areas. Zircon grains of Devonian age are scarce in populations of zircons analyzed, consistent with a passive margin and a lull of magmatic activity during this period in the paleo-Pacific border of Gondwana. The U-Pb detrital zircon data from the Quebrada del Carrizo Metamorphic Complex and El Jardín Schists coincide with detritalzircon U-Pb data previously published for other metamorphic complexes of central-northern Chile, which are part of a Late Paleozoic subduction complex or accretionary wedge developed in the western edge of Gondwana. Consequently, the Quebrada del Carrizo Metamorphic Complex and El Jardín Schists are relics of the same Paleozoic accretionary wedge, which constituted the substratum for the emplacement of the Permian plutons of the Sierra Castillo Batholith.

La Pacana (central Andes, Northern Chile) is one of the largest resurgent calderas in the world, formed 4 Ma ago during an eruption with a VEI of 8.7. We undertake a gravimetric study to contribute new insights into the inner structure and evolution of this caldera. The La Pacana Bouguer residual anomaly is asymmetric and has an average amplitude of -12 to -14 mGal, which we interpret as being produced by the low-density intracaldera ignimbrite infill. A reinterpretation of the caldera stratigraphy plus the available geochronology suggests that the current shape of La Pacana was produced by the collapse of two nested calderas, roughly limited by the axis where the resurgent dome changes its orientation, with the oldest eruption in the southern part of La Pacana. The gravity data suggests that these southern and northern nested structures would have collapsed with downsag and trap-door geometries respectively, evidence of asymmetric subsidence. Intra caldera ignimbrite thicknesses were calculated with 2.5D forward models and show that the ignimbrite infill in its southern and northern parts reach ∼0.6-1.1 km and ∼2.5-3 km respectively. Using Gauss’s theorem, we calculate the volume of the intra caldera ignimbrite infill is ∼3,400-3,500 km3, in agreement with previous estimates and with models that show that the larger the caldera diameter, the larger the erupted volume.

Two Holocene tephras encountered in outcrops, cores and trenches in bogs, and lake cores in the area around Cochrane, southern Chile, are identified (based on their age, tephra glass color and morphology, mineralogy, and both bulk and glass chemistry) as H1 derived from Hudson volcano, and MEN1 derived from Mentolat volcano. New AMS radiocarbon ages indicate systematic differences between those determined in lake cores (MEN1=7,689 and H1=8,440 cal yrs BP) and surface deposits (MEN1=7,471 and H1=7,891 cal yrs BP), with the lake cores being somewhat older. H1 tephra layers range from 8 to 18 cm thick, suggesting that both the area of the 10 cm isopach and the volume of this eruption were larger than previously suggested, but not greatly, and that the direction of maximum dispersion was more to the south. MEN1 tephra layers range from 1-4 cm in thickness, indicating that this was probably a reasonably large (>5 km3) eruption. Some of the lake cores also contain thin layers (<2 cm) of late Holocene H2 tephra and the recent H3 (1991 AD) tephra, both derived from the Hudson volcano. No tephra evidence has been observed for any late Pleistocene tephra, nor for the existence of the supposed Arenales volcano, proposed to be located west of Cochrane.

In the present study a first found of agates in Chile is reported from the Cordón de Lila region in northern Chile. The agates occur as veins and lenses in altered Permian volcanic rocks. The rock composition is rhyodacitic/ dacitic consisting of a fine-grained K-feldspar-quartz groundmass with phenocrysts of plagioclase (An50-60) and pyroxene (augite). The volcanic host rocks show strong features of alteration and brecciation. Results of XRD, polarizing microscopy and cathodoluminescence (CL) microscopy and spectroscopy point to a late- to postvolcanic formation of the agates by hydrothermal fluids and SiO2 which was released during the alteration of unstable minerals and volcanic glass. Strongly varying agate micro-textures and the appearance of euhedral quartz crystals with μm-sized growth lamellae (so called Bambauer quartz) indicate fluctuations in the physico-chemical conditions (SiO2 concentration, pH) during alteration and agate formation. Another indication for the alteration processes is the occurrence of secondary calcite in the agate-bearing rocks.

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