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

摘要

The Rio Damas Formation is an Upper Jurassic volcano-sedimentary succession, whose outcrops are largely distributed between 33S and 35.30S in Chile and Argentina, as part of the retroarc Neuquen Basin fill. This unit was deposited during an important marine regressive phase of the basin. We studied the Rio Damas Formation exposures at the Los Gateados creek valley, in the Alto Atuel depocenter (3440 ' S; westernmost Malargue 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 Neuquen Basin. This paper presents a lithofacial characterization of the Rio Damas Formation, a new U-Pb igneous crystallization age, and a paleographic reconstruction for this period. The Rio 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 Rio Damas Formation with a piedmont environment where sedimentary fluvial and alluvial setting coexisted with volcanic effusive and pyroclastic activity. A regional comparison of the Rio 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 Rio Damas Formation towards the west could be associated with the proximity to the Jurassic magmatic arc.