New insights on landscape and environmental evolution in the Subandean region, Southern Central Andes: Sedimentology and geochronology of Guanaco Formation in the Río Chico (Jujuy, Northwestern Argentina)

Abstract

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 Río 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.