Treptichnus pedum in the Três Marias Formation, south-central Brazil, and its implications for the Ediacaran-Cambrian transition in South America

doi:10.1016/j.jsames.2020.102983

Journal of South American Earth Sciences

Volume 105, January 2021, 102983

https://doi.org/10.1016/j.jsames.2020.102983 Get rights and content

Highlights

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An ichnofossil and MISS fossils have been discovered in the Três Marias Formation, the topmost unit of the Bambuí Group.
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These are the first fossils to be reported from the Três Marias Formation.
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T. pedum indicates an Early Paleozoic age for the uppermost Bambuí Group.
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The fossil record of Três Marias Formation contributes to a better constrained time frame for Bambuí Group.
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The Bambuí Group integrates the set of other six units hosting an Ediacaran-Cambrian boundary in South America.

Abstract

The index ichnofossil Treptichnus pedum Seilacher 1955 and four types of Microbially Induced Sedimentary Structures (MISS) - variants of wrinkle and elephant skin structures – are the first fossils described from the Três Marias Formation, topmost unit of the Bambuí Group in south-central Brazil. Despite the stratigraphic importance and widespread occurrence of this unit in the São Francisco Craton and adjacent Brasília Fold Belt, its age is still poorly constrained between the Neoproterozoic and Early Paleozoic. This discovery is significant because the first appearance datum (FAD) of T. pedum has been used to define the Ediacaran-Cambrian boundary. Hence, the presence of T. pedum indicates an early Paleozoic age for the uppermost Bambuí Group and means that the Ediacaran-Cambrian limit must lie deeper in the Bambuí Group, somewhere between this level and that of the terminal Ediacaran index-fossil Cloudina sp. in the Sete Lagoas Formation, near the base of the group. Available age determinations on detrital zircon grains and chemostratigraphic correlations are consistent with an Ediacaran to Cambrian age for this group. Detailed paleontological prospection throughout the group may now proceed within a better constrained time frame and hopefully reveal additional fossil evidence of Cambrian as well as Ediacaran life in the Bambuí Group.

Section snippets

Bambuí Group

The Bambuí Group, cropping out over 350.000 km² in the states of Minas Gerais, Bahia, Tocantins, and Goiás (Alkmim and Martins-Neto 2001), comprises the main Neoproterozoic-early Paleozoic cover of the São Francisco Craton of south-central Brazil (Fig. 1A). It evolved within a foreland basin system on the São Francisco-Congo paleocontinent, which was established in response to the development of the Brasília and Araçuaí orogens during the Brasiliano-PanAfrican West Gondwana assemblage, at the

Material and methods

The fossils discussed here were collected as loose blocks in a deactivated quarry in the city of Três Marias, Minas Gerais, Brazil. The quarry (Fig. 2A) shows reddish to brown sandstones intercalated with reddish siltstones. The feldspathic sandstones are fine to medium-grained, generally massive, with pelitic intraclasts and parallel (Fig. 2B) or truncated stratification. The reddish siltstones exhibit flaser-type lamination (Fig. 2C), load casts, flute casts (Fig. 2D), all indicative of

Microbially Induced Sedimentary Structures

Four types of Microbially Induced Sedimentary Structures (MISS) were recognized. According to the genetic scheme for MISS formation proposed by Schieber (2004) and Noffke (2009), the fossils reported here comprise structures related to the growth and diagenesis of microbial mats. Most are variations of wrinkle structures, and, thus, can be classified as i) wrinkle structures stricto sensu, ii) elephant skin, iii) fine wrinkle structures, and iv) fine wrinkle structures associated with bumps.

Testing biogenicity of Treptichnus of the Três Marias Formation

As the simple fossils described here may resemble abiogenic sedimentary structures, it is important to test their biogenicity. An alternative, abiogenic explanation for the Três Marias Treptichnus pedum is that it might represent a sedimentary structure made by water movement. For example, water channels can present similar series of connected branches in dendritic arrangements, such as rill marks, which forms miniature stream systems on intertidal surfaces when fresh sediments and water are

Conclusions

The fossil assemblage here reported is important for four reasons: i) it documents the first fossils known from the Três Marias Formation; ii) it adds fossil MISS and the Lower Cambrian to Ordovician ichnospecies Treptichnus pedum to the fossil record of the Bambuí Group, one the most widespread stratigraphic units in Brazil and an important sedimentary unit for the South American geology; iii) it sheds light on the age of the uppermost unit of the group; and, iv) it corroborates the presence

Author statement

AU, EAMS and TRF participated the fieldwork. AU indicated the quarry. TRF collected the T. pedum specimen. EAMS and TRF collected the MISS specimens. EAMS described the fossil assemblage. AU, EAMS and TRF prepared and revised the manuscript.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The research group thanks the CNPq - Conselho Nacional de Desenvolvimento Científico e Tecnológico (Brazil) for financial support (Project # 401815/2010-2) and FAPESP - Fundação de Amparo à Pesquisa do Estado de São Paulo (Project # 2011/07203-9), provided to EAMS for her doctorate. The authors also thank Nora Noffke (Old Dominion University) for discussion on MISS biogenicity, and two anonymous reviewers for their helpful criticism.

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The late Ediacaran record encompasses frequent occurrences of ³⁴S-enriched pyrites, which can surpass the coexisting sulfate isotope values. Deciphering what controls this marine isotope record is important to understand its biogeochemical implications, especially considering the early metazoan evolution. We present here multiple sulfur isotope compositions from pyrites of two distinct geotectonic settings within the late Ediacaran-early Cambrian West Gondwana, allowing to constraint spatial and temporal distribution of ³⁴S-enrichments. The Tamengo Formation represents a carbonate-siliciclastic ramp at Gondwana’s continental margin, whereas the Bambuí Group records an epeiric sea within the core of this supercontinent. Both units display positive δ³⁴S_py excursions matching regressive trends (δ³⁴S_py up to 51 ‰), with a drop in δ³⁴S_py values associated with transgressions. A scenario of sulfate-distillation events throughout regressions is supported by the multiple sulfur isotope data, after transgressive events that episodically recharged sulfate in these environments. However, in the epicontinental Bambuí sea, δ³⁴S_py, Δ³³S_py and Δ³⁶S_py values suggest almost quantitative conversion of the initial sulfate into sulfide at the end of regressive stage, forced by enhanced stratification and inefficient circulation within continental areas. In this case, widespread toxic euxinic conditions were developed in inland seas, accompanied by methanogenesis, which resulted in highly ¹³C-enriched DIC due to methanogenic CO₂, as recorded in the Bambuí Group. In the Tamengo ramp, on the other hand, sulfate levels would have not been completely consumed due to more frequent and efficient mixing with open waters, preventing perturbations in the carbon cycling and representing more favorable conditions to metazoan colonization, considering the abundant metazoan fossil record in this unit. The studied basins illustrate how 1^st-order tectonic cycles may have been the major player over remarkable ³⁴S-enrichments in pyrite for specific time intervals of the sedimentary record, through supercontinent assembly and expansion of restricted settings more prone to sulfate distillation.
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Stratigraphy, isotope geochemistry, seismic stratigraphy and paleogeography of the Lagoa do Jacaré Formation, Bambuí Foreland Basin (Ediacaran-Cambrian), Southeast Brazil
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Citation Excerpt :
The Três Marias Formation is deposited at the top of the succession, composed of arcoses and dark green siltstone (Dardenne, 1978; Chiavegatto, 1992), generally concordant with the underlying siltstones and, at the eastern border of the basin, it overlaps discordantly the siltstones and shales from the Serra da Saudade Formation (Rossi et al., 2019). This unit represents the last preserved stage of sedimentation of the Bambuí Group in the sedimentary record of the São Francisco Basin, already in the Cambrian (Tavares et al., 2020; Sanchez et al., 2021). Although the age of sedimentation of the Bambuí Group still raises some questions, important contributions have been made to this problem in recent years.
The Bambuí Group is a lithostratigraphic unit that comprises a mixed carbonate-siliciclastic platform deposited in a foreland basin. It is a 1st order sequence that filled the São Francisco Basin during the Ediacaran-Cambrian in southeast Brazil. The Lagoa do Jacaré Formation is the second carbonate unit of the Bambuí Group and is related to a 2nd-order regressive hemicycle composed of dark gray shallow water carbonates with high positive ẟ¹³C values (>+10‰) related to the upper part of the Middle Bambuí Excursion (MIBE). We described four outcrops in the center-north of Minas Gerais state (Paraopeba, Curvelo, Montes Claros and Ubaí areas) coupled with ẟ¹³C and ẟ¹⁸O, and SEM-EDS analysis in order to support detailed facies analyses. One seismic line and three well logs in the south part of the Januária High were also used to define the stratigraphic framework of the Lagoa do Jacaré Formation in the Bambuí Foreland Basin system. Eight facies were interpreted from the outcrop data and grouped into five facies associations, that represent peritidal zone (FA1), ooidal sand shoals (FA5), tempestites of mid ramp (FA3), fine-grained carbonate of outer ramp (FA4), and fine-grained siliciclastic from offshore (FA2). The strong presence of terrigenous material (quartz, feldspars, muscovite, biotite, apatite) with some authigenic/diagenetic minerals (pyrite, titanite, rutile) in all facies are the main petrographic aspect of the studied carbonate rocks, related to high sedimentary input and some events during early diagenesis. FA1 with lower mean values of ẟ¹³C (+9‰) predominates in the north, meanwhile FA3 is the main association in the south with higher ẟ¹³C mean values (+12‰). This difference is probably related to the stratigraphic differences between the basal section (at south) and upper section (at north) of the Lagoa do Jacaré Formation. The Januária High was the main controlling element for the deposition of the carbonate ramp in the north sector of the basin, close to the Ubaí and Montes Claros cities. Probably, this structural high also played an important role as source area in the forebulge sector, uplifted by flexure due to the orogeny of the Brasília and Araçuaí (still in development) Belts (∼540-520 Ma). To the east, a N–S carbonate ramp system was developed in the back-bulge sector according to the main paleocurrents measurements. This ramp is related to a 300 m thickness section of the Lagoa do Jacaré Formation that was firstly identified and correlated to the successions on the surface using well logs, possibly registering at the top the decreasing in the ẟ¹³C values, after the main stages of the MIBE probably in close association with Ediacaran-Cambrian boundary, related to the assembling of the Gondwana Supercontinent recorded in this region of southeast Brazil.
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2022, Journal of South American Earth Sciences
Citation Excerpt :
Occurrences of the index fossil Cloudina sp. (Warren et al., 2014; Perrella et al., 2017) and c. 550 Ma detrital zircons (e. g., Paula-Santos et al., 2015; Kuchenbecker et al., 2020) in the Sete Lagoas Formation constrain a late Ediacaran maximum depositional age for the basal Bambuí Group. A tuff layer yielding the age of 520.2 ± 5.3 Ma within the Serra da Saudade Formation (Moreira et al., 2020), the recent discovery of the Treptichnus Pedum ichnofossil (Sanchez et al., 2021), and detrital zircons yielding a maximum depositional age of 527 ± 5.3 Ma (Tavares et al., 2020) in the Três Marias Formation, indicate that the deposition spanned through the Cambrian period (Fig. 2). The known fossils from the Bambuí Group include, besides the above-mentioned taxa, microbialites, planktonic and benthic microfossils and trace fossils, mainly concentrated in the Sete Lagoas Formation (e.g. Cruz and Nobre-Lopes, 1992; Fairchild et al., 1996; Nobre-Lopes and Coimbra, 2000; Sanchez, 2014; Sanchez et al., 2018,2020).
The sedimentary evolution of the Bambuí foreland basin system in the interior of West Gondwana is marked by periods of connection and isolation from the global ocean during the late Neoproterozoic and early Paleozoic. To understand the link between these periods and seawater redox conditions, we present an integrated study of trace-metal geochemistry and stratigraphy of the carbonate-siliciclastic rocks from the two lowermost second order transgressive-regressive sequences of the Bambuí Group, east Brazil. The basal 2nd-order sequence trace-metal pattern shows a progressive decrease of mass fractions of Co, Cr, Ni, Cu, Mo, U, V, Zn, and Cd, concomitant with a progressive decrease of Al and Fe contents and Mo/TOC ratios. Among all these elements, only Cd, Mo and U mass fractions seem to be less or not affected by detrital influence, so they can be used as reliable redox proxies for the paleoenvironmental analysis of the studied Bambuí strata. Moreover, normalization to aluminum shows a progressive increase of trace-metal enrichments for Cd and Mo in the order of 0.1–10 times and for U in the order of 1–100 times, accompanied by a progressive increase of organic carbon content upward section. These changes in sedimentary trace-metal patterns provide evidence for the chemical evolution of basinal deep-waters, whose conditions changed progressively from suboxic-anoxic to anoxic-euxinic at the basal transgressive-regressive sequence. We suggest that the paleomarine system represented by the basal Bambuí Group has probably evolved as an intracontinental silled basin recording changes in seawater chemistry associated with redox variations and restricted hydrographic conditions. Sedimentary trace-metal patterns indicate that Bambuí epeiric sea was initially in communication with open ocean followed by the marine restriction during the transgressive and regressive cycles, respectively. This resulted in a long deepwater residence time and chemical evolution of deep watermass as a response to tectonic pulses and consequent sea-level variations during the restricted stage. Under anoxic-euxinic conditions of seawater, trace metals scarcity and micronutrient fixation limitation would have impacted nitrate bioavailability, preventing the evolution of early benthic metazoans in the Bambuí paleomarine system during the late Ediacaran and early Cambrian.
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Citation Excerpt :
Deformation associated with the mobile belts affected all units of the Bambui Group except the upper portion of the Tres Marias Formation (Zalán and Romeiro-Silva, 2005; Reis and Alkmim, 2015; Reis and Suss, 2016; Rossi et al., 2020; Tavares et al., 2020), indicating that nearly its entire thickness was deposited before the final compressive phases of the Brasiliano Orogeny at ca. 580–550 Ma (Teixeira et al., 2000; Alkmim et al., 2001; Sial et al., 2009; Misi et al., 2011; Heilbron et al., 2017a; Martins-Ferreira, 2019). The undeformed units of the Tres Marias Formation, however, form the present-day erosion surface and locally unconformably overlies older deformed units of the Bambui Group (Uhlein et al., 2019; Rossi et al., 2020; Tavares et al., 2020; Sanchez et al., 2021). The well-constrained ages of deformational events affecting the Bambui Group have been recently questioned and re-interpreted as the consequence of the far field effect of the collision between the Amazon and the Sao Francisco cratons on the western side of the Brasilia Belt during the Cambrian (Caxito et al., 2021).
The Bambui Group is a ca. 2,000 m-thick mixed biochemical-siliciclastic sedimentary succession that accumulated in the Sao Francisco Basin, central Brazil. Its age is contentious, but deposition during the Cryogenian-Ediacaran (ca. 635–550 Ma) is consistent with a new sequence stratigraphic model presented herein and previously published geochemical and structural data. Six stratigraphic sequences are recognized that record depositional evolution from an epeiric sea to foreland basin during the Marinoan snowball glaciation (ca. 635 Ma) to shortly after the Gaskiers (ca. 580 Ma) glaciation. Although this stratigraphic architecture differs from the classic lithostratigraphic and more recent chemostratigraphic and sequence stratigraphic interpretations of the Bambui Group, it is supported by correlation to other Cryogenian-Ediacaran successions globally. Collectively, these six sequences preserve a robust record of seawater redox structure leading up to the oxygenation threshold that triggered the evolution of multicellular animals in the Ediacaran.
Sequence 1 is composed of Marinoan glacial deposits and an overlying cap carbonate with aragonite fan pseudomorphs and barite. Sequence 2 is composed of lime-mudstone that changes laterally into loess-rich peritidal phosphorites, suggesting oxygen stratification in shallow paleoenvironments. Sequence 3 is a carbonate succession with abundant stromatolites implying widespread oxygen production. The top of Sequence 3 is marked by a regional subaerial unconformity developed during uplift associated with the onset of the Brasiliano-Pan African Orogeny. Sequence 4 signals a return to loess derived sedimentation. Distal deposits contain abundant organic matter with framboidal pyrite reflecting bacterial sulfate reduction in low sulfate Ediacaran seawater. Sequence 5 is composed of well-developed aggradational parasequences composed of alternating deep subtidal organic-rich siltstones and shallow-water carbonate grainstones. Their well-defined cyclicity is interpreted to record extreme sea level fluctuations during the onset of the Gaskiers glaciation. Stromatolites suggest an oxygen-stratified water column. Sequence 6 marks the change from an epeiric sea to a foreland basin. Interbedded basinal glauconitic siltstone and shale loess-rich are correlative with shoreface sandstones. Authigenic glauconite requires suboxic bottom waters to precipitate and thus reflects oxygenation of distal settings. Most of the deposition ended by ca. 560–550 Ma when the entire Bambui Group was deformed during the assembly of West Gondwana.
In sequences 1, 2, 3, 4, 5, and the lower portion of 6, the lateral continuity of lithofacies, predictable layer-cake stratigraphy, and lack of fluvial coarse siliciclastic deposits are characteristics of epeiric sea successions. The appearance of immature sandstone, poorly sorted conglomerate, braided fluvial deposits, and synsedimentary deformation in the upper part of Sequence 6 are interpreted as the onset of foreland basin development. Stacking relationships of redox sensitive, authigenic lithofacies in both the epeiric sea and foreland phases support recent research suggesting seawater redox conditions were in a state of flux during the apex of the purported Neoproterozoic Oxygenation Event. Organic matter and authigenic pyrite in Sequences 4 and 5 reflect increased primary productivity in surface waters fertilized by nutrients such as P and Fe adsorbed on loess particles. This mechanism could be expanded to a global scale and have contributed to the increase in atmospheric and oceanic oxygen levels. These findings redefine and highlight the significance of the Bambui Group for Neoproterozoic Earth history.
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Citation Excerpt :
The study area (Fig. 1C), to the west of the town of Coração de Jesus (Minas Gerais state), is located in the Alto do Paracatu region, characterized by a regional uplift of the basement that separates the Sanfranciscana Basin into the Urucuia and Abaeté sub-basins (Campos and Dardenne, 1997a). There, the Sanfranciscana units occur as small plateaus in an unconformable boundary with the lower Bambuí Group, a thick (up to 1000 m) carbonate-siliciclastic succession of Ediacaran to Cambrian age (Warren et al., 2014; Sanchez et al., 2021) that covers the southern region of the São Francisco Craton (Schobbenhaus et al., 1984; Alkmim and Martins-Neto, 2001). The stratigraphy of the Sanfranciscana Basin is comprised of the Santa Fé (Permo-Carboniferous), Areado (Lower Cretaceous), Mata da Corda (Upper Cretaceous), and Urucuia (Upper Cretaceous) groups (Fig. 1D; Campos and Dardenne, 1997a; Sgarbi et al., 2001).
The Coração de Jesus region, northern Minas Gerais state, Brazil, is highly relevant in the Sanfranciscana Basin context due to the presence of titanosaurs, abelisaurids, and noasaurids. However, there is a poor stratigraphic constrain of the occurrences and few paleoenvironmental investigations in the area. Herein, a detailed sedimentologic and stratigraphic investigation of the fossiliferous succession is conducted to position the vertebrate assemblage in the stratigraphy. The sedimentary rocks are interpreted as part of the Quiricó Formation (Areado Group), representing the thickest occurrence of the unit (ca. 56 m) to the north of the Abaeté sub-basin. There, two main sedimentary successions are recognized, with the lower succession dominated by mudstone facies, and the upper succession dominated by sandstone facies. Vertebrate fossils and trace fossils of the species Taenidium barretti, Skolithos isp. and Arenicolites isp. are recorded uniquely within the lower succession, where wetter environmental conditions prevailed. The sedimentary succession is interpreted as a lacustrine system with increasing influence of eolian processes, which suggests an intensification of the aridity in the region during Lower Cretaceous.

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Treptichnus pedum in the Três Marias Formation, south-central Brazil, and its implications for the Ediacaran-Cambrian transition in South America

Highlights

Abstract

Section snippets

Bambuí Group

Material and methods

Microbially Induced Sedimentary Structures

Testing biogenicity of Treptichnus of the Três Marias Formation

Conclusions

Author statement

Declaration of competing interest

Acknowledgments

Geobios

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A Bacia Intracratônica do São Francisco: arcabouço estrutural e cenários evolutivos

A beach structure due to wind-driven foam

J. Sediment. Petrol.

Sedimentary Structures, Their Character and Physical Basis

Direct dating of the Sete Lagoas cap carbonate (Bambuí Group, Brazil) and implications for the Neoproterozoic glacial events

Terra. Nova

Ecological interactions in Cloudina from the Ediacaran of Brazil: implications for the rise of animal biomineralization

Sci. Rep.

The Cambrian substrate revolution

GSA Today (Geol. Soc. Am.)

Decision on the Precambrian-Cambrian boundary stratotype

Episodes J. Int. Geosci.

Treptichnus pedum and the Ediacaran–Cambrian boundary: significance and caveats

Geol. Mag.

A large epeiric methanogenic Bambuí sea in the core of Gondwana supercontinent?

Geosci. Front.

Diverse biomineralizing animals in the terminal Ediacaran Period herald the Cambrian explosion

Geology

Análise estratigráfica das seqüências tempestíticas da Formação Três Marias (Proterozóico Superior), na porção meridional da Bacia do São Francisco

Surficial beach structures formed by wave-generated foam

J. Geol.

Identification of microbially induced sedimentary structures over a tidal flat

Lat. Am. J. Sedimentol. Basin Anal.

Síntese sobre a estratigrafia do Grupo Bambuí no Brasil Central

Boletim de trabalhos do 30° Congresso Brasileiro de Geologia