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During the Maastrichtian-Danian interval, southern South America was amply covered by the Atlantic Ocean. The land area was then reduced to an archipelago in which the central part of the Somun Curá Massif and the Deseado Massif were the main emergent zones. The large surface covered by the sea then was an important paleogeographic feature and played a crucial role in climatic and evolutionary processes, as it was,for some groups, a biogeographic barrier between Patagonia and the rest of South America (Pascual, 1998; Casadío, 1999). Towards the West, the Andean volcanic arc became an important feature interrupting the Atlantic-Pacific connection and also the major sediment source for the adjoining basins. Sedimentary rocks recording these geodynamic events in the Neuquén Basin are included in the Malargüe Group, and are well exposed in southern Mendoza, northeastern Neuquén, western La Pampa and northern Río Negro. The rocks involved were studied by numerous workers in the final part of the nineteenth and early part of the twentieth centuries. During the last decade, a renewed interest on them has surfaced because they contain the Cretaceous/Paleogene boundary, in both marine and continental facies. Therefore, they become an excellent opportunity to learn about the modifications undergone by ecosystems at the close of the Cretaceous in mid latitudes of the southern hemisphere. Sedimentological analysis of over thirty detailed sections spread over more than 600 km between Arroyo Las Aucas in Mendoza and El Matuasto in Río Negro, allowed recognition of differences in the vertical arrangement of facies and in the depositional paleoenvironments in different sectors of the basin. This, together with multidisciplinary studies on the macro- and microfaunas, made possible the pinpointing, with different degrees of accuracy, of the K/P boundary at several localities, and in different facies and/or lithostratigraphic units. The sediments containing the boundary are associated to a mix carbonatic-silicoclastic shelf developed during the Maastrichtian, which changed into a shallow carbonatic shelf during the late Maastrichtian-Danian. The K/P boundary coincides with an important change in the composition of invertebrate associations initiated during the late Maastrichtian. This change was coeval with a relative sea level rise in the basin and the onset of carbonatic sedimentation related to an increase in the water temperature in the South Atlantic (Casadío, 1998). The planktonic foraminifera mass extinction at the K/P boundary is recorded, although concealed by the high dominance of Guembelitria in the uppermost Maastrichtian beds, which reflects a shallow setting and a decreasing sea level after the major rise. In the central and eastern sector of the basin, characterized by cratonic influence, low slope values and reduced sedimentary supply, the K/P boundary lies within the shelf marine siltstones of the Jagüel Formation. It was located precisely at El Matuasto, in the province of Río Negro (Concheyro et al., 2002) and in a thin interval of centimeters in Bajada de Jagüel and Opaso, in the province of Neuquén (Náñez and Concheyro, 1997; Papú et al., 2000; Náñez et al., 2002). To the north, at Cerros Bayos, Barda Baya and La Amarga, in the province of La Pampa, and at Liu Malal in southern Mendoza, the boundary lies in shallower facies at the top of the Jagüel Formation or else at the contact between the siltstones of this unit and the limestone of the overlying Roca Formation (Casadío and Concheyro, 1992; Casadío, 1994; Concheyro and Villa, 1996; Pires et al., 1999). To the northwest, in the Andean region, the basin is placed in a retroarc position, with a sharper paleoslope and abundant sedimentary supply. Here, study of the macrofauna, 40K/40Ar dating, and magnetostratigraphy, all carried out at the locality named Cerro Butaló in southern Mendoza, indicate that the K/P boundary should be, with an uncertainty of several meters, within continental facies at the base of the Pircala Formation (Parras et al., 1998; Parras, 1999; Parras and Casadío, 1999). The Cretaceous/Paleogene boundary is difficult to precise, at a centimeter scale, in most of the basins of Argentina. However, it is closely constrained in several localities in the Neuquén Basin, mainly because of the quality of the exposures there and the excellent preservation and abundance of several biostratigraphically important fossil groups. In the central and eastern sectors of the basin, advances were especially based on micropaleontological studies (Casadío and Concheyro, 1992; Concheyro and Villa, 1996; Náñez and Concheyro, 1997; Papú et al., 2000; Concheyro et al., 2002; Náñez et al., 2002) and molluscs (Casadío, 1998). Contrarily, in northern localities a precise location of the boundary has not been quite as accurate because it lies in marginal marine or continental facies in which the fossil content is of limited biostratigraphic usefulness (Parras, 1999)

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