The Cerro Morado Group (Middle Ordovician) in Famatina, Andean foreland, Argentina.

Abstract

The new denomination of Cerro Morado Group is proposed for a thick volcanosedimentary succession (727 m) that unconformably overlies the Early Ordovician Famatina Group in the central region of the Sierra de Famatina, Andean foreland of western Argentina. From bottom to top a mainly volcanic unit, El Portillo Formation (580 m, former Cerro Morado Formation) is composed of acidic volcanic flows and succeeded by the volcanosedimentary La Escondida Formation (147 m). These records allow to expand the Ordovician record in Famatina to the Middle Ordovician and possibly younger. Alternations of regressivetransgressive cycles indicate recurrent volcanic activity, characteristic of volcanic arc settings. These cycles are interpreted as the flexural response to active volcanism followed by intervals of quiescence, where shallow marine environments interact with important volcaniclastic input. This episodic basin dynamics influenced relative sea level fluctuations and caused the cyclical pattern recognized throughout the Ordovician in Famatina. This study establishes a direct link to Ordovician subduction and later accretion of Precordillera. In this context, the intraordovician angular unconformity between the Cerro Morado Group and the Early Ordovician Famatina Group is related to the Ocloyic orogeny.