Ricardo N. Alonso

Giant evaporite belts of the Neogene central Andes

Large volumes of continental evaporites accumulated within the central Andes during Neogene uplift of the Altiplano-Puna plateau and development of the Andean volcanic arc. Halite and gypsum are dominant minerals, along with local and economically important borates. Playa conditions have existed since ca. 15 Ma; halite and borate deposition has occurred for the past 7 to 8 m.y. Evaporites formed in salar environments (e.g., playa lakes) and are characterized by complex mineral assemblages, occurrence, zonation, and geochemistry. Evaporite deposition was controlled by volcanism, geothermal activity, closed drainage, and climate. These Andean deposits, and their controls, differ from evaporites in other continental and marine environments.

Does the topographic distribution of the central Andean Puna Plateau result from climatic or geodynamic processes

Orogenic plateaus are extensive, high-elevation areas with low internal relief that have been attributed to deep-seated and/or climate-driven surface processes. In the latter case, models predict that lateral plateau growth results from increasing aridity along the margins as range uplift shields the orogen interior from precipitation. We analyze the spatiotemporal progression of basin isolation and fi lling at the eastern margin of the Puna Plateau of the Argentine Andes to determine if the topography predicted by such models is observed. We fithat the timing of basin fi lling and reexcavation is variable, suggesting nonsystematic plateau growth. Instead, the Airy isostatically compensated component of topography constitutes the majority of the mean elevation gain between the foreland and the plateau. This indicates that deep-seated phenomena, such as changes in crustal thickness and/or lateral density, are required to produce high plateau elevations. In contrast, the frequency of the uncompensated topography within the plateau and in the adjacent foreland that is interrupted by ranges appears similar, although the amplitude of this topographic component increases east of the plateau. Combined with sedimentologic observations, we infer that the low internal relief of the plateau likely results from increased aridity and sediment storage within the plateau and along its eastern margin.

Luminescence dating of alluvial fans in intramontane basins of NW Argentina

Abstract Alluvial fans are sensitive recorders of both climatic change and tectonic activity. The ability to constrain the age of alluvial-fan sequences, individual sedimentary events and the rates of sediment accumulation are key for constraining which mechanisms most control their formation. Recent advances in optically stimulated luminescence (OSL) measurement and analysis have resulted in vast improvements in the dating technique and reliability of age determinations, particularly for OSL dating of quartz grains, and routine application to a wide variety of depositional environments is now possible. Here we apply OSL methods to date a variety of deposits within Late Pleistocene conglomeratic alluvial sequences in NW Argentina. The ages obtained range from 39 to 83 ka and were determined from debris-flow- and fluvial-dominated deposits and lacustrine sequences in intramontane basins bounded by tectonically active mountain ranges with as much as 2 km of relief. With careful choice of facies and sample collection, OSL techniques can be used to date Late Pleistocene, predominately matrix-supported, cobble-conglomerate alluvial deposits.

Pliocene orographic barrier uplift in the southern Central Andes

Sedimentary basin fills along the windward flanks of orogenic plateaus are valuable archives of paleoenvironmental change with the potential to resolve the history of surface uplift and orographic barrier formation. The intermontane basins of the southern Central Andes contain thick successions of sedimentary material that are commonly interbedded with datable volcanic ashes. We relate variations in the hydrogen isotopic composition of hydrated volcanic glass (delta D-g) of Neogene to Quaternary fills in the semiarid intermontane Humahuaca Basin (Eastern Cordillera, northwest Argentina) to spatiotemporal changes in topography and associated orographic effects. delta D values from volcanic glass in the basin strata (-117 parts per thousand to -98 parts per thousand) show two main trends that accompany observed tectonosedimentary events in the study area. Between 6.0 and 3.5 Ma, delta D-g values decrease by similar to 17 parts per thousand; this is associated with surface uplift in the catchment area. After 3.5 Ma, delta D-g values show abrupt deuterium enrichment, which we associate with (1) the attainment of threshold elevations for blocking moisture transport in the basin-bounding ranges to the east, and (2) the onset of semiarid conditions in the basin. Such orographic barriers throughout the eastern flanks of the Central Andes have impeded moisture transport into the orogen interior; this has likely helped maintain aridity and internal drainage conditions on the adjacent Andean Plateau.

Tectonics, Climate, and Landscape Evolution of the Southern Central Andes: the Argentine Puna Plateau and Adjacent Regions between 22 and 30°S

The history of the Puna Plateau and its marginal basins and ranges in the Eastern Cordillera and the northern Sierras Pampeanas structural provinces in northwestern Argentina impressively documents the effects of tectonics and topography on atmospheric circulation patterns, the successive evolution of orographic barriers, as well as their influence on erosion and sedimentation processes. In addition, this region exemplifies that there are several pathways by which tectonic activity may be coupled to the effects of climate and erosion. Apatite fission track and sedimentologic data indicate that distributed, diachronous uplift of ranges within the present Puna Plateau of NW Argentina began as early as Oligocene time, compartmentalizing a foreland region similar to tectonically active sectors along the current eastern plateau margins. However, fission track data from detrital apatite in sedimentary basins and vertical profiles along the eastern plateau margin document that wholesale plateau uplift probably affected this region in mid to late Miocene time, which may have been associated with mantle delamination. This coincided with the establishment of humid conditions along the eastern Puna margin and a sustained arid to hyper-arid climate within the plateau region. A common feature of the Puna Plateau is that its location corresponds to hyper-arid areas of the landscape in which channels fail to incise deeply into basin sediments or surrounding basement ranges. Importantly, the local base-level is hydrologically isolated from the foreland. This isolation occurs where the incising power of regional drainage systems has been greatly reduced due to a combination of diminished precipitation related to regional climate and local orography, and exposure of resistant bedrock. Hydrologic isolation of the plateau from the foreland permits deposition within basins as material is eroded from the surrounding ranges, reducing the relief between basins and adjacent peaks. While a variety of deformation styles and possibly combinations of different processes may have generated the high elevations observed in the Puna Plateau, the observed low-relief morphology requires evacuation of material via regional fluvial systems to be restricted. Therefore, the low-relief character of the orogenic plateau may be a geomorphic, rather than a tectonic phenomenon. At the eastern plateau margins similar basin histories can be observed in fault-bounded intramontane depressions that straddle the eastern Puna border. However, these basins remain only transiently isolated and internally drained due to their proximity to the high precipitation gradients which were established by orographic barriers in the course of Pliocene uplift. These outlying barriers focus precipitation, erosion, promote headward erosion, stream capture, and ultimately basin exhumation. This conspiring set of processes thus prevents these areas to become incorporated into the plateau realm, while the interior of the orogen conserves mass and may influence deformation patterns in the foreland due to high lithostatic stresses. Sustained aridity in the core of the orogen may thus be responsible for the creation, maintenance and potential for future lateral growth of the plateau, thus emphasizing the coupling between tectonics, climate and erosion.

Landscape response to late Pleistocene climate change in NW Argentina: Sediment flux modulated by basin geometry and connectivity

Citation: Schildgen, T. F., Robinson, R. A. J., Savi, S., Phillips, W. M., Spencer, J. Q. G., Bookhagen, B., . . . Strecker, M. R. (2016). Landscape response to late Pleistocene climate change in NW Argentina: Sediment flux modulated by basin geometry and connectivity. Journal of Geophysical Research-Earth Surface, 121(2), 392-414. doi:10.1002/2015jf003607

Gypsum-Hydroboracite Association in the Sijes Formation (Miocene, NW Argentina): Implications for the Genesis of Mg-Bearing Borates

ABSTRACT This paper deals with sedimentologic and diagenetic aspects of the evaporitic facies of the Sijes Formation (Miocene, central Andes, NW Argentina), which contains the largest known hydroboracite reserves in the world. In outcrop, the sulfate minerals are secondary gypsum and minor anhydrite, and the borate minerals are hydroboracite with subordinate inyoite and colemanite, and some ulexite. In the Monte Amarillo Member of the Sijes Formation it is possible to distinguish two coeval, shallow lacustrine subbasins, in which the gypsum accumulated in the margins and the hydroboracite in the centers, the intermediate zones being characterized by mixed gypsum-hydroboracite layers. In the depositional sequence, primary gypsum (gypsarenite) and syndepositional anhydrite, in association with limited amounts of calcium borates (colemanite, inyoite) precipitated first, followed by hydroboracite (calcium/magnesium borate). Alternations of gypsum and hydroboracite layers also formed. Hydroboracite is mainly a primary mineral, although it replaced some gypsum under synsedimentary conditions. The formation of colemanite, which occurred during early diagenesis, is linked to the precipitation of calcium sulfates (gypsum and anhydrite), whereas inyoite coexists with both calcium sulfates and magnesium-bearing borates. Transformations among the various borate minerals during burial diagenesis were not detected. Primary gypsum was transformed into anhydrite from early diagenesis to moderate burial diagenesis. The boron source of these deposits seems to be related to the volcanic/hydrothermal activity in the central Andes during the Miocene.

Miocene orographic uplift forces rapid hydrological change in the southern central Andes

Rainfall in the central Andes associated with the South American Monsoon and the South American Low-Level Jet results from orographic effects on atmospheric circulation exerted by the Andean Plateau and the Eastern Cordillera. However, despite its importance for South American climate, no reliable records exist that allow decoding the evolution of thresholds and interactions between Andean topography and atmospheric circulation, especially regarding the onset of humid conditions in the inherently dry southern central Andes. Here, we employ multi-proxy isotope data of lipid biomarkers, pedogenic carbonates and volcanic glass from the Eastern Cordillera of NW Argentina and present the first long-term evapotranspiration record. We find that regional eco-hydrology and vegetation changes are associated with initiation of moisture transport via the South American Low-Level Jet at 7.6 Ma, and subsequent lateral growth of the orogen at 6.5 Ma. Our results highlight that topographically induced changes in atmospheric circulation patterns, not global climate change, were responsible for late Miocene environmental change in this part of the southern hemisphere. This suggests that mountain building over time fundamentally controlled habitat evolution along the central Andes.

Mio-Pliocene aridity in the south-central Andes associated with Southern Hemisphere cold periods

Significance This paper identifies two periods of enhanced aridity that are synchronous with faunal turnovers in southern South America. Close temporal coincidence with marine climate proxies suggests a period of latest Miocene aridity was associated with a global glacial period and the expansion of C4 vegetation. We thus argue that continental aridity in the south-central Andes is associated with cold periods at high southern latitudes and propose a model to link global and regional (continental) climate via shifting of the Southern Hemisphere westerlies. This paper also provides an example of how 10Be paleo-erosion rates can be used as a climate proxy and demonstrates how 10Be and 36Cl can be combined to reduce uncertainties associated with this method. Although Earth’s climate history is best known through marine records, the corresponding continental climatic conditions drive the evolution of terrestrial life. Continental conditions during the latest Miocene are of particular interest because global faunal turnover is roughly synchronous with a period of global glaciation from ∼6.2–5.5 Ma and with the Messinian Salinity Crisis from ∼6.0–5.3 Ma. Despite the climatic and ecological significance of this period, the continental climatic conditions associated with it remain unclear. We address this question using erosion rates of ancient watersheds to constrain Mio-Pliocene climatic conditions in the south-central Andes near 30° S. Our results show two slowdowns in erosion rate, one from ∼6.1–5.2 Ma and another from 3.6 to 3.3 Ma, which we attribute to periods of continental aridity. This view is supported by synchrony with other regional proxies for aridity and with the timing of glacial ‟cold” periods as recorded by marine proxies, such as the M2 isotope excursion. We thus conclude that aridity in the south-central Andes is associated with cold periods at high southern latitudes, perhaps due to a northward migration of the Southern Hemisphere westerlies, which disrupted the South American Low Level Jet that delivers moisture to southeastern South America. Colder glacial periods, and possibly associated reductions in atmospheric CO2, thus seem to be an important driver of Mio-Pliocene ecological transitions in the central Andes. Finally, this study demonstrates that paleo-erosion rates can be a powerful proxy for ancient continental climates that lie beyond the reach of most lacustrine and glacial archives.

Icno-asociaciones de la transición Precámbrico7Cámbrico en el noroeste de Argentina.

El descubrimiento de numerosos icnogeneros e impresiones de organismos de cuerpos blandos (Asaphoidichnus, Cochlichnus, Didymaulichnus, Dimorphichnus, Diplichnites, Glockeria, Gordia, Helmintoida, Helminthoidichnus, Helminthopsis, Helminthoraphe, Monomorphichnus, Multipodichnus, Neonereites, Nereites, Oldhamia, Paliella, Planolites, Protichnites, Protopaleodyction, Protovigularia, Scolicia, Squamodyction, Taphrohelminthopsis, Tasmanadia, Tiernavia, Torrowangea y Treptichnus),en pizarras y cuarcitas de la Formacion Puncoviscana s.l. de diferentes localidades del noroeste argentino, ha permitido tener un panorama mas claro acerca de las caracteristicas de la cuenca que conformo el orogeno Pampeano (Vendico/Cambrico inferior). El material es demostrativo de la amplia biodiversidad en diferentes niveles que representan la evolucion de la cuenca tanto en sus aspectos estructurales como temporales. La distribucion del registro paleoicnologico y su posicion relativa en la secuencia sedimentaria sugieren una particular disposicion del mismo, conforme a las caracteristicas evolutivas de la cuenca. Dicha disposicion queda expresada en tres icnofacies o asociaciones (Beltanelliformis, Nereites y Oldhamia) que representarian diferentes momentos de la evolucion del orogeno Pampeano INSUGEO (UNT-CONICET). Miguel Lillo 205.- 4000 Tucuman. Argentina. Correo: facenola@satlink.com (1) UNSa-CONICET. Buenos Aires 177, 4400 Salta. Argentina. Correo: ralonso@unas.edu.ar (2) 11 F. G. Acenolaza y R. N. Alonso