Seas to lakes and vice-versa: A test of facies versus elemental salinity proxies

Abstract

Recognizing marine-to-lacustrine (M-L) and lacustrine-to-marine (L-M) transitions can be difficult in stratigraphic successions, particularly in those composed of fine-grained sediments containing few fossils. Here, we test the utility of elemental salinity proxies (i.e., B/Ga and excess B) to assist in constraining the timing of such transitions in mixed marine-lacustrine strata of the Cretaceous (Valanginian-Hauterivian) Agrio Formation in Argentina. A combination of salinity and redox proxy data reveal that initial isolation of the Neuquén Basin watermass from seawater exchange resulted in transient hypersalinity and deepwater anoxia, followed by long-term freshening of the evolving lacustrine system. Subsequently, reconnection with the global ocean was marked by a large salinity increase prior to the appearance of marine fossils and transgressive surfaces. Our findings demonstrate the utility of elemental salinity proxies in identifying M-L and L-M transitions in fossil-poor, fine-grained successions, and they highlight the importance of using geochemical proxies in combination with sedimentological and paleontological information to understand salinity transitions in paleodepositional systems.