Permian endemic bivalves of the “Irati anoxic event”, Paraná Basin, Brazil: Taphonomical, paleogeographical and evolutionary implications

• Published in: Palaeogeography, palaeoclimatology, palaeoecology palaeoclimatology, palaeoecology, 2017-03, Vol.469, p.18-33
• Main Authors: Matos, Suzana Aparecida, Warren, Lucas Verissimo, Varejão, Filipe Giovanini, Assine, Mario Luis, Simões, Marcello Guimarães
• Format: Article
• Language: English
• Subjects: Allochthonous shells; Anoxic crisis; Black shales; Endemic bivalves; Permian; Southern Gondwana
• ISSN: 0031-0182; 1872-616X
• DOI: 10.1016/j.palaeo.2016.12.043

Anoxic to dysoxic conditions must have existed in the Paraná Basin during the deposition of the Permian oil-bearing shales of the Irati Formation, reflecting the marine isolation of the basin from the Panthalassic Ocean. In this environment, benthic invertebrates are extremely rare, in contrast to the overlying Serra Alta and Teresina formations, which were deposited in dysoxic/oxic waters, respectively. Hence, the abundant shallow-burrowing, endemic bivalves recorded in the shales of the basal portion of the Irati Formation are one of the most distinctive features of this stratigraphic interval. Their preservation in offshore deposits is, however, a product of storm flows in shallow waters that swept the shells to distal settings. Subsequently, these were sorted by long-lasting shelf currents that produced dense (2–5shells/20cm2) pavements in which shells in a hydrodynamically stable posture (convex-up) predominate, forming thin, complex shell concentrations despite their simple internal stratigraphy. The new data presented here have important paleoecologic, paleogeographic and evolutionary implications and indicate that (a) during the deposition of the oil-rich shales of the Irati Formation, numerous benthic bivalves thrived in the contemporary shallow-water bottoms of the Paraná Basin; (b) the mono- to paucispecific nature of the studied shell-rich pavements is, in part, due to hydrologic shell transport; and (c) restricted connections to open ocean waters (Panthalassa) existed during Irati times. Therefore, the origin of the endemic Passa Dois molluscan fauna occurred somewhere between the interval represented by the underlying Palermo Formation and the basal portion of the Irati Formation. Finally, these endemic bivalves appeared at least ~10millionyears earlier than previously thought. [Display omitted] •Dense concentrations of bivalves are, for the first time, documented in black shales of the Permian Irati Formation.•Shells found in offshore, anoxic deposits were assigned to shallow-water bivalves.•High energy processes in shallow water bottoms swept the shells to distal muddy settings.•Mono- to paucispecific nature of bivalve pavements is partly due to mechanical sorting.•Endemic bivalves in the Passa Dois Group appeared at least ~10millionyears earlier than previously thought.