Descripción: The Larsen Basin is the depositional area including all the upper Mesozoic-lower Cenozoic sedimentary rocks on the continental shelf of the northern Antarctic Peninsula. The western boundary between the basin fill and the rocks forming its technical basement, is not exposed, and was defined on the basis of aeromagnetic data. On petrological grounds, the Mesozoic marine sedimentary rocks exposed on western Tabarin Peninsula, at the north-eastern extremity of the Antarctic Peninsula, are assigned to the lower part of the Aptian-Eocene regressive megasequence that forms the basin filling. These rocks are faulted against sediments of the Antarctic Peninsula magmatic arc, suggesting that post- depositional tectonic movements probably occurred. This compressional deformation is assigned to mid-Cretaceous times, when the Coniacian partial basin inversion was accompanied by westward-verging deformation at the western basin margin. Although the original extent of the basin cannot be discerned from these data, the lithostratigraphical evidence and geographical position of the basin sediments, together with terrestrial magnetic data, confirm that the Larsen Basin extends onto the southern part of Tabarin Peninsula, and indicate that the western limit of the basin is located northward of the previously proposed limit. © 2004 Asociación Geológica Argentina.

Descripción: Although the depositational environment assigned by Darwin to the large erratic blocks and gravels in the Río Santa Cruz valley has been reinterpreted, his geomorphological and stratigraphic observations are still in force. The large erratic blocks he described as crowning the Condor Cliff terrace and spread at the bottom of the valley just east of this locality (Sites 2 and 3), are now interpreted as indicators of the maximum glacial expansion in Patagonia. Similar blocks, though of a different lithology, accumulated over a lower terrace located up-valley (Site 4), are now linked to moraines and glacifluvial terraces of the Penultimate Glaciation. Finally, in addition to the erratic block discovered by Darwin in the lower Río Santa Cruz valley (Site 1), there are others - recently discovered - which probably account for a catastrophic event ascribed to a big glacier-lake outburst during the last interglacial.

Descripción: In the cliffs of the Ladrillero cape, situated on the Atlantic coast of the Isla Grande de Tierra del Fuego, the uppermost records of the Austral or Magallanes foreland basin infill crop out. This locality contains Lower Miocene deposits informally named "Cabo Ladrillero beds" and "Cabo San Pablo beds" being the "upper Cabo Ladrillero beds" analyzed in this paper. This locality is situated around ten kilometers to the north of the emerging orogenic front (Punta Gruesa locality) where four facies association have been defined from gravitationally-driven supra batial to prograding and shallowing upward deltaic wedges. Deformed and massive beds by liquefaction processes and resedimentation are connected with abundant clastic dykes and synsedimentary faults. The statistic study suggests they are associated tectonic activity related to a transtensive episode rather than tectonic quiescence sometimes suggested in previous works. We conclude that microfossils documented in the area and used as a tool to support a deep marine environment are situated in underlying stratigraphic intervals that form part of the Desdémona Formation and the "lower Cabo Ladrillero beds" cropping out to the south of the study area. Sedimentological evidence are not conclusive respect to the batimetry but absence of classical turbidites, hyperpycnal flows and abundant deformed and resedimented deposits are more compatible with deltaic deposits, with high detritus supply generating unstable slopes rather than deep marine depositional settings, suggesting a more complex depositional evolution than the foraminiferal-based framework dominant in the literature.

Descripción: Samples obtained from four wells located at Bancalari, Buenos Aires Province, have been analysed in order to define textural parameters, petrofacies, palaeoenvironments and palaeoclimate evolution. The samples were obtained from the upper part of the Puelches Formation and from the Pampean and Postpampean sediments; in particular this paper highlights the informally named Preensenadense interval. Six textural groups (I to VI) and two petrofacies (A and B) have been recognised. Groups I to III represent fluvial sands (I), gradually overlain by backswamp and lake deposits (II -III), generated on very low slope and well-drained floodplains, with oxidizing conditions. Groups II and III are assigned to Preensenadense interval. The three groups are included in petrofacies A (Qt:F:L = 43:49:8), of sedimentary and basement provenance, which has an igneous metamorphic heavy suite, scarce hornblende and dominance of smectite over illite. By contrast the upper groups (IV to VI) show more varied depositional settings, including aeolian, fluvial and marine environments. Group IV is loessic and corresponds to the Ensenada Formation. Group V, the Lujan Formation (Río Salado Member), is composed of fluvial plain deposits deposited during a wet and temperate climate. Marine silts and clays (group VI) represent a transgression over terrestrial environments and record of the so-called Querandino event. The three upper groups are included in petrofacies B (12:56:32), and have a magmatic arc provenance, with plagioclase dominance and contributions of volcaniclastic and vitroclastic material. It contains a heavy-mineral suite mainly composed of hornblende and it has more illite than smectite. © 2005 Asociación Geológica Argentina.

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.

Descripción: A 25 m thick, rhythmic, limestone-marl alternation of Los Catutos Member (Vaca Muerta Formation, Neuquén Basin), was studied bed by hed in several quarries near Zapala. Beds up to 1 m thick are of massive or laminated, lithographic, fossiliferous limestone, locally mottled or weakly bioturbated (?Chondrites), interbedded with thinner marl beds. Limestones and marls can be classified as pelbiomicrites and biopelmicrites, mainly composed of pellets, radiolaria, forams, ostracods, equinoids, spicles of sponges and gastropods, with minor amounts of peloids, feldspar, chert, muscovite, glauconite and phosphates. Values for δ18O and δ13C are similar to those of lithographic limestones from the northern hemisphere. δ13O values are most probably result of diagenetic modification of the primary signal but the δ13C agrees well with the published curves for the Tithonian sea water and with other records from Tethyan limestones. A preliminary analysis of negative excursions of δ13C point to productivity crisis in the sedimentary system with a periodicity of 400 ka, which can be a result of changes in the orbital eccentricity of the earth. Smectite and interlayers reflect pedogenic processes developed on volcanogenic source-rocks, whereas more abundant kaolinite in marls might reflect more humid conditions. With TOC up to 1.95% the rocks offer prospects as a regular to good source rock for hydrocarbons, although thermally immature. A low-energy, dysaerobic, open-marine environment is inferred for Los Catutos Member, with high sedimentation rates as a result of high productivity at the sea-surface and incoming plumes of suspended sediment from the inner shelf. Sediment accumulation in wide and gently dipping calcareous bulges in an outer ramp system is envisaged.

Descripción: Detailed study of Callovian carbonates of the Calabozo Formation (30 m) in western Argentina, reveals the abundance of benthic carbonates particles produced by organisms that are light dependent, and non-skeletal grains such as ooids and peloids. The biota consists of bivalves, dasyclads (Salpingoporella annulata Carozzi, Cylindroporella annulata Carozzi, and Heteroporella sp.), scleractinian corals, echinoderms, cyanobacteria (Cayeuxia (Rivularia) piae Rech-Frollo, and Cayeuxia (Rivularia) kurdistanensis Elliot), oncoids, gastropods, forams, and calcispheres as well as ostracods, and bryozoans in relatively minor quantities. Allochemical particles suggest for the shallow marine limestones of the Calabozo Formation a Photozoan Association under subtropical seawater conditions. The petrographic features allow to recognize seven microfacies, differentiated by particle types, fossils and depositional texture: 1) criptalgal bindstone; 2) bioclastic-peloidal wackestone; 3) bioclastic-intraclastic wackestone; 4) bioclastic-peloidal-terrigenous wackestone, 5) coral framestone, 6) peloidal-bioclastic-intraclastic packstone, and 7) oolitic-bioclastic-peloidal packstone-grainstone. The recognition of microfacies groups constitute two palaeoenvironmental associations: inner ramp and middle ramp settings. Diagenetic studies are based on examination of stained thin sections, through conventional petrography and under cathodoluminescence. Early diagenetic processes are micritization and precipitation of peloidal micrite and acicular calcite. These cements predate granular and syntaxial calcite cements. Cathodoluminescent petrography allows to identify three morphologic types of clear cements: (1) fine acicular crystals, (2) granular cements, and (3) syntaxial cement rims on echinoderm particles. Description of the CL signature has evolved to discrimination as either, non-luminescent, dully luminescent red/orange, or brightly luminescent yellow. The earlier acicular calcite cement is non-luminescent, on the other hand, the predominantly dully luminescent red/orange, or brightly luminescent yellow of interparticular and intraparticular granular calcite and syntaxial cements indicate a shallow burial meteoric-phreatic diagenetic environment.

Descripción: The Cerro Cóndor depocenter represents the most complete stratigraphic sequence of the continental Jurassic of the Cañadón Asfalto basin and southern South America. It is situated in extraandean Patagonia, on both sides of the middle Chubut river valley. The sedimentation in this pull-apart basin begins early in the Middle Jurassic, accompanied by effusions of olivinic basalts. The prevailing lacustrine deposits are characterized by carbonatic and siliciclastic facies associations, interfingering with volcanic deposits grading from lavic in the base to predominantly pyroclastic towards the top. The facies evolution from north to south is described, embracing the sections of farm El Torito and the canyons of Los Loros, Las Chacritas, Carrizal, Asfalto and Lahuincó. The carbonatic facies/microfacies are represented by mudstones, wackestones, packstones, grainstones and microbialitic limestones, characteristic of littoral/marginal and palustrine environments. In the Cañadón Asfalto Formation are distinguished a lower member (Las Chacritas composed by limestones, shales, sandstones and conglomerates intercalated with olivinic basalts, and an upper member (Puesto Almada) composed by tuffs, tuffites, shales and sandstones. The first is bearing a palynologic assemblage of Bajocian-Bathonian age and the second dinosaurs of Tithonian age. The stratigraphic sections of both members of the formation are described and illustrated at their type localities cañadón Las Chacritas and farm El Torito and the paleoenvironments of the depocenter, from the Aalenian to the Tithonian are described and illustrated. The age of the Puesto Almada Member at its type locality was obtained by radiometric dating (K/Ar) of biotites from a thin layer of volcanic tuff (147.1 + 3.3 Ma, Tithonian) from the upper part of the unit.