Search

During the Early Cretaceous important changes occurred in South-western Gondwana related to its fragmentation, and South Atlantic Ocean opening. Then, two basins developed in southernmost Patagonia: the Austral-Magallanes Basin and the Rocas Verdes-Magallanes Basin. The Río Mayer Formation corresponds to the initial sedimentary stages of the Austral Basin including black shales, marls and fine-grained sandstones deposited in an outer shelf environment. We studied and sampled three detailed sedimentary logs at Río Guanaco Locality (S 49° 57′ 11"; W 72° 04′ 56″), dividing this unit into three informal sections (lower, middle and upper). Whole rock geochemical analyses were carried out over the three sections with the focus on trace elements, in particular, rare earth elements (REE). While, the lower section of the unit shows an affinity to acid volcanic source areas, the middle and upper sections show an affinity to intermediate source areas. These changes through the Río Mayer Formation can be correlated with the initial magmatic activity hosted in the Southern Patagonian Batholith during the Neocomian. Considering the REE patterns, the lower section of the Río Mayer Formation shows an affinity to the felsic compositions of the Tobífera Formation, while the upper section would be related to the erosion of the Punta Barrosa/Cerro Toro formations of the Rocas Verdes-Magallanes Basin. The clear distinction between the REE patterns of the Río Mayer and Zapata formations is considered evidence of the compartmentalization of these basins during the Early Cretaceous. Finally, the absence of correlation between the geochemistry of the studied unit and the Jurassic volcanic rocks of the Deseado Massif indicates that the north-eastern sector of the basin was not a source area for the Austral Basin, at least during the deposition of the Río Mayer Formation in the study area. • Geochemistry of the Río Mayer Formation reveals a clear trend for its provenance analysis. • During the Early Cretaceous the Rio Chico High was not a source area for the Austral Basin. • Austral and Rocas Verdes basins should have had different source areas during the Neocomian. [ABSTRACT FROM AUTHOR]

An integrated study combining sedimentological and ichnological data has been conducted in the lower wave-dominated deposits of the Calafate Formation. The study aimed to assess the variation in fair-weather and storm processes, tidal flux, and the coast configuration to correctly constrain sedimentary dynamics in complex depositional systems. The wave-dominated deposits comprise foreshore to upper shoreface (FA1), middle to lower shoreface (FA2) and offshore-transition deposits (FA3) moderately affected by storms. In FA1, the most proximal strata contain Macaronichnus isp. and occur within the foreshore, while the upper shoreface shows an increased ichnodiversity associated with the occurrence of the archetypal Skolithos ichnofacies evidencing heightened energies of the upper shoreface. FA2 display a fair-weather community assigned to the proximal Cruziana ichnofacies that shows a wider range and variability of living strategies, and a storm-related community represented by opportunistic colonizers which produced burrows assigned to Ophiomorpha nodosa. The monospecific occurrence of Thalassinoides isp. at the top of FA2 in CC02P section is assigned to the Glossifungites ichnofacies, developed on a firmground and autogenic surface. The ichnoassemblage in FA3 is attributable to the transitional Skolithos-Cruziana ichnofacies, that exhibits a slightly reduced diversity of the fair-weather community and a sparse bioturbation. The distribution and the general characteristics of the ichnological assemblages are consistent with deposition in fully marine conditions. The bioturbation intensity, trace-fossil diversity, maximum penetration, and burrow size distributions indicate fully oxygenated and clear waters, normal-marine salinity conditions, abundance and accessibility of food, and continuous and moderate sedimentation. Nevertheless, the ichnological signature in FA3 is related to minor physico-chemical stress produced by strong tidal currents originated during the southward expansion of the Magallanes embayment. • Integration of sedimentological and ichnological data allowed the differentiation between fair-weather and storms processes • The ichnological vertical trend shows a continuous of fully marine conditions along the shoreface depositional profile. • Tidal currents originated physico-chemical stress in FA3 deposits and later the tide-dominated deposits of the unit. [ABSTRACT FROM AUTHOR]

The Austral-Magallanes Basin covers the southernmost region of Patagonia, South America. South of Lago Argentino, the Calafate Formation documents part of the foreland stage of the evolution of this basin. Here, sandstones from the Calafate Formation were characterized using petrographical, X-ray diffraction (XRD) and geochemical data in order to establish their tectonic setting and sediment source areas. Based on our results, three intervals (lower, middle, and upper) were distinguished according to their composition. The lower and upper intervals are composed of felsic volcanic lithics, abundant quartz and orthoclase. A shift in the composition was observed in the middle interval, with an increase in the modal abundance of plagioclase as well as other accessory minerals such as pyroxene. In addition, XRD data reveal that the middle interval is dominated by smectite in the clay-fraction, whereas the lower and upper intervals present mixed-layer chlorite-smectite and chlorite assemblages. Similar differences are observed using major and trace elements: data from the lower and upper intervals point to source rocks of intermediate to acid composition, while the middle interval had source rocks of intermediate composition and a lower degree of weathering. With regard to the provenance studies, the three intervals have a strong affinity to intermediate compositions of the Tobífera Formation (El Quemado Complex) and with the diorite/andesite of the Patagonian Batholith, which implies an exclusive contribution from the western edge of the basin. However, the shift in the composition towards the middle interval indicates a sediment supply from the juvenile Andean arc, suggesting contemporaneous magmatic activity during the deposition of the Calafate Formation sandstones. This peak of magmatic activity may have been caused by an eastward migration of the arc, driven by a switch in the tectonic setting south of 48°S. • Petrographic and XRD analyses show changing compositional signatures throughout the unit. • Al 2 O 3 -(Na 2 O+CaO)–K 2 O diagram and CIA values indicate initial-moderately weathered sources. • Samples have Zr/Sc ratios of ~3 meaning that they are depleted compared to typical UCC. • In the Th/Sc–Zr/Sc and Co/Th–La/Sc diagram samples plot between the andesitic and granitic field. • Nb <20ppm and Zr <350ppm values indicate a tectonic affinity of a subduction-related volcanic arc. [ABSTRACT FROM AUTHOR]

Abstract: Variations in the composition, distribution, and diversity of molluscan assemblages from Patagonian marine terraces (MT) formed during the late Quaternary sea-level highstands and neotectonic events—between the late Pleistocene, mid-Holocene, and present—are discussed. Molluscs and associated macrofauna (balanids, brachiopods, polychaetes, bryozoans) from nine fossiliferous localities and three modern sites suggest paleoecological, paleobiogeographical, and paleoenvironmental implications for paleoclimatic interpretations of the coastal area of Bahia Bustamante–Caleta Malaspina (∼44.9 and 45.3°S) since marine oxygen isotope stage (MOIS) 7. Crepidula protea, Buccinanops paytensis, and Brachidontes rodriguezi are first recorded for the late Pleistocene MTIV (MOIS5-7); Nacella (Patinigera) deaurata, Epitonium magellanicum, B. paytensis, Aequipecten tehuelchus, and Clausinella gayi are recorded for the mid-Holocene MTVI (MOIS1) in the area. These molluscs currently live in the SW Atlantic, but Venericardia procera represents a northward migration and Tegula atra a faunal extinction in response to climate change. Overall, they indicate hard substrates, shallow waters, and truly marine conditions, similar to the modern littoral. With respect to temperature, the qualitative, quantitative, morphological, and distributional variations suggest slightly higher SST during the mid-Holocene (MTVI, MOIS1, hypsithermal), colder for MTV (MOIS5c?, 5a?), and warmer or similar for MTIV (MOIS5e, 7?). [Copyright &y& Elsevier]