Evidencias de sedimentación y volcanismo contemporáneo en la Formación Huitrera (Eoceno), arroyo Chacay, provincia de Río Negro

Abstract

Evidences of contemporaneous volcanism and sedimentation in the Huitrera Formation (Eocene), Arroyo Chacay, Río Negro province. The Pilcaniyeu Volcanic Belt is comprised by the Huitrera Formation (middle Paleocene-middle Eocene) and it is characterized by a bimodal composition, including mostly rhyolitic and minor basaltic volcanic rocks. Recent fieldwork along the Chacay stream lead us to separate the volcano-sedimentary sequence into two minor successions; a lower succession (SI) and an upper succession (SS). The lower succession, about 200 m thick, is composed by a peperitic and an autobrecciated facies, with subordinated volcanic debris avalanche facies. The upper succession includes sedimentary facies associated to synsedimentary normal faults indicating an ESE extension. Moreover, the upper succession also includes peperitic and lava facies, which were recognized reaching up to 80 m in thickness. Minor dikes intruding the SS are exposed, showing NNW to NNE trend. The contact relationships and lithological features along the lower succession evidence an intense volcanic activity that was coeval to subordinated sedimentary deposition. In this setting, the debris avalanche deposits occupy topographic depressions. Likewise, the peperitic facies evidence flooded hollows and sedimentary accumulations. On the other hand, the upper succession is dominated by syntectonic sedimentary facies, associated with minor lavas and peperitic facies, as evidence of magmatism and coeval sedimentation associated to an extensional tectonic setting.