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The James Ross Basin, in the northern Antarctic Peninsula, exposes which is probably the world thickest and most complete Late Cretaceous sedimentary succession of southern high latitudes. Despite its very good exposures and varied and abundant fossil fauna, precise chronological determination of its infill is still lacking. We report results from a magnetostratigraphic study on shelfal sedimentary rocks of the Marambio Group, southeastern James Ross Basin, Antarctica. The succession studied covers a ~1,200 m-thick stratigraphic interval within the Hamilton Point, Sanctuary Cliffs and Karlsen Cliffs Members of the Snow Hill Island Formation, the Haslum Crag Formation, and the lower López de Bertodano Formation. The basic chronological reference framework is given by ammonite assemblages, which indicate a Late Campanian – Early Maastrichtian age for the studied units. Magnetostratigraphic samples were obtained from five partial sections located on James Ross and Snow Hill islands, the results from which agree partially with this previous biostratigraphical framework. Seven geomagnetic polarity reversals are identified in this work, allowing to identify the Chron C32/C33 boundary in Ammonite Assemblage 8-1, confirming the Late Campanian age of the Hamilton Point Member. However, the identification of the Chron C32/C31 boundary in Ammonite Assemblage 8-2 assigns the base of the Sanctuary Cliffs Member to the early Maastrichtian, which differs from the Late Campanian age previously assigned by ammonite biostratigraphy. This magnetostratigraphy spans ~14 Ma of sedimentary succession and together with previous partial magnetostratigraphies on Early-Mid Campanian and Middle Maastrichtian to Danian columns permits a complete and continuous record of the Late Cretaceous distal deposits of the James Ross Basin. This provides the required chronological resolution to solve the intra-basin and global correlation problems of the Late Cretaceous in the Southern Hemisphere in general and in the Weddellian province in particular, given by endemism and diachronic extinctions on invertebrate fossils, including ammonites. The new chronostratigraphic scheme allowed us to calculate sediment accumulation rates for almost the entire Late Cretaceous infill of the distal James Ross Basin (the Marambio Group), showing a monotonous accumulation for more than 8 Myr during the upper Campanian and a dramatic increase during the early Maastrichtian, controlled by tectonic and/or eustatic causes. Fil: Milanese, Florencia Nidia. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires; Argentina Fil: Olivero, Eduardo Bernardo. Universidad Nacional de Tierra del Fuego; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Austral de Investigaciones Científicas; Argentina Fil: Raffi, María Eugenia. Universidad Nacional de Tierra del Fuego; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Austral de Investigaciones Científicas; Argentina Fil: Franceschinis, Pablo Reinaldo. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires; Argentina Fil: Gallo, Leandro César. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires; Argentina Fil: Skinner, Steven M.. California State University; Estados Unidos Fil: Mitchell, Ross N.. California Institute of Technology; Estados Unidos Fil: Kirschvink, Joseph L.. California Institute of Technology; Estados Unidos. Tokyo Institute of Technology; Japón Fil: Rapalini, Augusto Ernesto. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires; Argentina

The Ediacaran Period (635-542 Ma) witnessed a series of extraordinary events. It arises with the end of the Marinoan Glaciation and deposition of worldwide enigmatic cap carbonate deposits. This abrupt shift in paleoclimatic conditions coincides with major fluctuations in the isotope ratios of carbon and sulfur, and with significant changes in the concentration of redox-sensitive elements in marine sediments. The Ediacaran is also a period marked by rapid changes in geomagnetic polarity. Magnetostratigraphy may therefore provide high-resolution correlation between Ediacaran successions worldwide. Here, we combine stratigraphy logs, carbon isotopes and magnetostratigraphy on the Avellaneda Formation (590-560 Ma) which at the Rio La Plata Craton, eastern Argentina. We investigated two drill cores (TSE-34 and TSE-7) with a 0.3-0.7 m resolution covering the entire Avellaneda Formation, corresponding to 98 standard specimens (25 mm in diameter). The basal contact of the Avellaneda Formation with the underlying mudstone rocks from Loma Negra Formation (~ 590 Ma) is present in both cores. The upper contact with the Alicia Formation, only observed in TSE-34 core, is transitional. The TSE-7 displays an erosional contact between Avellenda and Cerro Negro Formations (~ 560 Ma). After stepwise thermal demagnetization up to 600°C, almost all samples provided a characteristic magnetization between 350°C and 600°C, therefore Ti-poor magnetite or titanohematite is likely the main carrier of the stable remanence in these rocks. A high-temperature, dual-polarity component is persistent and coherent in the two drill cores. The base of the unit is marked by normal polarity, followed by a reverse interval, followed by persistent normal polarity across to the upper part of the Avellaneda Formation. This magnetostratigraphic framework, together with the carbon isotope signal, will be compared with results recently obtained for potentially coeval successions in China, Canada and United States.

The Cambrian Meson Group represents the first infill stages of the NW Argentina Paleozoic basin, given after the major orogenic event represented by the Tilcaric orogenic phase. This unit extends for several hundred kilometers along the Eastern Cordillera in the provinces of Salta and Jujuy, but its age is poorly constrained due to the paucity of fossil evidence and scarce geochronological data. U–Pb LA-ICP-MS geochronological studies were carried out on detrital zircons from the Lizoite, Campanario and Chalhualmayoc formations, that make up the Meson Group, in outcrops in the Santa Victoria sub-basin in northernmost Argentina. Maximum depositional ages of 524.8 ± 4.1, 519.7 ± 2.4 and 508.6 ± 1.7 Ma were obtained for these formations, respectively. These new data, together with previous detrital zircon ages from southernmost outcrops of this Group, points to a possible diachronism in deposition, with younger ages towards the south. In our studied units, a dominant peak of Cambrian to late Ediacaran zircons is characteristic for the three formations, although a somewhat larger contribution from older crystals is observed in the Campanario and Chalhualmayoc formations. This pattern is similar to most other reported detrital zircon age spectra for the Meson Group along the basin. Published paleomagnetic data for the Campanario Formation indicates a very strong reverse magnetic polarity bias, which allows us to infer three more likely intervals for the deposition of this formation (roughly 512–510, 509–505 and 497-493 Ma). These three alternatives are analyzed with respect to either major diachronous or roughly synchronous deposition of the Meson Group sediments across the whole basin. Published magnetic fabric data for the Campanario Formation at three distant localities suggest possible paleocurrent directions from W to E.

A magnetofabric and paleomagnetic study was carried out in the Late Ediacaran - Early Cambrian Puncoviscana and the early Late Cambrian Campanario formations, exposed in Santa Victoria Oeste, in northwestern Argentina. Ten sites (93 samples) were located in tuffs and volcanic sills interbedded in the Puncoviscana Formation, one of which had been dated at 537 ± 0.9 Ma. On the other hand, 42 samples were collected at three sites from red to purple sandstones of the Campanario Formation. The analysis of anisotropy of magnetic susceptibility (AMS) allowed to recognize a pre-Andean fabric in both formations indicating that it is previous to the Andean cycle. In the paleomagnetic study reliable magnetic components were isolated in only two sites of the Puncoviscana formation whose virtual geomagnetic poles (VGPs) are close to but do not coincide with the apparent polar wander path of Río de la Plata craton - Gondwana for the late Neoproterozoic - Cambrian. A new paleomagnetic pole (18.2°S, 358.8°W, K: 27.9; A95: 3.9) was computed for the Campanario formation which is close to but does not coincide with those obtained in other locations for this unit and were considered anomalous respect to the expected position in the Gondwana path. The pole presented here is closer to the apparent polar wander path of Gondwana for the Late Cambrian-Early Ordovician suggesting two possible interpretations; the presence of different amounts of Andean tectonic rotations between different sampling locations of the Campanario Formation or the recording of a rapid Pampia dextral displacement along the Rio de la Plata craton margin in Cambrian times. Fil: Franceschinis, Pablo Reinaldo. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires; Argentina Fil: Rapalini, Augusto Ernesto. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Estudios Andinos "Don Pablo Groeber". Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Estudios Andinos "Don Pablo Groeber"; Argentina Fil: Escayola, Monica Patricia. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires; Argentina Fil: Luppo, Tomas. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires; Argentina

Formation of the south Central Andes basement cannot be unlinked from the final stages of Western Gondwana amalgamation. This process is still matter of much debate and continuous research. In the last two decades, a picture has emerged that suggests that the accretion and/or displacement of discrete crustal blocks in the Ediacaran and Early Paleozoic was a major process that finally configured the basement upon which three hundred million years later the Andean orogen developed. A main crustal fragment of the basement of central Argentina is the Pampia terrane. Recent, but still scarce, paleomagnetic results on Cambrian rocks from this terrane show anomalous pole positions whose interpretation is still ambiguous. Whether Pampia is a cortical fragment linked to the Amazonian craton that collided against the Rio de la Plata craton coetaneously with the closure of the hypothetical Clymene Ocean, whether it was attached previously and subsequently collided with a western crustal sliver (Western Pampia) associated with the Arequipa-Antofalla terrane, or whether it was a fragment detached from southern Kalahari that was displaced along the margin of the Rio de la Plata craton in the Cambrian, cannot be unambiguously solved due to the scarcity of paleomagnetic data. It is generally accepted that the very long magmatic belt along the western boundary of Pampia in the Ordovician (the Famatinian arc) developed on stretched continental crust. Systematic large clockwise rotation of Ordovician magmatic and sedimentary rocks along this belt has been confirmed in recent years, but the originally para-autochthonous rotated terrane model has been replaced by one in which a systematic pattern of small crustal block rotations accompanied deformation due to collision of the allochthnous Cuyania terrane. The very large counterclockwise rotations in the Western Puna (Antofalla terrane) of Chile and Argentina are still interpreted by some authors as evidence of closure of a V-shaped back-arc basin between Antofalla (and Arequipa?) and the Western Gondwana margin in the Late Ordovician. However, the interpretation of such rotations as due to tectonic escape of the Antofalla block during Cuyania collision may be more compatible with geochemical signatures that suggest crustal links between the Sierras Pampeanas and the Puna basements. Paleomagnetic support for the Laurentian origin of the Cuyania terrane has endured the significant improvement of the apparent polar wander path for Gondwana in the Ediacaran–Cambrian. Alternative models suggesting a para-autochthonous origin of Cuyania are difficult to reconcile with different lines of evidence (biogeographic, isotopic) and are not supported by available paleomagnetic data for this terrane. Whether Chilenia is a truly allochtonous terrane or a fragment of Cuyania that rifted apart to collide back in the Devonian is still controversial. Unfortunately, paleomagnetic data are not available to test these models yet.

The paleogeographic evolution of the Río de la Plata Craton during the Precambrian poses important questions regarding its participation in the Atlantica continent and Columbia supercontinent. With the aim of contributing to the paleogeographic evolution of the craton in mid-Paleoproterozoic times, systematic paleomagnetic studies were carried out in late- to post-tectonic plutons of the Piedra Alta terrane (Uruguay). These yielded six new paleomagnetic poles with ages between 2.1 and 2.0 Ga that can be interpreted as integrating a single apparent polar wander path together with three previous poles from the same terrane and similar ages. The positions of the poles along the proposed path are compatible with the known ages of the intrusives from which they were computed and show a smooth distribution of magnetic polarities. Comparison of this path with those coeval from other South American and African cratons confirms previous interpretations that if the Atlantica continent ever existed, had a significantly different configuration than the originally proposed. Based on the new apparent polar wander path proposed for this different continental configuration a model of its paleogeographic evolution that suggests fast displacements in (probably southern) polar latitudes during the study time lapse is presented. Fil: Franceschinis, Pablo Reinaldo. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires; Argentina Fil: Rapalini, Augusto Ernesto. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires; Argentina Fil: Sánchez Bettucci, Leda. Universidad de la República. Instituto de Ciencias Geológicas; Uruguay Fil: Martínez Dopico, Carmen Irene. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geocronología y Geología Isotópica. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geocronología y Geología Isotópica; Argentina Fil: Milanese, Florencia Nidia. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires; Argentina