Proterozoic ﬁrst-order sedimentary sequences of the São Francisco craton, eastern Brazil

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Proterozoic

ﬁ

rst-order sedimentary sequences of the São Francisco craton, eastern

Brazil

Fernando F. Alkmim

a,_*

_{, Marcelo A. Martins-Neto}

b

a_{Departamento de Geologia, Escola de Minas, Universidade Federal de Ouro Preto, Morro do Cruzeiro, 35.400.000 Ouro Preto, MG, Brazil} b_{Vicenza Mineração, Av. Agulhas Negras, 580, Mangabeiras, 30210-340 Belo Horizonte, MG, Brazil}

a r t i c l e

i n f o

Article history:

Received 4 May 2010 Received in revised form 15 August 2011 Accepted 18 August 2011 Available online 5 September 2011

Keywords:

First-order sequences Proterozoic São Francisco craton Brazil

a b s t r a c t

The São Francisco craton in eastern Brazil hosts sedimentary sequences deposited between the Paleo-archean (w3300 Ma) and Late Neoproterozoic (w580 Ma). Proterozoic successions occurring in this region compriseﬁve 1st-order sedimentary sequences, which besides episodes of global signiﬁcance record major basin-forming events. The ca. 8000 m-thick Minas-Itacolomi 1st-order sequence, exposed in the Brazilian mining district of the Quadrilátero Ferrífero and containing as marker bed the Lake Superior-type Cauê Banded Iron Formation, tracks the operation of a Wilson cycle in the Paleoproterozoic Era. The quartz-arenite dominated Espinhaço I and II sequences record at least two major rift-sag basin-forming events, which affected the host continent of the São Francisco craton at around 1.75 Ga and 1.57 Ga. The Macaúbas sequence and its correlatives in the extracratonic domains witness the individ-ualization of a São Francisco-Congo plate in synchronicity with the break-up of Rodinia in the Cryogenian period. The São Francisco-Congo plate together with various fragments derived from Rodinia reas-sembled to form Gondwana in the Ediacaran period. In the course of the Gondwana amalgamation, orogenic belts developed along the margins of the craton; its interior, converted into foreland basins, received the shallow water carbonates and pelites of the Bambuí 1st-order sequence and its correlatives.

1. Introduction

The best preserved and most intensively studied Precambrian sedimentary record of South America is exposed in a relatively small area of eastern Brazil, which corresponds to the São Francisco craton and its margins (Fig. 1). The São Francisco craton (Almeida et al., 2000) hosts sedimentary successions accumulated in the time interval between the Neoarchean (w2800 Ma) and Late Neoproterozoic (w580 Ma).

The Precambrian nucleus of the South American continent, consisting essentially of Archean to Paleoproterozoic rocks with minor additions of Meso- and Neoproterozoic juvenile material, comprises cratons and Neoproterozoic orogenic belts (Fig. 1). The São Francisco together with three other cratons of South America corresponds to the internal portions of the plates involved in the assembly of West Gondwana by the end of the Proterozoic Era. The Neoproterozoic or Brasiliano orogenic belts, on the other hand, encompass the margin of those plates and the intervening

accretionary material (Brito Neves et al., 1999; Almeida et al., 2000; Alkmim et al., 2001).

We selected for discussion in this paperﬁve Proterozoic 1st-order sequences exposed in the São Francisco craton region. The sequences chosen for discussion record not only the Proterozoic history of the São Francisco craton, but also tectonic and climatic events of global signiﬁcance. Archean sedimentary successions, although present in many sectors of the craton, are tectonically dismembered and demand further detailed studies. For this reason they will be not addressed in this paper.

As a consequence of its previous residence in Gondwana, the São Francisco craton has an African counterpart (Fig. 1), which is rep-resented by the Congo craton. The link between the São Francisco and Congo cratons is made by a crustal bridge that connects the Paleoproterozoic Eastern Bahia orogenic domain in Brazil to the Paleoproterozoic Francevillian or West Central African belt in Gabon (Fig. 1). This piece of crust remained intact from the middle Paleoproterozoic until the beginning of the Cretaceous, when West Gondwana started to split apart (Porada, 1989; Trompette, 1994; Ledru et al., 1994; Feybesse et al., 1998; Pedrosa-Soares et al., 1992, 2001). For this reason, correlatives of the focused Brazilian 1st-order sequences in the Congo craton are indicated in the text. *Corresponding author. Tel.:þ55 31 3559 1849; fax:þ55 31 3559 1600.

E-mail addresses:[email protected],[email protected](F.F. Alkmim).

Contents lists available atSciVerse ScienceDirect

Marine and Petroleum Geology

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / m a r p e t g e o

doi:10.1016/j.marpetgeo.2011.08.011

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Special emphasis is given to the correlations between the Neo-proterozoic successions exposed on both cratons.

The concept of 1st-order sequence adopted in the present paper was postulated byMartins-Neto et al. (2001)andCatuneanu et al. (2005). Since Precambrian basins are often characterized by poor stratal preservation and by a general lack of time control, an approach based on the stratigraphic hierarchy is considered the best way to interpret the stratigraphic framework. Stratigraphic hierarchy involves the separation of different orders of sequences and bounding surfaces based on their relative importance. In this context, changes in the tectonic setting provide the key criterion for the basic subdivision of the rock record. Thus, a first-order sequence corresponds to an entire sedimentary basin-fill succes-sion, regardless of the origin and life span of each particular basin. Thisfirst-order basin-fill succession can be in turn subdivided into second- and lower-order sequences, based on the steps involved in the tectonic evolution of the basin, irrespective of the time span between two same-order consecutive events.

2. The São Francisco craton

In the continent interior, the São Francisco craton is bounded by the Neoproterozoic Brasília, Araçuaí, Rio Preto, Riacho do Pontal and Sergipano orogenic belts (Fig. 2) (Almeida et al., 1981). Along the eastern Brazilian coast, the craton boundary is made up by Cretaceous passive margin basins, developed on the cratonic bridge that once linked the São Francisco and Congo blocks.

The basement of the São Francisco craton consists of an Archean nucleus and two segments of a Paleoproterozoic orogen (Alkmim and Marshak, 1998; Teixeira et al., 2000; Barbosa and Sabaté, 2004) (Fig. 1). The Archean block, which corresponds to the NS-elongated segment of the craton, is almost entirely covered by

Proterozoic and Phanerozoic sedimentary units. The ca. 2.1 Ga Mineiro belt (Teixeira and Figueiredo, 1991; Teixeira et al., 2000) that bounds the Archean block to the south involves the Archean basement, Paleoproterozoic passive margin to foreland basin units, and voluminous granitoids emplaced between 2.25 and 2.02 Ga (Machado et al., 1996; Teixeira et al., 2000; Noce et al., 2000; Ávila et al., 2010). The famous Brazilian mining district known as the Quadrilátero Ferrífero (“Iron Quadrangle”) (Dorr, 1969) is located in the foreland domain of the Mineiro belt and corresponds to the type area of the Paleoproterozoic Minas-Itacolomi sequence, dis-cussed in the next section.

The Proterozoic cover units of the São Francisco craton occur in two distinct tectonic domains: the São Francisco basin and the Paramirim aulacogen (Fig. 2). The poly-historic São Francisco basin extends over ca. 800 km along the NS-trending lobe of the craton and is bounded by emergent thrusts of the adjacent Neoproterozoic orogenic belts, except to the south and northeast (Dominguez, 1993; Alkmim and Martins-Neto, 2001; Zalán and Romeiro-Silva, 2007). The southern limit is erosional. To the northeast the basin is bounded by the Paramirim corridor, a Neoproterozoic intra-cratonic deformation zone, which affects the neighboring Para-mirim aulacogen (Alkmim and Martins-Neto, 2001; Cruz and Alkmim, 2006). The Precambrian basin-ﬁll units comprise the Espinhaço and São Francisco supergroups, of Paleo/Mesoproter-ozoic and NeoproterPaleo/Mesoproter-ozoic ages, respectively. The PhanerPaleo/Mesoproter-ozoic section contains the glacial sediments of the Permo-carboniferous Santa Fé Group (Campos and Dardenne, 1994), as well as the Cretaceous rift-sag succession of the Areado, Urucuia, and Mata da Corda Groups (Sgarbi et al., 2001).

The Paramirim aulacogen (sensu Cruz and Alkmim, 2006) encompasses two morphotectonic domains of the São Francisco craton: the northern Espinhaço range and a large plateau referred

Figure 1.The São Francisco craton together with its African counterpart, the Congo craton, in the tectonic scenario of West Gondwana. Cratons of South America and Africa: A Amazonian; P Rio de la Plata; WA West Africa; SFC São Francisco-Congo; K Kalahari. Cities: S Salvador and V Vitória (Brazil); Luanda (Angola); LB Lebreville (Gabon).

F.F. Alkmim, M.A. Martins-Neto / Marine and Petroleum Geology 33 (2012) 127e139

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to as the Chapada Diamantina (Fig. 2). Developed around 1.75 Ga, the Paramirim aulacogen experienced a long subsidence history in the course of the Proterozoic, which is also recorded by the Espin-haço and São Francisco supergroups (Schobbenhaus, 1996; Dardenne, 2000; Danderfer et al., 2009). During the Neo-proterozoic Brasiliano event, the Paramirim aulacogen underwent partial inversion. A system of double-verging and NNW-trending faults and folds developed in the aulacogen trough and became its most prominent fabric elements (Cruz and Alkmim, 2006).

The African counterpart of the São Francisco, the Congo craton, extends over the large area of the Congo, Cubango, Zambezi, and Cunene river basins in Central Africa. Fringed by the Oubanguides, Kibaran, Damara and West Congolian belts (Fig. 3), the Congo craton consists of a large Archean nucleus bounded to northwest by the Paleoproterozoic West Central African belt (Trompette, 1994; Feybesse et al., 1998). Approximately two thirds of the cratonic domain is covered by Phanerozoic and Precambrian sedimentary units. The Archean basement is exposed in the shield areas of Angola, Cameroon-Gabon-Congo, Zaire and Kasai (Fig. 3) (Trompette, 1994). The Precambrian cover section, which includes the Neoproterozoic

West Congolian Supergroup discussed further in this paper, occurs in the foreland domain of the West Congolian belt, as well as in the Sangha Aulacogen or Comba basin (Trompette, 1994; Alvarez and Maurin, 1991), a partially inverted, NE-trending rift (Fig. 3).

The 1st-order sequences addressed in the next sections are (Fig. 4): (i) the Paleoproterozoic Minas-Itacolomi sequence; (ii) the Paleo/Mesoproterozoic Espinhaço sequences; (iii) the Neo-proterozoic Macaúbas sequence, and (iv) the NeoNeo-proterozoic Bambuí sequence. For each sequence we describe the lithologic content, boundaries, sedimentologic aspects, and correlative units, discussing in addition their ages and tectonic signiﬁcance.

3. The Minas-Itacolomi sequence

Lying unconformably on top of an Archean greenstone belt, the Minas Supergroup together with the Itacolomi Group (Dorr, 1969) represents a ca. 8.000 m-thick 1st-order sequence, which tracks the operation of a Wilson cycle in the early Paleoproterozoic, between 2.5 and 2.0 Ga (Alkmim and Marshak, 1998). Minas and Itacolomi strata are deformed and metamorphosed to lower

Figure 2.Geological map of the São Francisco craton showing the bordering Neoproterozoic Brasiliano belts, the Proterozoic cover sequences (younger than 1.8 Ga), the São Francisco basin, and the Paramirim aulacogen. Morphotectonic domains of the Paramirim aulacogen: NE Northern Espinhaço range; CD Chapada Diamantina. The box indicates the area ofFig. 5; the dotted line the location of the seismic section shown onFig. 9.

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greenschist to lower amphibolites facies conditions in their type area, the mining district of the Quadrilátero Ferrífero (QF) (Herz, 1978) (Fig. 5). The succession comprises, as follows, three 2nd-order sequences (Fig. 6):

- (1) A ca. 1450 m-thick package of continental to marine sedi-ments (Dorr, 1969; Renger et al., 1995) that represents the development stage of the passive margin basin;

- (2) an up to 3500 m-thick package of turbiditic pelites, wackes, lithic conglomerates, and diamictites (Sabará Group), repre-senting syn-orogenic sediments (Dorr, 1969; Renger et al., 1995; Reis et al., 2002) shed from a colliding magmatic arc and spread over an evolving foreland basin onto the craton margin around 2.1 Ga;

- (3) an up to 1800 m-thick section of alluvial sandstones, conglomerates and minor pelites (Itacolomi Group) (Dorr, 1969) deposited in intramontane grabens, during the collapse phase of the Paleoproterozoic orogen developed along the margin of the Archean nucleus of the São Francisco craton (Alkmim and Marshak, 1998).

The basal 2nd-order succession can be subdivided into three 3rd-order sequences, which are (Fig. 6):

A ca. 600 m-thick package, consisting of alluvial to marine sandstones, conglomerates and subordinate pelites, comprising the Tamanduá and Caraça groups (Dorr, 1969), which represent the rift and transitional phases of the passive margin devel-opment (Renger et al., 1995; Alkmim and Marshak, 1998);

The ca. 400 m-thick package of marine sediments that includes banded iron formations and carbonates (Itabira Group) (Dorr, 1969) that record the full development the passive margin basin along the southern edge of the ancient craton (Alkmim and Marshak, 1998);

The ca. 450 m-thick pile of shallow marine to deep water sediments (Piracicaba Group) (Dorr, 1969; Renger et al., 1995), consisting mainly of siliciclastics with minor carbonates.

The regional 2nd-order unconformities that separate the three 2nd-order sequences mentioned above were recognized and mapped already in the pioneering work byDorr (1969).

The maximum age of the basal sandstones of the Minas-Itacolomi sequence, given by detrital zircons, is 2584 þ10 Ma (Hartmann et al., 2006). Limestones from the middle portion of the Gandarela Formation (Fig. 6) yielded a PbePb depositional age of 2420þ19 Ma (Babinski et al., 1995), whereas the ages obtained from detrital zircons extracted from the Sabará and Itacolomi groups were 2125 þ 4 Ma and 2059 þ 58 Ma, respectively (Machado et al., 1996). According to these data, the development of the Minas passive margin, as recorded by the two oldest 3rd-order sequences, took place in the time interval between 2.6 and 2.4 Ga and progressed afterward with deposition of another third 3rd-order sequence (Piracicaba Group,Fig. 6), whose depositional age is not constrained. The inversion of the Minas passive margin occurred only ca. 300 Ma later, as registered by the deposition of the syn-orogenic Sabará turbidites (Machado et al., 1996; Reis et al., 2002), which also mark a major change in the source of Minas sediments. Paleogeographic studies carried out in the QF (Dorr,

Figure 3. Geological map of the Congo craton emphasizing the marginal orogenic belts, the basement exposures, and the distribution of the Proterozoic cover units (younger than 1.8 Ga) (afterTrompette, 1994; Alvarez, 1995).

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1969; Renger et al., 1995; Machado et al., 1996) indicate Archean sources located to the north for the rift to passive margin 2nd-order sequence. A distinct and non-cratonic source located to south and southeast is required to explain the presence of 2.1 Ga old granitoid clasts and zircons in the Sabará and Itacolomi conglomerates (Dorr, 1969; Machado et al., 1996; Alkmim and Marshak, 1998; Reis et al., 2002).

The previously mentioned geochronologic data also imply that the Cauê Banded Iron Formation (Fig. 6), the marker bed of the QF region, was deposited in the time between 2.58 and 2.42 Ga. This estimation is in agreement with the depositional age interval of the Siderian banded iron formations worldwide (Klein, 2005; Clout and Simonson, 2005). According to several authors, the Siderian banded iron formations record the onset of the atmospheric oxygenation process (e.g.,Cloud, 1973; Konhauser et al., 2002; Guo et al., 2009). Also noteworthy, is a 20 cm to 50 m-thick conglomerate and breccia layer, consisting of carbonate, chert, banded iron formation, and pelite clasts, embedded in a carbonate/iron oxide-rich or pelitic matrix, that occurs in the upper portion of the Gandarela Formation (Fig. 6) throughout the whole QF region. Carbon isotope analyses performed in the type section of the Gandarela Formation by Bekker et al. (2003)yielded

d

13_{C positive values between 0.2 and}

0.4 permil for the dolomites below the breccia layer and negative

values in the range of 0.3 and 0.4 permil for the dolomites imme-diately beneath and above the breccia layer. Early Paleoproterozoic glacial events, some of them of global signiﬁcance, took place in the time interval of 2.4e2.2 Ga (e.g.,Hambrey and Harlam, 1981; Evans et al., 1997; Kirschvink et al., 2000). The Gandarela breccias are, in our view, candidates for representing one of these events, a matter that demands further investigation.

Correlatives of the Minas-Itacolomi sequence are:

- The Jacobina Group consisting of quartzites, conglomerates and subordinate pelites that occurs in the foreland domain of the Paleoproterozoic Eastern Bahia orogenic zone in the northern São Francisco craton (Ledru et al., 1994, 1997).

- The Ogooué, Nyong, Ayna, and Francevillian units, which comprise a 2.5 to 2.0 Ga succession exposed in the internal and foreland domains of the Paleoproterozoic West Central African belt in Gabon (Feybesse et al., 1998).

4. The Espinhaço sequences

The quartz-arenite dominated package of the Paleo/Meso-proterozoic Espinhaço Supergroup was deposited in a rift-sag

Figure 4.Stratigraphic correlation chart for the Proterozoic 1st-order sequences of the São Francisco and Congo cratons, and the marginal orogenic Brasilia, Araçuaí and West Congolian belts. Ages of tectonic and global glacial events are also indicated (see text for explanation and references).

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successor basin, nucleated in the continental mass that hosted the present-day São Francisco craton and its margins in the Staterian period, around 1.75 Ga (Uhlein et al., 1998; Brito Neves et al., 1996; Martins-Neto, 2000). The Espinhaço Supergroup is the mainﬁll unit of the Paramirim aulacogen (Fig. 7), occurring also in the core of some antiformal structures of the São Francisco basin. Its type area, the Southern Espinhaço range, however lies outside of the São Francisco craton. The Southern Espinhaço range corresponds to the NS-trending segment of the Neoproterozoic Araçuaí fold-thrust belt that fringes the craton to the east (Fig. 7).

Studies conducted byMartins-Neto (2000, 2007, 2009)in the southern Espinhaço range led to the conclusion that the Espinhaço Supergroup comprises a single 1st-order sequence, encompassing pre-rift, rift, transitional and sag evolutionary stages of an intra-continental basin. However, theﬁrst synthesis of the geology of the northern São Franciso craton (Inda and Barbosa, 1978) already suggested a more complex stratigraphic development for the Espinhaço Supergoup in the Paramirim aulacogen. In fact, sedi-mentological, stratigraphical and geochronological studies pub-lished in the last few years (Danderfer Filho and Dardenne, 2002; Guimarães et al., 2008; Danderfer et al., 2009), indicate that the Espinhaço package in the Paramirim aulacogen comprises at least two unconformity bounded 1st-order sequences, here referred to as Espinhaço I and II sequences (Fig. 8). Furthermore, a geochrono-logical investigation recently performed in the southern Espinhaço range byChemale et al. (2010)revealed that one of the unconfor-mities identiﬁed byMartins-Neto (2000)in the lower portion of the supergroup corresponds to an hiatus of at least 500 Ma.

The Espinhaço I and II 1st-order sequences can be in turn subdivided intoﬁve 2nd-order unconformity bounded sequences,

which reﬂect the evolution of two superimposed rift-sag basins in the time intervals of 1.75e1.57 Ga and 1.57e0.9 Ga (Figs. 4 and 8).

The Espinhaço I sequence occurs in the western Chapada Dia-mantina and along the eastern escarpment of the northern Espin-haço range (Fig. 7), regions that correspond to the ancient trough of the Paramirim aulacogen. A ca. 600 m-thick pile of rhyolites, dacites and volcaniclastics (Novo Horizonte and São Simão formations, Fig. 8) that grade upward into an 850 m-thick package of alluvial to lacustrine sediments forms the oldest 2nd-order rift sequence. Fluvial and lacustrine deposits (550e650 m) cover the volcanic succession, which is locally underlain by alluvial/aeolian sand-stones and conglomerates, representing pre-rift deposits (Algodão and Serra da Gameleira formations,Fig. 8) (Danderfer Filho and Dardenne, 2002; Guimarães et al., 2008; Loureiro et al., 2008). Separated from the previous sequences by an erosional unconfor-mity and marking a considerable expansion of the basin area, the 1000e2500-thick 2nd-order sag sequence of the Paraguaçu Group (Fig. 8) includes aeolian and marine deposits (Guimarães et al., 2008; Loureiro et al., 2008).

The nucleation of the Espinhaço rift took place at about 1.75 Ga, as indicated by the age of the basal volcanic rocks (Schobbenhaus et al., 1994; Brito Neves et al., 1996; Babinski et al., 1994; Danderfer et al., 2009) of the Novo Horizonte and São Simão formations (Fig. 8). The sag stage did not persist after 1.57 Ga, the age of a second episode of basic magmatism (Babinski et al., 1999; Battilani et al., 2007; Loureiro et al., 2008; Danderfer et al., 2009) recorded by the basal unit of the Espinhaço II sequence that marks the onset of renewed extensional tectonism in the Paramirim aulacogen.

Figure 5.Simpliﬁed geological map of the southern São Francisco craton showing the location of the Quadrilátero Ferrífero mining district and the distribution of the Paleo-proterozoic Minas-Itacolomi 1st-order sequence.

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Alluvial sandstones, diamond-bearing conglomerates and basic volcanics cut by dikes and sills of the same nature form the base of the Espinhaço II 1st-order sequence in the Chapada Diamantina (Dominguez, 1993; Battilani et al., 2007; Guimarães et al., 2008) (Figs. 7 and 8). This ca. 400 m-thick succession grades upward into aeolian deposits, which are in turn covered by marine pelites and sandstones containing intercalations of carbonates and minor evaporites. Fluvial to estuarine deposits lying on an erosional unconformity on top of the marine pelites (Dominguez, 1993) represent the youngest Espinhaço 2nd-order sequence in the Chapada Diamantina (Morro do Chapéu Formation,Fig. 8). Acid to intermediate volcanics (Bomba Formation,Danderfer et al., 2009) (Fig. 8), covered by alluvial/aeolian sandstones, form the base of the Espinhaço II 1st-order sequence in the northern Espinhaço range. These units are in turn overlain by a ca. 700 m-thick package of shallow marine and deltaic deposits.

Zircons extracted from the volcanics of the Bomba Formation yield UePb SHRIMP ages of 1582 þ 8 Ma and 1569 þ 14 Ma (Danderfer et al., 2009). According to these authors the dates obtained constrain the timing of a second phase of rifting acting upon the pre-existing Espinhaço basin at around 1570 Ma.

According to the geochronological data recently obtained by Chemale et al. (2010)in the type area of the Espinhaço Supergroup, the basal Bandeirinha and São João da Chapada formations (the syn-rift Olaria, Natureza and São João da Chapada 2nd-order sequences ofMartins-Neto, 2000, 2009) were deposited around 1.75 Ga. The age spectrum obtained from detrital zircons extracted from the overlaying Sopa-Brumadinho Formation, Galho do Miguel

Formation and Conselheiro Mata Group (corresponding to the homonymous 2nd-order sequences ofMartins-Neto, 2000, 2009) suggests that these units form a single rift-sag sequence, whose maximum age is 1.2 Ga (Chemale et al., 2010). Thus, these two groups of units represent the Espinhaço I and II 1st-order sequences, respectively.

The Araí and Rio Preto Groups (Brito Neves et al., 1996) exposed in the Brasília and Rio Preto belts on the northwestern margin of the São Francisco craton are correlatives to the Espinhaço I sequence.

In the Congo craton region, correlatives of the Espinhaço sequences are:

- The Chela Group, consisting of a ca. 600 m-thick succession of marine sandstones interbedded with acid volcanic, pelites, conglomerates and subordinate carbonates that cover Archean basement of the Angola block in the southwestern portion of the craton (Torquato and Fogaça, 1981; Pedreira and De Waele, 2008) (Figs. 3 and 4). Basal acid volcanics and volcaniclastics of the middle portion of the sequence have been dated at 1790þ17 Ma and 1718þ12 Ma, respectively (Mccourt et al., 2004).

- The Kibaran Supergroup exposed along the eastern margin of the craton (Fig. 3), which is made up of a very thick package of pelites that grade upward into sandstones. The maximum age of the middle portion of the supergroup is estimated at ca. 1.38 Ga (Kokonyangi et al., 2004; Pedreira and De Waele, 2008).

Figure 6.Stratigraphic chart of the Minas-Itacolomi 1st-order sequence in the Quadrilátero Ferrífero region. (Numbers marked with a square indicate depositional or intrusion ages; numbers marked with a star denote ages of the youngest detrital zircons found in the unit. See the text for explanations and references).

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5. The Macaúbas sequence

According toMartins-Neto (2007, 2009)andMartins-Neto and Hercos (2002), the Macaúbas 1st-order sequence is composed of continental glaciogenic rift to siliciclastic-carbonatic passive margin deposits in the São Francisco craton and its margins (Figs. 4, 9 and 10).

The Macaúbas sequence comprises the oldest known Neo-proterozoic sedimentary units of both the São Francisco basin and Paramirim aulacogen. In the exposures of the São Francisco basin the sequence is represented by the Jequitaí Formation, a relatively thin package (ca. 180 m) of glaciogenic deposits, including diamictites, sandstones and pelites (Dardenne, 1978; Montes et al., 1985; Karfunkel and Hoppe, 1988; Uhlein et al., 1999; Martins-Neto and Hercos, 2002). In the Chapada Diamantina (eastern Paramirim aulacogen) the sequence also consists of a relatively thin succession (ca. 230 m) of glaciomarine and lacustrine sediments (Bebedouro Formation) (Montes et al.,1985; Guimarães et al., 2008), whereas the correlative unit in the northern Espinhaço range (western Para-mirim aulacogen), the Santo Onofre Group, is made up of ca. 1500 m of siliciclastic turbidites (Danderfer Filho and Dardenne, 2002).

However, the full development of the Macaúbas sequence can be observed outside the craton, in the Araçuaí-West Congo orogen.

As the type unit of the external fold-thrust belt of the Araçuaí-West Congo orogen (Fig. 7), the Macaúbas sequence is made up of sandstones, pelites, diamictites, carbonates, basic volcanics, and banded iron formations metamorphosed under greenschist to amphibolite facies conditions (Uhlein et al., 1999; Pedrosa-Soares et al., 2001, 2008; Martins-Neto et al., 2001; Tack et al., 2001). In this context, the Macaúbas sequence represents pre-rift, rift, tran-sitional and passive margin stages of the development of a Red Sea type basin, the Macaúbas basin, formed between the São Francisco peninsula and the Congo continent in the course of the Cryogenian Period. Closure of the Macaúbas basin around 580 Ma led to the development of the Araçuaí-West Congo orogen (Fig. 2) (Uhlein et al., 1999; Pedrosa-Soares et al., 2001, 2008; Tack et al., 2001; Alkmim et al., 2006).

The onset of the Macaúbas rifting is associated with a ca. 850 Ma rift-related magmatic event documented in the craton interior and in the Araçuaí-West Congo Orogen (Pedrosa-Soares et al., 2008; Silva et al., 2008; Danderfer et al., 2009). Furthermore, glacio-genic sediments of the Macaúbas sequence contain detrital zircons dated at ca. 864 and 880 Ma byPedrosa-Soares et al. (2000)and Rodrigues (2008), respectively.

The Vazante and Canastra Groups as well as the Cubatão Dia-mictite (basal unit of Ibiá Group) (Figs. 4 and 7) (Dardenne, 2000;

Figure 7.Distribution of the Espinhaço (I and II), Macaúbas and Bambuí sequences in the Paramirim aulacogen and the Araçuaí belt. (a) Simplified geological map of the Northern Espinhaço range (NE) and Chapada Diamantina (CD) in the southern portion of the Paramirim aulacogen (Modified fromInda and Barbosa, 1978; Guimarães et al., 2008; Loureiro et al., 2008). (b) Geological map of the Southern Espinhaço range (SE) in the southwestern portion of the Araçuaí belt (Modified afterUhlein et al., 1998).

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Azmy et al., 2006, 2008) exposed along the western boundary of the São Francisco basin and in the adjacent Brasília belt, are potential correlatives of the Macaúbas 1st-order sequence. The Vazante Group is composed of intercalated carbonates and pelites containing sandstones and diamictites at the base. Available geochronologic and provenance studies (Pimentel et al., 2001, 2011; Coelho et al., 2008; Rodrigues, 2008) suggest that the Vazante sediments, shed from a cratonic source, accumulated in the time between 925 and 750 Ma. The Canastra Group, exposed in a relatively large area in the central and southern Brasilía belt consists of a ca. 800 m succession of greenschist facies pelites with carbonate lenses at the base, followed by black shales and pelite/

sandstone intercalations toward the top, characterizing an overall transgressive sequence (Dardenne, 2000). Provenance studies conducted in the Canastra Group indicated Mesoproterozoic rocks as its main source (Valeriano et al., 2004) and a maximum depo-sitional age of 1040 Ma (Rodrigues, 2008; Pimentel et al., 2011). The basal diamictites of the Ibiá Group, interpreted as glacial sediments (Pereira et al., 1994), were deposited between 850 and 640 Ma, as indicated by the ages of detrital zircons obtained by Rodrigues (2008)andPimentel et al. (2011).

Seismic data collected in the São Francisco basin indicate that the Macaúbas 1st-order sequence together with its correlatives occur regionally, covering pre-Neoproterozoic rocks (Fig. 9). These

Figure 8.Stratigraphic chart of the Espinhaço I and II sequences in the Paramirim aulacogen. Numbers marked with a square indicate deposition or intrusion ages; numbers marked with a star denote ages of the youngest detrital zircons found in the unit (see the text for explanations and references) (based onGuimarães et al., 2008; Loureiro et al., 2008; Danderfer et al., 2009).

Figure 9.Interpreted reﬂection seismic section across the São Francisco craton (seeFig. 2for location) showing the recognized 1st-order sequences, as well as lower-order sequences of the Macaúbas correlative units (modiﬁed afterMartins-Neto, 2005, 2009). See also the tectonic overprint of western Brasília and eastern Araçuaí fold-thrust belts, both verging to the craton interior (Vertical scale in two-way-travel time).

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data also allow the subdivision of the 1st-order sequence into two 2nd-order sequences: rift and passive margin. The passive margin 2nd-order sequence can be further subdivided into three 3rd-order sequences. The unit displays an overall wedge shape, with maximum thickness reaching ca. 6000 m in the westernmost part of the section, close to the Brasilia belt. The outcropping black shales/marls with intervals showing high TOC in the Canastra and Vazante groups comprise one of the highest potential hydrocarbon source rocks of the São Francisco basin (Martins-Neto, 2005, 2007, 2009).

The rift-related sediments of the Sansikwa Subgroup together with the glaciogenic Lower Mixtite Formation and the passive margin succession of the Haut Schiloango Subgroup (Tack et al., 2001; Frimmel et al., 2006) are the correlatives of the Macaúbas sequence in the West Congolian belt (Pedrosa-Soares et al., 2008). Detrital zircons extracted from the sandstones of the Sansikwa Subgroup yield a maximum depositional age of 923 þ 43 Ma (Frimmel et al., 2006).

The Macaúbas glaciogenic sediments, deposited between 864 Ma (age of the youngest detrital zircon so far found in the unit, Pedrosa-Soares et al., 2000) and 740 Ma (age of the basal carbon-ates of the overlying Bambuí sequence,Babinski et al., 2007), could be viewed as a manifestation of the Sturtian global glaciation, analogous to the wayFrimmel et al. (2006)interpreted the Lower Mixtite Formation of the West Congolian Supergroup.

6. The Bambuí sequence

The Bambuí Group, composed of alternating siliciclastic and carbonate deposits, is the mainﬁll unit of the São Francisco basin. Also occurring within large synformal structures of the Paramirim aulacogen (Fig. 7), the Bambuí Group comprises a 1st-order

sequence that marks the downwarping behavior of a substantial part of the São Francisco craton during a high sea-level event in the time after 630 Ma. As a consequence of the development of the Brasília belt, the craton domain was then converted into a foreland basin (Castro and Dardenne, 2000; Alkmim and Martins-Neto, 2001; Martins-Neto, 2007, 2009).

The Bambuí sequence can be subdivided into three 2nd-oder coarsening upward and progradational sequences (Fig. 10), whose facies architecture are distinct in the western and eastern sectors of São Francisco basin. The western portion, acting as a foredeep to the Brasília orogenic front, wasfilled mainly by siliciclastics, whilst the eastern sector, representing the flexural ramp, received fine-grained siliciclastics interbedded with shallow water carbonates (Martins-Neto, 2007, 2009). The succession exposed in the Para-mirim aulacogen is also made up of 2nd-order coarsening upward sequences of pelites and shallow water carbonates (Misi and Veizer, 1998; Misi et al., 2007).

Discontinuous beds of conglomerates, breccias and pelites (Carrancas Formation, Fig. 10) overlain by limestones showing pseudomorphs of aragonite fans (Sete Lagoas Formation,Fig. 10) mark the base of the Bambuí Group in some areas along the southern border of the São Francisco basin, where the Macaúbas sequence is absent. Sedimentological, geochemical and geochro-nological studies carried out in the last few years on these units led to the following conclusions:

- The Carrancas conglomerates, with a maximum depositional age of 1431 þ 68 Ma (Rodrigues, 2008) represent ﬂuvial depositsﬁlling valleys carved in the cratonic basement (Vieira et al., 2007).

- The limestones containing aragonite fan pseudomorphs at the base of the Sete Lagoas Formation, dated at 740 þ 22 Ma

Figure 10.Stratigraphic chart of the Macaúbas and Bambuí sequences in the São Francisco basin (Numbers marked with a square indicate deposition or intrusion ages; numbers marked with a star denote ages of the youngest detrital zircons found in the unit. See the text for explanations and references).

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(Babinski et al., 2007), show very low organic matter content and negative

d

13C values (Misi et al., 2007). They have been

interpreted as a Sturtian cap carbonate (Vieira et al., 2007). - The middle and upper portions of the Sete Lagoas Formation

constitute a distinct transgressive to regressive sequence, characterized by higher organic matter content and positive

d

13_{C values (}_{Martins and Lemos, 2007; Vieira et al., 2007}_).

Furthermore, detrital zircons extracted from these intervals yield a maximum depositional age of 610 Ma (Rodrigues, 2008; Pimentel et al., 2011).

Geochronological determinations on detrital zircons extracted from the middle and upper portions of the Bambuí Group resulted in maximum ages of 650 and 616 Ma, respectively (Rodrigues, 2008; Pimentel et al., 2011).

Correlatives of the Bambuí 1st-order sequence on the Congo craton and its western margin correspond to the upper section of the West Congolian Group that includes the glaciogenic Upper Mixtite Formation, the shallow marine carbonates of the Schisto-Calcaire Subgroup and the molasse deposits of the Mpioka Subgroup (Tack et al., 2001; Frimmel et al., 2006; Pedrosa-Soares et al., 2008). The Upper Mixtite Formation, which has no equiva-lents in the Bambuí sequence, was tentatively correlated to the Marinoan global glaciation deposits byFrimmel et al. (2006).

7. Tectonic signiﬁcance of the São Francisco craton 1st-order sequences

As described in the previous sections, the Proterozoic 1st-order sequences exposed in the São Francisco craton and its margins contain the record of basin-forming events that took place in distinct scenarios in the time interval between the beginning of the Paleoproterozoic (w2.5 Ga) and the end of the Ediacaran period of the Neoproterozoic Era (w580 Ma).

The oldest event, recorded by the lower portion of the Minas-Itacolomi sequence and its correlatives, corresponds to the devel-opment of passive margin basins along the borders of the Archean nuclei of the present-day São Francisco and Congo cratons, between 2.5 Ga and 2.1 Ga. The convergence and collision of these nuclei in the course of the ca. 2.1 Ga Transamazonian and Eburnean (Fig. 4) events of South America and Africa, respectively, are rep-resented by the upper 2nd-order sequences of the Minas-Itacolomi sequence and its correlatives. These events seem to reﬂect the assembly of a supercontinent during the Orosirian period of the Paleoproterozoic Era, the Atlantica supercontinent (Rogers, 1996), followed by Columbia (Rogers and Santosh, 2004; Zhao et al., 2004), whose reconstructions, though not fully accomplished, have progressed signiﬁcantly in the last few years.

A system of rift basins, among them the Paramirim aulacogen, nucleated in the continental plate that hosted the São Francisco and Congo cratons around 1.75 Ga. Currently referred to as the Espin-haço rifting or Staterian taphrogenesis (Brito Neves et al., 1996) in the Brazilian literature, this extensional episode is associated with the extrusion of bimodal volcanics and the deposition of the lower Espinhaço I sequence in the São Francisco craton and its margins. Renewed rifting around 1.57 Ga (Danderfer et al., 2009) led to reactivation of pre-existing extensional fabrics and deposition of the lower Espinhaço II sequence in the Paramirim aulacogen.

The 1.57 Ga rifting event was followed by sag basin development and marine incursions in the regions represented by the São Francisco craton and its margins, as recorded by the upper Espin-haço II sequence. However, the Mesoproterozoic history of the São Francisco-Congo craton and its relation to the Rodinia superconti-nent, assembled by the end of the era, are not well understood. Rodinia reconstructions available in the literature show the São

Francisco-Congo in the periphery of the supercontinent or as an isolated piece (e.g. Hoffman, 1991; Weil et al., 1998; Pisarevsky et al., 2003).

Regardless of its relationship to Rodinia, São Francisco-Congo became an individual continent surrounded by passive margins sometime during the Cryogenian period, as indicated by the sedi-mentary record of the Macaúbas sequence and its correlatives. The onset of the Macaúbas rifting in the São Francisco craton has been related to a ca. 850 Ma bimodal magmatic event documented in the São Francisco craton and its margins (Pedrosa-Soares et al., 2008). In the western margin of the Congo craton, this event was preceded by two magmatic episodes, the basic and acid volcanism of the Zadianian and Mayumbian groups, respectively, whose ages fall in the interval of 1000e920 Ma (Tack et al., 2001). The evolution of the Macaúbas rifting during the Cryogenian period resulted in the development of a large gulf, partiallyﬂoored by oceanic crust that separated the São Francisco peninsula from the Congo continent (Pedrosa-Soares et al., 2008).

The São Francisco-Congo continent and various fragments derived from Rodinia started to converge by the end of the Cry-ogenian period. Their reassembly resulted in the formation of Gondwana during the Ediacaran period. In this process the margins of the São Francisco-Congo were diachronically converted into the Brasiliano/PanAfrican orogenic belts; its interior, affected by a general marine transgression in the time after 610 Ma eventually behaved as a foreland basin, receiving the sediments of the Bambuí sequence and its correlatives.

8. Final remarks

The study of the Proterozoic sedimentary record of the São Francisco craton performed by several authors in the last few years allows the synthesis presented in the previous sections. However, many questions related to the geological history and trajectory of the São Francisco-Congo craton during the Proterozoic remain open, demanding further investigations. Some of these questions are: (i) does the breccia/conglomerate layer of the Gandarela Formation of the lower Minas Supergroup record the Paleoproter-ozoic global glaciation event?; (ii) what is the age and tectonic signiﬁcance of the unconformity bounded 2nd-order sequence represented by the Piracicaba Group of the middle Minas-Itacolomi sequence?; (iii) what is the age and tectonic signiﬁcance of the upper marine portion of the Espinhaço II 1st-order sequence?; (iv) do the glacial sediments of the Macaúbas sequence and its correl-atives represent the global Sturtian glacial event? In other words, are the cap carbonates found at the base of the Bambuí group related to the Macaúbas glacial sediments?

Acknowledgments

This paper resulted from studies conducted by the authors in the São Francisco craton region in the last 20 years. F.F.Alkmim (Grant #307531/2009-0) and M.Martins-Neto have been supported by the CNPq (Brazilian Council for Scientiﬁc and Technological Develop-ment) through these years. R Mazumder and an anonymous reviewer are thanked for their comments and suggestions. The authors thank specially P. Eriksson for the invitation to submit this paper to the Marine and Petroleum Geology special issue and also for his review that considerably improved the original manuscript.

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