The interplay between event and background sedimentation and the origin of fossil‐rich carbonate concretions: a case study in Permian rocks of the Paraná Basin, Brazil

Abstract

The Permian Serra Alta Formation was generated under transgressive conditions within a large, calm epeiric sea. A monotonous succession of ‘barren’, massive mudstones deposited under oxygen‐deficient conditions (mainly below storm wave base) is the main lithofacies of this unit. Fossils are generally rare and diluted in the matrix, but certain intervals contain shell‐rich concentrations with well‐preserved, closed articulated bivalves, mixed with shells and comminuted debris with variable quality of preservation, all encased in carbonate concretions. Two main scenarios may account for the origin of these bivalve‐rich concretions (i.e. unique events in sea‐water chemistry or unique burial‐starvation couplets). Sedimentological and taphonomic information indicates that the final deposition of the original shell‐rich mudstone intervals was probably tied to episodic influx of fine‐grained sediments in distal settings. Moderate bioturbation is also recorded suggesting low rates of sedimentation prior to early diagenesis. Hence, the fossil concentrations in concretions were formed due to the interplay of event and background sedimentation. These are internally simple concentrations with complex depositional histories. The concretion‐bearing beds are not randomly distributed in the Serra Alta Formation. Rather, they are found in the sparsely fossiliferous offshore deposits of the basal to intermediate portions of the unit. Thus, the concretionary mudstone beds and associated deposits are preserved in particular intervals and can be tracked for kilometres. This indicates that the conditions essential for concretion development existed only at particular stratigraphical intervals. Finally, our study strongly corroborates the idea that concretions are critical sources of sedimentological, taphonomic and stratigraphical information.

Septarian carbonate concretions in the Permian Rio do Rasto Formation: Birth, growth and implications for the early diagenetic history of southwestern Gondwana succession

“…In the same way, studying early diagenetic concretions is important and has helped expand our understanding of the paleobiological and paleoenvironmental reconstructions of the Paraná Basin and its chrono-correlated units across southwestern Gondwana (see Bondioli et al, 2015).…”

Le dévonien supérieur du sahara nord-occidental (algérie) : faciès, environnements et signification géodynamique des calcaires griottes

“…Ce mécanisme est compatible avec l'origine bactérienne de la pigmentation (MAMET & BOULVAIN, 1988 ;BOULVAIN, 1993 ;MAMET et al, 1997 ;PRÉAT et al, 1999 ;MAMET & PRÉAT, 2003). La présence de fossiles à l'intérieur des nodules coïncide avec le développement de concrétions autour des fossiles préalablement déposés et enfouis (quelques cm de profondeur) atteignant la zone de réduction des sulfates où s'effectuent des précipitations authigéniques (BONDIOLI et al, 2015). La présence d'imprégnations et de blocs d'asphaltites suggère également un suintement ou une mobilité des hydrocarbures (migration).…”

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

“…Therefore, the majority of the paleontological studies focused on those shell-rich deposits. In contrast, the monotonous and fossil-poor shales and mudstones of the Irati and Serra Alta formations are frequently overlooked for benthic macrofossils (see Bondioli, 2014;Bondioli et al, 2015;Warren et al, 2015 for exceptions). Hence, an analytical bias exists in the paleontological investigations of the sedimentary succession of the Passa Dois Group, i.e., the distal deposits are less explored.…”

“…The fossil (a single complete and poorly preserved left valve) was found 30 m above the basal portion of the Irati Formation (Beurlen, 1957b;Runnegar and Newell, 1971, p. 57). The rare specimen was assigned as a new species of Maackia (M. iratiensis), the dominant genus in the bivalve assemblages of the overlying Serra Alta Formation (Bondioli, 2014;Bondioli et al, 2015;Warren et al, 2015;Matos, 2016). Regrettably, repository information on the specimen (collection and reference number) is not available in Beurlen's brief note, and this fossil is probably lost.…”

“…During its~38 million years history (Artinskian-Changhsingian), this basin had limited or no connections with the Panthalassa Ocean and was subjected to important climatic oscillations (Hachiro et al, 1993). These effects produced third-order cycles of sea level fluctuations (Holz et al, 2010a) that periodically caused variations in salinity (hypersaline, brackish, and freshwater) and oxygen (anoxic to oxic bottoms) (Schneider et al, 1974;Zalán et al, 1991;Milani et al, 1994;Rohn, 1994;Wesselingh, 2007;Holz et al, 2010a;Bondioli et al, 2015;.…”