Draft:Calafate Formation

The Calafate Formation crops out in south-western Santa Cruz Province of Argentina and overlaps with the La Irene Formation. And the chorrillo formation which might mean they might of shared animals althought this is uncertain. It displays a stacking of asymmetrical coarsening–fining-upward cycles. These cycles are interpreted as the product of short-lived transgressive-regressive events in which the coarsening upward part represents sedimentary aggradation with a stable or decreasing sea level. Sedimentological and palynological analyses indicate nearshore marine conditions. Even though the existence of an estuary or incised valley cannot be determined, this is the most probable palaeogeographic model. Based on dinoflagellate cysts, the base of the section is considered to be not older than Maastrichtian. The presence of the oyster Ambigostrea clarae (Ihering) occurring together with the dinoflagellate cyst species Manumiella druggii (Stover) Bujak and Davies and Eisenackia circumtabulata Drugg in the middle part of the section indicates an age no older than late Maastrichtian. According to sedimentological data, deposits representing the Cretaceous–Palaeogene transition would have been eroded, which is confirmed by the presence of Grapnelispora loncochensis Papú. This megaspore is a consistent component of the Maastrichtian assemblages from Patagonia. The Austral or Magallanes Basin includes the portion of the Patagonian and Fueguian Andes from 45 to 54°S. The northern margin is coincident with the maximum extent of marine sedimentation during the Mesozoic (Riccardi, 1988, Ramos and Aguirre-Urreta, 1994). The basin boundaries are the Northern Patagonian Andes to the north, the Deseado Massif to the east and the magmatic arc to the west. The origin of the basin is linked to extensional episodes occurring during the separation of South America and Africa. This separation was also associated with the effusion of siliceous volcanic rocks during the Middle–Late Jurassic and the resulting rocks constitute the basin basement. Ramos and Aguirre-Urreta (1994) pointed out that the Mesozoic development of this basin could be divided into two marine cycles, the first extending from the Late Jurassic to Early Cretaceous and the second occurring during the Late Cretaceous, the latter being restricted to the southernmost part of the basin. The sedimentary infilling is represented first by the sandstones of the Springhill Formation and the Hauterivian–Barremian black shales in the Río Mayer Formation. During the Albian–Cenomanian a thick sedimentary sequence was deposited. This was associated with intense uplift in the western and southern areas of the basin. From the latest Campanian onwards until the end of the Palaeogene in northwestern areas there was a progradation of the depositional systems infilling the basin, most originating in shallow water. Malumián and Caramés (1997) stated that two main marine episodes characterize the Palaeogene. The first occurred during the Danian and the second, with several anthracogenic cycles, during the middle Eocene. According to Macellari et al., 1989, Manassero et al., 1990, Manassero and Merodio, 1992, the Late Cretaceous–Palaeogene sequence was deposited in a foreland basin and is almost entirely composed of sediments derived from an andesitic magmatic arc. In the Lago Argentino area Macellari et al. (1989) recorded three Late Cretaceous depositional sequences. The youngest of these is composed of rocks referred to the Calafate Formation. This formation progressively overlies older strata in a northward direction. It was dated as Maastrichtian–Palaeocene by Nullo et al., 1981, Macellari et al., 1989. However, Malumián and Caramés (1997, p. 198) suggested that in the area of Lago Argentino the Palaeocene was either missing or poorly developed. This was confirmed by Marenssi et al. (2002), who recognized that, in the locality of Cerro Calafate (south of Lago Argentino), the Eocene Man Aike Formation unconformably overlies the Late Cretaceous Calafate Formation. Thus, if the Cretaceous/Palaeogene boundary deposits were ever represented there, they have since been entirely eroded. This unconformity was recorded by Riggi (1979) in subsurface deposits of the eastern part of the basin. The present paper provides the sedimentological and biostratigraphical results of an analysis of the Calafate Formation at Estancia 25 de Mayo (Fig. 1), south of Lago Argentino (50° 23′ LS; 72° 12′ LW). The nearshore marine deposits of which the formation is composed contain both marine (dinoflagellate cysts, prasinophycean algae, acritarchs) and nonmarine (pollen, spores, Zygnematacean and Chlorococcalean algae) components, as well as other fossil groups, which are useful for independent age and palaeoenvironmental interpretations. A stratigraphic section was measured at Estancia 25 de Mayo using a Leica vector IV laser range-finder. The geometry of the beds, ranks of the bounding surfaces, lithology, texture, sedimentary structures and fossil content of the rocks were recorded. Samples were collected from fine-grained lithologies whenever possible because the predominantly coarse-grained deposits are not suitable for palynological studies. Twenty samples were processed for palynomorphs, nine of which proved to be. The Calafate Formation is bounded by unconformities and therefore at Estancia 25 de Mayo variable thicknesses have been recorded. In the section sampled we measured a 240-m-thick column. This is dominated by sandstones with occasional beds of conglomerates and very few mudstones (Fig. 2). The exposures show at least four sandy–conglomeratic beds, each 10–30 m thick, that form vertical cliffs. These intervals are separated by 30–50-m-thick intervals of finer-grained brownish sediments.