Abstract

VALLECILLO, Graciela; CABALLERO, María M; ROCHER, Sebastián  and  ESPIN, Eleonora. Analysis of the Choiyoi Group (Permian-Triassic), Cordillera Frontal of Calingasta, Province of San Juan. Rev. Asoc. Geol. Argent. [online]. 2010, vol.66, n.1-2, pp.238-252. ISSN 0004-4822.

This paper deals with the study of pyroclastic facies associated to interbedded clastic rocks and mesosilicic volcanic igneous rocks cropping out in the Frontal Cordillera, on both margins of the Calingasta River, in the San Juan Province of Argentina. These rocks are ascribed to the Choyoi Group, a volcanic, pyroclastic and clastic complex, assigned to Early Permian - Middle Triassic interval. Interpreted facies can be grouped in four categories: A) massive lithic breccias; B) autoclastic volcanic breccias; C) ignimbrites; D) stratified tuffs. Facies A are deposited by the action of block and ash flows, whereas facies B represent autoclastic brecciation processes in lavas. Facies C was the result of deposition from pumice-rich pyroclastic flows. Finally, facies D rocks represent deposits from ash fallout. Compositionally, there is a change from andesitic volcanism at the basal levels of the sequence, to rhyolitic volcanism at the top. Three main volcanic events are distinguished, the first one being represented by the discharge of andesitic lavas, the second one by effusion of rhyolitic lavas over a paleorelief, and the third, by the explosive eruption of vesiculated siliceous magma driving to the deposition of very thick ignimbrite sheets.

Keywords : El Palque Formation; Choiyoi; Frontal Cordillera; Pyroclastic and volcanic facies.

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