Sistema Regional de Información
en línea para Revistas Científicas de América Latina,
el Caribe, España y Portugal

The presence of stromatolites from Cretaceous outcrops at the headwaters of the río Chico locality in the Golfo San Jorge Basin is reported for the first time. They are present in the uppermost part of the Lago Colhué Huapi Formation (Coniacian-Maastrichtian) of the Chubut Group. The presence of, up to now, four structures interpreted as stromatolites in this locality are not only important because it is the first record in the basin, but because it contributes to the knowledge of these bioconstructions in continental environments. The stromatolite called E1, which is characterized and discussed in detail, is associated with an abundant and diverse fossil record represented by palynomorphs, fragments of silicified trunks, dinosaur remains and eggshells. The study, integrated mainly with the recorded pollen and spores, is important for paleoenvironmental interpretations of the site and contributes to the paleoecological and paleoclimatic understanding of the most modern stratigraphic interval of the Chubut Group.

Sedimentologic and ichnologic analysis of the Middle Jurassic Lajas Formation in Bajada de Los Molles area allow recognizing upper offshore-lower shoreface deposits in transition to prodelta, delta front and interdistributary bay succesions. This system is classified as a river-dominated delta due to the presence of distributary channel deposits with huge amounts of particulate organic matter and low diversity and abundance of trace fossils. Hyperpycnites are common in the basal and upper part of the studied section, and form channel-levee and distributary-channel systems, integrated by massive sandstones and load deformation structures, or transitional and recurrent passages of tractive sedimentary structures with abundant particulate organic matter on the foresets. The greatest diversity and abundance of trace fossils are recognized in the upper offshore-lower shoreface deposits where Skolithos and Cruziana ichnofacies occur. Deltaic deposits show trace fossil associations with lower diversity and abundance than the fully marine ones, whereas the hyperpycnite deposits are either unbioturbated or show the lowest diversity and abundance of trace fossils, reflecting the most stressed conditions within the system.

In the Argentinian Andes (Frontal Cordillera) the upper part of the late Carboniferous-early Permian San Ignacio Formation is made up of lacustrine-palustrine microbial carbonates and interbedded volcanic deposits. In this lacustrine-palustrine environment a natural monospecific forest was developed. The deposits of this sedimentary-volcanic succession were repeatedly subjected to subaerial exposure and modified by pedogenesis to varying degrees giving rise to paleosoils development. Diagenetic microfabrics were well preserved in the carbonates and volcanic rocks. The carbonate microfabrics comprise a wide spectrum of features consisting of root marks and stumps-related structures (rhizoliths, alveolar texture, tunnel-like structures and coprolites of arthropods), pisoids, coated grains and pseudomicrokarst, cracking, brecciated and nodular fabrics, and grainification also occur corresponding to different stages in the pedogenic evolution. Meteoric dissolution and cementation processes are observed; examples are well identified by scanning electron microscope showing silica-filled voids in partially dissolved carbonates and growths of inorganic carbonate microcrystals or of microbial origin in voids. Other different types of cements can be seen such as discontinuous carbonate crusts, ribbon spar, cavities with silt infillings and pendant cements. The whole set of these microfabrics are indicative of wetting, desiccation and meteoric conditions (vadose and phreatic). The abundance of plant roots and associated micro-organisms mainly of bacterial origin (micro-rods, short rod-shapes, nano-fibres, filaments and nano-spheres) played an important role in the pedogenic and subaerial diagenetic processes affecting these deposits. The immature character of the paleosoils and absence of calcretes point out to short intervals of subaerial exposure due to oscillating fluctuations in water level, intermittent volcanic supply, tectonic subsidence and oscillating climatic conditions. The whole of the macro and microfabrics reveals that the prevailing weather could correspond to an intermediate between semi-arid to sub-humid, however the alternating wetting and drying conditions in which the fossil forest developed and the abundance and diversity of micro-organisms, suggest a transition to sub-humid climate conditions.

The Ordovician System is extensively represented in the Precordillera of San Juan Province, Argentina. At the Cerro La Chilca in the Jáchal area, the limestone of the San Juan Formation is paraconformably overlain by interbedded limestone and shale of the Gualcamayo Formation. The present contribution reports new data on the conodont fauna and biostratigraphy of these darriwilian units, revising local and regional chronostratigraphic relationships. New information on the composition of conodont and graptolite associations through the stratigraphic sequence is presented. The presence of Paroistodus horridus horridus, Yangtzeplacognathus crassus, and Histiodella sinuosa constrain the uppermost strata of the San Juan Formation to the lower part of the Y. crassus Zone, according to the Baltoscandian scheme, and to the H. sinuosa Subzone of the Periodon macrodentatus Zone of the North American scheme. In the overlying Gualcamayo Formation the co-occurrence of Y. crassus with Histiodella holodentata enable the recognition of the Y. crassus Zone and the H. holodentata Subzone of the P. macrodentatus Zone. The identification of these zones allows for precise global and regional correlation. A graptolite assemblage that belongs to the epipelagic and deep-water biotopes with some components restricted to low paleolatitudes is recognized. This diverse assemblage is characteristic of the pelagic biofacies. The important diversity of graptolites in this section suggests a favorable environment for their development. Local changes in the taxonomic composition are recognized through the Gualcamayo Formation. When comparing this fauna with that of different study localities from the Central Precordillera (Cerro Potrerillo, Oculta Creek, Cerro Viejo de Huaco and Las Aguaditas Creek) slight differences in the generic composition are observed. Taxonomic differences support the preference of certain associations for particular environments; though, graptolites are more diverse in black shales facies, which represent deeper environments (the Los Azules Formation), in relation to the calcareous-shale facies of the Gualcamayo Formation from Cerro La Chilca and correlative unit at Las Aguaditas Creek.

The continental sedimentary rocks that constitute the Palo Pintado Formation of the late Miocene from Salta province, presents a great paleoclimatic interest due to the environmental conditions prevailing during this geochronologic interval. The geological and paleobotanical data suggest that during the sedimentary rocks accumulation of the Palo Pintado Formation (Angastaco Basin), wetter conditions would have existed comparing with other nearby and contemporary Formations, for example the Playa del Zorro Aloformation (late Miocene of Catamarca) and the Chiquimil (late Miocene of Tucumán), Salicas and the Toro Negro Formations (both from the late Miocene of La Rioja). In this study, the margin and the foliar area of the leaves contained on rocks from the Palo Pintado Formation are analyzed, in order to obtain the mean annual temperature (MAT) and the mean annual precipitation (MAP). The resulting values were: 23.98 °C and 330.8 mm. These results are coincident by the interpretation of different authors, who consider that the Palo Pintado Formation would have been deposited under a relatively humid environment, possibly as a consequence of the rains that affected locally the Angastaco basin región.

The Upper Cretaceous nonmarine deposits of the Neuquén Basin have an important regional exposure. These deposits are included in the Neuquén Group, a well-studied unit in both the south and central part of the basin. However, the northernmost exposed between the Laguna del Diamante and the Atuel River-assigned to the Diamante Formation-have not been studied in detail. In the studied area, the Diamante Formation corresponds to a braided fluvial system with moderate sinuosity evolving through time towards an anastomosing fluvial system. Petrographic analyses indicate that sandstones belong to feldspatic litharenites and litharenites, while the source area indicates provenance from both a recycled orogen and a transitional arc. The latter could be linked to the exhumation of the Andean orogen located to the west and to the input from the north-eastern border of the Neuquén Basin, represented by the rocks currently exposed in the San Rafael Block. On a macroscopic scale, the presence of calcareous lithic fragments in the mid and upper part of the surveyed stratigraphic section, strongly suggest an input from the west. This indicates an important change in the polarity of the basin and the presence of a deformation front located to the west of the study area in accordance to previous proposals in equivalent deposits to the south of study area.

Laguna Las Vueltas (LLV) area retains the morphology of a late Pleistocene watershed that was flooded during a mid-Holocene marine transgression. Sediments associated with a paleosol dated at 22,582 cal yr BP reflect subaerial exposure of the area prior to the submergence during the marine transgression. This transgression produced an extensive tidal flat near the mouth of the former LLV watershed by 7,477 cal yr BP. Subsequent decoupling of the Las Vueltas valley from the sea occurred through the growth of a baymouth barrier and a beach-ridge plain to the east. This decoupling turned the lagoon into a pan environment in which subsequent lake-level fluctuations were controlled by climate. A lunette dune developed at the pans in the former lagoon, providing a narrow corridor where humans trapped, killed and processed guanacos as early as 3,402 cal yr BP. Changes in aeolian sedimentation hint at increased aridity during the past 500 years.

Recent biostratigraphic studies on the western argentine Puna recorded the Middle Ordovician conodont Baltoniodus cf. B. navis (Lindström) for first time, related to key graptolite taxa of the Central Andean Basin. The analyzed material comes from the lower and middle thirds of the turbidite succession exposed at the Huaytiquina section, Salta Province, which was previously assigned to the “Coquena” Formation. The conodont fauna was recovered from the calcareous sandstone beds intercalated in the middle portion of this unit, and it is composed by species of the genera Baltoniodus, Gothodus, Trapezognathus, Drepanoistodus, Drepanodus, and Protopanderodus, among others. The conodont association indicates a middle Dapingian (Dp2) age, linking the conodonts of the Argentine Puna with those from Baltoscandinavia and South China. The conodont productive levels also contain graptolites assignable to Tetragraptus bigsbyi (Hall) and Isograptus sp. They are located overlying strata bearing Azygograptus lapworthi Nicholson and underlying deposits with Xiphograptus lofuensis (Lee). The graptolite associations are indicating a Dapingian age (Dp1-Dp2) for the lower and middle portions of the “Coquena” Formation. The current findings from the western Puna, as well as the record of Azygograptus lapworthi related to the early Dapingian (Dp1) index conodont Baltoniodus triangularis in the Argentine Cordillera Oriental, are suggesting that a high-resolution correlation between both geomorphological regions is possible. This also documents that the Cordillera Oriental as well as the Puna were connected parts of the Central Andean Basin, during the interval from the Lower Ordovician (Floian) to the Middle Ordovician (Dapingian), instead of corresponding to the source and infill sectors of the basin, respectively. Furthermore, the regional and global correlations are discussed, and the potential of the Ordovician successions of the Argentine Puna for future advances on conodont-graptolite high-resolution biostratigraphy is highlighted.

This study deals with the conodont biostratigraphy from the uppermost part of La Silla Formation (9.6 m) and the overlying San Juan Formation (264.7 m), at the Cerro La Silla section, Central Precordillera of San Juan, Argentina. The 41 samples of carbonate rocks that were digested for microfossils yielded 11,388 conodont elements corresponding to 78 species. The Paltodus deltifer deltifer Subzone of the Paltodus deltifer Zone from the Baltic biostratigraphic scheme is represented at the top stratum of the La Silla Formation and the basal part of the San Juan Formation (28.4 m), which correlates with the Macerodus dianae Zone (middle Tremadocian) of the Precordilleran and North American schemes. Following upwards, the Paroistodus proteus, Prioniodus elegans, Oepikodus evae, Oepikodus intermedius and Baltoniodus triangularis-Tripodus laevis zones (middle Tremadocian-lower Dapingian) are recorded in the San Juan Formation. The Baltoniodus triangularis-Tripodus laevis Zone is recognized from the second reef level (177.3 m from the base of the San Juan Formation) up to the top stratum in the section, in contrast to previous interpretations that assigned the referred interval to the Baltoniodus navis, Paroistodus originalis and Microzarkodina parva zones of the Baltic biostratigraphic scheme. The division of the Oepikodus evae Zone in subzones, according to its original definition for the Precordillera, is not applicable at the Cerro La Silla section due to the particular species distribution. The conodont elements show a brown alteration color (CAI 2-2.5), which indicates a burial paleotemperature of 60-155°C for the bearer strata.

In arid areas, the efficient management of scarce water resources is key for population survival and development. One of the oldest and greatest ancient water management system in drylands is the filtration gallery. Originated from ancient Persia, they were spread to other regions and cultures, and are found in the oasis of Pica, in the Atacama Desert. A filtration gallery consists of an almost horizontal tunnel dug underground until it reaches a water-bearing zone. It allows to tap and drains out groundwater, and thus a direct contact with groundwater table. With the objective to understand groundwater processes, preserve the water and geoheritage of one of the driest places on Earth and improve land-use planning, the present work explored and studied the filtration galleries, locally called socavones, of the oasis of Pica. Through direct exploration, topographical survey and geo-electrical prospection, 24 socavones were identified, mapped and their main physical features described, showing common traits with filtration galleries described worldwide, but also proper features highlighting their originality. The findings of the geological and hydrogeological studies of the socavones, complemented by physical and chemical analysis, allow to identify new groundwater recharge processes and, thus, to review and complete the hydrogeological model of the local aquifer of Pica. Most socavones are abandoned today, owing to physical and socioeconomic changes. Nevertheless, this study concludes that they can still have a role to play in the groundwater management of this arid area.