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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]

The reconstruction of palaeo-oxygenation levels in marine deposits from the Cretaceous has obtained a huge interest all around the world in recent years. This fascinating topic is here pointed out for the first time in the Austral Basin using the information provided by the black shales of the Río Mayer Formation, Patagonia, Argentina. The combination of sedimentology, ichnology and geochemistry (TOC, Ce anomaly and MnO content) allow the identification of three major intervals respecting the oxygen content. During the Berriasian and early Valanginian anoxic conditions prevail in the outer shelf. After that, between the late Valanginian and Hauterivian dysoxic palaeoenvironments were developed. Finally, a more oxygenated palaeoenvironment occurred since Aptian associated with a progradation of a proximal deltaic system. The identification of anoxic conditions is of much interest for the hydrocarbon research in this stratigraphical unit, which represents the most significant source rock of the Austral Basin. [ABSTRACT FROM AUTHOR]