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This paper is carried out in the coastal area of Bahía Creek and its surroundings, highlighting 15 km to the west Caleta de los Loros. This area is located on the coast of Río Negro province, which is part of the San Matías gulf. Bahía Creek is located precisely at 41º 05´S, 63º 56´ W. It belongs to the department of Adolfo Alsina and it is located 70 km SW of Viedma city, capital of the province and 70 km E of San Antonio Oeste port (Fig. 1).The main objective of this work is a morphological classification of the dunes present in the dunefield of Bahía Creek, based on the proposed classifications by McKee (1979), Lancaster (1995), Hesp (2011) and Sanjaume and Gracia (2011). For this it is necessary to observe satellite images and aerial and field photographs, together with the study of wind data. With this information it is also possible to know the factors that can condition the location of the dunes, their migration, their mobility index according to Lancaster (1988) and the sand drift potential (DP), the resulting drift direction (RDD), the resulting drift potential (RDP) and the directional variability of the wind through the RDP/DP relationship through the methodology proposed by Fryberger (1979). The used material consisted of aerial photographs from 1986 from the Servicio Geológico Minero Argentino, scale 1:80.000; the digital elevation model (DEM) of the ALOS satellite and Landsat 8 images from 2017, the latter of 30 m of spatial resolution. The images available in Google Earth, Earth Engine and Bing Maps were also used. The wind data used correspond to the records of the Servicio Meteorológico Nacional weather stations located in the towns of Viedma and San Antonio Oeste (SAO). Within these winds, those capable of mobilizing the material available in the study area (effective wind), of an average grain size of 0.25 mm (Toffani, 2018), are those that reach speeds equal to or greater than 6.7 m/s. This value was calculated using the equations proposed by Bagnold (1954).The effective wind was recorded in 34% of the total measurements within Viedma and SAO stations. In Viedma (Fig. 2) the main effective wind directions are NW (13.9%), and SW (10.1%), while in SAO (Fig. 2) the NW direction (18.8%) also predominates, followed by WNW (10.4%). These values suggest that the net displacement of the dunes occurs from NW/SW to SE/NE.Subsequently, applying the mobility index, it was obtained that the dunes, on average, correspond to active except in the interdune, with a value of 124.The annual value of DP is 1090, the RDP 392 and the annual RDD direction is 76 o, indicating a displacement of the sediments towards the ENE (Fig. 3). The directional variability of the wind regime, characterized by the RDP/DP ratio, shows the value of 0.36. This means that Bahía Creek is located within a high energy environment, with a wind regime between complex and bimodal.The annual rate of migration for the dunes studied, which are located within fields of dunes 2, 5 and 7, between 1986 and 2017 was 6.02 ± 0.3 m/year (Fig. 4), towards NE – SE.The dunefield is transgressive and is characterized by a gentle relief that goes from 0 to 60 – 90 meters above sea level from the beach towards N and E. The active dunes rest on a field of stabilized dunes and this in turn is located on an old dissected alluvial plain. The dunes can reach a relative height of 16 m and are composed of medium to fine sands, generally very well sorted, with the exception of some well sorted.Within the classification itself (Fig. 5), the dune fields 1 (Fig. 6a), 3 (Fig. 6c), 5 (Fig. 8a) and 6 (Fig. 8b) are characterized by the predominance of crescentic dunes. Field 2 has mainly linear and reversible crest dunes (Fig. 6b). Field 4 is characterized by star dunes (Fig. 6d). Field 7 is composed of symmetric, asymmetric and lateral coalescence parabolic dunes (Fig. 8c – d). While field 8 is represented by embryonic dunes (Fig. 9a – b), field 9 by climbing dunes (Fig. 9c) and field 10 by hanging dunes (Fig. 9d). Echo dunes (Fig. 10a), falling dunes from the cliff (Fig. 10b) and enveloping dunes (Fig. 10c) were also recorded in the field. In addition, within the different sectors mentioned, the formation of nebkas occurs (Fig. 10d).The wavelengths vary between 30 m and 350 m, the lowest values are after the creek, that is where the dunefield begins, and in the most distal part, east of the dune field 6. This sector is also the one that records the highest values. This distribution is coherent, since the first accumulations are of smaller size and close to each other, in general they are growing and having greater wavelength, until reaching a maximum. Where transport begins to decrease they acquire smaller sizes. The greatest lengths, up to 1 km, due to the development of its arms, are reached by the parabolic dunes. The smallest, 10 m, are those presented by the barchans. Regarding the width, the latter are also those that have smaller sizes, 10 m, while the largest correspond to coalescent parabolic dunes and transverse or barchanoid ridges, which are between 500 m and 600 m. It is also observed that the larger the size of the dunes in terms of length and width, the wavelength is greater. Variations in wind and sand transport rate at different time and space scales appear to be the most important control of their size and spacing. These factors, together with vegetation, humidity, input of sand, topography and constructions or anthropogenic processes, influence the distribution of the active dunes and at the same time in the location of the different types of dunes. The most relevant for the population of Bahía Creek are the constructions that act as an obstacle to the transport of sediments by the wind. They difficult the passage of winds coming mainly from the S, which contribute to the movement of the dunes towards the NE, leaving them more relative weight to the winds of the W and the NW. The latter cause the fastest advance of the dunes towards the E and SE, that is, where the buildings are.Bahía Creek dunefield, due the types of dunes and their distribution, shows great complexity, mainly due to the variations in the effective winds, and that over time generate compound and complex dunes. In Argentina, coastal dunefields with similar characteristics are develop, for example, around San Antonio Oeste and Este, in Río Negro province (Carbone et al., 2007); in the south and east of Buenos Aires province (Barrera Medanosa Austral y Oriental) (Bértola and Cortizo, 2005; Cortizo and Isla, 2007, 2012; Bértola et al., 2009); and in Península Valdés, Chubut province (del Valle et al., 2008). However, Bahía Creek has exceptional characteristics due to the possibility that offers to appreciate a natural environment with different geomorphological features with virtually no anthropic disturbance. Among these are beaches, cliffs and mainly the vast dunefield, which enters the continent with a concentration and striking diversity of dunes. It is a site that is not yet highly exploited by urbanization and tourism and allows visitors to access an almost virgin and pristine environment, which includes Caleta de los Loros, Pozo Salado and Punta Mejillón Protected Area. This last one is conformed to a greater extent by a tidal plain, which added to the rest of the coastal environment and to the dunefield, makes the area have a great diversity of species. In addition, the importance of the study of the dunefield lies in its proximity to the seaside town of Bahía Creek, which acquires more visitors over the years and is affected by the migration of the dunes, which sometimes have already covered houses or arrived at a distance very close to them and continue to move in that direction.

Origin of parent materials and genesis of soils in the northwestern edge of the Bajos Submeridionales Basin, Santiago Del Estero, Argentina The Bajos Submeridionales basin comprises an extensive hydrological system in the eastern Chaco region of Argentina, which occupies the northern part of Santa Fe, the southern part of Chaco and eastern part of Santiago del Estero (Fig. 1). It is a large plain characterized by its low slope and the presence of fluvial, lacustrine and eolian sediments. Different source areas for the sediments have been proposed, as well as geomorphological processes controlled by regional climatic and tectonic dynamics. The aim of this study was to evaluate the influence of these processes in the genesis and distribution of parent materials and soils in the northwestern edge of the Bajos Submeridionales basin. For this purpose micromorphological studies in soil thin-sections, mineralogical analyzes by X-ray diffractometry (XRD) in bulk samples and in the clay-fraction samples, magnetic susceptibility (MS) measurements and soil physical and chemical analyzes (texture, organic matter, cation exchange capacity (CEC) and soil moisture equivalent) were performed. Four soil profiles developed in different landscape positions were studied: 1) Entic Haplustoll (C-50) in the gently undulating plain unit; 2) Typic Argiustoll (C-16) in the slope unit; 3) Typic Haplustert (C-53) in the low plane unit; and 4) Typic Endoacuert (C-27) in the drainage network (Fig. 2 and Table 1). Unlike the loess of Chaco region (Urundel Formation), microscopic studies show a high proportion of fresh acid volcanic glass in the coarser fractions. This composition relates it to the loess of Tezanos Pinto Formation, which covers the central and southern sector of the Bajos Submeridionales basin.Granulometric analyses show that the A horizon of the Haplustoll, Argiustoll and Haplustert soils have a finer texture than the subsurface B and C horizons. This could be related to the presence of lithological discontinuities in the parent materials. Effectively, both the high values of the estimated CEC for the clay-fraction and the soil moisture equivalent suggest an underestimation of the clay percentage in the granulometric analysis of the subsurface horizons (Table 2). In turn, micromorphological studies showed the presence of pseudo-sands and pseudo-silts and a high proportion of larger microaggregates of reworked soil material (i.e. Fig. 3b, Fig. 4c-d, Fig. 5b-c and Fig. 6b) in the subsurface horizons, in addition to inherited pedofeatures such as fragmented and deformed clay coatings (Fig. 3c-d). These features suggest that the soils parent material is constituted by sediments that has been eroded from pre-existing soils and transported to its current position. In turn, the angular- and subangular-blocky structure observed at the base of the Haplustoll (Fig. 4a) and the Haplustert (Fig. 6c) profiles, seems to constitute in situ soil relicts. The set of these characteristics of inherited microstructure and pedofeatures indicate the presence of a pedosedimentary paleosurface at the base of these profiles.For all profiles, the magnetic susceptibility values show two main sections separated by the paleosurface (Fig. 10). These first magnitude differences in the MS values are related to the characteristics and composition of the parent material. In Section I, the highest values ??are related to the volcaniclastic minerals of the regional loessic sediments and to the pedosedimentary components. This Section I is subdivided into three parts (Sub-section Ia, Ib and Ic) according to smaller magnitude variations, which are can be related to pedogenetic processes. In turn, the magnetic signal in Section II shows sharp and progressive decreases towards the base of the profiles. This variation in depth could be explained both by the increase in calcium carbonate and by the presence of sediments from local supply areas, such as the sandstones and the alluvial deposits of the Salado River. The XRD diagrams show the presence of illite, kaolinite and irregular interstratified of illite-smectite minerals (I-S) (Fig. 9). These minerals are found in different proportions along the profiles. The superficial A-horizons of the Haplustoll, Haplustert and Endoacuert show a dominance of illite, which can be related to the regional loessic sediments. Meanwhile, the pedosedimentary materials show a higher proportion of I-S minerals with smectite dominance. This be would related to both previous pedogenetic processes as well as the process of enrichment in smectites by transformation in the present soil, favored by the slightly alkaline chemical environment in the middle and basal sections of the profiles. The origin of the pedosediment could be linked to the erosion of a paleosurface, given by the tectonic dynamics and the climatic changes occurred during the Pleistocene. On the other hand, the mineralogical composition of the superficial A-horizons suggests that they could derive from the aeolian accumulation of sediments eroded by wind from the river channels, during an arid Holocene event. Then, the pedogenesis of these deposits would have occurred in a more humid recent event. These interpretations allow linking the pedological processes with the tectonic, sedimentary and climatic recent Quaternary cycles that occurred in this sector of the Bajos Submeridionales basin.

The Neuquén Basin is one of the main hydrocarbon-generating basins in Argentina. This, along with a large sedimentary record, has made it the subject of numerous geological studies. Most of these studies focus on the central part of the basin, with very little background in the area of the high mountain ranges in Mendoza (Fig. 1). The rocks studied belong to the Bajada del Agrio Group, which includes the Huitrín and Rayoso formations, and the Diamante Formation equivalent to the Neuquén Group (Fig. 2 and 3). These units correspond to the transition between the back-arc and foreland stages (Lower Cretaceous-Upper Cretaceous).The goal of this work is to perform a detailed sedimentological, petrographic and stratigraphic study analyzing facies, stratigraphic relationships and provenance, with the aim of adjusting the depositional model of the succession representing the Lower Cretaceous-Upper Cretaceous transition.A sedimentological section was measured at Vega de los Patos (Fig. 4), approximately 100 km north of Malargüe. In that section fifteen facies were identified (Table 1), five facies associations (Fig. 5, 6, 7, 8 and 9) and two depositional cycles. The sedimentological analysis of the succession exposed at Vega de los Patos suggests that the analyzed section represents a transition between deposits in a marginal marine environment and fluvial fans (Fig. 15). This model allows relating the vertical variations of the facies with the progressive migration of the system, which could be related to tectonic uplift pulses. For the petrographic analysis included eight samples of medium sandstones, two of pelites, two of gypsum/anhydrite and one carbonate. The sandstones were classified as feldspathic lithoarenites and lithoarenites (Fig. 12). The petrographic analysis also showed that the sedimentary source of the analyzed sequence is mixed, between dissected arc and recycled orogen (Fig. 12).However, it is important to highlight the appearance of conglomerate levels with carbonate lithic fragments (Fig. 13) of the Agrio Formation (Lower Cretaceous) recognized in other sectors of the basin. This suggests a contribution of part of the lower Mesozoic carbonate sequence, probably as the result of a tectonic uplift pulse. Finally, the sandstones are affected by the precipitation of carbonate, zeolitic and to a lesser extent ferruginous cement (Fig. 10) and the carbonate rocks show an intense dolomitization and evidence of microfossils (Fig. 11).The analysis by X-ray diffractometry served to determine that the zeolitic cement observed petrographically was analcime (Fig. 14). It also revealed variations in the composition of clays along the section (Fig. 16). Such variations indicate arid and alkaline deposition conditions for the base of the Diamante Formation that change to a humid climate environment and acidic conditions in the middle sector.In the center of the basin, the transition between the bac-arc and foreland stages is related to the Intercenonian unconformity. However, there are no previous works that mention it in the study area. In previous works, an erosion or no deposit gap of 25 Ma was established, which include the entire Rayoso Formation, with the Huitrín Formation in contact with the Neuquén Group. Using a maximum age of deposition of the Huitrín Formation 100 km south of the study area (124 Ma) and those made in the first sandstone of the Diamante Formation in the study area (107 Ma) the gap would go down to 17Ma. Considering that there was no evidence of unconformity along the studied section, a transition or a paraconformity between the strata analyzed is assumed.

Dear LAJSBA readers, we want to stay in contact with you for the second time, in this case starting the 27th Volume and after two years as LAJSBA Editors. First of all, we would like to thank the support received by the broad sedimentological community in Argentina and Latin America, particularly to the affiliates of the Asociación Argentina de Sedimentología. The journal has experienced important advances in these two years which are our main focus on this communication. In our first Editorial, in Vol. 25.1, we put emphasis in some relevant topics to work for during this time. One of them was the possibility of incorporate the complete record of the AAS Revista (pre-LAJSBA) to SCOPUS. This finally was achieved by the end of 2019, and SCOPUS database included all the papers from AAS-Revista between 1996-2004. To do this, very important steps were done by members of the AAS board and previous editorial teams, so thanks to them! Another previously mentioned aspect was the incorporation of a Digital Object Identifier (DOI) for all the papers of the journal. This theme is on progress at this moment through Scielo Argentina, they will generate DOI’s for LAJSBA papers before 2020, after that we will continue the process. Apart from these two topics, may be the most important news for LAJSBA is the regularity of the volumes! After a 2018 with a record of papers submitted to the journal we could publish 4 full numbers in a year (25.1, 25.2, 26.1 and 26.2) which means that in December 2019 we publish on-line and on-time the Volume 26.2. In the same way, the present number (27.1) is fully available on-line in July 2020 on our final repository, Scielo Argentina.Special thanks are for Augusto Varela and José Cuitiño, guest editors of the two highly successful Special Volumes dedicated to the Austral Basin. We invite to our readers to propose new thematic volumes for LAJSBA. Nowadays the LAJSBA is an entirely online-only free-access journal, with a high level of scientific quality and design which is recognized for the Latin American Earth-science community. In terms of bibliographic indexes, LAJSBA is taken into account by SCOPUS, Scimago, Latindex, GeoRef, and SciELO. Considering the recent modification of SCOPUS’s CiteScore, LAJSBA maintain its status in the 3rd Quartile in the Scimago Journal Ranking in all the categories where it is included (Geology, Paleontology, and Stratigraphy). We strongly believed that LAJSBA is ready to jump to the next level, becoming more attractive for the international community. We will work to maintain the path marked by previous editorial teams for this journal. To conclude, we want to emphasize that the future of LAJSBA depends on all of us, readers, authors, reviewers, and editors. The journal needs the support of the Latin American sedimentological community, not just to maintain its importance in our region, but to cross our borders. A special message is for our readers; we are looking forward to receiving your contributions!

El 2 de noviembre de 2018 falleció a los 89 años de edad, en la ciudad de La Plata, el Dr. Julio César Merodio, con el profundo pesar de familiares, amigos y todos aquellos miembros del ámbito profesional y científico de las ciencias exactas y naturales, en especial la geoquímica, que tuvimos la enorme fortuna de conocerlo y compartir lindos momentos. Julio, o César (usaba ambos nombres de acuerdo al ámbito en donde se lo conocía, pero nunca dejó que lo llamasen Julio César como nombre compuesto, no le gustaba!), nació en 1929 en la ciudad de Tornquist, Provincia de Buenos Aires; se trasladó desde temprana edad a la ciudad de La Plata dónde cursó sus estudios secundarios en el Colegio Nacional de la UNLP, y luego se graduó de Licenciado en Química y posteriormente Doctor en Química, ambos títulos obtenidos en la Facultad de Ciencias Exactas de la Universidad Nacional de La Plata.

The Valentín Formation (late Pleistocene-middle Holocene) outcrops in the Ullum-Zonda valley located in the south-central region of the Province of San Juan, in the Argentine Republic, about 20 km west of the capital city of San Juan. It is a tectonic depression in which two fold and thrust belts with different levels of detachment and opposite vergence converge and collide: to the west, the east verging thin-skinned Central Precordillera, and to the east, the west verging thick-skinned Eastern Precordillera (Rolleri, 1969; Baldis et al., 1979; Ortiz and Zambrano, 1981; Ramos, 1988, Allmendinger et al., 1990; Jordan et al., 1993; among others). The outcrops of the Valentín Formation have a surface area of 3.6 km2 and are scattered in an area of ~110 km2 (Fig. 4) to the north, east, south, and southwest of the Ullum-Zonda valley. The paleoenvironmental evolution of these sequences is not yet clearly understood. Consequently, it has been almost impossible to determine the paleogeography and paleoclimatic conditions in this valley during the late Pleistocene-Holocene climatic transition. This study constitutes the first attempt to address this issue.Traditionally, the Valentín Formation has been interpreted as a lacustrine-palustrine environment associated with the San Juan River (e.g., Groeber and Tapia, 1926; Pandolfo, 1975; Salinas, 1979; García, 1996; Colombo et al., 2000; Suvires and Gamboa, 2011; Blanc, 2014; Blanc and Perucca, 2017; among others), an antecedent stream that descends from the Andes Cordillera. Following the three-component scheme to represent sediment movement along mountain valleys in Precordillera from Suriano et al. (2014) (Fig. 3), the San Juan River behaves as a sedimentary transference system for the intermountain basins developed along the narrow axial valleys between the numerous ranges of the Precordillera. This river has a glacial-nival regime with the peak of maximum runoff between December and January. It has an average annual flow of 60 m3/sec (Subsecretaría de Recursos Hídricos, 2004), registering millenary floods that can exceed 1,000 m3/sec (Perucca and Esper, 2009).In this study, we introduce a lithofacial, stratigraphic and paleoenvironmental analysis of the Valentín Formation together with a paleogeographic and paleoclimatic characterization of the Late Pleistocene and Early to Mid-Holocene in the Ullum-Zonda tectonic depression, based on sedimentological, geomorphological, and geochro­nological data (conventional radiocarbon, AMS, and OSL). Following the scheme proposed by Blanc and Perucca (2017; Fig. 4), the study area was divided into six sectors based on the spatial distribution of the Valentín Formation outcrops to facilitate a neat description and analysis. We carried out a detailed lithofacies analysis (Tables 2, 3, and 4) in five stratigraphic logs in sectors 1 (logs PB-VI and PB-COU), 2 (PB-FVZ log), and 5 (logs PB-DS1 and PB-DS2) (Fig. 5), and control observations in sectors 3, 4, and 6. Log description and analysis were made using the methodology proposed by Miall (1977), Allen (1983), Miall (1985; 1996; 2006) with modifications. The data obtained for the characterization of facies included: the thickness of horizons or layers, limiting surfaces, texture, structure, and carbonates. The texture was determined using classical soil manipulation methods to analyze its plasticity (Thien, 1979). The relative content of CaCO3 was determined from the sediment reaction (weak, moderate, or strong) in HCl diluted to 10%. Genetically related facies were grouped into facies associations (Table 3) considering as such a body of rock larger than a facies, characterized by its geometry (internal and external), the arrangement of its limiting surfaces (in the case of architectural elements), and the facies that compose it (Miall, 1977; Allen and Allen, 2005; Bridge and Demicco, 2008). Sixteen facies associations were defined and grouped into eight subenvironments representative of five sedimentary environments (Table 4). The geomorphological analysis consisted of the study of the landforms using satellite images, aerial photos, and digital elevation models (DEM of the Argentinian National Geographic Institute of 5 and 30 m resolution) together with field surveys where we observed the contact relationships between the different landforms and their morphogenetic processes. For the geochronological analysis, we used available numerical ages (Blanc and Perucca, 2017) (Table 1) from 14C dating of organic sediments by conventional and AMS techniques. Based on the obtained data, we constructed an artistic “paleo-satellite image” of the Ullum-Zonda valley using photographic montage techniques to approximately represent the paleogeography of this valley for the early Holocene (Greenlandian, ~ 9,300 years BP) (Fig. 10).Blanc and Perucca (2017) divided the Valentín Formation into two chronostratigraphic units: a late Pleistocene age (16,700–15,200 years BP) unit and an early to middle Holocene (9,475–7,685 years BP) unit. The stratigraphic relationship between these units has not been established. Results revealed that the Valentín Formation deposits show numerous flow sedimentary structures associated with rapidly aggrading anastomosed and ephemeral meandering river systems with thick unconfined mud-clay-sandy floodplains, vertical accretion, aeolian deposition, and a low gradient (Order C, Nanson and Croke, 1992). A subordinate share of these deposits indicated the existence of a small (<10 km2) and relatively shallow (<10 m) lake environment in the southeastern sector of the valley during the early to mid-Holocene. The lacustrine phase was initially characterized by calm, cold, and occasionally anoxic fresh waters that would have been associated with low flow and limited erosive capacity of the San Juan River. In Sector 1 (Pleistocene unit), the lower half of the Valentín Formation outcrop comprises the facies associations CH(SB), LV, FF1, CHA, FF2, and FE (Table 3). This sequence display channels characterized by monoepisodic filling immersed in thick floodplain facies, which would indicate the dominance of vertical aggradation. The floodplain (inter-channel areas) can be divided into three zones: a proximal zone, dominated by sandy facies deposited by main channels overflows, marked by the frequent presence of CS, CR, and to a lesser extent LV associations (Table 3); a transition zone, towards less energetic areas in an intermediate to a distal position, characterized by the FF1 association, with abundant vertical bioturbation and frequently affected by overflows from a network of secondary clay channels that cut through the flood plain (CHA); and finally a distal area marked by the associations FF2 and FE with deposition by settling in ephemeral pools and aeolian aggradation. The development of monoepisodic channels within massive flood­plains, the preservation of the upper surface of flow sedimentary structures, and the presence of several buried incipient paleosols suggest high sedimentation rates (Nadon, 1994; Gibling, 2006). From the numerical ages and the stratigraphic thickness, we estimated an average sedimentation rate of ~7 mm/year for the Pleistocene unit (16.770 a 15.160 cal. years BP). Some of these immature paleosols, called protosols according to the classification of Mack et al. (1993), showed accumulation of illuvial clay indicative of an argillic horizon, although poorly developed, therefore called argillic protosol. Other features observed were desiccation cracks, raindrop marks, and the development of cyclic greenish and reddish muds in the floodplain that, together with argillic protosols, would indicate semi-arid to possibly semi-humid seasonal conditions (Mack et al., 1993). However, the intercalation of the aeolian facies on top of aggradation cycles in the floodplain, and the presence of syngenetic gypsum, would reflect a greater degree of aridity and a marked seasonality (Tripaldi et al., 2001).The upper half of the Pleistocene unit composed of the LA, CS, CR, FF1, FF2, and FE facies associations represent an environmental change to more arid conditions. This sequence characterizes by lateral accretion bars (point bar, association LA) and greater development of aeolian facies (FE association) that are interrupted by the deposition of fine-grained alluvial facies in ephemeral pools and poodles (FF2 association). These fine-grained deposits show rainwater droplet marks and syngenetic gypsum in the form of veins and coatings, indicating seasonality. Salinas (1979) observed that grain size distributions (histograms) from some samples of the Valentín Formation were bimodal with the main mode in the very fine silt fraction and a secondary mode in very fine sands. Salinas (1979) interpreted this bimodality as a result of both fluvial and aeolian sedimentary inputs. The presence of gypsum in specks, veins, rosettes, and forming coatings on top of some channels, and the predominantly yellowish to reddish coloration of the deposits would be indicative of oxidizing conditions in a more arid and markedly seasonal climate (Watson, 1992).In Sector 5 (Fig. 4), close to the Zonda gorge, the Holocene unit of the Valentín Formation develops a shallowing lacustrine sequence characterized by the association of facies FLS (sub-littoral), FLM (sub-littoral to littoral), BD (littoral to supra-littoral), and FF2 (floodplain). The immediate passage from coarse alluvial channel facies (GB association) to a relatively deep lake environment (FLS association) suggests that the formation of the lake would have occurred abruptly, shortly before 9,475 years BP (Blanc and Perucca, 2017). The impressions of vegetal remains parallel to lamination found in the FLS association resemble the leaves and stems of reeds. The riparian nature of reeds and the integrity of their remains reflected in the clarity and detail of their casts would indicate a parautochthonous origin with little or no transport before deposition. The clear-cut lamination and the fine-grained size of sediments (silts and clays) would indicate deposition in calm waters. The greenish-white coloration of the deposit suggests slightly reducing conditions, possibly caused by localized anoxia related to the decomposition of organic matter (Bridge and Demicco, 2008). The passage to the reddish-brown FLM association, which conformably overlies the FLS association, would mark the opening of the system and the entry of a higher water and sediment input, causing the oxygenation of the water column. We observed no evidence of subaerial exposure in the FLS and FLM associations so that these deposits would have formed below the zone of annual fluctuation of the water level on the sublittoral belt. On top of the FLM association, sands with flow sedimentary structures interbedded with laminated muds showing load-casts linked to rapid sedimentation were interpreted as sandy bar deposits (BD association; Wright, 1977; Orton and Reading, 1993). The superimposed iron oxides on the lacustrine sequence deposits would indicate a change from reducing to oxidizing conditions, possibly as a result of an eventual lowering of the water table after the lake filled up with sediments. We estimated an average accumulation rate of ~2.1 mm/year for this lake, a value significantly lower than the estimations for the Pleistocene unit (7 mm/year). Towards the top of the sequence, the FF2 association represents the progradation of a distal fluvial plain as deduced from the presence of millimetric layers of organic matter, high bioturbation, and the development of incipient paleosols with clay translocation (argillic protosol). In Sector 2, the deposits of the Holocene unit show horizontal lamination and little development of lithosomes (some overflow lobes), indicating that deposition from torrential floods would be one of the dominant aggradation mechanisms. This sequence was interpreted as a fluvial system with multi-episodic channels and unconfined sandy floodplains, with significant vertical accretion and aeolian sedimentation (order C, Nanson and Croke, 1992).In summary, the paleoclimatic record of the Pleistocene indicates a progressive aridization, from seasonal semi-arid conditions (~16,800 years BP) to more arid conditions marked by a significant increase in aeolian deposition and the presence of syngenetic gypsum in channel facies (~15,000 years BP). The Holocene record shows arid to semi-arid seasonal climatic conditions (based on the accumulation of illuvial clay in some paleosols, fossil content, raindrop marks, and desiccation cracks) during the early to mid-Holocene (~8,300 to ~8,000 years AP). The Holocene unit would have started its deposition shortly before 9,475 years BP with a sudden increase in the local base level and the formation of a small lake in sector 5. In sector 2, the transition from alluvial fan facies to the fine-grained deposits of the Valentin Formation would have occurred ~1,100 years later, between 8,330 and 8,180 years BP under markedly seasonal arid climatic conditions. The beginning of the 8,200 BP cold period (Beget, 1983; Douglas et al., 2015; among others) would have caused an increase in river flow and sedimentary input to the lake. As a result, the lake in sector 5 filled up with sediment around ~8,100 years BP, giving way to a fluvial plain in a relatively cold, arid to semi-arid, and markedly seasonal climate. Finally, the maximum aggradation (registered in sector 6) would have occurred 600 years later, shortly after 7,685 years BP (Blanc and Perucca, 2017) in a seasonal arid climate.The deposition of the Valentín Formation would have occurred in episodic aggradation events partially coincident with the Last Glacial Maximum and the 8,200 BP cold period and possibly associated with non-catastrophic ruptures and reworking of the fine-grained deposits produced by natural damming of the San Juan River, upstream from the Ullum-Zonda valley (Colombo et al., 2000). This process would also have been favored by tectonic deformation and uplift in the eastern Precordillera, and during the Holocene, by an increase in seasonal torrential rainfalls, which would have contributed to obstructing with debris the San Juan river flow through the Zonda gorge. The alluvial fraction of the illite-chlorite-kaolinite clay assemblage described in the Valentín Formation by Salinas (1979) would locate the main provenance area of these sediments in the glaciated regions of the Andes Mountains and a secondary sedimentary input from the Precordillera.

Fecal pellets and cysts of the brine shrimp Artemia (Crustacea, Branchiopoda, Anostraca) constitute an important component of the biogenic carbonate sedimentation in many saline and hypersaline lakes bodies. However, in Argentina there are not studies of its presence in the fossil record, being this paradoxical given the abundance of large and diverse endorheic salt basins existing in the country. In this study we present the morphological and petrographic characteristics of fecal pellets and cysts of Artemia sp. and its association with evaporitic minerals occurring in a sedimentary core covering the last ca.1300 years (1220 years cal. BP) from the Chasicó Lake (38 ° 37 ‘S; 63 ° 05’ W) (Fig. 1) discussing the potential of these indicators in paleoenvironmental studies. Taking into account the ecological requirements of this crustacean, the abundance of fecal pellets and cyst in the studied deposits is discussed in relation to the physical-chemical changes that occurred during that time.The sedimentary section was carried out considering the changes of texture, structure and color of the layers to select 18 sampling intervals (Fig. 2). The qualitative determination of the total sample was performed by X-ray powder diffraction analysis. The fecal pellets are white to dry light gray and are presented in elongated cylindrical sections (Fig. 3), with one end straight and another in blunt point. They have a maximum average length of 680 µm (Fig. 4; Table 1) with a rounded net edge. They consist of a carbonaceous sludge, with a darker core of organic matter and clay, an edge with an outer shell of authigenic carbonate in very small crystals (Fig. 5). In the impregnated sections, light and dark sheets were differentiated. In light sheets, pellets with ovoidal and elongated shapes are concentrated. In contrast, in the dark layers, the pellets are presented in a smaller proportion and are distributed in a weave of gray fibers of organic matter that would correspond to a microbial mat. The cysts are reddish with an average size of 232.36 ± 8.7 µm (Table 1, Fig. 3) and are entire, broken (open) and invaginated.A succession of siliciclastic mud (mostly formed by extrabasinal clastic components) and carbonaceous peloidal micritic-aragonitic mud with evaporites minerals (mostly intrabasinal chemical and biochemical components) was recognized. The sequence was divided into four units (Fig. 2): (1) Unit C: grayish green (5Y 3/1; 5 and 4/2, 5Y 5/4) laminated and disturbed micritic-aragonitic mud with halite and thenardite (76-143 cm); (2) Unit B: gray (10YR 6/1 and 10YR 4/2) disturbed massive micritic-aragonitic mud with thenardite (Na2SO4) (57-76 cm); (3) Unit A: gray (10YR 5/1) micritic-aragonitic laminates mud with halite and thenardite subordinate (20-57 cm); (4) Unit H: gray (10YR 5/1) siliciclastic and micritic massive mud with halite (0-20 cm) (Fig. 2). Artemia sp. fecal pellets and cysts were registered between ca. 730 AD and 1978 AD and are absent after ca. 1978 AD in the lacustrine deposits of Chasicó Lake.The sedimentary section (Fig. 2) showed a predominance of biochemical and chemical deposition and very little terrigenous material between ca. 730 AD and 1411 AD (units B and C), suggesting that the accumulation would have occurred far from the coastal margins and at depths equal to or greater than 3 m (Last, 1994). The increase of terrigenous material in some samples and the presence of lamination in unit A (subunit A1) (between ca. 1411 AD and 1978 AD) would also support that the deposition of this unit occurred in the central areas of the lake and would indicate the absence of both bottom currents (Eardley and Gvosdetsky, 1960) and seasonality in the deposition, with intervals with greater extra-basinal contribution. From ca. 1978 AD (unit H) there is an increase in the extra-basin contribution coinciding with the greater contributions from the Arroyo Chasicó water basin.The differences in mineralogy of evaporitic species associated with the presence of fecal pellets and cysts of Artemia sp. in units A, B and C, and the absence of pellets and cysts in unit H allowed recognizing four different stages that suggest net changes in the chemical environment of the lake during the last 1300 years (Fig. 2). Between ca. 730 AD and 1233 AD (unit C) there was a greater proportion of fecal pellets and cysts associated with halite and thenardite, the later as a stable phase of the mirabilite. This mineralogical association would reflect periods with high salinities (extreme hypersaline environments) that allowed the formation of halite and intervals of lower salinity (hypersaline environments) with deposition of sodium sulfate. Between ca. 1233 AD and 1411 AD (unit B) the presence of free thenardite from other salts suggests the formation of mirabilite as primary sulfate, which occurred during cold periods and salinity below the saturation index of halite. Deposits with thenardite, as the only evaporitic mineral, suggest lower salinities than in unit C that would have occurred during or after its deposition, which prevented the precipitation of chlorides. Coincidentally, a lower representation of Artemia sp. cysts was observed. From ca. 1411 AD to 1978 AD (unit A) the sequence presents sectors with lamination and abundant of fecal pellets of Artemia sp. and very thin to thin stratification (subunit A1, Fig. 2). The presence of halite suggests a significant increase in salinity over the entire period that allowed the sulfate to be in solution, or that its deposition occurred only in the form of mirabilite in coastal areas by cooling during winters. Between ca. 1978 AD and 2010 AD (unit H) fecal pellets and cysts of Artemia sp. are absent and halite is the only evaporitic mineral. This apparent contradiction would be explained by the formation of interstitial brine and an intrasedimentary precipitation of chloride, under current salinity conditions and not the salinity that corresponds to the deposition of muds. The absence of Artemia sp. was related to a reduction in salinity and the entry of predators to the lake, such as the pejerrey (Odontesthes bonariensis) that created unfavorable conditions for the reproduction and proliferation of Artemia sp. The presence and/or absence of fecal pellets and cysts of Artemia sp. showed a positive correlation with the increase and/or decrease in salinity in the lake with respect to the estimated values from precipitated salts. Inferences for the 20th century are supported by the historical and instrumental data from the Chasicó Lake.Taking into account that the cyst size is a biometric characteristic of the Artemia sp. species, this measure was used to assign the fossil cysts to A. persimilis, although specific determinations are necessary to ensure it. The presence of this native species before ca. 1978 AD and its subsequent absence suggests ecological reasons related to the effects of climate change that occurred since the end of 1970s and raises questions about the current biogeographic distribution of Artemia sp. in Argentina in relation to these changes.

The Neogene Vinchina Basin developed between 27 and 33°S as a foreland basin as a response to the Andean deformation in the southern part of the Central Andes. The Miocene Vinchina Formation (Turner, 1964) was deposited mainly in fluvial, fluvial-eolian, eolian, and lacustrine environments, reaching up to 6400 m in thickness representing the main depositional unit in the basin. This unit is remarkably exposed along the Sierra de Los Colorados (La Rioja Province, Argentina), where this study was carried out.The results from statistical analyses from 33 gravel beds, their modal compositions, compositional trends, and distribution patterns, based on in-situ lithological clast-counting, are presented. The statistical approach was carried out by using hierarchical clustering and principal components analysis (PCA), which permitted differentiating three compositional clusters thought to represent different petrofacies. The first cluster involves samples with compositions widely dominated by neovolcanic andesitic clasts, where intrabasinal volcanic effusions were the main detrital source. The second one comprises samples with mixed compositions dominated by paleovolcanic clasts. Detrital sources of this cluster were primarily the cordilleran/precordilleran area, with subordinated contributions from the Western Sierras Pampeanas crystalline basement and intrabasinal volcanic deposits. The third cluster involves mixed samples with a dominance of crystalline-basement supply. The main source area for these samples was the Western Sierras Pampeanas, although the cordilleran/precordilleran supply is present in significant proportions. This study illustrates the merit of using non-parametric statistics in provenance studies, especially to detect internal compositional variations when multiple source areas are active.

It is commonly assumed in the high-resolution sequence stratigraphic analysis of shallow-marine deposits (e.g., deltaic and shoreface settings) that the depositional conditions of the system remain relatively constant during the transit of a shoreline that would eventually produce a single parasequence. However, based on the detailed sedimentary and architectural analysis of upper-shoreface and foreshore strata of two Early Cretaceous shoreface-shelf parasequences (Neuquén Basin, Argentina), it was possible to document a vertical change through the stratigraphy from deposits representing wave-dominated barred shorelines to deposits interpreted as representing a non-barred morphology. The presence of a well-defined limit between trough cross-bedded sandstones in the upper shoreface and planar laminated sandstones in the foreshore (and the presence of a surf diastem) characterize the development of barred shoreline conditions. Instead, planar lamination is ubiquitous within non-barred deposits, where trough cross-bedding is restricted to the bottomsets of the large-scale inclined beds that characterize this architectural style. Thickness, sediment composition and reconstructed shoreline trajectory also seemingly change vertically within the investigated parasequences. Collectively, these pieces of evidence suggest that the vertical transition from barred to non-barred deposits at this intra-parasequence scale could be related to wave-climate variations and the sequence-stratigraphic context. Specifically, changes in the prevailing wave behavior from dissipative to reflective conditions could be a feasible explanation for the morphological transformation of coastal systems through tens of thousands to hundreds of thousands years.