Temporal, Tectono-Sedimentary and Paleoclimate Constraints in Two Triassic Rift Basins in Sw Gondwana

During the Triassic Period, several extensional basins developed along the southwestern margin of Gondwana (western South America). The extension has been attributed to far-field stresses related to the breakup of Pangea, trench rollback along the subduction margin to the west, or a combination of these processes. From North to South those extensional basins are the Palanda rift (Colombia), the Payendé rift (Ecuador), the Mitu rift (Perú), the Domeyko rift (Chile), and the Cuyana and Ischigualasto-Villa Unión rifts (Argentina). Among these, the three largest basins are the Domeyko basin in northeastern Chile and the Cuyana and Ischigualasto-Villa Unión basins in western Argentina. The El Bordo sub-basin in the Domeyko basin and the Santa Clara sub-basin in the Cuyana basin, preserve thick and entirely continental deposits with abundant volcaniclastic levels that allow precise radioisotopic dating. The aim of this research is to provide a geochronologic framework for the most complete Triassic sedimentary successions in Gondwana to ultimately reconstruct climatic, environmental and biotic events and processes in the Southern Hemisphere, and correlate them on a regional to global scale. We studied Santa Clara sub-basin lithostratigraphic units representing alluvial-fluvial-deltaic and lacustrine systems to obtain radioisotopic ages (using the U-Pb zircon CA-TIMS technique) of volcaniclastic layers, from which a Bayesian age-stratigraphic model was produced. A subsidence curve for the Santa Clara sub-basin was obtained using Backstrip software, applying a 1D Airy-type backstripping method. We conducted facies analysis, clay composition, and continental carbonate petrographic, cathodoluminescence, and stable isotopic (δ13C and δ18O) analyses. Our ages for the stratigraphically lowest portion of the Santa Clara sub-basin suggest infill began during the Olenekian Stage (Cielo Formation) and continued through the earliest Anisian (lower Mollar Formation), from ca 248 Ma to 245 Ma. These new constraints, along with previous ages obtained for younger units in the sub-basin (lowermost Santa Clara Abajo Formation, 243.88 ± 0.41 Ma; and Santa Clara Arriba Formation, 242.78 ± 0.64 Ma, Anisian Stage), are a starting point for a basin-scale evolution analysis. The geometry of the resulting subsidence curve suggests an acceleration in subsidence rates during Santa Clara sub-basin evolution. Furthermore, ages obtained confirm that these deposits overlap in time with the Anisian Humid Phase, presenting the possibility of characterizing the event for southern mid-paleolatitudes. The stratigraphically lowest age obtained for the Cielo Formation in the Santa Clara is one of the oldest precise Triassic ages obtained for sedimentary deposits in the SW margin of Gondwana. This is in agreement with previous tectonic interpretations suggesting the extension of the breakup of SW Gondwana was diachronous and migrated from south to north. For the El Bordo sub-basin, El Bordo beds have been constrained to the Carnian stage confirming the overlying deposits potentially capture the Carnian Pluvial Episode with the possibility of filling a latitudinal gap for southern paleolatitudes regarding previous datasets obtained from the SW margin of Gondwana. Sedimentary records for the Domeyko and Cuyana basins will allow characterizing paleoclimate fluctuations regionally from Olenekian (Early Triassic) to Carnian (Upper Triassic) times and contribute to a refinement and better understanding of near- and far-field paleoenvironmental records.