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A-B, columnar stromatolites found at the São João da Ponte site. Drawing enables the observation of uniform columns with sinuous growth vectors and alpha, beta and parallel branching patterns. Column with vertically increasing width is also present. C-D, large, meter-long stromatolitic dome composed of light-dark alternation of rhombic-shaped and moderately convex laminae. Geological hammer is approximately 28 cm in length.
Source publication
Here we catalog, for the first time, the known stromatolite-containing localities and morphologies found in outcrops previously attributed, by past mapping efforts, to the Lagoa do Jacaré Formation, Bambuí Group. Unlike the Sete Lagoas Formation, which has already provided several crucial samples containing a substantial variety of microbialitic fo...
Contexts in source publication
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... were commonly associated with intraclastic calcirudites. The columnar stromatolites occurred as isolated fragmented bioherm blocks alongside the road cut presenting poor to moderate preservation ( Figure 5A). Columns are closely arranged, decimetric, cylindrical, and uniform in shape with sinuous growth vectors. ...
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... laminae are rhombic-shaped, overlap each other forming a partial wall and vertically transition to symmetrical and moderately convex laminations with two orders of laminar curvature (convex or curved laminations which themselves are crenulated). A light-dark alternation pattern is clearly visible (Figures 5D-C). Vertical inheritance is good. ...
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... were commonly associated with intraclastic calcirudites. The columnar stromatolites occurred as isolated fragmented bioherm blocks alongside the road cut presenting poor to moderate preservation ( Figure 5A). Columns are closely arranged, decimetric, cylindrical, and uniform in shape with sinuous growth vectors. ...
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... laminae are rhombic-shaped, overlap each other forming a partial wall and vertically transition to symmetrical and moderately convex laminations with two orders of laminar curvature (convex or curved laminations which themselves are crenulated). A light-dark alternation pattern is clearly visible (Figures 5D-C). Vertical inheritance is good. ...
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... were commonly associated with intraclastic calcirudites. The columnar stromatolites occurred as isolated fragmented bioherm blocks alongside the road cut presenting poor to moderate preservation ( Figure 5A). Columns are closely arranged, decimetric, cylindrical, and uniform in shape with sinuous growth vectors. ...
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... laminae are rhombic-shaped, overlap each other forming a partial wall and vertically transition to symmetrical and moderately convex laminations with two orders of laminar curvature (convex or curved laminations which themselves are crenulated). A light-dark alternation pattern is clearly visible (Figures 5D-C). Vertical inheritance is good. ...
Citations
... Recently, an occurrence of putative trace fossils identified as Treptichnus pedum in association with MISS has been described in the Três Marias Formation, the uppermost unit of the Bambuí Group (Sanchez et al. 2021). Microbial structures are less conspicuous and generally poorly preserved in predominantly siliciclastic units of the Bambuí Group Santos et al. 2018;Uhlein et al. 2020), hampering their identification and interpretation. ...
... The presence of mixed carbonate-siliciclastic facies with local evidence of subaerial exposure (e.g., mudcracks) and salt pseudomorphs indicates deposition in inter-to supratidal settings under transitory evaporitic conditions . Domal and columnar stromatolites, as well as laminated microbialites, have also been described in the overlying Lagoa do Jacaré Formation (Iglesias and Uhlein 2009;Fragoso et al. 2011;Santos et al. 2018;Uhlein et al. 2019). ...
... Abundant stromatolites are known from the Bambuí Group, particularly the Sete Lagoas, Lagoa do Jacaré, and upper Serra da Saudade formations (Vieira et al. 2007;Santos et al. 2018;Uhlein et al. 2020). However, eukaryotic fossils and particularly macroscopic animal fossils are rare in the Bambuí Group, either because of their poor preservation (Simonetti and Fairchild 2000;Warren et al. 2014), their rarity due to ecologically stressful environment (Caetano-Filho et al. 2019Hippertt et al. 2019;Caxito et al. 2021), or inadequate sampling. ...
Microbially induced sedimentary structures (MISS) are abundant in Ediacaran and lower Cambrian
successions. However, the relationship between MISS distribution and facies has not been thoroughly explored in
Ediacaran–Cambrian successions in South America. This study documents the occurrence of MISS and other
potential biogenic structures from the late Ediacaran Serra de Santa Helena Formation in the Bambu´ı Group of
eastern Brazil. This unit overlies the Cloudina-bearing Sete Lagoas Formation and is a mixed carbonate-siliciclastic
succession devoid of macroscopic body fossils. Potential microbial structures include wrinkled structures such as
‘‘elephant-skin’’ and Kinneyia-like textures, as well as pustular structures and abundant positive epirelief discoidal
structures. Another putative biogenic structure is a mm-wide meandering groove resembling a simple locomotion trail
of a small vagile benthic metazoan. Microbial surface textures (i.e., ‘‘elephant skin’’ and Kinneyia-type wrinkles) were
mainly observed in heterolithic deposits, usually at the interface between sandstone and siltstone/shale. On the other
hand, discs show a facies-independent distribution, observed in heterolithic as well as carbonate and marl deposits.
Petrographic analyses of these discs suggest that they have complex origins and some of them may be diagenetic
structures. Thus, while facies may have strongly controlled the preservation of MISS-related structures and textures
in the Serra de Santa Helena Formation, their abundance and diversity in tidal flat deposits indicate the wide
distribution of matgrounds in these shallow marine paleoenvironments. Also, we demonstrate how detailed
description and classification of simple features, such as discoidal structures, is an important task for
paleoenvironmental reconstruction of marine ecosystems at the Ediacaran–Cambrian transition when the microbially
bounded substrates played important roles in the dynamics of coastal environments.
... Although it was never investigated in detail in terms of depositional facies, the Lagoa do Jacaré Formation shows features from storm-deposited beds to peritidal microbialites (e.g., Santos et al. 2018, Freitas et al. 2021, depicting a completely structured ancient carbonate ramp that likely covered hundreds of thousands of square kilometers. A restrictive, anoxic, and methanogenic basin during this time is hypothesized by many authors (e.g. ...
Abstract A section investigated in the region of Paraopeba, Minas Gerais, provided detailed sedimentologic, stratigraphic and chemostratigraphic data from the Ediacaran Lagoa do Jacaré Formation, Bambuí Group, Southeast Brazil. This information allowed interpretation of tempestite facies in transgressive-regressive cycles, reinforcing the previously proposed storm-influenced sedimentation model and clarifying how it is associated with unusually high C-isotope values related to the Middle Bambuí Excursion (MIBE). Facies analysis of nine lithofacies identified at the GMD quarry based on field and petrographic descriptions showed distal tempestite facies grading upwards to a shallower, oncoidal/ooidal carbonate marine environment, then to basinal shales. The entire succession comprises a low-order transgressive hemicycle, recording the transition from a storm-influenced carbonate ramp to a siliciclastic-dominated platform. Chemostratigraphic data yielded high δ¹³C values ranging between +11.11‰ and +13.94‰. Our data contribute to the revision of the previously proposed interpretation and as well as to the understanding of paleoenvironmental conditions and C isotope signatures across the MIBE, near the Ediacaran-Cambrian boundary in the Bambuí Basin.
... There are few microbialite occurrences in the Lagoa do Jacaré Formation (Ribeiro et al. 2008, Iglesias and Uhlein 2009, Signorelli 2009, Kuchenbecker and Pedrosa-Soares 2010, Atman 2011, Fragoso et al. 2011. The morphotypes suggest a relation with a small barrier reef and a wave-protected peritidal environment (Santos et al. 2018) formed during the end of a regional regression trend (Uhlein et al. 2019). On the Januária high basement, the Bambuí Group may be subdivided into five transgressive-regressive 2 nd -order sequences. ...
... The apex of regression is in the middle Lagoa do Jacaré Formation, which is marked by the first appearance of microbialite beds and tidal-related structures before the deepening of the basin (Uhlein et al. 2019). Other studied sections, such as the KM7-14 of Cui et al. (2020) or the Bom Despacho section from Santos et al. (2018) present low resolution stratigraphic data in a basin scale, and both are located in the southern basin approximately 600 km far south from Ubaí area. Therefore, we are able to tentatively correlate the Ubaí section with the beginning of the retrogradation tendency described by Uhlein et al. (2019) on the Januária paleo-high. ...
... Attempts since the 19th century to find macrofossils and microfossils in the Bambuí Group have generally met with limited results, although diversified microbialites are known from at least seventeen localities (Sanchez 2014;Fairchild and Sanchez 2015;Santos et al. 2018). Studies have concentrated mostly upon the Sete Lagoas Formation, at the base of the group, because of its post-glacial context and widespread occurrence, but the only well-preserved fossils described from the formation to date are microbialites and a very significant occurrence of biomineralized tubes of the terminal Ediacaran index fossil Cloudina in northern Minas Gerais (Warren et al. 2014). ...
... The formation has also yielded organic-walled microfossils and acritarchs, but these are generally very poorly preserved and, thus, not suitable for detailed taxonomic classification or biostratigraphic correlation (Simonetti and Fairchild 2000;Hidalgo 2007;Sanchez and Fairchild 2017). A sparse microbialitic record is known from the middle of the Bambuí Group in the Lagoa do Jacaré Formation (Santos et al. 2018), and at the top, in the Jaíba Formation, a lateral equivalent of the Três Marias Formation . ...
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 of 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.
... The presence of resistive carbonate and calcareous rocks was inferred from~320 m to~480 m followed by a layer of intercalated shales and sandstones (Solon et al., 2015 [23]). This carbonate layer, which corresponds to the Lagoa do Jacaré carbonate layer, exhibits a potential karst system according to the outcrops located East of the São Francisco Basin (Dos Santos et al., 2018 [57]). Assuming a karst system at depth, this could imply a high level of porosity favorable for a massive storage volume of an aquifer. ...
Hydrogen gas is seeping from the sedimentary basin of São Franciso, Brazil. The seepages of H2 are accompanied by helium, whose isotopes reveal a strong crustal signature. Geophysical data indicates that this intra-cratonic basin is characterized by (i) a relatively high geothermal gradient, (ii) deep faults delineating a horst and graben structure and affecting the entire sedimentary sequence, (iii) archean to paleoproterozoïc basements enriched in radiogenic elements and displaying mafic and ultramafic units, and (iv) a possible karstic reservoir located 400 m below the surface. The high geothermal gradient could be due to a thin lithosphere enriched in radiogenic elements, which can also contribute to a massive radiolysis process of water at depth, releasing a significant amount of H2. Alternatively, ultramafic rocks that may have generated H2 during their serpentinization are also documented in the basement. The seismic profiles show that the faults seen at the surface are deeply rooted in the basement, and can drain deep fluids to shallow depths in a short time scale. The carbonate reservoirs within the Bambuí group which forms the main part of the sedimentary layers, are crossed by the fault system and represent good candidates for temporary H2 accumulation zones. The formation by chemical dissolution of sinkholes located at 400 m depth might explain the presence of sub-circular depressions seen at the surface. These sinkholes might control the migration of gas from temporary storage reservoirs in the upper layer of the Bambuí formation to the surface. The fluxes of H2 escaping out of these structures, which have been recently documented, are discussed in light of the newly developed H2 production model in the Precambrian continental crust.
... The presence of resistive carbonate and calcareous rocks was inferred from ~320 m to ~480 m followed by a layer of intercalated shales and sandstones (Solon et al., 2015 [20]). This carbonate layer, which corresponds to the Lagoa do Jacaré carbonate layer, exhibits a potential karst system according to the outcrops located East of the São Francisco Basin (Dos Santos et al., 2018 [58]). Assuming a karst system at depth, this could imply a high level of porosity favorable for a massive storage volume of an aquifer. ...
Hydrogen gas is seeping from the sedimentary basin of São Franciso, Brazil. The seepages of H2 are accompanied by helium, whose isotopes reveal a strong crustal signature. Geophysical data indicates that this intra-cratonic basin is characterized by (i) a relatively high geothermal gradient, (ii) deep faults delineating a horst and graben structure and affecting the entire sedimentary sequence, (iii) archean to paleoproterozoïc basements enriched in radiogenic elements and displaying mafic and ultramafic units, and (iv) a possible karstic reservoir located 400 m below the surface. The high geothermal gradient could be due to a thin lithosphere enriched in radiogenic elements, which can also contribute to a massive radiolysis process of water at depth, releasing a significant amount of H2. Alternatively, ultramafic rocks that may have generated H2 during their serpentinization are also documented in the basement. The seismic profiles show that the faults seen at the surface are deeply rooted in the basement, and can drain deep fluids to shallow depths in a short time scale. The carbonate reservoirs within the Bambuí group which forms the main part of the sedimentary layers, are crossed by the fault system and represent good candidates for temporary H2 accumulation zones. The formation by chemical dissolution of sinkholes located at 400 m depth might explain the presence of sub-circular depressions seen at the surface. These sinkholes might control the migration of gas from temporary storage reservoirs in the upper layer of the Bambuí formation to the surface. The fluxes of H2 escaping out of these structures, which have been recently documented, are discussed in light of the newly developed H2 production model in the Precambrian continental crust.
... The basal part of the Lagoa do Jacaré Formation consists primarily of microbialite facies and domal or columnar stromatolites (dos Santos et al., 2018) (Fig. 4C-E). These microbial facies are commonly interbedded with massive mudstone and wave-ripple cross-laminated grainstone ( Fig. 4G-L) with locally preserved syneresis cracks ( Fig. 5A-D) (Iglesias and Uhlein, 2009). ...
... First, abundant oolites and stromatolites (Figs. 4,6,10) have been found in the MIBE (Misi and Veizer, 1998;Reis and Suss, 2016;Paula-Santos et al., 2017;dos Santos et al., 2018;Caetano-Filho et al., 2019;this study). Published studies suggest that ooid formation typically occurs in local environments with carbonate saturated, normal to elevated salinity, warm and agitated aqueous conditions (> 20 °C) (Lees, 1975;Paul and Peryt, 2000;Siewers, 2003;Lehrmann et al., 2012;Trower et al., 2017). ...
The Ediacaran-Cambrian Bambuí Group in Brazil records an anomalously positive excursion in carbonate carbon isotopes (δ¹³Ccarb) with a sustained plateau of ca. +15‰ (aka the Middle Bambuí positive Excursion–MIBE). Considering that the δ¹³Ccarb signals in Ediacaran-Cambrian seawaters do not typically exceed +6‰, the MIBE therefore represents a profound carbon cycle anomaly in Earth’s history. Although intensive studies have been done on the Bambuí Group, origins of the MIBE remain enigmatic. In order to better constrain the biogeochemical carbon and sulfur cycles during the MIBE, high-resolution chemostratigraphic analysis was conducted for both the plateau (i.e., Lagoa do Jacaré Formation) and the recovering part (i.e., lower Serra da Saudade Formation) of the MIBE. Chemostratigraphic profiles reveal remarkably different values in δ¹³Ccarb, δ¹³Corg, δ¹⁸Ocarb, and δ³⁴Spyrite between these two studied MIBE intervals. The new data show that the plateau of the MIBE is characterized by coupled higher δ¹³Ccarb, higher δ¹³Corg, and higher δ³⁴Spyrite signals compared with the recovering part of the MIBE. Based on multiple lines of sedimentological, geochemical, and modelling evidence, we propose that the possibilities of enhanced organic carbon burial and porewater methanogenesis are insufficient to explain the MIBE. Instead, local controlling factors, including water-column methanogenesis, low-sulfate conditions, and enhanced carbonate recycling in a restricted basin may have played a role, independently or in unison, in generating this profound positive δ¹³Ccarb excursion. Therefore, the MIBE may reflect a local or regional event, instead of a global carbon cycle anomaly. We caution against the use of the MIBE in chemostratigraphic correlations on a global scale or any other attempt to infer global carbon cycling at that time. The biogeochemical landscape of the late Ediacaran-Cambrian basins and ocean margins may be more heterogeneous than previously thought.
... 615 Ma), suggesting that a large Ediacaran Clymene Ocean existed between Amazonia and the Congo-Sao Francisco-Río de la Plata block (Rapalini, 2018), that probably closed at the early Cambrian (Tohver et al., 2012). The extensive record of microbialites in clean carbonate environments related to the Clymene Ocean (635-540 Ma; Trindade et al., 2006;Gaucher and Poiré, 2009;Poiré and Gaucher, 2009;Warren et al., 2011Warren et al., , 2014Warren et al., , 2017Warren et al., , 2019Poiré et al., 2018;dos Santos et al., 2018;Uhlein et al., 2019) is a valuable seawater proxy for both secular and geographic variations in ancient seawater. Rare earth elements plus yttrium (REY) were recently investigated, as well as the key issue of the oxygenation states of Precambrian seas (Frimmel, 2009;Wang et al., 2014;Zhang et al., 2014;Hohl et al., 2015;Paula-Santos et al., 2018;Wei et al., 2018Wei et al., , 2019. ...
The limestones of the Loma Negra Formation provide an excellent opportunity to examine the behaviour of rare earth elements plus yttrium (REY) due to the near-zero diagenetic alteration of the carbonate system. The terrigenous supply to this unit was minimal or scarce in relation to the depth of seawater in which the limestones accumulated. The interpreted palaeoenvironment suggests that limestones could have originated by suspension fall-out in a shallow, open marine ramp. The diagenetic features, composition, and original microtextures are considered good criteria for investigating their REY composition. Burial diagenesis products were limited and limestones largely preserved their original texture and organic matter, as well as their isotope and trace element composition. The study of REY distribution in the Loma Negra Formation in different settings of the Tandilia System revealed systematic chemostratigraphic patterns related to uniform palaeo-redox conditions. The general light rare-earth element depletion, marked positive La, Y, and Gd anomalies, as well as the clear negative Ce anomalies, are further features typically ascribed to oxidizing seawater conditions. From these results, it can be concluded that the Loma Negra Formation documents an oxygenation event that stands out in the overall anoxic Ediacaran ocean. This record may be correlated with the mid-Ediacaran oxygenation event (ca. 580Ma). This may support the hypothesis that the Clymene Ocean was well oxygenated and connected during the Ediacaran, at least in the southwestern border of Gondwana.
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.
