Geochemical characterization of black shales from the Rio Mayer Formation (Early Cretaceous), Austral-Magallanes Basin, Argentina: Provenance response during Gondwana break-up

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 Rio 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 Rio Guanaco Locality (S 49 degrees 57' 11 ''; W 72 degrees 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 Rio 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 Rio Mayer Formation shows an affinity to the felsic compositions of the Tobifera 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 Rio 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 Rio Mayer Formation in the study area.