Search

Two end-member models have been proposed for the Paleogene Andean foreland: a simple W-E migrating foreland model and a broken-foreland model. We present new stratigraphic, sedimentological and structural data from the Paleogene Quebrada de los Colorados ( QLC) Formation, in the Eastern Cordillera, with which to test these two different models. Basin-wide unconformities, growthstrata and changes in provenance indicate deposition of the QLC Formation in a tectonically active basin. Both west- and east-vergent structures, rooted in the basement, controlled the deposition and distribution of the QLC Formation from the Middle Eocene to the Early Miocene. The provenance analysis indicates that the main source areas were basement blocks, like the Paleozoic Oire Eruptive Complex, uplifted during Paleogene shortening, and that delimits the eastern boundary of the present-day intraorogenic Puna plateau. A comparison of the QLC sedimentary basin-fill pattern with those of adjacent Paleogene basins in the Puna plateau and in the Santa Bárbara System highlights the presence of discrete depozones. These reflect the early compartmentalization of the foreland, rather than a stepwise advance of the deformation front of a thrust belt. The early Tertiary foreland of the southern central Andes is represented by a ca. 250-km-wide area comprising several deformation zones (Arizaro, Macón, Copalayo and Calchaquí) in which doubly vergent or asymmetric structures, rooted in the basement, were generated. Hence, classical foreland model is difficult to apply in this Paleogene basin; and our data and interpretation agree with a broken-foreland model. [ABSTRACT FROM AUTHOR]

The Quebrada de los Colorados Formation, at the north end of Calchaquí Valley in Salta Province, northwest Argentina, preserves evidence of syndepositional deformation since the middle Eocene (ca. 40 Ma) that includes (1) an angular unconformity with the underlying Salta Group (Paleogene), and (2) internal unconformities and changes in vertical facies succession and provenance. Its fossil record [mammalian (notoungulates), middle Eocene] is correlatable to the Casa Grande Formation, which also unconformably overlies the Salta Group; both units record middle Eocene deformation along the eastern border of the Puna Plateau and outline previous first-order mechanical heterogeneities related to the Cretaceous rift basin border. Along the western margin of the Puna, Eocene deformation coincides with thermal (magmatic arc) and mechanical (basin inversion) heterogeneities. Thus, the distribution of Eocene deformation followed an irregular pattern as a consequence of the heritage of preexisting heterogeneities. [ABSTRACT FROM AUTHOR]

Abstract: The southern portion of the Cobres Range is studied with an interdisciplinary, petrological–geochemical and structural-cartographic approach. The Cobres Plutonic Complex (CPC) consists of two main plutons, a granodiorite and a monzogranite, as well as small bodies of gabbros, acidic dikes, and episyenite bands. Their mineralogy and geochemistry suggest a comagmatic character for both plutons, which are strongly peraluminous and define a typical calc–alkaline trend. Harker diagrams indicate a fractionated crystallization trend. The REEs have similar patterns for enclaves, granodiorites, monzogranites, aplites, and acidic dikes, also similar to those from the upper crust rocks. Gabbros included in the CPC show geochemical signatures compatible with the mantle. The CPC is syntectonic, though the granodioritic and monzogranitic plutons represent different stages in the tectomagmatic history of the region. The geology of the CPC and its regional framework show characteristics—thermal anomalies, alternation of contractional and extensional deformational events, magmatic rocks with crustal and mantle affiliations, and contemporaneous extensional basins—that indicate intrusion of the CPC in an extensional accretionary orogen. [Copyright &y& Elsevier]

Abstract: An Eocene foreland basin linked to the Andean uplift in northwestern Argentina has recently been proposed. The basin is divided and partially eroded due to subsequent Neogene orogenic phases, so that a simple reconstruction is insufficient to describe complex field relationships. This presents a new challenge in understanding the initial phases of Central Andean evolution. We propose a multidisciplinary approach in key locations and/or at key geological features as a way to reconstruct the Paleogene basin. In this contribution, we report on sedimentological and geochemical evidence of a conspicuous weathering surface in the continental Eocene Lumbrera Formation and provide an age estimate based on vertebrate mammalian biostratigraphy and an absolute U/Pb zircon age of 39.9Ma. Weathering surfaces become evident when diagnostic features like paleosols, karsts, and trace fossils are distinctive but, in our case, these characteristics only emerge through detailed sedimentological and geochemical surveys. The Lumbrera paleosurface is represented by a hardened level (20–30cm thick) characterized by moderately developed reddish paleosols. Moreover, major and trace element profiles show inflections at the top and/or base of the weathered horizon delineating it. A modified form of the chemical index of alteration shows that chemical leaching was moderate and not extensive. In addition, mammalian fossil records substantially differ below and above the weathered paleosol-bearing surface. We conclude that this horizon represents a Middle Eocene omission surface and represents a key level marking a major basin change in northwest Argentina, adding a new constraints for Eocene foreland reconstruction. [Copyright &y& Elsevier]