Sedimentary evolution and sequence stratigraphy of Ediacaran high‐grade phosphorite–dolomite–shale successions of the Bocaina Formation (Corumbá Group), Central Brazil: Implications for the Neoproterozoic phosphogenic event

Abstract

Extensive phosphorite deposition is observed in the Neoproterozoic after a prolonged hiatus during most of the Mesoproterozoic era. This event is thought to represent an important record of major palaeoenvironmental, palaeoceanographic and biotic changes that shaped Neoproterozoic ecosystems, suggesting close relationships between phosphogenesis and the preservation of key Ediacaran biotas. However, high‐grade Ediacaran phosphorite deposits are relatively uncommon, diminishing the opportunity to test current phosphate mineralization–deposition models and their implications for Neoproterozoic research. In this scenario, widespread Ediacaran phosphorite–dolomite–shale successions of the Bocaina Formation (Corumbá Group – Central Brazil) are poorly explored in international literature. Nevertheless, recent advances in phosphate exploration gave access to continuous drill core sections and freshly opened mine pits, revealing an unprecedented record of complex phosphatic successions featuring the occurrence of Ediacaran microfossils assigned to the Doushantuo–Pertatataka assemblages. This work seeks to constrain main lithofacies, sequence stratigraphy and depositional settings from these phosphatic successions in order to analyse the sedimentary evolution of the unit under the current Neoproterozoic phosphorite research framework. These results indicate that the Bocaina Formation records secular sustained phosphate deposition. These deposits are related to unprecedented, microbialite reef rim phosphorites deposited during a lower accretionary rimmed platform stage, followed by the deposition of Doushantuo‐like, whole platform phosphorites associated with a later, drowned platform stage, therefore, reinforcing the evidence for the operation of strong allogeneic controls on phosphate mineralization–concentration. In addition, this study concludes that fossiliferous Ediacaran phosphatic deposits such as the Bocaina Formation are important to understanding Neoproterozoic phosphogenic events, because they may record the transition from a Precambrian to Phanerozoic‐like phosphogenesis associated with the instauration of the Ediacaran–Cambrian phosphatic taphonomic window. This evidence hints that the growing dataset from the Bocaina Formation may bring new, exciting perspectives for Neoproterozoic research as a whole.

Dawn of diverse shelled and carbonaceous animal microfossils at ~ 571 Ma

“…Refs. 44 , 47 ), occur in a slightly asymmetrical, west verging syncline, bounded to the west and east by inverse faults (Fig. 2 b).…”

“…Truncated by regional erosional unconformities at both its top 43 , 44 , 55 and base 44 , 47 , 53 , this work), the Bocaina Formation overlies Paleoproterozoic crystalline basement and Cryogenian to Early Ediacaran post-glacial deposits 44 , 47 , 51 , 53 (Figs. 1 , 2 a,b).…”

“…In both regions, the Bocaina Formation is virtually unmetamorphosed. It forms part of the carbonate-dominated succession of the Corumbá Group, consisting of dolomitic grainstones and stromatolitic deposits with shale locally present in its upper part 43 , 45 – 47 (Figs. 1 , 2 ).…”

“…1 , 2 ). Significant bedded and nodular phosphate deposits occur at intermediate to upper stratigraphic positions, as well as phosphorite clast accumulations 43 , 45 – 47 .

Figure 1 Geological setting of the study area.

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“…( c ) Neoproterozoic stratigraphy in the Urucum district and Serra da Bodoquena *Parry et al 48 , § Piacentini et al 49 ; † Frei et al 50 ; ϕ Freitas et al 51 ; ∂ McGee et al 52 ; Ω This work. Geology after Campanha et al 44 ; Romero et al 53 , McGee et al 2018; Warren et al 54 ; Freitas et al 51 , Hippert et al 47 . Figure created using Corel X8.…”