The neopaleozoic of the Sierra de Castaño (Andean Cordillera Frontal, San Juan, Argentina): Tectonic and paleoenviromental reconstruction.

Abstract

This paper addresses the stratigraphic analysis of the Late Paleozoicsequence outcroping in the Sierra de Castaño area (Cordillera Frontal, San Juan Province, Argentina). In this region three major stratigraphic units have been recognized. The lower one corresponds to the Cerro Agua Negra Formation (Late Carboniferous-Early Permian?) composed of sandstones and mudstones, mainly deposited in shallow marine environments. The San Ignacio Formation (Early Permian) covers unconformably the Cerro Agua Negra Formation and it is formed by conglomerates, breccias, sandstones, mudstones, stromatolitic limestones, volcanic and pyroclastic rocks. Three facies associations were recognized in the San Ignacio Formation. The lower (AFa) is composed of breccias, conglomerates and lithic sandstones which were very probably deposited in alluvial and fluvial (braided) systems that filled the irregular topography carved into the Cerro Agua Negra Formation. The facies association AFb, formed by conglomerates,sandstones and mudstones, characteristically develops metric-scale finning-upward cycles probably formed in high-sinuosity fluvial systems. Regarding the facies association AFc comprises stromatolitic limestones, mudstones, volcanics and pyroclastic rocks which are interpreted as deposited in shallow water bodies under recurrent volcanic activity. The San Ignacio Formation allows characterizing the transition from the prevolcanic sequences belonging tothe Cerro Agua Negra Formation to the large volcanic activity represented in the Choiyoi Group (Middle Permian-EarlyTriassic). Moreover the analyzed sequence demonstrates the existence of two mayor deformational events during theCarboniferous and Permian in the Andean region. The older one took place close to the Carboniferous-Permian boundary or during the earliest Permian. The second tectonic event, occurred at the end of the early Permian, correlates with the San Rafael Tectonic Phase that pointed out the onset of the large permotriassic volcanism (Choiyoi Group) extensively developed along the Andean region.