A review on depositional systems, bioevents and paleogeography of the Valanginian-Hauterivian Neuquén Sea: Refining sedimentary and biological signals linked to the dynamics of epeiric seas

Abstract

Epeiric seas were common in deep times, but modern analogues are rare. Large-scale reconstruction of ancient examples is usually limited by available data, which also hampers the conceptualization of their variability and key controlling parameters. In this study we summarize and integrate a large stratigraphic dataset of a relatively small, semi-restricted Cretaceous epeiric sea in southwestern Gondwana to identify key sedimentary and biological signals that are used to discuss oceanographic connection with adjacent oceans, areal distribution of sediments entering the marine basin, sediment dispersal pathways within it, and the resulting stratigraphy. The Valanginian–Hauterivian Pilmatué-Lower Centenario study interval represents an exceptional record of well-dated marine and continental sediments deposited in the Neuquén Sea, which occupied a back-arc setting and was semi-connected with the proto-Pacific Ocean across a volcanic arc. The study interval is organized in three transgressive-regressive sequences (Lower, Middle, and Upper) defined by the integration of genetically linked depositional systems, macrobenthic bioevents, and stratal patterns. Detailed paleogeographic reconstructions show a consistent east-west proximal-distal depositional trend and a major fluvial system located in its southeastern apex. This continental-scale delivery system was responsible for the vast volume of sands and muds brought into the sea. The degree of connection between the Neuquén Sea and the adjacent ocean was evaluated by considering onlap stratal relationships, incursion of pandemic fauna, onset and deactivation of tidal offshore fields, and contribution of plankton-derived carbonate mud (a proxy to paleoproductivity). These criteria indicate that the maximum volume of watermasses within the semi-restricted sea was reached not during initial flooding (i.e., base of Lower Sequence), but most probably near the Valanginian/Hauterivian boundary (base of Middle Sequence). Dispersal pathways within the distal marine settings of the Neuquén Sea were reconstructed by integrating geographical thickness variations, composition of muds, and seafloor oxygenation levels. Results allow defining a non-uniform, along-depositional strike scenario, with a locus of siliciclastic deposition westwards of major deltaic systems, and lower accumulation towards the northwestern and southwestern areas. The northwestern region, having maximum carbonate contribution and lower oxygen levels, is inferred to have been more prone to water-column stratification over time. Some of the key attributes reconstructed for the Neuquén Sea (namely significant restriction, major sediment supply in one apex, asymmetric distribution of sediments in the distal settings) are comparable with configurations observed in the present Persian Gulf and the Adriatic Sea and departing from others such as the Baltic Sea and the Gulf of Carpentaria. Comparisons with ancient examples such as the Western Interior Sea and the North American Seaway suggest contrasting paleogeographic models, but at the same time reinforce the value of these key parameters to produce better discrimination between the possible spectrum of epeiric seas. In that sense, building classifications for epeiric seas based on these key parameters, such us size, degree of connection with the open ocean, sediment entry points, and sediment dispersal pathways, appears as the next challenge for more comprehensive reconstructions of these environments worldwide.