ELEMENTAL PALEOSALINITY PROXIES DEMONSTRATE FRESHWATER CONDITIONS IN TRIASSIC-JURASSIC BOUNDARY STRATA PREVIOUSLY ASSUMED TO BE MARINE (ARROYO MALO FORMATION, ARGENTINA)

The salinity of marginal-marine water bodies can range from freshwater to fully marine, depending on the strength of freshwater fluxes and degree of connectivity to the ocean. In ancient marginal-marine systems, fully marine salinity is often inferred from marine fossils, even when limited in abundance and stratigraphic distribution, potentially leading to erroneous interpretations regarding time-averaged salinity conditions of the basin of interest. In the present case study, we evaluate the salinity of the Neuquén Basin (Argentina) during deposition of shales of the Arroyo Malo Formation spanning the Triassic-Jurassic boundary. The ~300-m-thick study section has long been regarded as a fully marine deposit based on bivalves and scarce ammonites and brachiopods. Here, we show that elemental salinity proxies (B/Ga, Sr/Ba, and S/TOC) uniformly indicate dominantly freshwater conditions throughout the study interval: (1) B/Ga values are 1.59-2.43 (mean 1.99; n = 80), below the freshwater/brackish (FW/Br) threshold of 3.0; (2) Sr/Ba ratios are 0.11-0.17 (mean 0.14; n = 80), below the FW/Br threshold of 0.20; and (3) S/TOC values are 0.04-0.17 (mean 0.11; n = 130), with half below the FW/Br threshold of 0.10 (see Wei and Algeo, 2020, GCA for discussion of threshold values for each proxy). Redox proxies (Mo_EF, U_EF and C_org:P) confirm a well-oxygenated water column without evidence of stratification.

The Neuquén Basin was located on the southwestern margin of Gondwana. During the Rhaetian-Hettangian, it was characterized by an extensional regime related to the collapse of the Gondwanan Orogen, producing a series of subparallel, elongate half-grabens long assumed to have been connected to the Panthalassic Ocean at their northern end. However, our elemental evidence of freshwater conditions strongly implies that the Neuquén Basin was hydrographically isolated from Panthalassa despite its geographic proximity. The fossil content of the Arroyo Malo Formation, although of marine origin, is sparse and heavily fragmented. It is likely to have been transported into the basin by transient marine incursions and therefore is unrepresentative of average salinity conditions within the Neuquén Basin during the Triassic-Jurassic transition. This case study demonstrates the need for geochemical assessment of salinity conditions in ancient marginal-marine systems.