The development of the Colorado Basin, like that of other Southwest Atlantic basins, is related to the Late Jurassic-Early Cretaceous rifting and subsequent break up of Gondwana (Juan et al ., 1996). All these Southwest Atlantic basins were large areas of deposition during the Late Cretaceous and Cenozoic. Fryklund et al . (1996, fig. 5, p. 143) identified tectono-stratigraphic sequences characteristic of a passive margin setting in the Colorado Basin, including rift, sag and drift deposits. The Pedro Luro Formation constitutes the base of the drift sequence and was deposited during a widespread marine transgression of Maastrichtian-Danian age. The formation consists of marine siltstones and claystones with subordinate, sometimes glauconitic sandstones.
In the Punta del Este Basin, the Gaviotín Formation represents partially the passive margin deposits (Ucha et al ., 2003). Gaviotín Formation was defined by Ucha et al . (2003), which consists of a marine succession of dark-greenish to gray claystones, siltstones, and sandstones with occasional intervals of glauconitic beds. The dinocyst assemblages from the lower part of the formation suggest a Maastrichtian to Danian age (Daners and Guerstein, 2004).
In the Austral Basin, the age of the Calafate Formation has been widely discussed. Nullo et al . (1981) and Macellari et al . (1989) suggested a Maastrichtian-Paleocene age for this unit, whereas Malumián and Caramés (1997) indicate that in the area of Lago Argentino the Paleocene deposits were either missing or poorly developed. According to Marenssi et al . (2004), the age of the Calafate Formation south of Lago Argentino is Maastrichtian to late Maastrichtian. The studied section at this area is characterized by a predominance of marine sandstones with minor conglomerate beds and a few mudstone beds.