Limnogeology of the Agua de la Zorra Formation (Middle–Upper Triassic), Cuyana Basin, NW Argentina: Emphasising lacustrine carbonates

Abstract

Lacustrine systems are complex and highly sensitive in responding to allogenic factors like tectonics, climate and volcanism but also present within lake processes that are difficult to disentangle. The Agua de la Zorra Formation (Middle–Upper Triassic) outcropping in the Paramillos de Uspallata sub-basin, Cuyana rift Basin (SW Gondwana), is composed of deltaic–fluvial–lacustrine–volcaniclastic deposits. The lacustrine system presents carbonate deposits, as almost every other sub-basin within the Cuyana rift. This makes it an exceptional case for analysing limnogeological questions in an extensional tectonic regime linked to volcanism (basalt flow and tuff deposits) and studying lacustrine carbonate in that particular setting. This study focuses on a high-resolution stratigraphic, sedimentologic, petrographic and limnogeological investigation of the Agua de la Zorra depositional system to understand its evolution through space and time; and to reconstruct the development of the carbonate factory within the palaeolake. Stratigraphic correlation of five sedimentary logs across the sub-basin allowed the identification of a deltaic–lacustrine facies association and a progradational–retrogradational stacking pattern interpreting the palaeolake as an overfilled lake basin type. Five facies associations represent a delta front, prodelta, lake margin, lake centre and volcanic episodes. The stratigraphic analysis allowed interpreting that volcanic episodes were diachronous during the evolution of the Agua de la Zorra system; the development of a well-established axial fluvial system upsection and the coeval migration of the highest accommodation point and lake centre northwards. Carbonate deposits are part of the lake margin subenvironment and immediately overlie volcanic basalts and are laterally linked to altered sandstones in contact with basalts. Their microfabric is characterised by fibro-radial spar mosaics and laminated carbonate, interpreted as inorganic authigenic calcite precipitation. This and their restricted extent support that alkaline lava flows in contact with the palaeolake water column triggered carbonate precipitation. Stratigraphic, sedimentologic, limnogeological and petrographic analyses strongly support volcanism and tectonics as the main allogenic factors controlling the Agua de La Zorra lacustrine system evolution.