Sedimentary facies, fossil distribution and depositional setting of the late Ediacaran Tamengo Formation (Brazil)

Abstract

Mixed carbonate-siliciclastic deposits of the Tamengo Formation (terminal Ediacaran), record the rise of calcifying metazoans and the origin of exoskeletons in animals. To explore the relationships between environmental setting and the first appearance of calcified metazoans and their ecology, this study presents detailed sedimentological and stratigraphic data of eight sections that capture the final stages of the Ediacaran (550 to 543 Ma). This study combines stratigraphic characterization with detailed facies descriptions and evaluates lateral heterogeneity and overall ramp sedimentation integrated with fossil distribution. The Tamengo Formation represents a storm-dominated ramp. The outer to mid-ramp is composed of very finegrained siliciclastic rocks containing Corumbella body fossils and thinbedded mudstone/wackestone containing Cloudina. The mid-inner ramp is dominated by wackestone/packstone with abundant Cloudina skeletal debris and ooid packstone/grainstone shoal deposits. Locally, fragments of Corumbella and Cloudina are found on the same horizon, which is a result of their high accumulation rate, resulting from the reworking and mixing of epifaunal organisms. In spite of taphonomic biases, the general distribution of Corumbella and Cloudina across the unit suggests that these organisms have been transported across ramp and/or probably show a differential response of fauna preservation. When compared to other occurrences worldwide, this dataset indicates an already complex ecosystem in the Ediacaran, where these early animals were capable of adapting to specific environmental niches.

New Species of Macroalgae from Tamengo Formation, Ediacaran, Brazil

“…In addition, the preservation occurred because of a transformation of the non-mineralized soft tissues (organic matter) to a stable kerogen is a process named kerogenization (Cai et al, 2012). Amorim et al (2020) propose a facies association in the deposits from the Tamengo Fm. where the macroalgae were found.…”

“…It is intercalated with carbonated facies such as massive to poorly laminated calcimudstone, laminated wackestone, and hummocky/swaley cross-stratified packstone. The alternation between fossiliferous pelites with tempestites (carbonates) suggested the preservation of the fragile macroalgae during the interval of the storms, during the suspension fallout of fine sediments (Labaj and Pratt, 2016;Amorim et al, 2020).…”

“…Other fossils of metazoan are preserved in the same stratigraphic level such as Corumbella werneri; they also miss the basal part but present an overall well-preserved level in their carapace (Amorim et al, 2020). Corumbella werneri possess a thin organic and delicate wall (Pacheco et al, 2015;Walde et al, 2019), so the well-preserved fossil associated with the silty shale facies can indicate a minimal transportation or rework at a low-energy environment (Warren et al, 2012;Amorim et al, 2020).…”

“…The Tamengo Formation (Figure 1C) is marked at the base by carbonatic breccia, overlain by carbonate facies of grainstones and mudstones with sedimentary structures, such as hummocky cross-stratification and wave marks with a volcanic ash at the top, in which zircons are dated at 541.85 ± 0.75 Ma. Packages of shales, siltstones, and fine sandstones with parallel plane lamination are interspersed with the carbonate facies (Boggiani et al, 2010;Parry et al, 2017;Amorim et al, 2020).…”

“…The facies characterized by siliciclastic rocks, metazoans, trace fossils, and plane parallel structures suggest an environment of low energy, associated with the offshore zone. The facies with limestone, hummocky cross-stratification, wave ripples, and Cloudina sp., is associated with the shoreface zone (Boggiani et al, 2010;Oliveira et al, 2019;Amorim et al, 2020).…”