Chapter 14, 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, C

BENAVENTE, CECILIA ANDREA; MANCUSO, ADRIANA CECILIA; BOHACS, KEVIN M.

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

springer

Año: 2020; p. 220 - 240

Reconstructing continental paleo-environmental conditions (temperature, precipitation, hydrology, and hydrography) is essential for constraining the influences on terrestrial ecosystems, sediment and solute yields to the ocean, and carbon cycles, as well as for calibrating numeric paleoclimate models. Making such reconstructions from lacustrine strata, however, is quite challenging because of varying water sources, flow paths, and residence times, paleogeographic effects, and eodiagenesis. Indeed, even stratigraphic evidence of fluctuations in lake level cannot be directly interpreted in terms of changing precipitation because of the complex response of lakes to changing input conditioned by upstream and downstream factors. Such challenges can be addressed by thorough integration of paleogeography, stratigraphy, sedimentology, mineralogy, and stable isotope data of lacustrine carbonates within a framework that accounts for the many and convoluted controls on lake systems. We illustrate this approach using data from three lacustrine units from the Cuyana Basin (Argentina) of early Middle Triassic age?the Santa Clara Arriba, Cerro de las Cabras, and Cerro Puntudo formations. They represent sedimentation in carbonate-rich lacustrine systems during a time for which information on continental paleoenvironments is sparse, especially for interior Pangea. Our detailed study of the stable carbon and oxygen isotope composition of the carbonate beds of these paleolakes, integrated with other geologic evidence, enabled interpretation of their complex hydrology and paleoclimate conditions. Sedimentologic, stratigraphic, and mineralogic data suggest surface water flow ranged from intermittently open to persistently closed, whereas C and O stable isotope values indicate that both Cerro de las Cabras and Cerro Puntudo paleolakes had open groundwater flow. The Santa Clara Arriba paleolake, although intermittently open to surface flow, had long water residence time, based on high correlation of C and O stable isotopes. This evidence of the hydrographic-hydrologic complexity of these continental interior basins obviates confident quantitative estimates 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-exposure features, vertisols, and clay-mineral assemblages dominated by smectite/illite suggest a seasonally varying warm semi-arid to sub-humid paleoclimate. This is in agreement with paleoclimate models for the Triassic that pointed to marked seasonality in the Cuyana Rift Basin.