Descripción: Fil:Litvak, V.D. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.

Descripción: In this paper new geologic evidences allow us to complete the stratigraphy of Ramez de Velasco ranges. Granitoids of the igneous basement are included into the Ojo de Agua-Ambargasta complex as the Sumampa Granite which is composed of three facies: facies Para Yacu, Sumampa Viejo and Los Telares. Thin aplitic dykes were included into the same basement as the last magmatic event of the same geological setting. The geochemical signature of these rocks shows meta to peraluminous compositions, which evidence magmatic arc granitoides. Basic dykes, few meters wide and more than one kilometer long, also intrude these granitoids. They are composed of basalts and andesites with calcoalkaline signatures that were correlated with Balbuena formation due to their petrological features. Finally, few and small outcrops of sandstones appears as the only expression of the Neopaleozoic sedimentation. On the basis of all the data obtained we suggest that the Ramíez de Velasco ranges preserve evidences of several geologic processes that took place since the Paleozoic.

Descripción: Isotopic age determinations (40Ar/39Ar) and associated magnetic polarity stratigraphy for Casamayoran age fauna at Gran Barranca (Chubut, Argentina) indicate that the Barrancan 'subage' of the Casamayoran South American Land Mammal 'Age' is late Eocene, 18 to 20 million years younger than hitherto supposed. Correlations of the radioisotopically dated magnetic polarity stratigraphy at Gran Barranca with the Cenozoic geomagnetic polarity time scale indicate that Barrancan faunal levels at the Gran Barranca date to within the magnetochronologic interval from 35.34 to 36.62 megannums (Ma) or 35.69 to 37.60 Ma. This age revision constrains the timing of an adaptive shift in mammalian herbivores toward hypsodonty. Specifically, the appearance of large numbers of hypsodont taxa in South America occurred sometime between 36 and 32 Ma (late Eocene-early Oligocene), at approximately the same time that other biotic and geologic evidence has suggested the Southern high latitudes experienced climatic cooling associated with Antarctic glaciation.

Descripción: Guido bonarelli is an important figure in the field of geology in Argentina and he constituted one of the most relevant geologists in the oil exploration in our country. of Italian origin, he was born in Ancona on July 25, 1871 and he died in montereano, January 11, 1951. He excelled in stratigraphy, geological survey, historical geology and paleontology, becoming a specialist in the taxonomy of Jurassic ammonites and the stratigraphy of the Central Apennines. His professional work began in Italy and continued in southeast Asia, south America, bolivia, Chile, Argentina and uruguay. During 1911-1918, he worked in the Dirección General de minas, Geología e Hidrografía of Argentina. He defined the sub-Andean ranges as an orographic complex and he pointed out the locations of oil and natural gas reservoirs. He developed his researches in several Argentinean provinces and in the pacific Chilean coast. During his second period of research in Argentina (1923 -1927), Guido bonarelli organized the Geological Division of YpF, and he intensified the oil exploration in the northwestern of Argentina, as well as in the provinces of mendoza, neuquén and the Comodoro rivadavia region. He developed a professional and academic team that dealt with the geology and the hydrocarbon exploration in the country. bonarelli discovered in 1891 a sedimentary horizon that has great importance and it serves as a baseline to characterize the global anoxic event known to the Cenomanian-turonian transition. this world-famous skyline has been named Livello Bonarelli as a posthumous tribute to his discoverer.

Descripción: The stratigraphy of Cretaceous marine mudstones in the Fuegian Andes, roughly equivalent to Charles Darwin's clay-slate formation, remains a still unsolved problem. Previous records of Albian, Turonian-Coniacian, and Santonian-Campanian bivalves are combined with new findings of the Late Albian inoceramid Inoceramus anglicus Woods, and the Maastrichtian ammonites Diplomoceras sp., Anagaudryceras sp., Maorites densicostatus (Kilian and Reboul), Maorites sp., and Pachydiscus (Neodesmoceras) sp. to further constrain the Cretaceous stratigraphy of the eastern Fuegian Andes. In addition, new records of distinctive trace fossils and ichnofabric are meaningful for stratigraphic division and delineation of paleoenvironmental settings in these Cretaceous mudstones. The Lower Cretaceous ichnoassemblage of Chondrites targioni (Brongniart) and Zoophycos isp. is consistent with the inferred slope-volcaniclastic apron settings of the Yahgan Formation; Nereites missouriensis (Weller) reflects distal basin plain depositional settings for the Beauvoir Formation. In the Upper Cretaceous, the "Estratos de Buen Suceso" record the earliest extensively bioturbated horizons, reflecting prolonged well-oxygenated bottom conditions. In the Bahía Thetis Formation, organic-rich, channel margin or distal basin slaty mudstones record the last occurrence of inoceramid bivalves in the Austral Basin; the generalized absence of trace fossils is consistent with dysoxic bottom conditions. The thoroughly bioturbated Policarpo Formation, records a marked change in paleoceanographic conditions. The strong contrast in the intensity of bioturbation between the Upper Campanian-Maastrichtian Bahía Thetis Formation, almost devoid of trace fossils, and the highly bioturbated Maastrichtian-Danian Policarpo Formation reflects a change from dysoxic-anoxic to well ventilated conditions, probably associated with a cooling trend of bottom waters in the austral deep oceans.

Descripción: The geological observations made by Darwin in 1835 during his crossing of the Andes from Santiago to Mendoza via the Piuquenes Pass and Cordón del Portillo are compared with the present geological knowledge of the Cordillera Principal and Cordillera Frontal at 33°-34°S. The analysis of the complex stratigraphy of the Cordillera Principal, the imbricated structure of the Aconcagua fold and thrust belt, as well as the stratigraphy and structure of the inter mountain foreland Tunuyán Basin, allows to assess the pioneer observations of Darwin. He recognized the old metamorphic basement and the granitoids and volcanic sequences of late Paleozoic to Triassic age of the Cordillera Frontal, established the Cretaceous age of the marine successions cropping out along the eastern Cordillera Principal and studied the conglomeratic deposits associated with the uplift of the Cordillera in the Alto Tunuyán Basin. Based on the study of clast provenance of the synorogenic deposits of the Alto Tunuyán Basin, Darwin recognized that the Cordillera Frontal was uplifted later than the Cordillera Principal. The present knowledge of this sector of the Andean Cordillera confirms his pioneer observations and show that Darwin was one of the first scientists ever in realizing that in an orogenic system the sequence of uplift and deformation proceeds from hinterland towards foreland, according to a process that is exceptionally well-illustrated along the Piuquenes-Cordón del Portillo transect.

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: Between 1870 and 1884, Florentino Ameghino consecrated all his efforts to document and to prove the coexistence of man and Pampean megafauna in the Luján valley. He described several sites in channel and alluvial plain settings in valleys incised on Pampean loess. The geoarchaeological context of these sites, characterized by the abundance of megafauna modified bones, were never studied again after the publication of La Antigüedad del Hombre en el Plata in 1880-81. Here we present a stratigraphical reevaluation and reinterpretation of his "Piso Lujanense" based on a sequence stratigraphy analysis and AMS, OSL and ESR dating. We conclude that the Pleistocene valley fill spans from OIS 4 to OIS 2 with sequence boundaries at 75, 55, 30, 17 and 13 ky BP, reflecting the OIS 4 to OIS 2 glacioeustatic falls in the pampas valleys. These deposits were sealed by a rich organic matter layer or black mats, indicating an abrupt climatic change with a dramatic increase of humidity starting sometime between 12.5 and 13 ky BP. The associated ecological changes probably triggered the extinction process.

Descripción: In the right bank of the Arroyo Tapalqué (Salto de Piedra, Olavarría, Buenos Aires province) is a section through fluvial sediments of the Luján Formation (upper and lower members) and aeolian deposits of La Postrera Formation. A geological and geophysical study was carried out to determination the magnetic parameters of the sediments of both formations, in order to evaluate the environmental and climatic conditions at the time of deposition. Levels with high concentrations of ferromagnetic minerals are thought to be related to an increase in capacity of river transport probably due to higher rainfall. By contrast, a particular magnetic behaviour was detected in levels closely associated with pedogenetic processes (Suelo Puesto Callejón Viejo, SPCV, Suelo Puesto Berrondo, SPB and present soils). This behaviour is attributed to the partial dissolution of the ferromagnetic fraction (magnetic/titanomagnetite) and to possible formation of a high c oercitivity magnetic fraction (hematite?) as an associated process. These results are similar to those obtained for paleosols from the Buenos Aires Formation, probably because of parental material common to both. On the basis of present field and laboratory observations, together with previously available information, a detailed correlation is proposed between features observed in this section and palaeoclimatic fluctuations identified for Pleistocene/Holocene.

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: Results of a detailed geologic survey of the Río Borbollón area, located in the Cordillera Principal of the province of Mendoza are presented. The stratigraphic record of the area corresponds to a Jurassic-early Cretaceous sedimentary succession, the products of Neogene igneous activity and Quaternary deposits. The Tres Esquinas and La Manga Formations of Jurassic age were recognized for the first time in the study area. The main structural features of the area are described and interpreted based on the obtained data and previous works. A balanced structural cross section of the Malargüe fold and thrust belt at 34°15'S depicts the structural style and the amount of orogenic shortening in this region of the Andes.

Descripción: A composite section, 1400 m thick, is established for the Upper Cretaceous-Paleogene strata in the fold and thrust belt of the Andes Fueguinos, south of 54° 30′S. The basal Policarpo Formation, > 350 m thick, consists of tuffaceous sandstones and bioturbated sandstones and siltstones. The ammonites Maorites densicostatus and Diplomoceras sp.; foraminifera common to the Gaudryina healyi Zone; and the dynocysts Manumiella seelandica and Operculodium cf. azcaratei indicate a Maastrichtian age. The Paleocene to lower Eocene include four new formations. The Cabo Leticia Formation, c.150 m, Paleocene, consists of gravity flows deposits: brecchias; conglomerates; and massive, tuffaceous sandstones. La Barca Formation, c. 220 m, includes two members: LB1, tuffaceous sandstones and intercalated carbonaceous siltstones; and LB2, black mudstones with Palaeocystodinium golzowense and Spiroplectammina spectabilis. P. golzowense and the Bulimina karpatica Assemblage in LB1 indicate a late Paleocene age. Punta Noguera Formation, 380 m, is dominated by glauconite rich, massive, tuffaceous sandstones with interbedded turbidite packages. The dynocyst group Apectodinium, Deflandrea robusta, Palaeocystodinium sp., and Odontodinium askinae; the foraminifera Alabamina creta, Charltonina acutimarginata, Valvulineria teurensis and the first ocurrence of Elphidium and Cribrorotalia suggest an age near the Paleocene/Eocene boundary. The Cerro Ruperto Formation, 200 m, is dominated by glauconite rich, silty very fine sandstones and siltstones; dominance of Deflandrea dartmooria indicate an early Eocene age. Resting on angular unconformity, the Río Bueno Formation, c. 60-80 m, consists of carbonate rocks; its member RB1, rhythmically bedded grainstones, with the planktic foraminifera Planorotalites australiformis and Subbotina linaperta is of early middle Eocene age; and its member RB2, regular alternation of grainstones and bioturbated marls and micrites, with Elphidium saginatum and Bulimina cf. bortonica, is assigned to the mid middle Eocene. The sedimentary and stratigraphic features of the Maastrichtian/Eocene have a strong tectonic control, suggesting that the foreland phase of evolution of the basin has been active at least since the Maastrichtian.

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.

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: 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: The southern part of the Ñirihuau basin at the valley of the Cushamen River region, is deformed and uplifted forming a prominent Precordillera Patagónica fold and thrust belt, east of the Patagonian Andes. The Ñirihuau Basin infill is represented by Oligocene to Miocene sedimentary to volcaniclastic rocks gathered in eleven litho types belonging to four members. These foreland sequences are exposed by a combination of thin-skinned deformation whose shortening is transferred to the basement in the west beneath the Cordón del Maitén. The entire Ñirihuau section as well as the overlying Collón Cura Formation is characterized by the occurrence of progressive unconformities, fact that implies synorogenic sedimentation at the time of the Cordón del Maitén range uplift. This belt is formed by an east-verging basement wedge associated with a series of backthrusts. The reactivation of the deformation associated with growth strata, indicates that the basin has evolved in a wedge-top of a foreland system in different pulses of deformation.

Descripción: In 1833 during his journey across the Buenos Aires Pampas, Charles Darwin made observations that reflected his thoughts on two major landscape units, Pampa interserrana and Pampa deprimida, later identified by other authors. Darwin grouped the Pampean sediments into a single unit, the Pampean Formation, based upon the lithological homogeneity and the large extension of the deposits; the unit was thought to be of estuarine-marine origin and attributed to the Recent Epoch considering the paleontological content (vertebrates and mollusks). At present, the Pampean sedimentary succession, which accumulated approximately during the last 11-12 Ma, is interpreted as a pedosedimentary sequence due to the ubiquity of pedogenetic features throughout the deposits. Four main subcycles of sedimentation are identified related to reactivations of the Pampean landscape. At a regional scale, the outcrop distribution of Pampean sediments of different ages suggests the dominance of more stable conditions since the late Miocene-Pliocene in a vast area of Pampa interserrana, documented by the formation of calcretes. However, sedimentation during the late Pliocene-Pleistocene was active within the domain of the Salado tectonic basin and Sierras de Tandil. The regional disparity shown by the Pampean stratigraphic record reveals the major morphostructural differences of its basement.

Descripción: The Huecú depocenter is located in the central sector of the Loncopué trough in western Neuquén. This basin is composed of volcanic, volcaniclastic, fluvial and lacustrine deposits, younger than 1.6 Ma encompassing postglacial and even historical sediments and lavas. During most of its evolution, the Huecú depocenter has constituted a small closed depression (pullapart basin) restricted to the Loncopué trough, a major extensional retroarc basin in the Central and Patagonian Andes. This basin was fed from the north since the beginning of its evolution by a series of big amalgamated post-Pliocene stratovolcanoes located in the Mandolegüe Cordillera. The basin was also filled by fissural lava flows originated in the Agrio caldera located to the east grouped in the Escorial Basalt of 1.6 to 0.8 Ma. Polygenetic volcanic products form in the region big stratovolcanoes among which the Trolón and Colorado volcanic centers were the most prominent. These centers of Pleistocene age were highly eroded by the last glacial activity that affected the region. Nearly at the end of the glacial period two fissural volcanic fields covered the Huecú depocenter corresponding to the Rankül-Lom Basalt and preglacial Tres Hermanos basalts. Finally postglacial volcanic lavas covered most of the Huecú depocenter. These were grouped in the Cerro Artillería Basalts and postglacial Tres Hermanos basalt, which have recurrently dammed the main fluvial basins in the area. The fluvial and lacustrine deposits are represented by El Huecú and Mar Pequeño Formations.