Descripción: Sedimentology and paleoenvironments of the Patagoniano and its transition to the Santa Cruz Formation south of Lago Argentino, southern Patagonia. Detailed sedimentological analysis of the Patagoniano and its transition to the Santa Cruz Formation has been carried out south of Lago Argentino, in southern Patagonia. The 180 meters thick succession of fossiliferous sandstones was subdivided into twelve lithofacies, grouped in three facies associations. From base to top we defined the facies association I, dominated by massive, bioturbated and fossiliferous sandstones; the facies association II, dominated by fining upward cycles of cross-stratified sandstones and heterolithic facies; and the facies association III dominated by thin conglomeratic levels interbedded with mudstones rich in plant remains and paleosoils. The facies association I is interpreted as the deposit of a transgressive, shallow sea with oyster rich biogenic accumulations, finally evolving to a coastal plain paleoenvironment. A new transgression originated the discontinuity underlying the facies association II, which was interpreted as a progradational estuarine system. Finally, the ongoing progradation produced the fluvial meandering system of facies association III, assigned to the Santa Cruz Formation. The new name "Estancia 25 de Mayo" is formalized here for the Patagoniano in the southernmost Cordillera Patagónica, instead of the problematic name Centinela Formation. The Estancia 25 de Mayo Formation was subdivided in two members: the lower Quién Sabe Member, equivalent to the facies association I, and the upper Bandurrias Member, equivalent to the (facies association II).

Descripción: Considering the high concentration of rock avalanches in the northern part of the Neuquén province of Argentina (36°-38°S and 70°-71°W) and their association with lacustrine basins forming natural dams, their catastrophic collapse seems not to be such a common process as could be expected for these cases. The present work depicts the area immediately affected by the outburst flood corresponding to the Navarrete dam catastrophic collapse (175 × 106 m3), through a sedimentological analysis, generated since the arroyo Colorado was blocked due to a rock avalanche deposit during not well constrained postglacial times. The outburst flood dispersion is initially inferred based on morphological criteria and then determined from a sedimentological analysis of its matrix. Furthermore, the matrix is compared to that of the fluvial terraces, which have been sampled along the arroyo Colorado. This comparison shows that even though differences among them are not so significant through most of the study area, matrix frequency starts to be distinctive from the most distant point of sampling.

Descripción: The study of the carbonate-ramp deposits of La Manga Formation has allowed us to recognize three facies associations. The lithofacies association A is composed by five lithofacies (A1-A5). This association represents part of the transgressive systems tract. It corresponds to external ramp deposits, accumulated below the storm-wave base. The A facies association appears in cycles of centimetric to decimetric thickness and belong to a retrograding parasequence set of 5th order. The lithofacies B association starts after an abrupt facies contact. It is characterized by six lithofacies (B1-B6). They are coarsening upward beds that form metric-scale cycles. These are interpreted as middle-ramp deposits dominated by storms. It corresponds to 4th order sequences. The lithofacies C includes three lithofacies (C1-C3). The small scale cycles were controlled by storm events, while the medium and large scale ones were likely originated by eustasy. The studied stratigraphic sections of La Manga Formation are included in two depostitional sequences SD-2 and SD-3. An abrupt discontinuity marks the beginning of the depositional sequence SD-2, which constitutes a transgressive system-tract, composed by a coarsening upward parasequence stacking set. Above a maximum flooding surface, manifested by a drastic change of facies, a low rate accommodation space period and a sea-level fall (characterized by a lowstand systems tract) are evidenced. The third depositional sequence SD-3 is composed by lagoon deposits. It starts after a paleokarstic surface, characterized by the development of a sheet-cake breccia and an intense vadose diagenesis.

Descripción: Late Cretaceous and Paleogene sedimentary rocks of the Austral Basin crop out on the south-eastern margin of Lake Viedma. In the Barrancas Blancas section, 28 m of yellowish sandstone and mudstone of the La Asunción Member of the Anita Formation are transitionally covered by 390 m of greyish sandstone and mudstone of the Cerro Fortaleza Formation. The former represent a prograding barred nearshore system, whereas the latter correspond to paralic, fluvial and fluvial-tidal sedimentation. The evolution of the depositional sequences, sedimentary palaeoenvironments, sandstone provenance and palaeocurrents indicate that the sediments were deposited during the back-arc (sag) stage of the basin. At Cerro Piramides, Tertiary sedimentary rocks rest with fault contact on top of the Cerro Fortaleza Formation. The limited thickness (<1m) preserved of greenish sandstone of the Man Aike Formation precludes any attempt to interpret its depositional environment. Conglomerate, sandstone and mudstone (locally carbonaceous) of the Río Leona Formation (75 m) rest with erosional contact on the former. These rocks represent a rapid transition from a high energy to low energy fluvial systems that eventually graded into marginal marine environments represented by the fossiliferous sandstones of the Centinela Formation (<3m). The facies arrangement, evolution of the palaeoenvironments, and palaeocurrents suggest that these sediments were deposited during the foreland basin stage. © 2003 Asociación Geológica Argentina.