The development of miocene extensional and short-lived basin in the Andean broken foreland: The Conglomerado Los Patos, Northwestern Argentina

Abstract

The Conglomerado Los Patos is a coarse-grained clastic unitthat crops out irregularly in the San Antonio de los Cobres Valley in thePuna, Northwestern Argentina. It covers different units of theCretaceous-Paleogene Salta Group by means of an angular unconformity and,in turn, is overlaid in angular unconformity by the ViscachayocIgnimbrite (13±0.3 Ma) or by late Miocene tuffs.Three lithofacies have been identified in the Corte Blanco locality; 1)Bouldery matrix-supported conglomerate (Gmm); 2) Clast-supportedconglomerate (Gch) and 3) Imbricated clast-supported conglomerate (Gci).The stratigraphic pattern displays a general fining upward trend. Thesedimentary facies association suggests gravitational flow processes andsedimentation in alluvial fan settings, from proximal to medial fanpositions, together with a slope decrease upsection. Provenance studiesreveal sediments sourced from Precambrian to Ordovician units located tothe southwest, except for volcanic clasts in the Gmm facies that showsU/Pb age of 14.5±0.5 Ma. This new age represents the maximum depositionalage for the Conglomerado Los Patos, and it documents that deposition tookplace simultaneously during a period of increased tectonic and volcanicactivity in the area.The structural analysis of the San Antonio de los Cobres Valley and theavailable thermochronological ages, indicate active N-S main thrusts andNW-SE transpressive and locally normal faults during the middle Miocene.In this context, we interpret the Conglomerado Los Patos to representsedimentation in a small, extensional and short-lived basin associatedwith the compressional Andean setting.