Stratigraphy and paleoenvironment of the Upper Jurassic Río Damas formation in the Alto Atuel depocenter (Malargüe fold and thrust belt)

Abstract

The Río Damas Formation is an Upper Jurassic volcano-sedimentary succession, whose outcrops are largely distributed between 33°S and 35.30°S in Chile and Argentina, as part of the retroarc Neuquén Basin fill. This unit was deposited during an important marine regressive phase of the basin. We studied the Río Damas Formation exposures at the Los Gateados creek valley, in the Alto Atuel depocenter (34°40′S; westernmost Malargüe fold and thrust belt), next to the Argentinean-Chilean border. This area represents a key location where the formation interdigitates with the nonmarine red beds of the time equivalent Tordillo Formation, representing a continentalization episode of the Neuquén Basin. This paper presents a lithofacial characterization of the Río Damas Formation, a new U–Pb igneous crystallization age, and a paleographic reconstruction for this period. The Río Damas Formation in the Alto Atuel depocenter consists of porphyritic basaltic and andesitic lavas, interbedded with fine-grained sandstones, conglomerates, volcanic lithic breccias, and pyroclastic deposits. Based on field and petrographic analyses, we associate the Río Damas Formation with a piedmont environment where sedimentary fluvial and alluvial setting coexisted with volcanic effusive and pyroclastic activity. A regional comparison of the Río Damas and Tordillo Formations along the Argentinian-Chilean border shows that the composition and thickness of these units present important latitudinal and longitudinal variations. The increase in thickness and the volcanic component of the Río Damas Formation towards the west could be associated with the proximity to the Jurassic magmatic arc.