New detrital zircon U-Pb geochronology data from the Cenozoic Magallanes-Austral Basin in Argentina and Chile ~51° S establish a revised chronostratigraphy of Paleocene-Miocene foreland synorogenic strata and document the rise and subsequent isolation of hinterland sources in the Patagonian Andes from the continental margin. The upsection loss of zircons derived from the hinterland Paleozoic and Late Jurassic sources between ca. 60 and 44 Ma documents a major shift in sediment routing due to Paleogene orogenesis in the greater Patagonian-Fuegian Andes. Changes in the proportion of grains from hinterland thrust sheets, comprised of Jurassic volcanics and Paleozoic metasedimentary rocks, provide a trackable signal of long-term shifts in orogenic drainage divide and topographic isolation due to widening of the retroarc fold-thrust belt. The youngest detrital zircon U-Pb ages confirm timing of Maastrichtian-Eocene strata but require substantial age revisions for part of the overlying Cenozoic basinfill during the late Eocene and Oligocene. The upper Río Turbio Formation, previously mapped as middle to late Eocene in the published literature, records a newly recognized latest Eocene-Oligocene (37-27 Ma) marine incursion along the basin margin. We suggest that these deposits could be genetically linked to the distally placed units along the Atlantic coast, including the El Huemul Formation and the younger San Julián Formation, via an eastward deepening within the foreland basin system that culminated in a basin-wide Oligocene marine incursion in the Southern Andes. The overlying Río Guillermo Formation records onset of tectonically generated coarse-grained detritus ca. 24.3 Ma and a transition to the first fully nonmarine conditions on the proximal Patagonian platform since Late Cretaceous time, perhaps signaling a Cordilleran-scale upper plate response to increased plate convergence and tectonic plate reorganization.
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.
ABSTRACT. 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. RESUMEN. Depósitos estuarinos en la Formación Río Baguales (Chattiano-Aquitaniano), Provincia de Magallanes, Chile. Hay muy pocos trabajos sobre la estratigrafía y sedimentología de la Formación Río Baguales del Oligoceno superior-Mioceno inferior en la Provincia de Magallanes del sur de Chile, luego de su primera definición en 1957. Se presenta un perfil estratigráfico detallado de la parte superior de la formación al oeste del río Baguales, con una interpretación de los ambientes de depositación. Se identificó una costa progradante con facies de boca de estuario, medio de estuario (laguna) y delta de cabecera. Las grandes capas frontales del talud deltaico indican progradación hacia el norte y noroeste, mientras que la presencia de laminación cruzada inclinada hacia el sureste sugiere la acción de oleaje. La sucesión en general refleja una caída gradual del nivel del mar, acompañada por subsidencia tectónica con la creación del espacio de acomodación necesario para la acumulación de potentes facies mareales y submareales. Un período de transgresión hacia la parte superior de la sucesión indica una subida del nivel del mar o una aceleración en la tasa de subsidencia tectónica, seguida por una regresión mayor que comenzó alrededor de 23,3 Ma, probablemente coincidente con la apertura del Pasaje de Drake y el crecimiento de la calota de hielo de la Antártica del este. Oligoceno-Mioceneo, Polykladichnus. Palabras clave: Estuario, Delta tipo Gilbert, Cambios de nivel del mar del
RESUMENArqueros, Algodones y Rodaíto son tres distritos argentíferos del norte de Chile que tuvieron gran importancia económica en el siglo XIX. Dado que fueron intensamente explotados en el siglo pasado, en la actualidad el principal testimonio mineralógico lo constituye la Colección Mineralógica Ignacio Domeyko del Museo Mineralógico de La Serena (Chile).El estudio de esta colección, junto con la revisión de la geología distrital de estos yacimientos, permite establecer las siguientes conclusiones: 1) los tres distritos son mineralizaciones filoDianas epitermales encajadas en rocas volcánicas y sedimentarias del Cretácico, y son asignables a los modelos de yacimientos argentíferos tipo «Comstock» o «Creede»; 2) las paragénesis estudiadas pueden asimilarse a la asociación Cu-Pb-Zn-Ag (e.g. Creede, Colorado, USA; Zacatecas, México); 3) la existencia de skutterudita y eritrina sugiere, sin embargo, un cierto carácter transicional respecto a otras asociaciones: Ag-Bi-Co-Ni-As (e.g. Cobalt, Ontario, Canadá) y Pb-Zn-Ag (Sb-As) (Coeur d'Alene, Idaho, USA: Freiberg, Alemania). Palabras clave: Colección mineralógica, Chile, plata y yacimientos epitermales.ABSTRACT Arqueros, Algodones and Rodaíto are silver districts from northern Chile that were of major economic importance during the XIX century. Since the deposits were extensively mined during the last century, at present, the main mineralogical record is kept at the «Co-lección Ignacio Domeyko», a major collection belonging to the Mineralogical Museum of La Serena (Chile).The study of the collection together with a revision of the local geology of the districts led to the following conclusions: 1) the districts are formed by Comstock/Creede-type epithermal silver-base metals deposits, which are hosted by volcanic and sedimentary rocks of Cretaceous age: 2) the studied parageneses can be ascribed to the Cu-Pb-Zn-Ag association (e.g. Creede, Colorado, USA; Zacatecas, Mexico); 3) the presence of skutterudite and erythrite suggests, however, a transitional character towards other mineralogical associations: Ag-Bi-Co-Ni-As (e.g. Cobalt, Ontario, Canada) and Pb-Zn-Ag (Sb-As) (Coeur d'Alene, Idaho, USA).
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