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

• Published in: Lethaia Vol. 48; no. 4; pp. 522 - 539
• Main Authors: Bondioli, João G., Matos, Suzana A., Warren, Lucas V., Assine, Mário L., Riccomini, Cláudio, Simões, Marcello G.
• Format: Journal Article
• Language: English
• Published: Oslo Wiley 01.10.2015; Blackwell Publishing Ltd; Scandinavian University Press
• Subjects: Biostratinomy; bivalves; carbonate concretions; fossil diagenesis; Permian
• ISSN: 0024-1164; 1502-3931
• DOI: 10.1111/let.12124

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.