Paleohydrogeologic reconstruction of Triassic carbonate paleolakes from stable isotopes: Encompassing two lacustrine models

Abstract

Reconciling interpretations of geochemical, sedimentologic, and stratigraphic observations is a common challenge with many datasets, especially those from paleolake systems. The Triassic Cerro de las Cabras and Cerro Puntudo formations, part of the sedimentary fill of the Cuyana rift Basin in central-west Argentina, represent carbonate-rich playa-lake systems with abundant microbialites, but different sedimentologic and stratigraphic attributes such as presence or absence of efflorescent crystalline crusts and aggrading versus prograding-aggrading stacking patterns. A detailed study of the stable-carbon and oxygen-isotope composition of the limestones deposited in these paleolakes, together with sedimentological and mineralogical evidence as petrography, microprobe, XRD and cathodoluminescence data, enabled interpretation of their complex hydrology.Lack of correlation between carbon and oxygen stable-isotope values for both Cerro de las Cabras (δ13C χ–6.03‰; σ 0.67‰/δ18O χ −12.64‰; σ 0.55‰) and Cerro Puntudo (δ13C χ −3.10‰; σ 1.15‰/δ18O χ −12.58‰; σ 1.8‰) paleolakes strongly indicates open hydrology (underground flow), in apparent contradiction with sedimentologic and stratigraphic evidence for intermittent to persistently closed hydrography (overland flow). Stable-isotopic values for the Cerro de las Cabras paleolake together with sedimentology data indicate a recharge (hydrologically open) lake, and for the Cerro Puntudo a through-flow (hydrologically open) playa-lake. To resolve the apparent contradictions, we propose an integration of playa-lake hydrology (groundwater) with lake-basin type sedimentology, stratigraphy, and hydrography. This allowed identifying and proposing a balanced-fill–through-flow lake subtype for the Cerro Puntudo system and an underfilled–recharge-lake subtype for the Cerro de las Cabras paleolake; both with hydrology open to groundwater flow. This work suggests a starting point for expanding the lake-basin-type model to explicitly incorporate different groundwater and overland flow pathways.