LATE CRETACEOUS-NEOGENE COMPARATIVE STRATIGRAPHY BETWEEN THE CRATONIC AND OROGENIC MARGINS OF THE AUSTRAL-MALVINAS FORELAND BASINS, TIERRA DEL FUEGO ISLAND, ARGENTINA

In southernmost South America, the Austral/Malvinas basins developed from the Late Cretaceous/ Neogene as perisutural depressions, tectonically attached to the Fuegian Andes-North Scotia Ridge orogen, and extended for more than 300 km towards the South American continent. The Late Cretaceous/Neogene filling is best known outside the fold/thrust belt, offshore and in the subsurface of central Tierra del Fuego. Here, published results by oil companies differentiate three megasecuences: MS3, Aptian-Maastrichtian, distal shelf mudstones, representing a period of tectonic quiescence; MS4, Paleocene-Mid Eocene, distal shelf to basinal glauconitic sandstones, mudstones, and limestones representing localized tectonism and early development of the foredeep; and MS5, late Eocene-Miocene deep to shallow marine sandstones and mudstones, representing a strong deepening event and initiation of the main foredeep phase. New stratigraphic data from the innermost fold/thrust belt offer a different picture. Here, the Late Cretaceous/Neogene filling consists of four packages. 1) Campanian/ Danian, resedimented conglomerates breccias, turbidite sandstones, and bioturbated mudstones. Clast lithologies (foliated, acidic metavolcanites and radiolarian slates) and northward directed paleocurrents indicate an Andean source area located to the south. 2) Late Paleocene/early Eocene, sediment gravity flow conglomerates, breccias, sandstones and mudstones. Fresh, andesitic, volcanic fragments dominate clast composition, and the dispersal pattern is complex and variable. 3) Late Mid Eocene/late Eocene, limestones, estuarine sandstones, and turbidite sandstones and mudstones. Clast composition is dominated by quartz and lithic fragments and sediment dispersal is to the north and east. 4) Oligocene/Miocene mudstones, sandstones, and localized conglomerates. Packages 1) to 4) are exposed in different, successive, subparallel bands, with 1) occupying the southernmost area. These new data indicate that the evolution of the Austral-Malvinas foreland basins is more complex than the simple pattern emerging from subsurface studies in their cratonic margins. Our data from the orogenic margin, suggest that the foreland phase already started in the Campanian/Maastrichtian, and that several Paleogene depocenters were formed, accompanying the northward propagation of the fold and thrust belt.