The Carrancas conglomerate, also called Carrancas diamictite, is found in section CR. It is the lowermost unit in the study area (Fig. 1) and is preserved in isolated channels incised in the Palaeoproterozoic basement. Composed of normally grading polymictic conglomerates (Cm facies) interbedded with pebbly sandstone lens with incipient parallel lamination (SMp facies), it defines a metre-scale finning-upward cycle, possibly of coastal to alluvial origin.

It consists of coarse-grained rounded pebbles of carbonate, granite, and quartz composition fragments, within a micrite to microsparite matrix with sparse sand-sized grains of quartz, plagioclase, biotite, and granitic lithoclasts (Fig. 2A). Under the microscope, secondary carbonates are observed in microveins cutting across the clasts or in plagioclase (saussuritisation). The textural similarity of these secondary carbonates with the matrix casts doubts on the primary origin of the carbonatic matrix of the Carrancas conglomerate. Contrasting to previous reports [19,50], no clear evidence of glacial influence could be found in these deposits during the course of this study.

Outer ramp deposits are found at sections CR, SA and TA, implying a regional-scale lateral extension (Fig. 1). They form tabular layers up to 30 m thick, organized in centimetre-scale cycles of lime mudstone (Mp) and calcite crystal-fans (Mc), interpreted as aragonite pseudomorphs (Figs. 2B and C). The abundance of crystal-fans is the most striking feature of the Sete Lagoas carbonate succession. Aragonite-pseudomorph crystals are black to dark grey. They show acicular morphology with straight terminations, forming bottom-nucleated, upward-radiating fans up to 15 cm tall, laterally connected to thin, millimetric cement-crusts (Fig. 2C). Crystal layers are covered by light-grey or red lime mudstones showing parallel to undulating lamination reflecting the irregular surface at the top of underlying crystals, which is often truncated by stylolites. The morphology of the fans and the fact that micritic layers often onlap crystal fans indicate that these features are primary, and not a diagenetic imprint. At some places, however, blades of crystal overgrowth intersect the upper layers of aragonite pseudomorph, truncating the lamination of the upper lime mudstone beds. These overgrowths are interpreted as a diagenetic modification. In outcrop, these overgrowths are highlighted by their white-to-red colour (arrow in Fig. 2D). Occasionally, deformational features are observed in the crystal beds, such as synsedimentary faults that displace lime mudstone/cementstone cycles for some centimetres (arrow in Fig. 2B), and crystal bends toward the west. The upward decrease in frequency of crystal fans in both TA and SA sections is accompanied by the replacement of micrites of the Mp facies by terrigenous crystalline limestone with quasi-planar to supercritical climbing wave-ripple cross lamination of the Lc facies.

The FA2 covers thousands of square kilometres in the study area, suggesting a wide platform setting, free from the action of strong currents in the initial stages of the Sete Lagoas succession. Preserved crystal fans are concentrated in beds, without any evidence of either wave or storm action corroborating a sub-wave storm environment. Aragonite crystal growth at the ocean floor has been ascribed to deep-platform environments associated with CaCO₃ oversaturation in the ocean [36,46,60]. Interestingly, periodic cycles of lime mudstone/cementstone precipitation could imply a recurrence in oversaturation events. The waning of crystal fans upsection, accompanied by the increase of wave structures, suggests a progressive change from a CaCO₃-oversaturated deep-water setting to the more energetic, moderately deep environment that characterizes the transition to FA3.

Storm-dominated middle ramp deposits comprise crystalline limestone with hummocky cross-stratification (Lh) and megaripple bedding (Lm).

Facies Lh and Lm are better exposed at the lower 20 m of the section PA and at the upper 40 m of section TA. They correspond to a set of undulating beds of grey and red crystalline limestone with mud drapes (Fig. 3A). Hummocky cross-stratification (wavelength up to 1.5 m) is observed, associated with pinch and swell lamination, quasi-planar lamination, and subordinately swaley cross-stratification. In plane view, hummocky domes are crowned by ripple marks with interference pattern (Fig. 3B). These deposits grade laterally to facies Lm exhibiting megaripple bedding with low-angle cross-stratification parted by mud drapes. The base of the beds is planar to erosive, with centimetre-scale gutter casts occurring frequently.

The coarse-grained nature of the deposits and the ubiquitous presence of terrigenous grains (up to 10%) associated with several types of lamination (planar, hummocky, and swaley) suggest a high-energy carbonate facies. The presence of hummocky cross-stratification and associated wave structures indicate storm-related, powered combined flow [9]. Mud partitions suggest deposition in lower shoreface. Large-scale bed forms with preserved morphology and scoured base (gutter cast) indicate periodic migration of megaripples or bars under current action with alternation of suspension, and sporadic storm events.

Tidal-dominated inner ramp deposits comprise crystalline limestone with evenly parallel lamination (Lp), climbing ripples and quasi-planar lamination (Lc), forming rhythmite with pelite (Rh).

Facies Rh is characterized by heterolithic bedding of the reddish crystalline limestone and pelite rhythmite. This facies is observed exclusively at the PA section, distributed in tabular beds up to 40 cm thick, laterally extensive for more than hundreds of metres. Main structures are: wavy bedding, asymmetric ripple marks, and supercritical climbing-ripple cross lamination, generally outlined by mud drapes (Fig. 3C). The planar bedding is characterized by a strong regularity of mud drapes separating thin and thick limestone laminae, interpreted as mud couplets (Fig. 3D). Facies Rh alternates or is interbedded with an evenly parallel laminated-crystalline limestone (facies Lp), sometimes filling channels. Channels are tens of meters wide and less than five metres deep, asymmetric with planar-erosive, mud-draped base; some of them show a preserved flank. Inclined beds at the flank limb are conformable with the horizontal beds away from the channel. Locally, millimetre-scale gutter casts erode the planar lamination (Fig. 3E). Facies Lc is found above facies Rh and Lp. It consists of crystalline limestone with supercritical wave climbing ripples (up to 20 cm in thickness) and quasi-planar lamination. Aragonite-pseudomorph crystal fans are found associated with intertidal deposits.

Heterolithic beds indicate the alternation of traction and suspension controlled by tidal currents. The regular arrangement of rhythmite beds is interpreted as due to the alternation of slack waters and currents probably related to ebb/flood diary cycles, the thicker beds separated by mudstone layers being the highest tidal cycles. Migratory channels filled by shoals (even parallel laminated limestone) were incisive into the subtidal zone. The presence of aragonite-pseudomorph crystal fans at the top of the FA4 is interpreted as a shallow-water, saturation event in a restricted bay.

Mixed outer ramp deposits of FA5 comprise grey lime mudstone–pelite rhythmite (Rmp) and black crystalline limestone (Bp)s. They are the most expressive facies association in the study area, reaching up to 160 m in thickness at some sections, mostly in the southeastern sector of the study area. The Rmp is formed by the alternation of carbonate and pelite, and reaches more than 12 m in thickness (Fig. 4A and B). Individual beds vary from 2 to 15 cm in thickness. This facies grades upward to the thick succession of black limestones of facies Bp (Fig. 4A), which is characterized by planar to low-angle lamination and high organic matter content.

The large surface covered by FA5 throughout the study area indicates a wide platform setting. The abundance of fine siliclastic at the basal unit suggests a deepest-offshore zone where carbonate precipitation is limited. Moreover, the lack of wave structures and the upward presence of laminated crystalline limestone (originally mudstone), rich in organic matter, corroborates the continuous deposition in deep waters.

Deposits of FA6 comprise two facies: black crystalline limestone with planar and wave truncated lamination (Bp), and microbial boundstone (Mb). Facies Bp is characterized by wave-truncated lamination (wavelength up to 60 cm and 5 cm high) with undulated internal lamination forming pinch and swell arrays. Individual beds are 15 cm thick, and exhibit in the base a small-scale gutter cast. Convolute bedding occurs locally. Facies Mb was observed only in section PR, and corresponds to dark grey to black columnar, rarely branching stromatolites (Fig. 4C). Individual columns are up to 40 cm and can be as large as 10 cm in diameter, with convex internal lamination. In transverse section, they display a polygonal morphology, the inter-column spaces being filled by black lime mudstone or locally brecciated.

Ubiquitous truncated laminations and planar laminations suggest wave action in upper regime flow (e.g., [28]), probably induced by storms in the shoreface zone. Liquefaction processes are attributed to the wave impact in unconsolidated sediments. Microbial mat colonization occurred in low-energy zones with deposition of carbonate muds related to shallow subtidal or lagoon environment. Locally, the collapse of stromatolites generated intraformational breccias filling inter-column depressions.

Deposits of FA7 represent a deeper, lateral variation of FA6. They comprise black crystalline limestone with convolute and planar lamination (Bw). Deformational features include also slump folds and slight undulation of planar lamination. Convoluted layers are 2 to 10 cm thick. These layers are interbedded with non-deformed layers, forming deposits of tens of metres in thickness. The abundance of deformational features (convolute lamination, slump folds) suggests a rapid deposition in a steepened ramp setting.

It is worth noting that black limestones are found into facies associations FA5, FA6, and FA7 (comprising facies Bp, Bw and Mb), and may reach altogether up to 200 m in thickness. Obliteration of primary structures by recrystallization may hinder individualization of such a facies in the field.

The carbon isotope record shows coherent and phased changes corresponding to the stratigraphic variations (Fig. 5). The δ¹³C-δ¹⁸O cross-plot in Fig. 6 clearly shows a net increase in δ¹³C towards the top of the succession (from FA1 to FA7), with distinct clusters of samples for contemporary facies association. The carbonate matrix of the Carrancas conglomerate (FA1) shows the most depleted δ¹³C, ranging from –5.1 to –3.3‰. The lowermost sectors of sections CR, SA and TA (FA2), comprising lime mudstones with aragonite pseudormorphs, show depleted δ¹³C values around –4‰. Upsection, these values increase from –4.5 to + 0.8‰. In sections TA and CE, δ¹³C values increase sharply to around 0‰ within the first metre of the facies association FA2, and remain constant within facies association FA3. In section SA, δ¹³C values increase gradually (within 20 m), reaching the value of 0‰ at the base of the facies association FA3 (Fig. 5). The presence of rocks from FA2 with values around 0‰ is responsible for the overlapping of isotopic values between FA2 and FA3 (oversaturated outer ramp and storm-dominated middle ramp) in Fig. 6. Values of δ¹³C for FA3 and FA4 are limited to a narrow range around 0‰ (Fig. 5). Facies association FA5 is characterized by positive δ¹³C values. These values increase from around +2‰ in the lowermost mudstone–pelite rhythmite to values in excess of +8‰ in the uppermost black mudstone of FA6 and FA7 (Fig. 5). The microbial boundstone of FA6, which caps the carbonate succession of the Sete Lagoas Formation, shows high δ¹³C values varying from +10.5 up to +11.2‰, and the highest δ¹³C value of +14.3‰ is found in the black limestone of FA5.

The oxygen record shows a large spread with δ¹⁸O varying from –14 up to –6‰. But, in contrast to the carbon isotopes, it shows large variations and little overlap among the sections (Fig. 6). For instance, facies association FA2 was sampled in three sections (CR, SA, and TA) and shows a large variation in δ¹⁸O values (ranging from –13.1 to –7.3‰), but values for each sampled outcrop seem to cluster within 2‰: in the CR section, δ¹⁸O values vary between –13.1 and –11.2‰, whereas values for section TA are between –10.6 and 7.5‰, and values for section SA are between –9.9 and –7.3‰. Similarly, facies associations FA6 and FA7 show a wide range of δ¹⁸O values, from –12.0 to –5.7‰, with narrower and specific ranges for PR, CA, and MG sections. Nonetheless, some sections show a coherent trend in δ¹⁸O values, with less negative values towards the top. This is the case of section SA, for which values of –9.9‰ were obtained on limestones with aragonite-pseudomorph crystal fans at the base of the outcrop, contrasting with the values of –5.7‰ obtained on the storm-dominated crystalline limestone at the top.

In summary, in the Sete Lagoas Formation, the oxygen isotope ratios (δ¹⁸O) for most sections seem to record local signatures. This and the much depleted values, between –13.5 to –4.5‰, suggest some degree of diagenetic imprint on the oxygen isotopic signal, despite the apparent pristine state of the samples. Therefore, the O isotope values will not be used in the discussion.

The stratigraphic and sedimentologic data presented above allowed us to develop a depositional model for the Sete Lagoas carbonate platform and thus to establish a framework for regional correlation. The carbonate platform developed on the Palaeoproterozoic basement, preserving the Carrancas conglomerate in isolated channels. The Carrancas conglomerate (FA1) defines a metre-scale thinning-upward cycle, possibly of coastal to alluvial origin, with no evidence of a glaciogenic environment. It is interpreted as representing the initial stages of platform development. Then, the palaeoenvironmental interpretation reveals a deep- to shallow-platform setting, with a depocentre towards the southeastern border of the São Francisco craton, as indicated by the thickness increase of deep-water deposits (and the thinning of shallow-water deposits) to the southeast (Fig. 5). Preservation of a thick shallow-water succession in some sections suggests a strongly subsiding basin, and the presence of storm- and tide-influenced deposits indicates an oceanic connection.

Two shallowing-upward megacycles are recognized, comprising more than 200 m in the stratigraphic record. Each cycle is limited by a flooding surface amalgamated with a third-order sequence boundary [52].

The first megacycle (Fig. 7a) begins with deep-water deposition in an aragonitic, oversaturated ocean (FA2). It marks a pronounced transgression over the São Francisco craton, depositing carbonates over the Carrancas conglomerate or the basement rocks. The oversaturated waters enabled the periodic nucleation of seafloor precipitates, including crystal fans and crusts, covered by micrite. Furthermore, during the highstand, tide- and storm-influenced deposition (FA3 and FA4) prevailed. At this stage, deposition along the coast of the Sete Lagoas platform seems to have been strongly controlled by storm waves, but tidal processes dominated in locally protected zones (Fig. 7b).

The second megacycle (Fig. 7c) starts with a transgression, drowning the platform and allowing deposition of a thick succession of mixed, sub-storm wave-base deposits (FA5). It contrasts sharply with the first one by the lack of oversaturation events and the larger volume of deposits. We note also the lack of cyclicity and exposure surfaces throughout more than 200 m in the sedimentary record, suggesting high subsidence rates of the basin at this stage. This deep-water environment was prone to organic-matter preservation in the black limestones (upper FA5, FA6, and FA7). Coastal environments were developed after the progressive shallowing of the basin, forming fair-weather wave and storm deposits, associated with microbial building (FA6). Towards the depocentre, these rocks present ubiquitous deformational features, such as convolute and undulated laminations (FA7), related to the steepening of the ramp morphology. This megacycle ends with a rapid transgression marked by the deposition of siltstones and sandstones of the Santa Helena Formation.

In the Neoproterozoic successions, correlations rely mostly on the carbonate δ¹³C variations, on the basis of the hypothesis that the δ¹³C of carbonates is primary and records the δ¹³C of dissolved inorganic carbon in the surface oceans, considered to be homogeneous at the geological scale. Although the δ¹³C composition in carbonate rocks is quite resistant to chemical overprinting, as many studies have indicated that even diagenetically altered carbonates appear to preserve their original δ¹³C [7,20,27,39,40], care must be taken to ensure that it is the case.

As already mentioned, the Sete Lagoas Formation has not undergone metamorphism and the selected samples show little evidence for carbonate mineral neomorphism, except in the Carrancas conglomerate (FA1), which will not be considered in the following discussion. Here, the absence of covariations of δ¹³C with δ¹⁸O within and between each facies association, and the fact that δ¹³C presents very similar values for each facies association, independently of the section (Fig. 6), strongly suggest that δ¹³C records a primary signature. Moreover, the carbon isotope difference between the carbonate and TOC of the Sete Lagoas Formation is constant and of 28 ± 2‰ [34], reinforcing the hypothesis of primary carbon signature preservation. The carbon isotope data reported here can therefore be interpreted as representing the original depositional signature and can be used for correlations with other Neoproterozoic sections worldwide.

The high-resolution δ¹³C stratigraphy reported in this study shows a strong increase from the base to the top (between –4.5 and + 14‰) of the Sete Lagoas Formation, consistent with previous data [34]. Such an isotopic variation is in agreement with the δ¹³C signature found in the post-Sturtian sequences, where negative values in the cap carbonates directly overlying the diamictites are followed by extreme positive values upsection – see review in [27]. In fact, to our knowledge, such high δ¹³C values have never been found in post-Marinoan sequences, which usually reach maximum values of +7 up to +10‰ [27,29,48,59].

The δ¹³C stratigraphic record of the Sete Lagoas platform compares very well with other post-Sturtian successions, suggesting that the negative and positive excursions identified elsewhere are also recorded in the São Francisco Craton [61], and supporting the hypothesis that they are of global extent.

Considering a steady-state carbon cycle [29,45,58], some mechanisms have been invoked to explain the anomalous positive and negative shifts found in the δ¹³C Neoproterozoic signature.

The global extension of the negative excursion that follows the Sturtian deglaciation seems to be quite established and usually occurs within cap carbonates. The mechanisms responsible for it are still a matter of debate – see review in [32].

The possible factors responsible for the positive excursion have been reviewed by Shields et al. [58]. The first hypothesis is anomalously high burial rates of isotopically depleted carbon as organic matter (enriching the shallow ocean in ¹³C), but it has not been supported so far by known outcrops of organic-rich formations at this period. The second hypothesis is an increase in the isotopic composition of carbon entering the ocean due to the repetition of glacially related eustatic regressions. The third hypothesis is an increase in the average isotopic fractionation between sedimentary carbonates and total organic carbon (Δ¹³C_carb org comprised between 33 and 37‰), but it is insufficient on its own to explain the positive δ¹³C excursions. The fourth hypothesis is that the high δ¹³C is the result of unrepresentative sampling in restricted basins with locally elevated δ¹³C.

The Sete Lagoas formation is located in a different continent/sedimentary basin than those previously studied are [61], and it provides an additional record of the positive excursion, suggesting that this excursion is of global significance. In the Sete Lagoas Formation, δ¹³C reaches unusually high values (up to + 14‰), which suggests an additional local control. For the first time, this unit provides geological evidence for high organic carbon burial rates, associated with high δ¹³C values (up to 5% TOC [34]). Due to this high TOC, anoxia can be suspected and, indeed, some authors [34,58] have pointed out that the positive C isotope excursion seems to coincide with ocean redox stratification, with a causal mechanism remaining to decipher.

The presence of aragonite-pseudomorph crystal fans and other sedimentary features, associated with negative C-isotope excursions in Neoproterozoic transgressive carbonate successions following glacial deposition, has been used as a diagnostic of cap-carbonate deposits worldwide [33,40], despite some criticisms [46].

Cap-carbonate sequences found in different countries (Australia, Africa, China, Canada, India) display successions that present similar structures, depositional environments and isotopic signature (e.g. [27,32]). These successions typically show a transgressive cycle, which starts with cap dolostones constituted by fine, deep-water deposits [3,36,37,41]. They grade upward to disturbed platform facies [37,38] or grainstones and stromatolites near to the coast [13,36,41]. The δ¹³C curves obtained on the cap carbonates show depleted δ¹³C values (down to –6‰) at the bottom of the sequence, which increase upward to positive values [15,17,23,27,32,35,37,63].

Although cap-carbonate features have been described in a generical way [41] for sequences correlated to all Neoproterozoic glacial events (Sturtian, Marinoan, and Gaskiers), several differences have been observed between the cap carbonates following each glacial interval [42]. Most cap carbonates described in detail in the literature are correlated to post-Marinoan deposition [3,26,36,64]. There are quite few detailed descriptions of cap carbonates related to the Sturtian event. Most post-Sturtian sequences are truncated at the bottom and lack a cap dolostone unit [24,32,42]. Post-Sturtian cap carbonates studied in northern Canada, southern Australia, and northern Namibia present successions initiated with a record of maximum flooding, which comprise either marly shales or rhythmites of black limestones rich in organic-matter or sulphide. They grade into allodapic dolomites, thin sandstone layers and slump breccias (southern Australia and Canada), and may also comprise stromatolites (Namibia). The carbon isotopic signature of post-Sturtian cap carbonates is also different from the post-Marinoan one due to their basal truncation [15,27,30]. Normally, the older (Sturtian) cap carbonates initiate with δ¹³C values around –3‰ to –4‰, which rise quickly upward to positive values.

There is no firm evidence for glacial deposition below the carbonate deposits in the study area, the Carrancas conglomerate being interpreted here as a coastal-to-alluvial deposit infilling incised valleys. However, glaciogenic or glacially influenced units were recognized below the Bambuí Group in other regions of the São Francisco Craton, including the Jequitaí Formation [50,62] and the lower successions of the Macaúbas Group [38,62]. These units are probably a lateral equivalent to the basal unconformity of the Bambuí Group in the studied area. The FA2 may therefore represent a true cap carbonate, due to this regional correlation. In addition, the Sete Lagoas cap carbonate constitutes a transgressive sequence, beginning with the FA2 deep platform deposits characterized by δ¹³C values of –4.5‰. These deposits grade upward to shallow platform deposits influenced by storm and wave, with δ¹³C values around 0‰ (FA3 and FA4). All these features allow us to propose that the Sete Lagoas formation is a cap-carbonate sequence.

Because the rocks of the Sete Lagoas Formation were deformed during the Brasiliano event (630 Ma: [12]), which is older or contemporary with the recently obtained ages of the Marinoan glaciations (630 Ma: [11,31,64]), a Sturtian correlation seems better for the Sete Lagaos cap carbonate, and is also consistent with the δ¹³C stratigraphy and the Pb–Pb age cited above (section 2).

However, comparison of the Sete Lagoas sedimentary and isotopic records with other Sturtian units raises some problems. The sedimentary record of Sete Lagoas resembles more that of post-Marinoan deposits. The base of the studied succession is marked by a thick interval (up to 16 m) with aragonite-pseudomorph crystal fans that do not occur frequently in post-Sturtian cap carbonates. Aragonite-pseudomorph crystal fans in purported post-Sturtian deposits are described only in the Pocatello Formation, for which no basal glacial layer has been identified [22]. Moreover, in Sete Lagoas, the whole transgressive cycle is recorded; the maximum-flooding surface occupies the middle of the succession, where C-isotopic values shift from negative to positive. In other post-Sturtian carbonates, the record initiates with the maximum-flooding deposits and the transition of negative to positive C-isotopic values is located above the maximum-flooding surface.

Recent geochronological data available for ash beds within Neoproterozoic deposits point to a possible diachronism of glacial units usually correlated to the Sturtian event (709 ± 5 Ma, southeastern Idaho; 711.8 ± 1.6 Ma, Oman; 685 ± 7 Ma and 684 ± 4 Ma, central Idaho; 761 ± 8 Ma, southern China: see [22]). In this case, the Sete Lagoas carbonate platform does not necessarily correlate with other carbonate sequences in a worldwide basis, and the negative C anomaly could be of local significance. On the other hand, if we take the post-Sturtian sequences as coeval, differences in depositional settings may indicate different position/depths of sediment accumulation across the basin. Then, the Sete Lagoas succession would correspond to a shallower deposition with regard to other described post-Sturtian cap carbonates, and, because of that, it would present the record of wave-storm influenced deposition, which is not normally found in other post-Sturtian successions. In such case, the negative anomaly found at the base of Sete Lagoas succession would correlate with that found in southern Australia, Canada and Namibia.