Stratigraphy and volcanic facies of the of the Valle del Cura Formation: Paleogene volcanism in the Frontal Cordillera of San Juan

Abstract

A thick volcanic and volcaniclastic sequence, which includes epiclastic and pyroclastic facies, was recognized in the Valle del Cura region in the Frontal Cordillera of San Juan. The Valle del Cura Formation represent an important Eocene volcano-sedimentary episode composed of thick beds of tuff, coarse conglomerate, coarse to middle lithic sandstone, volcanic sandstone and scarce shale. The volcanic levels include rhyolitic lavas and high-grade ignimbrites that represent strongly welded and high-temperature pyroclastic flows. The formation also includes thick pyroclastic flow deposits composed of crystalline, lithic and/ or glassy tuff, lapilli tuff, and even coarse facies such as volcanic breccia and agglomerate. Volcanic epiclastic facies interbedded within the pyroclastic deposit represent the reworked primary pyroclastic material. These pyroclastic flow deposits show features that reflect the proximity of its eruptive centre, which might be located in the southern end of the cordillera de la Brea. The Valle del Cura Formation can be correlated with the equivalent units exposed farther north at Río de la Sal and Río Blanco (Macho Muerto) and, chronologically, with the Bocatoma Unit intrusives that crop out to the west in Chile, emplaced along the Eocene magmatic front. By contrast, the Valle del Cura Formation represents intense volcanic activity developed in a back-arc setting.