Low-salinity conditions in the “marine” Late Triassic-Early Jurassic Neuquén Basin of Argentina: Challenges in paleosalinity interpretation

Salinity, a key property of watermasses, is difficult to reconstruct in paleodepositional systems, and errors in assigning salinity facies to epicratonic and marginal-marine deposits may be common. In this study, we evaluate the salinity conditions of mudstones of the Arroyo Malo Formation (AMF), spanning the Triassic-Jurassic transition (TJT) of the Neuquén Basin, Argentina. Although the AMF has long been considered marine based on its fossil fauna, we find that three elemental salinity proxies (B/Ga, Sr/Ba, S/TOC) are consistent in indicating dominantly freshwater influence, with occasional excursions to low-brackish conditions. In this context, the sparse, fragmented, and poorly preserved assemblage of bioclasts in the AMF do not accurately represent its salinity facies. We infer that, during the TJT, the Neuquén Basin was largely isolated from the Panthalassic Ocean, or perhaps intermittently weakly connected to it. The same elemental salinity proxies indicate low-brackish conditions in a Lower Jurassic unit previously assumed to be marine and fully marine conditions in Lower Cretaceous mudstone formations of the Neuquén Basin. This trend documents the gradual establishment of open communication with the Panthalassic Ocean, with fully marine conditions prevailing by the Middle to Late Jurassic. Standard redox proxies show that the AMF watermass was not markedly reducing, demonstrating that the impoverishment of the fossil biota was not due to unfavorable redox conditions. These findings demonstrate that inferences of marine conditions based on fossil content are potentially erroneous, and they emphasize the general need to make use of salinity proxies in analysis of deep-time depositional systems. •New assessment of salinity in Mesozoic strata of Neuquén Basin of Argentina•Triassic-Jurassic transition shales yield freshwater values for B/Ga, Sr/Ba and S/TOC•Freshwater interpretation challenges previous assumption of marine conditions•Intermittent connection of basin to Panthalassa during Triassic-Jurassic transition•Fossil content alone can yield incorrect interpretations of marine salinity