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The Triassic successions of western Argentina commonly show thin pyroclastic levels intercalated within thick fluvial and lacustrine terrigenous deposits. The Potrerillos Formation is the thickest Triassic unit in the Cuyo Basin. It is composed of alternating cycles of gravelly- sandy- and muddy-dominated intervals, in which several laterally-continuous tuff horizons occur. U-Pb SHRIMP ages were determined on zircon grains from three tuff levels located between the lowermost and the middle sections of the Potrerillos Formation. The ages for the time of deposition of the tuffs are 239.2 ± 4.5 Ma, 239.7 ± 2.2 Ma and 230.3 ± 2.3 Ma (Middle Triassic). Chemical data indicate that these acid to intermediate volcaniclastic rocks are derived from coeval basic magmas displaying tholeiitic to slightly alkaline signatures. They are associated with the rift stage that followed the extensive post-orogenic volcanism of the Choiyoi Group, that in turn has been ascribed to slab break-off in neighbouring areas. Two of the studied samples also record a subpopulation of inherited zircon grains with crystallisation ages of 260-270 Ma. The latter are considered to be an indirect measurement for the age of the Choiyoi Group in the Cuyo basin. The rift-related Triassic event represents the culmination of the Gondwanian magmatic cycle, and is interpreted as the result of subduction cessation and anomalous heating of the upper mantle previous to the western Gondwana break-up.

The northernmost outcrops of the Triassic extensional Cuyo basin in western Argentina reveal a basal coarse conglomerate overlain by a thinning-fining upward siliciclastic red-bedded section with subordinated tuffs and microbial-rich carbonates (Cerro Puntudo Formation), unconformably covered by a second coarse conglomerate (El Relincho Formation), which were previously interpreted as two rifting events. An integrated analysis of facies associations, stacking patterns, paleocurrent and provenance was carried out in order to reveal the rift sedimentation using a source-to-sink approach. Besides the traditional controls in continental basins (e.g., sedimentation, tectonics and climate), this study considers possible drainage pattern evolution in order to differentiate isolated versus integrated depocenter stages. U-Pb zircon data corroborate our analysis and reveal a volcanic basement source of 256.8 ± 3.5 Ma (Permian/Lopingian) and a sedimentation age of 249.8 ± 2.5 Ma (Early Triassic) for a tuff interbedded within the Cerro Puntudo Formation, complementing previous age of 243.8 ± 1.9 Ma (Middle Triassic) at the top of this unit. Stratigraphic stacking allows the interpretation of three evolutionary stages. A first stage, represented by thick massive boulder-volcanic-rich conglomerates characterizing alluvial fan deposition, indicates strong tectonic activity of border faults (first rifting event). These features suggest high surface gradient and high available space controlling accommodation and dispersal of gravity and debris-flow deposits. Paleocurrents to the east reveal largely north-south normal faults located to the west and sediment supply from a proximal volcanic source area. A second stage, represented by a notable fining-thinning upward facies, including fluvial and palustrine (stromatolites with pedogenetic features) deposits, indicates a transition to tectonic quiescence. These facies associations developed in a context of low sedimentation rate, limited tectonic relief and low accommodation space. A third stage is represented by greenish well-rounded coarse-grained polymictic conglomerates of El Relincho Formation that unconformably truncate the Cerro Puntudo Formation. These conglomerates are interpreted as perennial braided fluvial systems (with paleocurrents to the northwest) indicating drainage reorganization primarily influenced by climate change and combined tectonic reactivation, previously considered the second rifting event. Climate change affected sediment delivery and allowed regionally connectivity and overfilling of the depocenters. In this last stage, northwest axial transport from variable sources within the Precordillera (to the southeast) is inferred from provenance analysis indicating open linked-depocenters. This study suggests a new stratigraphic correlation for the northern part of the Cuyo basin, implying that during the first rifting event a semi-arid climate prevailed with development of isolated and independent depocenters (first and second stages), whereas above the unconformity initiating the second rifting event (third stage) depocenters were interconnected. In the light of our source-to-sink analysis, such paleogeographic change allowed overfilling and laterally linkage of separate half-grabens along the Cuyo basin. Whereas tectonic reactivation may better explain renewed conglomerate deposition, climate change toward more humid conditions allowed previously separated depocenters to be connected through axial drainages. Fil: Teixeira, Bárbara M. N.. Petrobras. Petróleo Brasileiro S. A; Argentina Fil: Astini, Ricardo Alfredo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; Argentina Fil: Gomez, Fernando Javier. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; Argentina Fil: Morales, Norberto. Universidade Estadual Paulista. Instituto de Geociências e Ciências Exatas; Brasil Fil: Pimentel, Marcio. Universidade do Brasília; Brasil

The northern area of Cuyo Basin (west‐central Argentina) corresponds to the Rincón Blanco half-graben, whose filling is arranged into the Rincón Blanco and Sorocayense groups. In the present study, we propose a new stratigraphic scheme for the Sorocayense Group in the Barreal depocenter (San Juan Province), revise the palaeoenvironmental interpretations, and organize and analyse the plant assemblages of previous and new fossiliferous levels. We defined three tectosedimentary sequences. The basal sequence represents the initial graben filling with pyroclastic flows, alluvial fan, and ephemeral fluvial systems, and is arranged in a new unit, the Cerro Colorado del Cementerio Formation. The middle sequence, including Barreal and Cortaderita formations, is characterized by sediment gravity flow deposits and different fluvial systems, with development of floodplains with vertisols and calcisols, and temporal ponds/lakes. In turn, the Cortaderita Formation was divided into the following lithostratigraphic members: Don Raúl and La Emilia. The upper sequence, represented by Cepeda Formation, was deposited by distributary fluvial and ephemeral fluvial systems. Twelve fossiliferous strata (EF) were recognized, which were all identified in the middle sequence: EF1 to EF3 in the Barreal Formation, EF4 to EF8 in the Don Raúl Member, and EF9 to EF12 in the La Emilia Member, both of Cortaderita Formation. The taphocoenosis found in the Barreal and Cortaderita formations were dominated by corystosperms, with conifers, cycadales, and peltasperms as subordinate forms. The available evidence from the taphofloras, palaeosols and palaeonvironments indicate the development of seasonal subtropical climates, which vary from arid or semi-arid in the basal sequence, sub-humid to semi-arid in the middle sequence, and arid or semi-arid in the upper sequence. On the basis of the new information, geological correlation and age of the lithostratigraphic units were accurately re-evaluated. Based on the palaeofloristic content and the correlation with the Rincón Blanco Group, it is possible to infer that the basal sequence was accumulated during late Early Triassic-early Middle Triassic, the middle sequence during the Middle Triassic, while the upper sequence was deposited in the early Late Triassic., El área norte de la cuenca Cuyana (centro-oeste de la Argentina) corresponde al hemigraben de Rincón Blanco, cuyo relleno comprende los grupos Rincón Blanco y Sorocayense. En este estudio, se propone un nuevo esquema estratigráfico para el Grupo Sorocayense en el depocentro de Barreal, se reinterpretan los paleoambientes, y se organizan y analizan las asociaciones plantíferas de niveles fosilíferos nuevos y previamente estudiados. Se definieron tres secuencias tecto-sedimentarias. La secuencia basal representa el relleno inicial del hemigraben con depósitos de flujos piroclásticos, abanicos aluviales y sistemas fluviales efímeros, y es incluida en una nueva unidad: Formación Cerro Colorado del Cementerio. La secuencia media, que comprende a las formaciones Barreal y Cortaderita, se caracteriza por depósitos de flujos gravitacionales de sedimentos, y diferentes sistemas fluviales, con planicies de inundación en las que se desarrollaron vertisoles y calcisoles, y estanques o lagunas temporales. La Formación Cortaderita fue dividida en los siguientes miembros litoestratigráficos: Don Raúl y La Emilia. La secuencia superior, representada por la Formación Cepeda, fue depositada por sistemas fluviales distributarios y efímeros. Se reconocieron doce estratos fosilíferos (EF), todos identificados en la secuencia media: EF1 al EF3 en la Formación Barreal; EF4 al EF8 en el Miembro Don Raúl; y EF9 al EF12 en el Miembro La Emilia, ambos de la Formación Cortaderita. Las tafocenosis halladas están dominadas por corystospermas, con las coníferas, cycadales y peltaspermas como formas subordinadas. La evidencia disponible acerca de las tafofloras, paleosuelos y paleoambientes indican el desarrollo de climas subtropicales estacionales, que varían de áridos a semiáridos en la secuencia basal, subhúmedos a semiáridos en la secuencia media, y áridos o semiáridos en la secuencia superior. Con la nueva información, se reevaluaron las correlaciones y la edad de las unidades en forma más precisa. Sobre la base del contenido paleoflorístico y la correlación con el Grupo Rincón Blanco, se infiere que la secuencia basal se depositó durante el Triásico Temprano tardío–Triásico Medio temprano, la secuencia media durante el Triásico Medio y la secuencia superior durante el Triásico Tardío temprano., Facultad de Ciencias Naturales y Museo

A new actinopterygian, Calaichthys tehul gen. et sp. nov. is described on the basis of a few, well-preserved specimens from the Anisian Cerro de Las Cabras Formation, Cuyo Basin in Mendoza Province. The new genus shows a combination of primitive characters (e.g. deep posterior region of the maxilla contacting the preopercle, a suspensorium backwardly oriented) and more advanced characters (e.g. distally segmented fin rays, hemiheterocercal caudal fin) and is thus considered to be a ‘subholostean’ fish. The new taxon is assigned to Redfieldiiformes on the basis of its single branchiostegal plate-like ray, a median gular plate, pectoral fins with stout, stiff, mainly non-segmented and distally branched fin rays, long and robust fringing fulcra in all fins, basal fulcra bordering both lobes of the caudal fin, and opposite dorsal and anal fins. Calaichthys is characterized by a combination of characters some of which are unique among redfieldiiforms (i.e. strongly ornamented but thin scales with a serrated posterior margin, smooth skull roof bones, extrascapulars with a bifid posterior margin, three suborbital bones, a hatchet-shaped preopercle, rectangular and slender opercle, deep mouth gape, entopterygoid and ectopterygoid with small pointed teeth disposed in several series, unornamented rostral bone, massive pectoral fins and delicate dorsal, pelvic and anal fins). Calaichthys provides novel information about the anatomy of redfieldiiforms, in particular in relation with the fins and associated scaly elements, as well as the scale morphology and their disposition over the body. Our work confirms for the first time that redfieldiiforms occur in South America. Redfieldiiforms seem to have originated in Gondwana and shown two diversity peaks: in the Anisian and the Carnian. Calaichthys is coeval with those redfieldiiforms of the Middle Triassic (Anisian) of Africa and Australia dwelling in ephemeral lakes. Fil: Gouiric Cavalli, Soledad. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Departamento Científico de Paleontología de Vertebrados; Argentina Fil: Zavattieri, Ana Maria. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Provincia de Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Universidad Nacional de Cuyo. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; Argentina Fil: Gutierrez, Pedro Raul. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”. Área de Paleobotánica y Paleopalinología ; Argentina Fil: Cariglino, Barbara. Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”. Área de Paleobotánica y Paleopalinología ; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Balarino, María Lucía. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”. Área de Paleobotánica y Paleopalinología ; Argentina

Potrerilloxyela menendezi gen. et sp. nov. from the Potrerillos Formation, Cuyo Basin, early Upper Triassic, Mendoza Province, Argentina is described. It is included in the living family Xyelidae and tentatively placed in Liadoxyelini of Xyelinae, which is otherwise known only from the Jurassic of Asia. This record is the first for Hymenoptera in the Triassic of the New World and provides new evidence about the close faunal connections between Laurasia and Gondwana during Mesozoic times. The phylogenetic position of the new genus is discussed. Fil: Lara, Maria Belén. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Nordeste. Centro de Ecología Aplicada del Litoral. Universidad Nacional del Nordeste. Centro de Ecología Aplicada del Litoral; Argentina Fil: Rasnitsyn, A. P.. Natural History Museum; Reino Unido. Paleontological Institute; Rusia Fil: Zavattieri, Ana Maria. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Provincia de Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Universidad Nacional de Cuyo. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; Argentina

Se presenta el estudio sistemático de la fauna de Spinicaudata de la Formación Cerro de Las Cabras (Triásico Medio), norte del cerro Bayo, en la clásica localidad de Potrerillos, Precordillera de Mendoza, Cuenca cuyana, Argentina. Se describen dos nuevas especies: Estheriellites zavattieriae sp. nov. y Euestheria menendezi sp. nov., se enmienda la diagnosis del género y de la especie Challaolimnadiopsis mendozaensis Shen and Gallego, y se describe la especie Euestheria martinsnetoi Gallego et al., previamente citada para la Formación Río Mendoza y que ahora se considera perteneciente a los estratos de Cerro de Las Cabras. La sección estudiada consiste de finas areniscas tufáceas y pelitas interpretadas como depósitos de barreales, ríos y/ o arroyos efímeros de la planicie fluvial. Lagos efímeros o ambientes lacustres someros están caracterizados por una sucesión de limolitas interestratificadas con calizas nodulares-moteadas y estromatolíticas con exposición sub-aérea e incipiente desarrollo de suelos en áreas deprimidas y anegadas bajo condiciones de clima estacional semiárido. Los espinicaudados probablemente habitaron los ambientes de las costas de los lagos y los pequeños pantanos de la planicie fluvial. La profusa presencia de depósitos piroclásticos a lo largo de la secuencia indica una apreciable actividad volcánica explosiva, coetánea con la sedimentación. Sobre la base de consideraciones litoestratigráficas, paleontológicas y dataciones realizadas previamente en esta unidad, la edad de los depósitos estudiados fueron limitados al Triásico Medio (equivalente al Anisiano Tardío-Ladiniano Tardío de las secuencias marinas) y por lo tanto, la asociación faunística E. zavattieriae-E. martinsnetoi es considerada como indicadora para tal edad. This is a systematic study of the Spinicaudata fauna of the Cerro de Las Cabras Formation (Middle Triassic), north of Cerro Bayo, in the classical Potrerillos area, Precordillera of Mendoza, Cuyo Basin, Argentina. Two new species are described: Estheriellites zavattieriae sp. nov. and Euestheria menendezi sp. nov.. The diagnosis of the genus and species of Challaolimnadiopsis mendozaensis Shen and Gallego, is emended; and Euestheria martinsnetoi Gallego et al., previously described for the Río Mendoza Formation is now considered as coming from the Cerro de Las Cabras strata. The studied section consists mainly of fine-grained tuffaceous sandstones and interbedded mudstones interpreted as playa lake, ephemeral rivers and/or streams developed in the floodplain. Ephemeral to shallow lacustrine environments are characterized by interbedded siltstones, nodular-mottle and stromatolitic limestones with sub-aerial exposures and incipient soil development in depressed and flood-prone areas under seasonal semi-arid conditions. Spinicaudatans probably inhabited lacustrine shorelines and small ponds in the floodplain where plant remains (arthrophyte) and rare fish scales were also recorded. The profuse presence of pyroclastic deposits along the sequence indicates considerable explosive volcanic activity coeval with sedimentation. Based on previous lithostratigraphical, paleontological and chronological data, the age of the studied deposits was constrained to Middle Triassic (equivalent to Late Anisian—Late Ladinian of marine sequences) and therefore the E. zavattieriae-E. martinsnetoi Faunal Association is considered as a marker for such age range. Fil: Vaz Tassi, Lara. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Nordeste. Centro de Ecología Aplicada del Litoral. Universidad Nacional del Nordeste. Centro de Ecología Aplicada del Litoral; Argentina Fil: Zavattieri, Ana Maria. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Provincia de Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Universidad Nacional de Cuyo. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; Argentina Fil: Gallego, Oscar Florencio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Nordeste. Centro de Ecología Aplicada del Litoral. Universidad Nacional del Nordeste. Centro de Ecología Aplicada del Litoral; Argentina

The Triassic successions of western Argentina commonly show thin pyroclastic levels intercalated within thick fluvial and lacustrine terrigenous deposits. The Potrerillos Formation is the thickest Triassic unit in the Cuyo Basin. It is composed of alternating cycles of gravelly- sandy- and muddy-dominated intervals, in which several laterally-continuous tuff horizons occur. U-Pb SHRIMP ages were determined on zircon grains from three tuff levels located between the lowermost and the middle sections of the Potrerillos Formation. The ages for the time of deposition of the tuffs are 239.2 ± 4.5 Ma, 239.7 ± 2.2 Ma and 230.3 ± 2.3 Ma (Middle Triassic). Chemical data indicate that these acid to intermediate volcaniclastic rocks are derived from coeval basic magmas displaying tholeiitic to slightly alkaline signatures. They are associated with the rift stage that followed the extensive post-orogenic volcanism of the Choiyoi Group, that in turn has been ascribed to slab break-off in neighbouring areas. Two of the studied samples also record a subpopulation of inherited zircon grains with crystallisation ages of 260-270 Ma. The latter are considered to be an indirect measurement for the age of the Choiyoi Group in the Cuyo basin. The rift-related Triassic event represents the culmination of the Gondwanian magmatic cycle, and is interpreted as the result of subduction cessation and anomalous heating of the upper mantle previous to the western Gondwana break-up.

The Triassic successions of western Argentina commonly show thin pyroclastic levels intercalated within thick fluvial and lacustrine terrigenous deposits. The Potrerillos Formation is the thickest Triassic unit in the Cuyo Basin. It is composed of alternating cycles of gravelly- sandy- and muddy-dominated intervals, in which several laterally-continuous tuff horizons occur. U-Pb SHRIMP ages were determined on zircon grains from three tuff levels located between the lowermost and the middle sections of the Potrerillos Formation. The ages for the time of deposition of the tuffs are 239.2 ± 4.5 Ma, 239.7 ± 2.2 Ma and 230.3 ± 2.3 Ma (Middle Triassic). Chemical data indicate that these acid to intermediate volcaniclastic rocks are derived from coeval basic magmas displaying tholeiitic to slightly alkaline signatures. They are associated with the rift stage that followed the extensive post-orogenic volcanism of the Choiyoi Group, that in turn has been ascribed to slab break-off in neighbouring areas. Two of the studied samples also record a subpopulation of inherited zircon grains with crystallisation ages of 260-270 Ma. The latter are considered to be an indirect measurement for the age of the Choiyoi Group in the Cuyo basin. The rift-related Triassic event represents the culmination of the Gondwanian magmatic cycle, and is interpreted as the result of subduction cessation and anomalous heating of the upper mantle previous to the western Gondwana break-up., Centro de Investigaciones Geológicas

Nuevas exposiciones en localidades triásicas clásicas como las de los cerros Bayo y Cocodrilo (Potrerillos, noroeste de la Provincia de Mendoza, Argentina) permitieron (1) describir y caracterizar las formaciones Río Mendoza, Cerro de Las Cabras, Potrerillos y Cacheuta; y (2) mejorar el conocimiento bioestratigráfico de las sucesiones triásicas basándose en el hallazgo de nuevos registros de plantas fósiles. Dos secciones verticales fueron estudiadas en detalle, y el análisis de las litofacies ha sido usado para interpretar los sistemas de deposición. Facies gruesas de abanico aluvial caracterizan a la Formación Río Mendoza y a la sección inferior de la Formación Potrerillos. Sistemas fluviales traccionales gravosos y arenosos aparecen hacia el techo de la Formación Río Mendoza y en distintos intervalos de la Formación Potrerillos. En el tramo basal de la Formación Cerro de Las Cabras y en la mayor parte de la Formación Potrerillos se identificaron depósitos de sistemas fluviales de alta sinuosidad y de planicies de inundación. Finalmente, algunos intervalos de la Formación Potrerillos y más específicamente la Formación Cacheuta están dominados por sucesiones de lutitas negras generadas por procesos suspensivos en lagos meromícticos. En asociación con los depósitos mencionados, secuencias progradacionales discretas de areniscas y conglomerados sugieren el desarrollo de barras de desembocadura hipopicnales y de canales distributarios. Se individualizaron 16 niveles fosilíferos con 23 taxones: Equisetites fertilis, Neocalamites carrerei, Cladophlebis mesozoica, C. mendozaensis, Dicroidium argenteum, D. dubium, D. odontopteroides, Johnstonia coriacea, J. stelzneriana, Xylopteris elongata, Zuberia feistmanteli, Feruglioa samaroides, Pachydermophyllum praecordillerae, Kurtziana cacheutensis, Sphenobaiera argentinae, Baiera africana, B. cuyana, Yabeiella mareyesiaca, Y. spathulata, Y. brackebuschiana, Fraxinopsis andium y Gontriglossa sp. y Rochipteris sp. Se describieron 16 tafocenosis que se relacionaron con los respectivos ambientes de sedimentación. El análisis de las tafocenosis y su vinculación con las facies y paleoambientes de acumulación permitió reconocer en la localidad tipo de la Formación Potrerillos a dos biozonas de asociación en continuidad estratigráfica (Yabeiella mereyesiaca-Scytophyllum bonettiae-Protophyllocladoxylon cortaderitaensis -MBC- y Yabeiella brackebuschiana-Scytophyllum neuburgianum-Rhexoxylon piatnitzkyi -BNP-). La Formación Potrerillos en este sector de la cuenca se desarrolló durante el Triásico Medio tardío-Triásico Tardío temprano New exposures of the classic Triassic localities of cerro Bayo and cerro Cocodrilo (Potrerillos, northwestern Mendoza province, Argentina) allowed to: (1) describe the stratigraphic architecture of the Río Mendoza, Cerro de Las Cabras, Potrerillos and Cacheuta formations; and (2) to improve the biostratigraphic knowledge of the Triassic succession based on new findings of fossil plants. Two vertical sections were measured in detail, and lithofacies analysis has been used to interpret depositional systems. Alluvial fan deposits are recognized in the Río Mendoza Formation and in the lower part of the Potrerillos Formation. Gravely and sandy bedload fluvial deposits appear towards the top of the Río Mendoza Formation and in several intervals of the Potrerillos Formation, whereas the basal part of the Cerro de Las Cabras Formation and almost all the Potrerillos Formation are characterised by high sinuosity stream and flood plain deposits. Some intervals of the Potrerillos Formation, but specifically the Cacheuta Formation, are dominated by black shale successions deposited from suspension fallout in meromictic lakes. Progradational sandstone and conglomerate intercalations within black shale successions suggest the development of hypopycnal mouth bars and distributary channel deposits. Fossil plants within the studied succession are located in 16 fossiliferous levels, and 23 taxa were identified: Equisetites fertilis, Neocalamites carrerei, Cladophlebis mesozoica, C. mendozaensis, Dicroidium argenteum, D. dubium, D. odontopteroides, Johnstonia coriacea, J. stelzneriana, Xylopteris elongata, Zuberia feistmanteli, Feruglioa samaroides, Pachydermophyllum praecordillerae, Kurtziana cacheutensis, Sphenobaiera argentinae, Baiera africana, B. cuyana, Yabeiella mareyesiaca, Y. spathulata, Y. brackebuschiana, Fraxinopsis andium and Gontriglossa sp. and Rochipteris sp. Based on the relationship between the paleobotanical composition of the fossiliferous levels, sedimentary facies and depositional settings, 16 taphocenoses are defined and described. Finally, two assemblage biozones (Yabeiella mereyesiaca- Scytophyllum bonettiae-Protophyllocladoxylon cortaderitaensis -MBC- and Yabeiella brackebuschiana - Scytophyllum neuburgianum-Rhexoxylon piatnitzkyi -BNP-) are identified in the type locality of the Potrerillos Formation. This biostratigraphic information allows to suggest that the Potrerillos Formation was deposited during the late Middle Triassic - early Late Triassic

Nuevas exposiciones en localidades triásicas clásicas como las de los cerros Bayo y Cocodrilo (Potrerillos, noroeste de la Provincia de Mendoza, Argentina) permitieron (1) describir y caracterizar las formaciones Río Mendoza, Cerro de Las Cabras, Potrerillos y Cacheuta; y (2) mejorar el conocimiento bioestratigráfico de las sucesiones triásicas basándose en el hallazgo de nuevos registros de plantas fósiles. Dos secciones verticales fueron estudiadas en detalle, y el análisis de las litofacies ha sido usado para interpretar los sistemas de deposición. Facies gruesas de abanico aluvial caracterizan a la Formación Río Mendoza y a la sección inferior de la Formación Potrerillos. Sistemas fluviales traccionales gravosos y arenosos aparecen hacia el techo de la Formación Río Mendoza y en distintos intervalos de la Formación Potrerillos. En el tramo basal de la Formación Cerro de Las Cabras y en la mayor parte de la Formación Potrerillos se identificaron depósitos de sistemas fluviales de alta sinuosidad y de planicies de inundación. Finalmente, algunos intervalos de la Formación Potrerillos y más específicamente la Formación Cacheuta están dominados por sucesiones de lutitas negras generadas por procesos suspensivos en lagos meromícticos. En asociación con los depósitos mencionados, secuencias progradacionales discretas de areniscas y conglomerados sugieren el desarrollo de barras de desembocadura hipopicnales y de canales distributarios. Se individualizaron 16 niveles fosilíferos con 23 taxones: Equisetites fertilis, Neocalamites carrerei, Cladophlebis mesozoica, C. mendozaensis, Dicroidium argenteum, D. dubium, D. odontopteroides, Johnstonia coriacea, J. stelzneriana, Xylopteris elongata, Zuberia feistmanteli, Feruglioa samaroides, Pachydermophyllum praecordillerae, Kurtziana cacheutensis, Sphenobaiera argentinae, Baiera africana, B. cuyana, Yabeiella mareyesiaca, Y. spathulata, Y. brackebuschiana, Fraxinopsis andium y Gontriglossa sp. y Rochipteris sp. Se describieron 16 tafocenosis que se relacionaron con los respectivos ambientes de sedimentación. El análisis de las tafocenosis y su vinculación con las facies y paleoambientes de acumulación permitió reconocer en la localidad tipo de la Formación Potrerillos a dos biozonas de asociación en continuidad estratigráfica (Yabeiella mereyesiaca-Scytophyllum bonettiae-Protophyllocladoxylon cortaderitaensis -MBC- y Yabeiella brackebuschiana- Scytophyllum neuburgianum-Rhexoxylon piatnitzkyi -BNP-). La Formación Potrerillos en este sector de la cuenca se desarrolló durante el Triásico Medio tardío-Triásico Tardío temprano, New exposures of the classic Triassic localities of cerro Bayo and cerro Cocodrilo (Potrerillos, northwestern Mendoza province, Argentina) allowed to: (1) describe the stratigraphic architecture of the Río Mendoza, Cerro de Las Cabras, Potrerillos and Cacheuta formations; and (2) to improve the biostratigraphic knowledge of the Triassic succession based on new findings of fossil plants. Two vertical sections were measured in detail, and lithofacies analysis has been used to interpret depositional systems. Alluvial fan deposits are recognized in the Río Mendoza Formation and in the lower part of the Potrerillos Formation. Gravely and sandy bedload fluvial deposits appear towards the top of the Río Mendoza Formation and in several intervals of the Potrerillos Formation, whereas the basal part of the Cerro de Las Cabras Formation and almost all the Potrerillos Formation are characterised by high sinuosity stream and flood plain deposits. Some intervals of the Potrerillos Formation, but specifically the Cacheuta Formation, are dominated by black shale successions deposited from suspension fallout in meromictic lakes. Progradational sandstone and conglomerate intercalations within black shale successions suggest the development of hypopycnal mouth bars and distributary channel deposits. Fossil plants within the studied succession are located in 16 fossiliferous levels, and 23 taxa were identified: Equisetites fertilis, Neocalamites carrerei, Cladophlebis mesozoica, C. mendozaensis, Dicroidium argenteum, D. dubium, D. odontopteroides, Johnstonia coriacea, J. stelzneriana, Xylopteris elongata, Zuberia feistmanteli, Feruglioa samaroides, Pachydermophyllum praecordillerae, Kurtziana cacheutensis, Sphenobaiera argentinae, Baiera africana, B. cuyana, Yabeiella mareyesiaca, Y. spathulata, Y. brackebuschiana, Fraxinopsis andium and Gontriglossa sp. and Rochipteris sp. Based on the relationship between the paleobotanical composition of the fossiliferous levels, sedimentary facies and depositional settings, 16 taphocenoses are defined and described. Finally, two assemblage biozones (Yabeiella mereyesiaca- Scytophyllum bonettiae-Protophyllocladoxylon cortaderitaensis -MBC- and Yabeiella brackebuschiana - Scytophyllum neuburgianum-Rhexoxylon piatnitzkyi -BNP-) are identified in the type locality of the Potrerillos Formation. This biostratigraphic information allows to suggest that the Potrerillos Formation was deposited during the late Middle Triassic - early Late Triassic., Facultad de Ciencias Naturales y Museo