Paleolimnology evolution in rift basins: the Ischigualasto–Villa Unión Basin (Central-Western Argentina) during the Triassic

Abstract

Extensional basins offer a high preservation potential for sedimentary and fossil records. Nonmarine rift basins contain diverse environments, including fluvial and lacustrine. Lake deposits have long been recognized as significant archives of the paleoenvironmental record. The Ischigualasto–Villa Unión Basin represents the only nearly continuous continental Triassic record with up to 6000m of alluvial, fluvial, and lacustrine deposits. The deep lacustrine interval is known as the Los Rastros Formation and consists of several lacustrine–deltaic cycles. A detailed facies analysis allowed the characterization of five facies associations and two types of cycles: (1) a shoreline-dominated, progradational Type I cycle containing Facies association A (offshore lacustrine deposits) and Facies association B (marginal palustrine deposits along the shoreline of the lake) or Facies association C (lacustrine siliciclastic coastal deposits) and (2) a delta-dominated, progradational Type II cycle characterized by Facies association A (offshore lacustrine and prodelta deposits), Facies association D (delta front deposits), and Facies association E (delta plain deposits). The sedimentary evolution of the Chañares–Los Rastros sequence is defined within this extensional setting as one depositional sequence with three system tracts (lowstand, transgressive, and highstand system tract). Biotic, abiotic, and sedimentologic data allowed the interpretation of the limnologic dynamics of the Los Rastros paleolake as a deep freshwater lake within a half-graben, exhibiting meromixis, and showing a change from moderate eutrophic conditions with low oxygen content to relatively oxygenated oligotrophic conditions. The successive fluctuations recorded in lake level are considered in the sequence stratigraphic framework of Bohacs et al. to reflect the relative balance of sediment and water input versus accommodation in an overfilled lacustrine basin.