Reconstructing paleoenvironmental conditions through integration of paleogeography, stratigraphy, sedimentology, mineralogy, and stable isotope data of lacustrine carbonates?an example from early Middle Triassic strata of southwest Gondwana, Cuyana Rift,

BENAVENTE, CECILIA ANDREA; MANCUSO, ADRIANA CECILIA; BOHACS, KEVIN MICHAEL

Limnogeology: Progress, challenges and opportunities. A tribute to Beth Gierlowski-Kordesch

Springer

Año: 2021; p. 471 - 509

Reconstructing continentalpaleo-environmental conditions (temperature, precipitation, hydrology, andhydrography) is essential for constraining the influences on terrestrialecosystems, sediment and solute yields to the ocean, and carbon cycles, as wellas for calibrating numerical paleoclimate models. Making such reconstructionsfrom lacustrine strata, however, is quite challenging because of varying watersources, flow paths, and residence times, paleogeographic effects, andeodiagenesis. Indeed, even stratigraphic evidence of fluctuations in lake levelcannot be directly interpreted in terms of changing precipitation because ofthe complex response of lakes to changing input conditioned by upstream anddownstream factors. Such challenges can be addressed by thorough integration ofpaleogeography, stratigraphy, sedimentology, mineralogy, and stable isotopedata of lacustrine carbonates within a framework that accounts for the many andconvoluted controls on lake systems. We illustrate this approach usingdata from three lacustrine units from the Cuyana Basin (Argentina) of earlyMiddle Triassic age?the Santa Clara Arriba, Cerro de las Cabras, and CerroPuntudo formations. They represent sedimentation in carbonate-rich lacustrinesystems during a time for which information on continental paleoenvironments issparse, especially for interior Pangea. Our detailed study of the stable carbonand oxygen isotope composition of the carbonate beds of these paleolakes, integratedwith other geological evidence, enabled interpretation of their complexhydrology and paleoclimate conditions. Sedimentological, stratigraphic, and mineralogicaldata suggest surface water flow ranged from intermittently open to persistentlyclosed, whereas C and O stable isotope values indicate that both Cerro de lasCabras and Cerro Puntudo paleolakes had open groundwater flow. The Santa ClaraArriba paleolake, although intermittently open to surface flow, had long water residencetime, based on high correlation of C and O stable isotopes. This evidence of the hydrographic-hydrologiccomplexity of these continental interior basins obviates confident quantitativeestimates of paleotemperature from oxygen isotopes. The isotope data do,however, indicate significant evaporation and frequently varying precipitation,runoff, and nutrient supply. These data, along with ephemeral-stream strata, subaerial-exposurefeatures, vertisols, and clay-mineral assemblages dominated by smectite/illite suggesta seasonally varying warm semi-arid tosub-humid paleoclimate. This is in agreement with paleoclimate models for theTriassic that pointed to marked seasonality in the Cuyana Rift Basin.