Early to Middle Devonian ironstone and phosphorite in the northwestern Gondwana Parnaiba Basin, Brazil: A record of an epeiric margin paleoceanographic changes

Pragian to Givetian successions in the Caninde group, Parnaiba Basin (Brazil), present phosphorites, ironstone and organic rich sediments that globally coincide with greenhouse conditions and an increase in continental weathering, ocean oxygen levels and black shale deposition events. These fades are associated with the evolution of the northern Gondwana epeiric sea margin of the Rheic Ocean and can tell us how ocean chemistry changed and the paleoceanographic factors involved in this particular transition of the ocean record. We conducted an integrated study based on sedimentology, sequence stratigraphy, petrography, scanning electron microscopy and X-ray diffraction to understand the paleoenvironmental, stratigraphic and diagenetic controls. Lithofacies stacking patterns indicate that deposition occurred during marine transgressions resulting in four depositional sequences. Late Eifelian and Givetian maximum flooding surfaces are represented by phosphogenesis and francolite precipitation associated with condensed zones. Pragian-Emsian to Givetian ironstones were represented by authigenic coated phosphatic iron silicate (odinite, chamosite/dinochlore, francolite) and siderite precipitation. These are related to normal regressions under lowstand and highstand systems tract. Oxic waters prevailed and episodic upwelling is interpreted as the source of nutrients that maintained Fe, Si and P supply on a storm-influenced to storm-dominated shelf with an additional source of iron also associated with river discharges. Phosphorite was formed in a stratified ocean, near storm wave base, under anoxic (sulfide reducing bacteria microbial communities) and suboxic conditions (productivity fueled by recycled P). Shallower anoxic conditions (near the fair weather wave base) existed, possible related to a shift in the locus of upwelling, and then resumption to oxic conditions indicate that water column stratification was not a stable feature due to variations in base level, ventilation, terrigenous or marine contribution and progressive microbial degradation. At the beginning of late Eifelian transgression, an event of increased delivery of continental flux and eutrophication can also be proposed. The link between organic-rich intervals and third order black shale events (Daleje, Choi& and KaZak) suggests that extinction events can be tracked by analyzing sections in South America. Global correlation with ironstone precipitation and organic rich sediments shows that stepped periods of C sink conditions probably contributed to the ocean oxygenation. (C) 2020 Elsevier B.V. All rights reserved.