A multidisciplinary analysis of palynofacies in a sequence-stratigraphical framework, TOC (total organic carbon) and TAI (thermal alteration index) analyses, has been applied to outcrop samples of the uppermost Los Molles Formation (Middle Jurassic), Neuquén Basin, Argentina, in order to characterise the palaeoenvironmental and palaeoclimatic conditions during the deposition of this unit.Five types of palynofacies (P-1 to P-5) have been identified and are interpreted to indicate a restricted marine to inner neritic environment.In P-1 and P-4, a marine environment close to the terrestrial source area with moderate oxidizing conditions and energy, is suggested.In P-2, the assemblage of freshwater algae, acritarchs and prasinophytes suggest a marginal marine environment influenced by fluvial discharge.In P-3, the marine microplankton content reflects conditions ranging from a marginal-marine (sub-normal salinity) to an inner neritic environment.In P-5, a dysoxic marine environment (probably marginal) is suggested.Warm and relatively humid (abundance of Cheirolepidiaceae in association with Araucariaceae) and locally humid (presence of swamps or ponds, on the alluvial plains) conditions are inferred.TOC content (65% of the analyzed samples reach TOC values over 1%), TAI values (2 to 2+, transitional between an immature phase and the window of liquid petroleum generation) and kerogen type (P-1, P-2 and P-5 show transitional characteristics between kerogen type II and III, while P-3 and P-4 show characteristics of kerogen type III and occasionally type IV), suggest that, in the studied area, the Los Molles Formation has some hydrocarbon potential.However, kerogen state suggests that the anoxic-dysoxic conditions of the site of deposition were not optimum for its preservation.
RESUMEN.El estudio geológico y estructural detallado de sedimentos recientes en el valle del río Maipo (Cajón del Maipo), al este de Santiago, muestra la existencia de una deformación en compresión con fallas inversas y pliegues de arrastre contemporáneos.Estas estructuras son cinemáticamente compatibles con una dirección de esfuerzo cercana a N-S (N338°E) que afecta también a las unidades de roca más antiguas (Formación Abanico), que constituyen el sustrato de las terrazas aluviales cuaternarias.En estas rocas del sustrato, se superponen dos episodios de deformación, el primero de edad neógena (post 9 Ma), caracterizado por un σ1 de dirección E-O; el segundo cuaternario y con un σ1 cercano a N-S.Este último episodio de deformación resultaría de la partición regional del vector de convergencia oblicua entre las placas de Nazca y Sudamérica; un 'buttress' a los 33°S produciría el gradiente de velocidad latitudinal necesario en el antearco para conducir a un acortamiento norte-sur.La sismicidad cortical superficial en la parte interna del antearco (Cordillera Principal) es -al menos en parte-compatible con la dirección
Structural and microstructural observations combined with apatite and zircon fission-track thermochronotogy within two sectors of the Main Andean Range in the Los Lagos Region of Chile reveal an episodic history of intrusion and deformation in the North Patagonian Batholith (NPB). A dextral displacement of similar to 30 km along the Liquine-Ofqui fault zone (LOFZ) is inferred from the correlation of corresponding Cretaceous intrusions of the NPB across the fault zone at Reloncavi. Relative uplift of the western block in the late Miocene is indicated by apatite and zircon fission-track cooling histories. Microstructures in samples from Miocene and Cretaceous plutons along the fault zone generally indicate deformation at temperatures below similar to 300 degrees C, with the exception of some samples from Cretaceous intrusions showing deformation at higher temperatures. In the Hornopiren area, significant relative uplift of the eastern block is indicated by 1. the different styles of deformation observed across the fault zone, 2. kinematic analysis of a shear zone in tonalite and 3. geobarometry of contact metamorphic mineral assemblages. For the plutonic rocks in the Hornopiren area, extremely rapid cooling is indicated by thermochronometry, suggesting the activity of hydrothermal systems or thermal effects of late shallow intrusions.
RESUMEN.En el cinturón de Famatina, ubicado entre las provincias geológicas argentinas de Sierras Pampeanas y Precordillera, se describe y analiza una sucesión de conglomerados de ~400 m, considerada en trabajos previos como la sección superior del intervalo postglacial neopaleozoico del Grupo Paganzo (Pensilvaniano tardío).En este trabajo estos conglomerados (Formación Las Pircas nom.nov.), extensamente desarrollados en Famatina, son separados como una unidad litoestratigráfica diferente asociada a un episodio de deformación, sobre la base de: 1. la presencia de una discordancia angular en su base, 2. un fuerte contraste litofacial con la unidad glacial subyacente y 3. su organización estratigráfica interna.El análisis de facies indica un predominio de depósitos de abanicos aluviales proximales, que pasan en transición hacia sistemas fluviales entrelazados y desarrollo local
As the Beagle Channel lies between the Atlantic and the Pacific oceans, this is a critical region for the interpretation of faunal distributions in the Magellan Region. This work proposes a paleoenvironmental interpretation of the Holocene, inferred from malacological data. The development of different local benthic paleocommunities during the mid-Holocene is associated with the diversity of habitats inside this channel. The analysis of records of Venus antigua and Ensis macha through time and their geographic links to ecological factors showed that these taxa are typical elements of the cold-temperate seas throughout the Magellan Region, showing a similar range of distribution and paleodistribution. The presence of these taxa in life position (AMS 14C of 6,276±41 years BP) suggests that during the mid-Holocene the Beagle Channel was a path for dispersion of these taxa between the two oceans. Whether Venus antigua andEnsis macha are Pleistocene survivors or Holocene migrants is herein discussed. In the future, integrated cross-disciplinary studies will be necessary for a better understanding of the biogeographic relationships among magellanic mollusks.
Muchos de los episodios de metamorfismo y deformación asociados a las rocas de bajo grado del NO argentino fueron tradicionalmente vinculados con orógenos antiguos como el pampeano (Cámbrico) y el oclóyico (Ordovícico).En el cinturón del Famatina, la falta de dataciones precisas y de un análisis estratigráfico de detalle en las sucesiones con bajo grado, condujeron a interpretaciones ambiguas respecto a la evolución paleozoica del segmento sur de los Andes Centrales.Estudios recientes establecen que el 'basamento metamórfico' de bajo grado de dicha región estaría conformado por las formaciones Negro Peinado y Achavil, cuyos análisis de procedencia las vinculan con la finalización del ciclo pampeano.Por su parte, la Formación La Aguadita, también afectada por metamorfismo de bajo grado, correspondería a depósitos sinorogénicos ordovícicos (oclóyicos).En consecuencia, el conjunto sería más joven que la Formación Puncoviscana (prepampeana) con la que fue habitualmente correlacionado.Las edades K-Ar presentadas en este trabajo permiten establecer que el metamorfismo y la deformación de estas unidades son Ordovícicos y no más antiguos (Proterozoico-Cámbrico Medio) como se interpretó durante muchos años.Edades de 457±9 Ma (roca total) y 463±14 Ma (fracción <2µm) obtenidas para la Formación Negro Peinado, permiten su vinculación con el clímax de la orogenia oclóyica, desarrollada en el margen occidental de Gondwana durante el Paleozoico inferior.Las edades de 435±12 Ma (fracción <2µm) en la Formación La Aguadita y de 444±8 Ma (roca total) en filonitas de la faja de deformación Angulos, son consistentes con la extensión de este ciclo orogénico hasta el Silúrico Inferior.Adicionalmente, se obtuvo una edad de 359±7 Ma (fracción <2µm) en rocas de la Formación Negro Peinado (faja de deformación Las Trancas), que evidencia un reequilibrio local del sistema isotópico K-Ar parcialmente coincidente con el rango temporal asignado para la orogenia achaliana (Silúrico Tardío-Carbonífero temprano).
Stelloglyphus llicoensis isp. nov. is a large radial, discoidal to ellipsoidal trace fossil with a central shaft and single to bifurcating branches radiating from different levels. A 30 m thick measured section of the Ranquil For- mation at Punta Litre contains an associated trace fossil assemblage including Zoophycos, Chondrites, Phycosiphon, Nereites missouriensis, Lockeia siliquaria, Psammichnites(?), Parataenidium, Ophiomorpha, and Rhizocorallium, some of which reworked the Stelloglyphus traces. The sedimentology, together with micro- and macrofossils and the associated trace fossil assemblage, suggest that the succession was deposited in an outer continental shelf to slope environment in subtropical to tropical waters.
Many of the metamorphic and deformational events associated to low-grade units in NW Argentina have been linked with ancient orogenies, like the Pampean (Cambrian) and the Ocloyic (Ordovician) cycles. The lack of specific ages in the low-grade metamorphic rocks of the Famatina belt, as well as the absence of a detailed stratigraphic analysis, have led to ambiguous interpretations respect to the Paleozoic evolution of the southern segment of the Central Andes. Recent work allows recognition of two units within the low-grade 'metamorphic basement'; the Negro Peinado and the Achavil formations. Provenance analysis relates deposition of both units to the final stages of the Pampean cycle during the Cambrian, whereas an Ordovician (Ocloyic) synorogenic nature is suggested for the La Aguadita Formation, also traditionally included within the low-grade 'metamorphic basement'. Thus, as a whole, these units are younger than the traditionally correlated pre-Pampean Puncoviscana Fort nation. K-Ar ages obtained for Famatina indicate that low-grade metamorphism and deformation of these units are Ordovician, in contrast to a much older age (Proterozoic-Middle Cambrian) previously suggested. Ages of 457 +/- 9 Ma (whole rock) and 463 +/- 14 Ma (<2 mu m fraction) for the Negro Peinado Formation allow establishing a link with the climax of the Ocloyic Orogeny developed along western Gondwana during the early Paleozoic. Ages of 435 +/- 12 Ma (<2 mu m fraction) in the La Aguadita Formation and of 444 +/- 8 Ma (whole rock) in phyllonites of the Angulos shear zone are consistent with an extent into the earliest Silurian for this cycle. Additionally, a 359 +/- 7 Ma (<2 mu m fraction) age for the Negro Peinado Formation (Las Trancas shear zone) indicates local resetting within the age range assigned to the Achalian Orogeny (Late Silurian-early Carboniferous).
This work documents fault activity and the Neogene's strain field in northern Chilean Coastal Cordillera. Fault activity is expressed as a group of fault scarps and fault-bend fold scarps whose orientation defines three main domains WNW-ESE, N-S and NNW-SSE. The WNW-ESE and N-S faults show reverse kinematics, and NNW-SSE faults shows dextral-reverse kinematics. Exposure ages using cosmogenic Ne-21 show that the faults disrupt an Oligocene-Miocene landscape preserved at the Coastal Cordillera. Inactive valleys incised in this landscape are offset by the faults showing that faults were active after 4 and 2 Ma. Ar-40/Ar-39 chronology of displaced volcanic tuffs and the deformation of Late Pleistocene sediments indicate that fault activity remain still active during the Quaternary. The deformation regime is constrictional and characterized by subhorizontal shortening in all directions, that is explained by oblique convergence along an active curved continental margin.
Ar-40/Ar-39 age data (laser and furnace step heating) on plagioclase from Lower Cretaceous volcanic sequences from the Arqueros Formation in two sections of the Coastal Range at the latitude of La Serena (approximate to 29 degrees S) have been obtained. Due to the partial alteration of plagioclase crystals, disturbed age spectra in the furnace experiments have been observed, whereas laser heating determinations involving a much smaller quantity of grains carefully selected, could display plateau ages corresponding to pure plagioclase, as demonstrated by a constant Ar-37(Ca)/Ar-39(K) ratio. Plateau ages of 114.1 +/- 0.5 Ma (sample ARQ99-4), 111.3 +/- 0.9 Ma (sample TC99-5a), and 91.0 +/- 0.6 Ma (sample TC99-2) were found in lava flows, and 84.3 +/- 1.3 Ma on a dyke (sample ARQ99-7). These new Ar-40/Ar-39 ages, together with those previously published in central Chile, allow a constriction of the extensional magmatism during the Early Cretaceous in the Coastal Range of central and north-central Chile. All these data are in accordance with a long lived Early Cretaceous Magmatic Province (119-84 Ma), that could have started with a brief and huge magmatic event, mostly developed in the central part of the Coastal Range, followed by discrete magmatic pulses at further northern latitudes.
In the present study, we describe and analyze the structure of a Late Pleistocene (likely last interglacial) marine macrobenthic assemblage in Caleta Patillos (20°45'S, 70°12'W), northern Chile. A taphonomic analysis sug- gests the existence of a shallow subtidal paleonvironment, mainly soft-bottom, of high energy and intense mixing from several benthic habitats. The total number of 38 taxa recorded, mainly gastropods and bivalves, was not different than the ones reported in other Late Pleistocene assemblages in northern Chile. At a biogeographic scale, the composition of mollusk species showed remarkable similarities with present-day fauna, and no extralimital species were recorded. At local scale, however, a comparison with the living assemblage in the same area of study showed a dramatic shift in the species' composition, a result that cannot be explained by sampling bias. A deep and perhaps very recent (i.e., at historical times) alteration of the structure of local macrobenthic assemblages is hypothesized.
The Coastal Ranges in the western part of the Chiloe Archipelago represent an emerged forearc high at the subduction front of south-central Chile. Prior to the Cenozoic framework of the subducting Farallon and Nazca plates beneath the South American plate, the history of the metamorphic basement in the Coastal Ranges involves episodes of subduction and/or accretion of oceanic and ensialic material along the proto-Pacific margin of Gondwana. Along different segments of the Main Chiloe Island the thermal record of the metamorphic basement rocks includes regional metamorphism, exhumation and finally magmatism. Detrital zircons conventional U-Pb ages from a pelitic schist of the Central Segment of the Main Chiloe Island constrain the maximum possible sedimentation age to the Carboniferous (310 Ma). Other concordant zircon fractions (360, 390 and 412 Ma) suggest Ordovician primary sources. The sedimentary provenance of zircons could correspond to continental magmatic rocks of similar ages presently exposed in adjacent areas such as Nahuelbuta Mountains and Principal Cordillera and, additionally, from distal areas in the North-Patagonian and Deseado massifs. Widespread metamorphism in greenschist facies is well represented in the basement unit of the Main Chiloe Island. K-Ar and Ar-40/Ar-39 cooling ages in white mica range between 245 to 220 Ma, interpreted as near the peak of metamorphism during Middle to early Late Triassic times. Zircon and apatite fission-track (FT) central ages in metamorphic rocks range from Late Jurassic (ca. 156 Ma) to Eocene (ca. 38 Ma) and are accompanied by relatively large dispersions. The modelling of the zircon FT single-grain age distributions depicts younger deconvoluted Late Cretaceous peak (ca. 80 Ma) and the apatite modelling shows two populations, an older Late Cretaceous peak (ca. 64-91 Ma) and a younger Eocene peak (ca. 38-53 Ma). The zircon and apatite Late Cretaceous peaks together with the presence of a marine sedimentary succession of presumably later Late Cretaceous age found at the forearc slope of the Main Chiloe Island, suggest exhumation of the metamorphic unit during the early Late Cretaceous. Magmatic zircons from a granodioritic body (Metalqui Pluton) emplaced in pelitic schists within the Central Segment of the Main Chiloe Island, a differentially uplifted block in relation to the northern and southern segments, gave an Eocene U-Pb crystallization age (39.6 +/- 0.3 Ma). Upper Eocene (ca. 37 Ma) biotite-bearing porphyric dacitic sills and dikes (Gamboa Dacite) also occur within the Central Segment. Zircon and apatite FT concordant ages (ca. 36 Ma) indicated rapid cooling for these subvolcanic rocks during Eocene times. According to this data, Eocene apatite central and modelling single-grain FT ages detected in the metamorphic rocks probably represent thermal resetting by shallow magmatism, coeval with the Gamboa Dacite and possibly the Metalqui Pluton.
In the Famatina belt, western Argentina, a similar to 400 m thick conglomerate succession is extensively developed and has been previously considered as the uppermost section of a postglacial interval (late Pennsylvanian) of the Paganzo Group. Here we separate it as a different lithostratigraphic unit (Las Pircas Formation, nom. nov.) with a significant tectonic meaning based on: 1. the angular discordance at its base; 2. the strong litofacial contrast with the underlying glacial unit and 3. its internal stratigraphic organization. Facies analyses indicate topographically controlled proximal alluvial fans succeeded by braided fluvial systems, and local development of lacustrine deposits. Granite bearing-megaboulder beds and conglomerates indicate significant basement exhumation and tectonic unroofing. Clast composition also depicts early Pennsylvanian sandstones and Ordovician volcanic rocks mainly exposed in the central part of Famatina (at present). Paleocurrent analyses are compatible with a dispersal pattern mostly to the west. The position of basement topographic high further east from the main contemporaneous arc together with evidences of folding allow us to interpret previously unrecognized basement thrusting within the foreland. This may have been the main source for the westward coarse arkosic elastic wedge known in the Argentine Precordillera as the Tupe Formation and equivalents. This late Pennsylvanian-Early Permian foreland model can be compared with the modem broken foreland of the Central Andes.
A detailed study of recent sediments in the Maipo River Valley (Cajon del Maipo), east of Santiago, shows a compressive deformation documented by coeval reverse faults and drag-folds. These structures are characteristic of a principal stress direction trending roughly N-S (N338 degrees E) that also affects the older sediments of the Abanico Formation constituting the bedrock of the Quaternary alluvial terraces. Two deformation episodes are superposed in the bedrock: a Neogene deformation event (post 9 Ma), having an E-W-trending sigma 1, and a late Quaternary event with sigma 1 roughly trending N-S. The latter would be the result of regional-scale partitioning of the oblique convergence vector between the Nazca and South American Plates; a buttress at 33 degrees S would produce a north-south velocity gradient in the forearc leading to regional, north-south shortening. Shallow crustal seismicity in the inner forearc (Main Cordillera) is -at least in part- compatible with the compression direction as obtained from the kinematic analysis of mesoscopic fault-slip data.
In this contribution, we present a reassessment of the Pular Formation age (Sierra de Almeida, Antofagasta) that in previous papers was attributed to either the Late Paleozoic or Triassic.At Quebrada del Medio, the Pular Formation includes a 379-meter-thick stratigraphic section formed from base to top by conglomerates, sandstones, black bituminous shales and limestones, tuffaceous sandstones and a poorly welded ignimbrite that accumulated in fl uvial and lacustrine environments.A multidisciplinary study of the fl ora and fauna was undertaken.The existence of diagnostic Gondwana microfl ora and macrofl ora allows attributing a Middle Triassic to early Late Triassic age to the Pular Formation.
A methodology for determining long-term denudation rates from morphologic markers in a Digital Eleva- tion Model (DEM) is checked by a comparative study of two drainage basins in the Precordillera of the Central Andes. In both cases the initial confi guration of an incised pediment surface has been restored by using two different methods: the Black Top Hat (BTH) function and the Inverse Distance Weighting (IDW) interpolation. Where vertical incision and hillslope erosion are recorded, the IDW appears to be the most adequate to reconstitute the pediment surfaces. Conversely, where only vertical incision is observed, the BTH describes more precisely the former pediment surfaces and it is easier to solve. By subtracting the DEM from the reconstructed marker we calculated an eroded volume, and estimated its uncertainty by considering Root Mean Square Error (RMSE) and DEM grid error. For the last ~10 Myr we obtained long-term denudation rates of 7.33±1.6 m/Myr in the San Andres drainage basin and 13.59±1.9 m/Myr in the El Salado drainage basin. These estimations are largely in agreement with other reported estimates of long-term denudation rates in the Atacama Desert. Comparison with long-term denudation rates reported in a wide range of climatic regimes suggests that our estimates cannot be explained by the current rainfall in the Precordillera. However they could be explained by a rainfall similar to that reported 40 km to the east in the Puna. This suggests that during the time span concerned the geomorphologic evolution of the study area, this evolution is dominated by an orographically controlled rainfall pattern. The preserved pediment surface and the small long term denudation rates determined in this study also indicate that the Precordillera was never reached by humid tropical air masses and precipitation as currently observed in the Altiplano during the summer months.
New field data, specially the discovery of the contact between Upper Cretaceous-Paleocene rock units in a previously unexplored area in southeast Tierra del Fuego, allowed us to estimate the amount of strike-slip displacement accommodated by the Fagnano Transform System (FTS). This stratigraphic contact is exposed both to the north and south of the FTS and it is deformed by a NW-SE compressive regional structure that forms part of the Fuegian thrust-fold belt, called the Cerro Piramide-Cerro Malvinera Thrust (PMT). The compressive structure is laterally offset by the FTS. The precise mapping of the PMT and the Upper Cretaceous-Paleocene contact shows a horizontal offset of ca. 48 km along the transform system in the Isla Grande de Tierra del Fuego. A maximum age of similar to 7 Ma for the beginning of the strike-slip in this sector is estimated by combining the obtained offset with published data on the slip-rate along the FTS in Tierra del Fuego. This Late Miocene age coincides with the creation of the divergent plate boundary between Sandwich and Scotia Plates, which has been proposed as responsible for the beginning of the strike-slip activity between South America and Scotia Plates.
We describe sixteen theropod dinosaur trackways from site III (Lower Cretaceous?) of the Quebrada Cha- carilla in the Chacarilla Formation (Jurassic-Lower Cretaceous), northern Chile. The trackbed belongs to a meandering river environment, with recurrent track assemblages. We fi nd that the main direction of the ichnites from site III parallels that of water fl ow. Two theropod footprint morphotypes are recognized: A) digit II and IV occupying about 0.5 times the total footprint length, and B) digit II and IV occupying less than 0.35 times the total footprint length. We estimate similar speed (~7 km/h) for the two parallel theropod trackways of morphotype A, suggesting synchro nized walking. We also describe some of the largest theropod footprints from South America (>60 cm), which are all of morphotype B.
