Los Plesiosaurios (Reptilia, Sauropterygia) más antiguos del Cretácico Superior bajo de la Patagonia Austral, Argentina

Plesiosaurs are recorded for the first time from the lower section of Mata Amarilla Formation, Santa Cruz Province, Patagonia, Argentina. The stratigraphic succession consists of mudstones and siltstones interbedded with medium to fine-grained sandstone, deposited in a littoral environment during the Cenomanian-Santonian; therefore the material is the oldest record of plesiosaurs from the lower Late Cretaceous rocks of Argentina. The remains include teeth, some vertebrae, and one propodium assigned to Elasmosauridae indet. and Plesiosauria indet. The status of Polyptychodon patagonicus Ameghino, 1893, as well as its stratigraphic position are discussed, leading to the conclusion that the material described by Ameghino is probably from the Mata Amarilla Formation and can only be referred to Plesiosauria indet. Analysis of sedimentologic features suggests that the material described here was deposited in an estuarine environment, strongly influenced by tides. The characters of the inferred environment are consistent with the type of preservation of the materials.Se dan a conocer los primeros registros claramente referibles a plesiosaurios de la sección inferior de la Formación Mata Amarilla en la provincia de Santa Cruz, Argentina. Los niveles portadores están compuestos por limolitas y arcillitas grises y negruzcas, que alternan con bancos de areniscas finas y medianas. Dichos niveles se depositaron en un ambiente litoral durante el intervalo Cenomaniano-Santoniano, siendo por tanto el registro más antiguo de plesiosaurios del Cretácico Superior de Argentina. Los restos hallados corresponden a dientes, vértebras y un propodio asignados a Elasmosauridae indet. y a Plesiosauria indet. Se discute el estatus de Polyptychodon patagonicus Ameghino, 1893 y su posición estratigráfica, concluyendo que el material descripto por Ameghino es sólo referible a Plesiosauria indet. y que proviene de la sección inferior de la Formación Mata Amarilla. El análisis sedimentológico indica que los materiales que se describen aquí fueron depositados en un ambiente estuarino con fuerte influencia mareal. Las características del ambiente de depositación de esta sección son consistentes con el estado de preservación de los materiales.Facultad de Ciencias Naturales y MuseoCentro de Investigaciones Geológica

Los Plesiosaurios (Reptilia, Sauropterygia) más antiguos del Cretácico Superior bajo de la Patagonia Austral, Argentina

Plesiosaurs are recorded for the first time from the lower section of Mata Amarilla Formation, Santa Cruz Province, Patagonia, Argentina. The stratigraphic succession consists of mudstones and siltstones interbedded with medium to fine-grained sandstone, deposited in a littoral environment during the Cenomanian-Santonian; therefore the material is the oldest record of plesiosaurs from the lower Late Cretaceous rocks of Argentina. The remains include teeth, some vertebrae, and one propodium assigned to Elasmosauridae indet. and Plesiosauria indet. The status of Polyptychodon patagonicus Ameghino, 1893, as well as its stratigraphic position are discussed, leading to the conclusion that the material described by Ameghino is probably from the Mata Amarilla Formation and can only be referred to Plesiosauria indet. Analysis of sedimentologic features suggests that the material described here was deposited in an estuarine environment, strongly influenced by tides. The characters of the inferred environment are consistent with the type of preservation of the materials.Se dan a conocer los primeros registros claramente referibles a plesiosaurios de la sección inferior de la Formación Mata Amarilla en la provincia de Santa Cruz, Argentina. Los niveles portadores están compuestos por limolitas y arcillitas grises y negruzcas, que alternan con bancos de areniscas finas y medianas. Dichos niveles se depositaron en un ambiente litoral durante el intervalo Cenomaniano-Santoniano, siendo por tanto el registro más antiguo de plesiosaurios del Cretácico Superior de Argentina. Los restos hallados corresponden a dientes, vértebras y un propodio asignados a Elasmosauridae indet. y a Plesiosauria indet. Se discute el estatus de Polyptychodon patagonicus Ameghino, 1893 y su posición estratigráfica, concluyendo que el material descripto por Ameghino es sólo referible a Plesiosauria indet. y que proviene de la sección inferior de la Formación Mata Amarilla. El análisis sedimentológico indica que los materiales que se describen aquí fueron depositados en un ambiente estuarino con fuerte influencia mareal. Las características del ambiente de depositación de esta sección son consistentes con el estado de preservación de los materiales.Facultad de Ciencias Naturales y MuseoCentro de Investigaciones Geológica

Depositional processes and stratigraphic evolution of the Campanian deltaic system of La Anita Formation, Austral-Magallanes Basin, Patagonia, Argentina

Coastal depositional systems are commonly classified in terms of the relative interaction of wave, tide and fluvial processes. The La Anita Formation represents the opportunity to study and better understand coastal sedimentary systems. It is a poorly studied prograding siliciclastic deltaic-coastal wedge accumulated in the Campanian during the foreland stage of the Austral-Magallanes Basin. A detailed depositional process-based facies analysis have allowed the definition of 13 sedimentary facies and 9 facies associations for the La Anita Formation, ranging from prodelta to interdistributary delta-channel deposits. According to the spatial distribution of these facies associations, the La Anita Formation was divided into two informal units bounded by a regional erosion surface. The lower unit shows abundant hummocky cross-bedded and bioturbated sandstones, coarseningupward trends and mainly aggradational to progradational vertical stacking pattern, and it was interpreted as a wave-dominated fluvial-influenced delta. The upper unit is characterized by unidirectional dune cross-bedding, coarseningupward trend and a progradational vertical stacking pattern, and was interpreted as a fluvio-dominated delta with no evidence of tide or wave influence. These two units represent two genetically unrelated depositional sequences bounded by a regional erosion surface, which is interpreted as a sequence boundary triggered by a relative sea-level fall. The lower unit is part of a progradational highstand systems tract which involves the underlying deep-marine Alta Vista Formation. The upper unit deposits reflect a complete relative sea-level cycle which includes an undifferentiated lowstand and transgressive systems tracts and, toward the top, highstand systems tract.Fil: Moyano Paz, Damián. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Geológicas. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Centro de Investigaciones Geológicas; ArgentinaFil: Tettamanti, Camila. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Geológicas. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Centro de Investigaciones Geológicas; ArgentinaFil: Varela, Augusto Nicolás. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Geológicas. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Centro de Investigaciones Geológicas; ArgentinaFil: Cereceda, Abril. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Geológicas. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Centro de Investigaciones Geológicas; ArgentinaFil: Poire, Daniel Gustavo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Geológicas. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Centro de Investigaciones Geológicas; Argentin

Trace fossils as mechanical discontinuities in shales, insight for the generation of bedding-parallel veins (BPV)

Understanding shale petrophysical parameters is of interest due to its direct implications as cap rocks for CO2 or hydrogen storage, waste depositions, and as unconventional reservoirs. The generation and propagation of natural and induced fracture networks in such rocks is highly dependent on the mechanical behavior linked to several sedimentological parameters, as lithological discontinuities or bioturbation. This study is focused on a diferent sedimentological parameter that consists of trace fossils and their implication on the generation of fuid-assisted fractures, called bedding-parallel veins. In the Austral-Magallanes Basin, Southern Patagonia, Argentina, both geological features, Skolithos Ichnofacies (doomed pioneers trace fossils) and bedding-parallel veins, are numerous, especially at the top of the turbiditic bodies. The trace fossils exhibit U-shaped vertically oriented burrows composed of clean sandstone, partially cemented by calcite, and a spreite in the central part with heterogenous laminated siltstone. Bedding-parallel veins are composed of calcite fbers with some pyrite grains and bitumen. They are located on the top of the trace fossils along the lithological discontinuity between the turbiditic bodies and the impermeable shales. On their surfaces, a radial pattern starts growing from the trace fossils. Moreover, the number of bedding-parallel veins is dependent on the bioturbation intensity. With this study, we infer that trace fossils represent ichnological mechanical discontinuities (IMD) that have a key role in the generation and development of bedding-parallel veins. By correlation, we also suggest that these geological features must be thoroughly studied, especially regarding their potential for the development of induced fracturing networks.Fil: Zanella, Alain. Le Mans Université.; FranciaFil: Ruiz, Remigio. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. YPF - Tecnología; ArgentinaFil: Varela, Augusto Nicolás. YPF - Tecnología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Arregui, Mariano Germán. YPF - Tecnología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Evolución paleoambiental de la formación Río Mayer, Cretácico inferior, Cuenca Austral, provincia de Santa Cruz, Argentina

La Formación Río Mayer es parte del relleno inicial de la Cuenca Austral, la cual se ubica en el sector más austral de la Patagonia. El área de estudio se encuentra entre los lagos San Martín y Argentino (Provincia de Santa Cruz), donde se relevaron diez perfiles sedimentológicos de detalle en los que se describieron 18 facies sedimentarias. Estas facies responden a diferentes procesos sedimentarios, principalmente autigénicos, hemipelágicos y episódicos. A partir de la distribución de las facies y del origen del material fósil se reconocieron tres paleoambientes sedimentarios: marino de plataforma externa, marino de plataforma externa con influencia deltaica y prodelta. El análisis de la distribución espacial y temporal de las facies condujo a la elaboración de un esquema de evolución paleoambiental para el Cretácico Inferior de la Cuenca Austral en la zona estudiada. La depositación de la Formación Río Mayer, por encima de la Formación Springhill, en el área de la Seccional Río Guanaco, comenzó durante el Berriasiano. En el Valanginiano la transgresión generó los depósitos incluidos en la Formación Springhill en la región de Lago San Martín, mientras que en el intervalo Hauteriviano-Barremiano toda el área de estudio presentaba características de plataforma externa distal. Durante el lapso temporal Aptiano-Albiano, se produjo la instalación de un sistema deltaico (Formación Piedra Clavada / Kachaike) en el norte del área de estudio entre los lagos San Martín y Viedma. Para este intervalo temporal la Formación Río Mayer en las regiones de Lago San Martín y Estancia La Vega, representa un ambiente de prodelta. Paralelamente, en la región sur del área de estudio se produjo la sedimentación de los niveles incluidos en un ambiente de plataforma externa influenciada por flujos turbidíticos distales de frente deltaico. Finalmente durante el Albiano superior-Cenomaniano inferior se desarrolló una transgresión marina, que marca el comienzo de la etapa de antepaís de la Cuenca Austral. En la región de los lagos San Martín y Viedma, la transgresión está evidenciada por los depósitos litorales de la Formación Mata Amarilla, mientras que en la región de Río Guanaco es representada por los depósitos de la Formación Cerro Toro.The marine deposits of the Río Mayer Formation are part of the initial filling of the Austral Basin, which is developed in the southernmost part of South America, including the south of Chile and Argentina (southwestern Patagonia, Santa Cruz and Tierra del Fuego Provinces, Figs. 1, 2). The study area is located between the San Martín and Argentino lakes in the Santa Cruz Province (Fig. 3). The Río Mayer Formation (Fig. 2) was studied in three main localities, where detailed sedimentary logging was undertaken (Figs. 4, 5), taking into account the lithology, the primary sedimentary structures, the palaeontological content, rock bodies geometry, their orientation and the hierarchy as well as the contacts with underlying and overlying units. Eighteen sedimentary facies were defined (Figs. 6, 7; Table 1), which were grouped into three facies association representing different sedimentary palaeoenvironments, like marine outer platform (AF 1), marine outer platform influenced by a deltaic system (AF 2) and prodelta (AF 3) (Fig. 8, Tables 1, 2). The facies of the outer platform palaeoenvironment corresponds to the rocks traditionally assigned to the Río Mayer Formation, according to the available bibliography, and deposited by hemipelagic and authigenic processes interrupted by episodic sedimentation. The deposits of the outer platform influenced by a deltaic system palaeoenvironment are similar to the above mentioned but presents continental fossils that indicate a deltaic source. Finally, AF 3 of a prodelta palaeoenvironment is dominated by sandy/heterolithics levels associated with turbidity currents and hyperpycnal flows originated in a delta front. The best and most complete exposures of the unit were observed in the Río Guanaco locality, where palaeoenvironmental interpretations were performed, allowing the division of the 350 m-thick unit in three sections (Fig. 9). The lower section is composed mainly by black shales with both tabular and concretional marls levels (AF 1). Trace fossils are not recorded in this section, but ammonites, belemnites and bivalves are frequent. It has been interpreted as accumulated in a distal platform, under anoxic conditions. The middle section is characterized by bioturbated black marls and shales, with wellpreserved Zoophycos ichnofacies (Richiano et al., in press), occurring in a distal platform depositional environment. Body fossils are frequent, especially belemnites. The upper section is composed of massive black mudstones with intercalations of very fine- to fine-grained sandstones and less frequent conglomerates. In this section both debris flows and distal low-density turbidity current deposits were identified. Zoophycos ichnofacies and Ophiomorpha rudis ichnosubfacies were recognized in association with molds of petrified wood containing Teredolites isp. (Richiano et al., in press). The presence of turbulent currents and large trace fossils suggest a more oxygenated conditions. At Lago San Martín and Estancia La Vega localities the most significant characteristic is the presence of prodeltaic deposits in the transition between Río Mayer Formation and Piedra Clavada formations (Fig. 9). To sum up, after the Upper Jurassic rifting stage of the Austral Basin, the Springhill Formation represents the initial infill of the extensional structures. After that, during the Berriasian (Fig. 10a), the levels of the lower section of the Río Mayer Formation started to accumulate in the Río Guanaco area. During the Valanginian (Fig. 10b), an acceleration in the transgression generated the accumulation of the Springhill Formation in the Lago San Martín area while in the Río Guanaco region the middle section of the Río Mayer Formation developed. Between the Hauterivian and the Barremian all the study area was under distal platform conditions (Fig. 10c). A major change in the sedimentary systems of the Austral Basin during the Lower Cretaceous is the development of the deltaic Piedra Clavada Formation (Fig. 10d). This event generated two different palaeoenvironments in the Río Mayer Formation. Prodeltaic deposits are present to the north, while sediments of distal platform conditions with highly frequent sandy levels prevail southwards. Finally a new transgression indicates the beginning of the Cerro Toro and Mata Amarilla formations in the middle Albian- lower Cenomanian (Fig. 10e).Fil: Richiano, Sebastián Miguel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Geológicas. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Centro de Investigaciones Geológicas; Argentina. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo; ArgentinaFil: Varela, Augusto Nicolás. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Geológicas. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Centro de Investigaciones Geológicas; Argentina. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo; ArgentinaFil: Cereceda, Abril. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Geológicas. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Centro de Investigaciones Geológicas; Argentina. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo; ArgentinaFil: Poire, Daniel Gustavo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Geológicas. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Centro de Investigaciones Geológicas; Argentina. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo; Argentin

Salvinialean megaspores in the Late Cretaceous of southern Patagonia, Argentina

We report here two megaspores species related to the aquatic ferns of the Order Salviniales from the Late Cretaceous Mata Amarilla Formation (Austral Basin), southern Santa Cruz Province, Argentina. They were assigned to the species Arcellites disciformis and Balmeisporites cf. B. holodictyus. The presence of A. disciformis, in particular, is significant not only because it represents the first record for the Southern Hemisphere, indicating a bi-hemispheric distribution for the species, but it also increases the diversity of this genus in Patagonia. The new record enlarge the importance of water ferns in the Late Cretaceous landscapes of southern South America by adding the presence of a new species. The common occurrences of Arcellites and Balmeisporites, whether in shallow, fresh or brackish water facies, support a local aquatic paleoenvironment for the Mata Amarilla Formation, as was also suggested by the sedimentological evidence. They also allow referring the lower and middle levels of the Mata Amarilla Formation to the Chronostratigraphic Zone M3 (Albian-Cenomanian) defined for the cretaceous of Patagonia on the bases of its megaspore content.Centro de Investigaciones Geológica

Salvinialean megaspores in the Late Cretaceous of southern Patagonia, Argentina

We report here two megaspores species related to the aquatic ferns of the Order Salviniales from the Late Cretaceous Mata Amarilla Formation (Austral Basin), southern Santa Cruz Province, Argentina. They were assigned to the species Arcellites disciformis and Balmeisporites cf. B. holodictyus. The presence of A. disciformis, in particular, is significant not only because it represents the first record for the Southern Hemisphere, indicating a bi-hemispheric distribution for the species, but it also increases the diversity of this genus in Patagonia. The new record enlarge the importance of water ferns in the Late Cretaceous landscapes of southern South America by adding the presence of a new species. The common occurrences of Arcellites and Balmeisporites, whether in shallow, fresh or brackish water facies, support a local aquatic paleoenvironment for the Mata Amarilla Formation, as was also suggested by the sedimentological evidence. They also allow referring the lower and middle levels of the Mata Amarilla Formation to the Chronostratigraphic Zone M3 (Albian-Cenomanian) defined for the cretaceous of Patagonia on the bases of its megaspore content.Centro de Investigaciones Geológica

Diplodon CF. Colhuapiensis (Bivalvia, Hyriidae) in the Santa Cruz Formation (Early-Middle Miocene) at the Río Santa Cruz, Patagonia, Argentina. Stratigraphic and paleoenvironmental considerations

La Formación Santa Cruz (Mioceno Temprano–Medio), es una de las unidades sedimentarias más extendidas de la Patagonia argentina. Esta unidad contiene una gran abundancia y diversidad taxonómica de vertebrados fósiles, especialmente de mamíferos. De esta manera, la información paleoecológica y paleoambiental proviene principalmente del análisis de la asociación de vertebrados, como también la evidencia icnológica y paleobotánica. El registro de bivalvos de agua dulce asignados a la especie Diplodon cf. colhuapiensis Ihering, 1903 de la Formación Santa Cruz, recolectado en la localidad Barrancas Blancas (Estancia Santa Lucía), cerca del Río Santa Cruz, permite inferir las condiciones paleoambientales de la depositación de sedimentos de los niveles portadores de los especímenes. Teniendo en cuenta estos restos, proponemos que el material de Diplodon originalmente asignado al “piso Sehuenense” (piso sehuenense de F. Ameghino), podrían provenir del Mioceno Temprano–Medio de la Formación Santa Cruz. En este sentido, el registro de Diplodon cf. colhuapiensis confirma la existencia de una población establecida de moluscos de la familia Hyriidae en los niveles medio-altos de la Formación Santa Cruz. La presencia de bivalvos de agua dulce, probablemente habitando canales fluviales, sugiere la existencia de cursos de agua bien desarrollados en el ambiente depositacional de la unidad. La identificación del género en la Formación Santa Cruz valida su presencia en el Mioceno Temprano y extiende su distribución sur a la latitud actual del Río Santa Cruz (~ 50o S).The Santa Cruz Formation (Early-Middle Miocene) is one of the most widespread sedimentary units of the Argentine Patagonia. This unit contains an abundant and taxonomically diverse fossil vertebrate fauna, especially in mammals. Thus, the paleoecological and paleoenvironmental information derives mainly from the analysis of the vertebrate assemblages, as well as from the ichnological and paleobotanical evidence. The record of freshwater bivalves assigned to the species Diplodon cf. colhuapiensis Ihering, 1903 from the Santa Cruz Formation, collected in the locality of Barrancas Blancas (Estancia Santa Lucía), at Río Santa Cruz, allows us to infer the particular paleoenvironmental conditions setting during the deposition of the bearing levels. Considering this record, we propose that Diplodon, which was originally assigned to the “Sehuenense stage” (piso sehuenense of F. Ameghino), could have come from the Early-Middle Miocene of the Santa Cruz Formation. In this sense, the specimens referred to Diplodon cf. colhuapiensis suggest the existence of an established community of Hyriidae mollusks at the upper-middle levels of the Santa Cruz Formation. The presence of freshwater bivalves suggests that the depositional environment of this unit included the existence of water courses. The identification of the genus in the Santa Cruz Formation validates its presence in the Early Miocene and extends its southern distribution to the latitude of Río Santa Cruz (~ 50ºS).Fil: Pérez, Leandro Martín. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. División Paleozoología Invertebrados; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata; ArgentinaFil: Cuitiño, José Ignacio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Centro Nacional Patagónico. Instituto Patagónico de Geología y Paleontología; ArgentinaFil: Varela, Augusto Nicolás. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Geológicas. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Centro de Investigaciones Geológicas; ArgentinaFil: Muñoz, Nahuel Antu. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. División Paleontología Vertebrados; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata; Argentin

Palaeoenvironmental evolution of Holocene littoral ridges during a sea level fall at Bahía Samborombón, Buenos Aires, Argentina

Las grandes fluctuaciones climáticas cíclicas ocurridas durante el Cuaternario tuvieron un efecto muy marcado en las regiones costeras, afectando principalmente el ambiente geomorfológico y la sedimentación de los depósitos allí acumulados. El mejor entendimiento de los sistemas depositacionales litorales generados en el pasado más reciente representa una herramienta fundamental para reconstruir y comprender otros depósitos similares del registro geológico, así como para desarrollar predicciones sobre posibles escenarios de cambios climáticos futuros. En el área costera de la Bahía Samborombón, noreste de la provincia de Buenos Aires, se encuentran preservados excelentes depósitos litorales acumulados con posterioridad al Último Máximo Glacial, durante la transgresión holocena. Sobre la base del análisis de facies se reconocieron cinco unidades de acumulación: cordón arenoso, cordón bioclástico, depósitos de sobrelavado, planicie de mareas y albufera. El análisis arquitectural y la jerarquización de superficies de discontinuidad permitieron elaborar un modelo de acumulación dinámico de los cordones de playa y ambientes asociados en diferentes etapas durante el Holoceno (ca. 8000 a A.P.- presente). El análisis estratigráfico secuencial permitió establecer la interacción de dos controles alocíclicos predominantes: eustático (regresión forzada) y climático (Óptimo Climático del Holoceno medio). La interacción de ambos factores extrínsecos no sólo condicionó la acumulación y amalgamación de geoformas cordoniformes, sino también, la relación entre aporte terrígeno y productividad carbonática de los depósitos.In the Argentinean littoral zone several transgressive - regressive events took place during the Quaternary. Excellent examples of these events are recorded in the Bahía Samborombón coastal area in northeastern Buenos Aires province (Fig. 1). Following the stratigraphic scheme of Fidalgo et al. (1973) modified by Fucks et al. (2010), deposits of the Canal de las Escobas Formation (Holocene) were analyzed (Figs. 2, 3). The Canal de las Escobas Formation was discriminated into 4 members (Fucks et al., 2010), the Destacamento Río Salado Member (tidal flat, 5.8-7 ka B.P.), the Cerro de la Gloria Member (beach ridge, ca. 3-8 ka B.P.), the Canal 18 Member (coastal plain, 6-7 ka B.P.) and the Canal 15 Member (coastal plain, 3 ka B.P. to present). Fourteen detailed sedimentary sections were logged in the study area (Fig. 4), taking into account lithology, primary sedimentary structures, geometries of the sedimentary bodies with their orientation and the hierarchy of the bounding surfaces, as well as the palaeontological (predominantly molluscan) content. Thirteen sedimentary facies were defined (Figs. 5, 6) which were grouped into three major divisions: siliciclastic sandstones, fine-grained siliciclastic and carbonatic bioclastic rocks. The excellent preservation of the Canal de las Escobas Formation in the study area allowed the recognition of five sedimentary units based on the spatial facies arrangement and the hierarchy of internal surfaces. These units are: sand and bioclastic ridge, washover, lagoon and coastal plain units (Figs. 7, 8, 9, 10, 11). In order to determine the palaeoenvironmental evolution, based on the stacking patterns of the sedimentary units together with the hierarchy of the sedimentary discontinuity surfaces, six stages were devised after the maximum Holocene transgression (Fig. 12). Stages 1 and 2 correspond to the development of a sand ridge as a response to the erosion and reworking of a previous spit system (Fig. 3; Facies Pinamar). Subsequently, during stages 3 to 5, a bioclastic ridge was amalgamated with the sand ridge (Fig. 12). Simultaneously to these five stages, in a landward position, lagoon and washover deposits were developed (Fig. 12). Finally, during stage 6 the continuous sea level fall generated the actual coastal plain deposits (Fig. 12). The sudden change in the ridge deposits composition, from sandy sediments to almost exclusively bioclastic (carbonate) between stages 2 and 3 can be interpreted as the combined result of an increase in carbonate productivity along with a decrease of siliciclastic supply together the coast. This stage would have been developed approximately 5-6 ka B.P. (Marine Isotope Stage, MIS, 1), in coincidence with the Holocene Climatic Optimum or Mid-Holocene Thermal Maximum (sensu Briner et al., 2006) during which the very special climatic conditions may have led to the proliferation of large communities of benthic organisms, particularly bivalve mollusks, which are not so abundant today. The high occurrence of Mactra isabelleana in the beach ridges of Bahía Samborombón, including all dimensions and ontogenetic stages and excellent preservation of the shells, indicates that this bivalve represents an autochthonous element of the coastal native communities, suggesting high adaptive radiation of this species in the area and time span considered, when atmospheric-oceanic circulation patterns / changes in the prevalent shallow oceanic currents are documented by different sources of evidence (palaeobiodiversity, palaeobiogeography, stable isotopes) and the warm Brazilian current (moving southwards) prevailed over the cold Malvinas current (flowing northwards) (Fig. 3). This change in the intensities of the ocean currents would have originated a dual effect: firstly, a greater contribution of warm-temperature waters to the bay and secondly, the change in the terrigenous input. The contribution of sand, due to erosion and reworking of the previous spit facies, would have drastically decreased, whereas the contribution of terrigenous may have been restricted from the north coming from the Río de la Plata (Fig. 3). Nevertheless, no changes in composition and/or texture were observed in siliciclastic material between the sand and bioclastic ridges units. This would be related to the sort of material transported by the Río de la Plata, which is mostly pelitic and potentially washed by tidal activity as it is well documented for similar Quaternary deposits from other areas (Meldahl, 1995). Considering the whole dataset gathered, we propose an alternative sequence stratigraphic scheme for the Holocene ridge development from Bahía Samborombón (Fig. 13). This ridge is now considered originated as a consequence of a forced regression (ie. part of a falling stage system tract) instead of the previously interpretation during the late stage of sea level rise (ie. part of a highstand system tract). Both eustatic and climatic controls can be recognized, although with different orders of magnitude, which controlled the genesis of the studied deposits. In turn, this study provides an example of the strong changes occurred in coastal environments as a result of climate change, particularly in the context of global warming episodes which characterized interglacial periods of the Quaternary in South America (e.g. MIS 1, 5, 11).Fil: Richiano, Sebastián Miguel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Geológicas. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Centro de Investigaciones Geológicas; ArgentinaFil: Varela, Augusto Nicolás. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Geológicas. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Centro de Investigaciones Geológicas; ArgentinaFil: D'Elia, Leandro. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Geológicas. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Centro de Investigaciones Geológicas; ArgentinaFil: Bilmes, Andrés. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Geológicas. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Centro de Investigaciones Geológicas; ArgentinaFil: Aguirre, Marina Laura. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de La Plata; Argentin

U-Pb zircon constraints on the age of the Cretaceous Mata Amarilla formation, southern Patagonia, Argentina: Its relationship with the evolution of the Austral Basin

A pesar del abundante contenido fosilífero de la Formación Mata Amarilla (Patagonia Austral, Provincia de Santa Cruz, Argentina), siempre se generaron abundantes dudas y debates acerca de cuál es la edad de esta formación. Los datos cronológicos aportados por los invertebrados, los dinosaurios, peces, tortugas, plesiosaurios y flora fósil son dispares. En el presente trabajo se obtuvo una edad U-Pb concordia por la metodología de ablación láser aplicada a 20 puntos de la parte externa de circones zonados provenientes de un nivel tobáceo hallado en la sección media de la Formación Mata Amarilla, lo cual arrojó una edad de 96,23±0,71 Ma, que corresponde al Cenomaniano medio. La depositación de la sección inferior de la Formación Mata Amarilla marca el inicio del estadio de antepaís de la Cuenca Austral (también conocida como Cuenca de Magallanes); este pasaje está signado por el cambio oeste-este de los sistemas depositacionales, los cuales se encuentran en concordancia con la progradación de la faja plegada y corrida cretácica. Así, el comienzo del estadio de antepaís quedaría comprendido entre el Albiano superior y el Cenomaniano inferior, debido a que la subyacente Formación Piedra Clavada posee una edad Albiano inferior. Al comparar estos resultados con los datos de la Provincia de Última Esperanza en Chile, se sugiere que el comienzo del cierre de la cuenca marginal de Rocas Verdes se produjo en forma simultánea.Despite the abundant fossil content of the Mata Amarilla Formation (Southern Patagonia, Santa Cruz Province, Argentina), its age has always generated a considerable number of questions and debates. The chronological data provided by invertebrates, dinosaurs, fish, turtles, plesiosaurs and fossil flora are contradictory. In this work, twenty U-Pb spot analyses by laser ablation were carried out on the outer parts of the zoned zircon crystals from a tuff layer of the middle section of the Mata Amarilla Formation, yielding a U-Pb concordia age of 96.23±0.71 Ma, which corresponds to the middle Cenomanian. The deposition of the lower section of the Mata Amarilla Formation marks the onset of the foreland stage of the Austral Basin (also known as Magallanes Basin); this transition is characterized by the west-east shift of the depositional systems, which is consistent with the progradation of the Cretaceous fold-and-thrust belt. Thus, the onset of the foreland stage could have occurred between the upper Albian and lower Cenomanian, as the underlying Piedra Clavada Formation is lower Albian in age. On comparing the data obtained with information from the Última Esperanza Province in Chile, it can be suggested that the initiation of the closure of the Rocas Verdes Marginal Basin occurred simultaneously.Facultad de Ciencias Naturales y Muse