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During the late Miocene, the development of topography of the eastern margin of the Puna Plateau of the Central Andes, was related to the onset of the eastward migration and foreland fragmentation. The Guanaco and Piquete formations (late Miocene-early Pleistocene, Jujuy Subgroup) represent a thick succession of the adjacent foreland. These units outcrop in the Rio Chico (=Xibi Xibi) locality (Jujuy in north-west Argentina), a transition zone between the Eastern Cordillera (EC) and the Subandean Ranges (SR). We combine a detailed stratigraphic section, paleocurrent measurements, provenance and facies analyses, as well as a new absolute age near to the top of the Guanaco Formation (5.7 ± 0.8 Ma), and new data on the Neogene environmental changes in this region to characterize these units in the context of the regional tectono-sedimentary evolution. Our analyses suggest a progradation of an alluvial-fan system over a braided-fluvial system, linked to an increase in relief due to rapid shortening pulses and associated to the regional climatic changes in this part of the Central Andes. In the studied area, the earliest uplift of the eastern margin of the Puna Plateau and other areas of the EC generated higher topographic relief to the north and north-west, which increased the relevance of local sources and caused the deflection of paleo-rivers toward the south. The limit between the Guanaco and Piquete formations was reinterpreted to be located 1000 m higher in the stratigraphic section compared to previous proposals, in agreement with the age of the newly dated tuff. Our analysis improves the temporal and spatial characteristics of tectonic processes in the southern Central Andes and provides a framework for the study of landscape as well as biotic, and environmental changes during the late Miocene within the context of the eastward migration of the orogenic front and coeval foreland evolution in the Central Andes.

The Tres Cruces basin presents an excellent record of deposits linked to the evolution of the foreland in the context of the Andean orogeny in the Eastern Cordillera, Central Andes. The Cenozoic synorogenic deposits are represented by Casa Grande, Rio Grande, and Pisungo formations with thicknesses up to 5000 m. The detailed sedimentological and paleoenvironmental analysis on the north-central sector of the Tres Cruces basin allowed the recognition of an unconformity over post-rift Salta Group, marked by the progradation of an ephemeral fluvial system (Casa Grande Formation) over the Lumbrera Formation during the middle Eocene. The establishment of a meandering fluvial system with flow directions to the east, and an increase of lithic content versus quartz content, support the idea of a distal orogenic front uplifting to the west. During the late Oligocene, an erosive unconformity and the change to a braided fluvial system characterize the Rio Grande Formation, where the coarsening-upwards arrangement, paleocurrents with a distributary pattern, and the increase of volcanic fonts are related to the approach of the orogenic front. From the late Miocene, Pisungo Formation deposits are restricted to local syncline depressions, characterized by diffuse geometries and boulder clast size with a rotation of the paleocurrent directions, indicating the development of local highs and the low capacity of river systems. Our analyses suggest an environmental progradation from meandering to braided and later to an alluvial fan fluvial system linked with a change from distal to proximal foreland between Casa Grande and Rio Grande formations, to an intermontane system in Pisungo Formation. These results provide an adequate temporal framework for the study of environmental and landscape evolution of Eastern Cordillera and have direct implications for the local and regional development of the Andean orogenesis.

The Guanaco Formation (Oran Group) is one of the Neogene units outcropping in the Subandean region of northwestern Argentina. The fossil assemblage from the middle section was studied in a few contributions. Here we present new remains for the middle section (~6.3 Ma) and the first records for the upper section (

Very scarce information is available about the extinct Neogene communities of the Subandean valleys of Jujuy province, Northwestern Argentina. A new vertebrate fossiliferous locality (Los Alisos), and the first mammalian assemblage from Guanaco Formation, is described. The taxa recorded include mainly caviomorph rodents (Hystricognathi)represented by Orthomyctera sp., Lagostomus sp., and an Octodontoidea and cingulates (Xenarthra)represented by the armadillos aff. Stenotatus, Dasypus sp. nov., two Euphractinae, and a Glyptodontidae. A single remain of a turtle (Testudines)is also described. The composition of the faunistic record has similitude with Andalhuala, Chiquimil and Salicas formations, but some taxa differ from all other late Miocene Northwestern Argentina localities. The most outstanding feature of Los Alisos paleocommunity is the presence of Dasypus, which confirms the existence of the genus in times as old as the late Miocene. The presence of a tuff with known age (5.9–6.4 Ma)immediately below to the fossiliferous level gives a lower temporal constraint for the association, and this absolute age as well as the biochrons of the identified taxa are compatible with an Huayquerian stage. The integrative analysis of the mammals of Los Alisos allowed preliminary paleoecological and paleoenvironmental inferences for Guanaco Formation, indicating a landscape composed by open areas, whether extensive or arranged in patches, including grasslands, under warm and seasonal climatic conditions. Fil: Ercoli, Marcos Darío. Universidad Nacional de Jujuy. Instituto de Ecorregiones Andinas. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Salta. Instituto de Ecorregiones Andinas; Argentina. Universidad Nacional de Jujuy. Instituto de Geología Minera; Argentina Fil: Alvarez, Alicia. Universidad Nacional de Jujuy. Instituto de Ecorregiones Andinas. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Salta. Instituto de Ecorregiones Andinas; Argentina. Universidad Nacional de Jujuy. Instituto de Geología Minera; Argentina Fil: Santamans, Carla Daniela. Universidad Nacional de Jujuy. Instituto de Ecorregiones Andinas. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Salta. Instituto de Ecorregiones Andinas; Argentina. Universidad Nacional de Jujuy. Instituto de Geología Minera; Argentina Fil: González Patagua, Sonia A.. Universidad Nacional de Jujuy. Instituto de Geología Minera; Argentina Fil: Villalba Ulberich, Juan Pablo. Universidad Nacional de Jujuy. Instituto de Ecorregiones Andinas. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Salta. Instituto de Ecorregiones Andinas; Argentina. Universidad Nacional de Jujuy. Instituto de Geología Minera; Argentina Fil: Constantini, Ornela Estefania. Universidad Nacional de Jujuy. Instituto de Ecorregiones Andinas. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Salta. Instituto de Ecorregiones Andinas; Argentina. Universidad Nacional de Jujuy. Instituto de Geología Minera; Argentina