Andean

The Andean Paleozoic basement of the Cordón del Plata (Argentina) consists of two sets of rocks showing different stratigraphy, structure and metamorphism. The lower one is represented by the pre-Carboniferous (Devonian?) Vallecitos beds. These rocks have been affected by folds and associated cleavage, developed under low-grade metamorphic conditions and related to the Chanic orogenic event of the Famatinian Orogenic Cycle (Upper Devonian-Lower Carboniferous). The Vallecitos beds is pre-orogenic to the Chanic deformation event and must have been deposited in a fore-arc basin located on the active margin of Chilenia, before its collision with the passive margin of Gondwana. The upper set unconformably rests on the Vallecitos beds and consists of the Late Carboniferous El Plata Formation, and the Río Blanco Conglomerates, probably Late Carboniferous-Permian in age. These rocks have been affected by eastdirected thrusts and associated folds formed under very low-grade to non-metamorphic conditions. This deformation can be related to the San Rafael phase (Gondwanan Orogen). The El Plata Formation was deposited in an extensional back-arc basin while the Río Blanco conglomerates must have been deposited in a retro-arc basin, both of them on the active margin of Gondwana. The Choiyoi Group, essentially volcanic and Permo-Triasic in age, rests unconformably on the previously described successions and was deposited in an extensional setting (pre-orogenic stage) associated with the beginning of the Andean Cycle. This cycle lead to the uplift of the Frontal Cordillera during the Cenozoic and the deposition of thick continental units at the base of the Cordón del Plata in Neogene-Quaternary times.

Table 8. RepResenTaTive elecTRon micRopRobe analyses of mineRals in coRdieRiTe-sillimaniTe-gaRneT gneiss (sample pb1 137 m2). mineral-garnet mineral-biotite mineral-plagioclase mineral-K-feldspar mineral-sillimanite mineral-chlorite mineral-cordierite (pinnitized) mineral-magnetite analysis no.

REsuMEN. El sistema de cuenca de antepaís Austral-Malvinas adyacente al orógeno Fueguino comprende sedimentitas marinas expuestas en el norte de Península Mitre, incluyendo 550 m de la antefosa del Paleoceno tardío-Eoceno temprano, y más de 2.900 m de la subcuenca María Luisa, depocentro de techo de cuña ('wedge-top') del Eoceno medio tardío-?Mioceno. La columna se subdivide en las formaciones La Barca (100 m, Paleoceno superior), Punta Noguera (450 m, Eoceno inferior), Leticia (30 m, Eoceno medio superior), Cerro Colorado (935 m, Eoceno medio superior-Eoceno superior), Puesto José (nom. nov., 1.600 m, Oligoceno) y Malengüena (nom. nov., 216 m, ?Mioceno). La microfauna de la Formación Malengüena se distingue por el dominio de Globocassidulina cf. brocha, y por su peculiar preservación, ambas características no reconocidas previamente en la porción argentina de la cuenca Austral. Se verifican discordancias sintectónicas entre el límite Paleoceno-Eoceno y el Eoceno medio, asociadas a una etapa de orogénesis regional: en el Eoceno más alto-Oligoceno basal, relacionada localmente con el anticlinal Campo del Medio; en el Oligoceno 'medio', asociada al retrocorrimiento Malengüena; y en el Mioceno ?inferior, relacionada con estructuras profundas de la faja corrida y plegada. Palabras claves: Paleoceno-Mioceno, Cuenca de antepaís, Techo de cuña, ABsTRACT. The Paleocene-Miocene of Península Mitre: foredeep and wedge-top of the Austral Basin, Tierra del Fuego, Argentina. The orogenic margin of the Austral-Malvinas foreland basin system is formed by a marine succession exposed at northern Península Mitre, composed of 550 m of the late Paleocene-early Eocene foredeep, and more than 2,900 m of the late middle Eocene-?Miocene María Luisa wedge-top sub-basin. The succession is subdivided in the following formations: La Barca (100 m, upper Paleocene), Punta Noguera (450 m, lower Eocene), Leticia (30 m, upper middle Eocene), Cerro Colorado (935 m, upper middle Eocene-upper Eocene), Puesto José (nom. nov., 1,600 m, Oligocene) and Malengüena (nom. nov., 216 m, ?Miocene). The foraminiferal content of the Malengüena Formation is distinguished by the dominance of Globocassidulina cf. brocha, and by its particular preservation, features not previously recognized in the Argentinean part of the Austral Basin. Syntectonic unconformities are recognized between the Paleocene-Eocene boundary and the middle Eocene, related to regional orogenesis; in the uppermost Eocene-lowermost Oligocene, locally related to the Campo del Medio anticline; in the 'mid' Oligocene, associated with the Malengüena backthrust; and in the ?lower Miocene, related to deep structures of the thrust-fold belt.

Very little work has been done on the stratigraphy and sedimentology of the upper Oligocene-Lower Miocene Río Baguales Formation in the Magallanes Province of southern Chile since its original definition in 1957. A detailed stratigraphic section of the upper part of the formation exposed west of the Baguales River is presented, with an interpretation of depositional environments. This indicates a prograding shoreline in which estuary mouth, middle estuary (lagoon) and bay head delta facies are represented. Large-scale delta slope foresets indicate progradation towards the north and northwest, whereas southeast-directed cross-beds on the delta front probably suggest wave action. The general sequence reflects a gradual sea-level fall largely counteracted by tectonic subsidence, which provided the necessary accommodation space for thick tidal flat and subtidal deposits to accumulate. A longer period of transgression towards the top of the succession may be related to a rise in sea-level or accelerated tectonic subsidence, whereas a major regression commencing at around 23.3 Ma probably coincides with the opening of the Drake Passage and the growth of the East Antarctic Ice Sheet.

ABSTRACT. At Domeyko, 40 km south of Vallenar in northern Chile (28 57'S-70 53'W), the Dos Amigos and Tricolor porphyry copper centers are located within a north-south-elongated hydrothermal alteration zone 6x1. 5 km of surface dimensions. The centers are ...

We present new data on the paleoichthyology, paleobotany and radiometric results of the Loreto Formation in the Brunswick Peninsula of southernmost Chile, that allow us to propose a Late Eocene age. The rich diversity of fossil cartilaginous fishes (Chondrichthyes, Elasmobranchii) recognized in upper levels of this unit includes the taxa Carcharias aff. 'hopei' (Agassiz), Odontaspis sp., Carcharoides catticus (Philippi), Striatolamia macrota (Agassiz), Anomotodon sp., Macrorhizodus praecursor (Leriche), Galeorhinus sp., Abdounia sp., Hexanchus sp., Squatina sp., Hexanchidae indet., Myliobatis sp., Myliobatoidea indet., and Ischyodus dolloi Leriche. This assemblage has clear ecological affinities with Eocene Tethyan fauna previously described in the Northern Hemisphere, and also has common elements with Eocene cartilaginous fishes from Antarctica. Additionally, a paleobotanic study of this unit identified leaf imprints of Asplenium sp., Pteris sp., Podocarpus sp., and abundant angiosperms including Nothofagus lanceolata Dusén, N. simplicidens Dusén, N. variabilis Dusén, N. cf. alessandri Espinosa, N. subferruginea (Dusén), Hydrangea sp. and Phyllites spp. Wood remains of Nothofagoxylon scalariforme Gothan and Araucariaceae cf. Araucarioxylon Kraus were also identified. Additionally, pollen grains indicate gymnosperms and angiosperms: Podocarpidites otagoensis Couper, Retitricolpites sp., Tricolpites sp., Liliacidites sp., Polyporina sp., Nothofagidites cincta Cookson, and Nothofagidites cranwellae Couper, having affinities with Eocene florae, and being consistent with the age of the fossil fishes. Finally, a SHRIMP U-Th-Pb analysis of two samples collected from the studied beds provided thirty-eight and sixty zircon grains, indicating a clear main peak at 36.48±0.47 Ma (MSWD=1.5) and 36,73±0.50 Ma (MSWD=0.65). The integrated results indicate that the upper part of the Loreto Formation has a minimum Priabonian age, supporting previous reassignations of this part of the formation into the Late Eocene, and differing from the Oligocene age proposed in its original definition.