Stratigraphic revision of the Angulos Group (Neogene) Famatina Ranges, La Rioja province: Sedimentary environements and its bearing on the Central Andean broken foreland till

Abstract

Thick synorogenic Tertiary strata are exposed on the eastern flank of the Famatina Ranges, largely known as the "Estratos Calchaquenses" and formally named as the Angulos Group (>1500 m thick). This group was originally divided into three Formations: Del Abra, Del Buey and El Durazno. This work carries out a detail revision of the Angulos Group stratigraphy with emphasis on the paleoenvironmental evolution. The El Durazno Formation is emended and subdivided, redefining its lower large-scale cross-stratified section as the Santo Domingo Formation. On the base of its stratigraphic position, together to the paleontological record and regional correlations, the Angulos Group is reassigned between the Middle Miocene and the Early Pliocene. Eleven facies were distinguished that are dominated by epiclastic and volcanoclastic sediments onto the carbonate rocks. The Del Abra Formation facies association allows interpreting a proximal alluvial fan setting which evolved toward more distal fan deposits. Low-efficiency processes dominated the basal section, which are gradually replaced by high-efficiency alluvial facies toward the top of the formation. Transitionally, the Del Buey Formation is characterized by distal alluvial fan facies, and records biolaminated carbonate beds and calcrete horizons, indicating low-gradient environments as well as low sedimentation rates. Alternation of large-scale cross-stratified sandstones and strongly incised sandy conglomerates compound the Santo Domingo Formation, indicating eolian-fluvial interaction systems. This succesion is unconformably overlain by the El Durazno Formation conglomerates that show a paleoenvironment evolution from braided fluvial plains with pyroclastic influence, toward high-efficiency alluvial fan systems. These units are disposed into three megasequences, a basal fining-upward megasequence followed by two coarsening-upward megasequences. A conglomerate provenance analysis shows shifting in the source input and paleocurrent inversion, suggesting noteworthy changes in the Middle Miocene-Pliocene foreland evolution. The tectonostratigraphic evolution of the succession allows differentiating three episodes, from a retroarc extensional basin stage with a terminal sag period, followed by two later broken foreland stages.