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Marine and Petroleum Geology 33 (2012) 127e139

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Marine and Petroleum Geology

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

Fernando F. Alkmima,*, Marcelo A. Martins-Netob

aDepartamento de Geologia, Escola de Minas, Universidade Federal de Ouro Preto, Morro do Cruzeiro, 35.400.000 Ouro Preto, MG, BrazilbVicenza 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 2010Received in revised form15 August 2011Accepted 18 August 2011Available online 5 September 2011

Keywords:First-order sequencesProterozoicSão Francisco cratonBrazil

* Corresponding author. Tel.: þ55 31 3559 1849; faE-mail addresses: ffalkmim@gmail.com, alkmim@d

0264-8172/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.marpetgeo.2011.08.011

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 thisregion comprise five 1st-order sedimentary sequences, which besides episodes of global significancerecord major basin-forming events. The ca. 8000 m-thick Minas-Itacolomi 1st-order sequence, exposedin the Brazilian mining district of the Quadrilátero Ferrífero and containing as marker bed the LakeSuperior-type Cauê Banded Iron Formation, tracks the operation of a Wilson cycle in the PaleoproterozoicEra. 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 and1.57 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 Cryogenianperiod. 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.

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1. Introduction

The best preserved and most intensively studied Precambriansedimentary record of South America is exposed in a relativelysmall area of eastern Brazil, which corresponds to the São Franciscocraton and its margins (Fig. 1). The São Francisco craton (Almeidaet al., 2000) hosts sedimentary successions accumulated in thetime interval between the Neoarchean (w2800 Ma) and LateNeoproterozoic (w580 Ma).

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

x: þ55 31 3559 1600.egeo.ufop.br (F.F. Alkmim).

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

We selected for discussion in this paper five Proterozoic 1st-order sequences exposed in the São Francisco craton region. Thesequences chosen for discussion record not only the Proterozoichistory of the São Francisco craton, but also tectonic and climaticevents of global significance. Archean sedimentary successions,although present in many sectors of the craton, are tectonicallydismembered and demand further detailed studies. For this reasonthey will be not addressed in this paper.

As a consequence of its previous residence in Gondwana, the SãoFrancisco craton has an African counterpart (Fig. 1), which is rep-resented by the Congo craton. The link between the São Franciscoand Congo cratons is made by a crustal bridge that connects thePaleoproterozoic Eastern Bahia orogenic domain in Brazil to thePaleoproterozoic Francevillian or West Central African belt inGabon (Fig. 1). This piece of crust remained intact from the middlePaleoproterozoic until the beginning of the Cretaceous, when WestGondwana 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 Brazilian1st-order sequences in the Congo craton are indicated in the text.

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: AAmazonian; 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).

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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 paperwas postulated by Martins-Neto et al. (2001) and Catuneanu et al.(2005). Since Precambrian basins are often characterized by poorstratal preservation and by a general lack of time control, anapproach based on the stratigraphic hierarchy is considered thebest way to interpret the stratigraphic framework. Stratigraphichierarchy involves the separation of different orders of sequencesand bounding surfaces based on their relative importance. In thiscontext, changes in the tectonic setting provide the key criterion forthe basic subdivision of the rock record. Thus, a first-ordersequence corresponds to an entire sedimentary basin-fill succes-sion, regardless of the origin and life span of each particular basin.This first-order basin-fill succession can be in turn subdivided intosecond- and lower-order sequences, based on the steps involved inthe tectonic evolution of the basin, irrespective of the time spanbetween two same-order consecutive events.

2. The São Francisco craton

In the continent interior, the São Francisco craton is bounded bythe Neoproterozoic Brasília, Araçuaí, Rio Preto, Riacho do Pontaland Sergipano orogenic belts (Fig. 2) (Almeida et al., 1981). Alongthe eastern Brazilian coast, the craton boundary is made up byCretaceous passivemargin basins, developed on the cratonic bridgethat once linked the São Francisco and Congo blocks.

The basement of the São Francisco craton consists of an Archeannucleus and two segments of a Paleoproterozoic orogen (Alkmimand 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 GaMineiro belt (Teixeira and Figueiredo, 1991; Teixeira et al., 2000)that bounds the Archean block to the south involves the Archeanbasement, 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; Ávilaet al., 2010). The famous Brazilian mining district known as theQuadrilátero Ferrífero (“Iron Quadrangle”) (Dorr, 1969) is located inthe foreland domain of the Mineiro belt and corresponds to thetype area of the Paleoproterozoic Minas-Itacolomi sequence, dis-cussed in the next section.

The Proterozoic cover units of the São Francisco craton occur intwo distinct tectonic domains: the São Francisco basin and theParamirim aulacogen (Fig. 2). The poly-historic São Francisco basinextends over ca. 800 km along the NS-trending lobe of the cratonand is bounded by emergent thrusts of the adjacent Neoproterozoicorogenic 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 basinis bounded by the Paramirim corridor, a Neoproterozoic intra-cratonic deformation zone, which affects the neighboring Para-mirim aulacogen (Alkmim and Martins-Neto, 2001; Cruz andAlkmim, 2006). The Precambrian basin-fill units comprise theEspinhaço and São Francisco supergroups, of Paleo/Mesoproter-ozoic and Neoproterozoic ages, respectively. The Phanerozoicsection contains the glacial sediments of the Permo-carboniferousSanta Fé Group (Campos and Dardenne, 1994), as well as theCretaceous rift-sag succession of the Areado, Urucuia, and Mata daCorda Groups (Sgarbi et al., 2001).

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

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 SãoFrancisco basin, and the Paramirim aulacogen. Morphotectonic domains of the Paramirim aulacogen: NE Northern Espinhaço range; CD Chapada Diamantina. The box indicates thearea of Fig. 5; the dotted line the location of the seismic section shown on Fig. 9.

F.F. Alkmim, M.A. Martins-Neto / Marine and Petroleum Geology 33 (2012) 127e139 129

to as the Chapada Diamantina (Fig. 2). Developed around 1.75 Ga,the Paramirim aulacogen experienced a long subsidence history inthe 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 underwentpartial inversion. A system of double-verging and NNW-trendingfaults and folds developed in the aulacogen trough and becameits 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, andCunene river basins in Central Africa. Fringed by the Oubanguides,Kibaran, Damara andWest Congolian belts (Fig. 3), the Congo cratonconsists of a large Archean nucleus bounded to northwest by thePaleoproterozoic West Central African belt (Trompette, 1994;Feybesse et al., 1998). Approximately two thirds of the cratonicdomain is covered by Phanerozoic and Precambrian sedimentaryunits. TheArcheanbasement 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 Supergroupdiscussed further in this paper, occurs inthe foreland domain of the West Congolian belt, as well as in theSangha Aulacogen or Comba basin (Trompette, 1994; Alvarez andMaurin, 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) thePaleo/Mesoproterozoic Espinhaço sequences; (iii) the Neo-proterozoic Macaúbas sequence, and (iv) the NeoproterozoicBambuí sequence. For each sequence we describe the lithologiccontent, 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, whichtracks the operation of aWilson cycle in the early Paleoproterozoic,between 2.5 and 2.0 Ga (Alkmim and Marshak, 1998). Minas andItacolomi strata are deformed and metamorphosed to lower

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 than1.8 Ga) (after Trompette, 1994; Alvarez, 1995).

F.F. Alkmim, M.A. Martins-Neto / Marine and Petroleum Geology 33 (2012) 127e139130

greenschist to lower amphibolites facies conditions in their typearea, 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 thedevelopment 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 arcand spread over an evolving foreland basin onto the cratonmargin 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 collapsephase of the Paleoproterozoic orogen developed along themargin of the Archean nucleus of the São Francisco craton(Alkmim and Marshak, 1998).

The basal 2nd-order succession can be subdivided into three3rd-order sequences, which are (Fig. 6):

� A ca. 600 m-thick package, consisting of alluvial to marinesandstones, conglomerates and subordinate pelites, comprisingthe Tamanduá and Caraça groups (Dorr, 1969), which representthe 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 includesbanded iron formations and carbonates (Itabira Group) (Dorr,1969) that record the full development the passive marginbasin along the southern edge of the ancient craton (Alkmimand Marshak, 1998);

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

The regional 2nd-order unconformities that separate the three2nd-order sequences mentioned above were recognized andmapped already in the pioneering work by Dorr (1969).

The maximum age of the basal sandstones of the Minas-Itacolomi sequence, given by detrital zircons, is 2584 þ 10 Ma(Hartmann et al., 2006). Limestones from the middle portion ofthe Gandarela Formation (Fig. 6) yielded a PbePb depositional ageof 2420 þ 19 Ma (Babinski et al., 1995), whereas the ages obtainedfrom detrital zircons extracted from the Sabará and Itacolomigroups were 2125 þ 4 Ma and 2059 þ 58 Ma, respectively(Machado et al., 1996). According to these data, the development ofthe Minas passive margin, as recorded by the two oldest 3rd-ordersequences, took place in the time interval between 2.6 and 2.4 Gaand progressed afterward with deposition of another third 3rd-order sequence (Piracicaba Group, Fig. 6), whose depositional age isnot constrained. The inversion of the Minas passive marginoccurred only ca. 300 Ma later, as registered by the deposition ofthe syn-orogenic Sabará turbidites (Machado et al., 1996; Reis et al.,2002), which also mark a major change in the source of Minassediments. Paleogeographic studies carried out in the QF (Dorr,

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 WestCongolian belts. Ages of tectonic and global glacial events are also indicated (see text for explanation and references).

F.F. Alkmim, M.A. Martins-Neto / Marine and Petroleum Geology 33 (2012) 127e139 131

1969; Renger et al., 1995; Machado et al., 1996) indicate Archeansources located to the north for the rift to passive margin 2nd-order sequence. A distinct and non-cratonic source located tosouth and southeast is required to explain the presence of 2.1 Gaold granitoid clasts and zircons in the Sabará and Itacolomiconglomerates (Dorr, 1969; Machado et al., 1996; Alkmim andMarshak, 1998; Reis et al., 2002).

The previously mentioned geochronologic data also imply thatthe Cauê Banded Iron Formation (Fig. 6), the marker bed of the QFregion, was deposited in the time between 2.58 and 2.42 Ga. Thisestimation is in agreement with the depositional age interval of theSiderian banded iron formations worldwide (Klein, 2005; Clout andSimonson, 2005). According to several authors, the Siderian bandediron formations record the onset of the atmospheric oxygenationprocess (e.g., Cloud, 1973; Konhauser et al., 2002; Guo et al., 2009).

Also noteworthy, is a 20 cm to 50 m-thick conglomerate andbreccia layer, consisting of carbonate, chert, banded iron formation,and pelite clasts, embedded in a carbonate/iron oxide-rich or peliticmatrix, that occurs in the upper portion of the Gandarela Formation(Fig. 6) throughout the whole QF region. Carbon isotope analysesperformed in the type section of the Gandarela Formation byBekker et al. (2003) yielded d13C positive values between 0.2 and0.4 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 Paleoproterozoicglacial events, some of them of global significance, took place in thetime interval of 2.4e2.2 Ga (e.g., Hambrey and Harlam, 1981; Evanset al., 1997; Kirschvink et al., 2000). The Gandarela breccias are, inour view, candidates for representing one of these events, a matterthat demands further investigation.

Correlatives of the Minas-Itacolomi sequence are:

- The Jacobina Group consisting of quartzites, conglomerates andsubordinate pelites that occurs in the foreland domain of thePaleoproterozoic Eastern Bahia orogenic zone in the northernSão Francisco craton (Ledru et al., 1994, 1997).

- The Ogooué, Nyong, Ayna, and Francevillian units, whichcomprise a 2.5 to 2.0 Ga succession exposed in the internal andforeland domains of the Paleoproterozoic West Central Africanbelt 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 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.

F.F. Alkmim, M.A. Martins-Neto / Marine and Petroleum Geology 33 (2012) 127e139132

successor basin, nucleated in the continental mass that hosted thepresent-day São Francisco craton and its margins in the Staterianperiod, around 1.75 Ga (Uhlein et al., 1998; Brito Neves et al., 1996;Martins-Neto, 2000). The Espinhaço Supergroup is themain fill unitof the Paramirim aulacogen (Fig. 7), occurring also in the core ofsome antiformal structures of the São Francisco basin. Its type area,the Southern Espinhaço range, however lies outside of the SãoFrancisco craton. The Southern Espinhaço range corresponds to theNS-trending segment of the Neoproterozoic Araçuaí fold-thrustbelt that fringes the craton to the east (Fig. 7).

Studies conducted by Martins-Neto (2000, 2007, 2009) in thesouthern Espinhaço range led to the conclusion that the EspinhaçoSupergroup comprises a single 1st-order sequence, encompassingpre-rift, rift, transitional and sag evolutionary stages of an intra-continental basin. However, the first synthesis of the geology of thenorthern São Franciso craton (Inda and Barbosa, 1978) alreadysuggested a more complex stratigraphic development for theEspinhaço 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 theEspinhaço package in the Paramirim aulacogen comprises at leasttwo unconformity bounded 1st-order sequences, here referred to asEspinhaço I and II sequences (Fig. 8). Furthermore, a geochrono-logical investigation recently performed in the southern Espinhaçorange by Chemale et al. (2010) revealed that one of the unconfor-mities identified byMartins-Neto (2000) in the lower portion of thesupergroup corresponds to an hiatus of at least 500 Ma.

The Espinhaço I and II 1st-order sequences can be in turnsubdivided into five 2nd-order unconformity bounded sequences,

which reflect the evolution of two superimposed rift-sag basinsin the time intervals of 1.75e1.57 Ga and 1.57e0.9 Ga (Figs. 4 and8).

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 ofthe Paramirim aulacogen. A ca. 600m-thick pile of rhyolites, dacitesand volcaniclastics (Novo Horizonte and São Simão formations,Fig. 8) that grade upward into an 850 m-thick package of alluvial tolacustrine sediments forms the oldest 2nd-order rift sequence.Fluvial and lacustrine deposits (550e650 m) cover the volcanicsuccession, which is locally underlain by alluvial/aeolian sand-stones and conglomerates, representing pre-rift deposits (Algodãoand Serra da Gameleira formations, Fig. 8) (Danderfer Filho andDardenne, 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, the1000e2500-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 (Schobbenhauset al., 1994; Brito Neves et al., 1996; Babinski et al., 1994;Danderfer et al., 2009) of the Novo Horizonte and São Simãoformations (Fig. 8). The sag stage did not persist after 1.57 Ga, theage 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 marksthe onset of renewed extensional tectonism in the Paramirimaulacogen.

Figure 6. Stratigraphic chart of the Minas-Itacolomi 1st-order sequence in the Quadrilátero Ferrífero region. (Numbers marked with a square indicate depositional or intrusionages; 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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Alluvial sandstones, diamond-bearing conglomerates and basicvolcanics cut by dikes and sills of the same nature form the base ofthe 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 intoaeolian deposits, which are in turn covered by marine pelites andsandstones containing intercalations of carbonates and minorevaporites. Fluvial to estuarine deposits lying on an erosionalunconformity on top of the marine pelites (Dominguez, 1993)represent the youngest Espinhaço 2nd-order sequence in theChapada Diamantina (Morro do Chapéu Formation, Fig. 8). Acid tointermediate volcanics (Bomba Formation, Danderfer et al., 2009)(Fig. 8), covered by alluvial/aeolian sandstones, form the base of theEspinhaço II 1st-order sequence in the northern Espinhaço range.These units are in turn overlain by a ca. 700 m-thick package ofshallow marine and deltaic deposits.

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

According to the geochronological data recently obtained byChemale et al. (2010) in the type area of the Espinhaço Supergroup,the basal Bandeirinha and São João da Chapada formations (thesyn-rift Olaria, Natureza and São João da Chapada 2nd-ordersequences of Martins-Neto, 2000, 2009) were deposited around1.75 Ga. The age spectrum obtained from detrital zircons extractedfrom the overlaying Sopa-Brumadinho Formation, Galho do Miguel

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

The Araí and Rio Preto Groups (Brito Neves et al., 1996) exposedin the Brasília and Rio Preto belts on the northwestern margin ofthe São Francisco craton are correlatives to the Espinhaço Isequence.

In the Congo craton region, correlatives of the Espinhaçosequences are:

- The Chela Group, consisting of a ca. 600 m-thick succession ofmarine sandstones interbedded with acid volcanic, pelites,conglomerates and subordinate carbonates that cover Archeanbasement of the Angola block in the southwestern portion ofthe craton (Torquato and Fogaça, 1981; Pedreira and De Waele,2008) (Figs. 3 and 4). Basal acid volcanics and volcaniclastics ofthe middle portion of the sequence have been dated at1790 þ 17 Ma and 1718 þ 12 Ma, respectively (Mccourt et al.,2004).

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

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 NorthernEspinhaço range (NE) and Chapada Diamantina (CD) in the southern portion of the Paramirim aulacogen (Modified from Inda and Barbosa, 1978; Guimarães et al., 2008; Loureiroet al., 2008). (b) Geological map of the Southern Espinhaço range (SE) in the southwestern portion of the Araçuaí belt (Modified after Uhlein et al., 1998).

F.F. Alkmim, M.A. Martins-Neto / Marine and Petroleum Geology 33 (2012) 127e139134

5. The Macaúbas sequence

According to Martins-Neto (2007, 2009) and Martins-Neto andHercos (2002), the Macaúbas 1st-order sequence is composed ofcontinental glaciogenic rift to siliciclastic-carbonatic passivemargin 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 andParamirim aulacogen. In the exposures of the São Francisco basin thesequence is represented by the Jequitaí Formation, a relatively thinpackage (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 andHercos, 2002). In the Chapada Diamantina (eastern Paramirimaulacogen) the sequence also consists of a relatively thin succession(ca. 230 m) of glaciomarine and lacustrine sediments (BebedouroFormation) (Montes et al.,1985; Guimarães et al., 2008),whereas thecorrelative unit in the northern Espinhaço range (western Para-mirim aulacogen), the Santo Onofre Group, is made up of ca.1500mof siliciclastic turbidites (Danderfer Filho and Dardenne, 2002).

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

As the type unit of the external fold-thrust belt of the Araçuaí-WestCongo orogen (Fig. 7), the Macaúbas sequence is made up ofsandstones, pelites, diamictites, carbonates, basic volcanics, andbanded iron formations metamorphosed under greenschist toamphibolite facies conditions (Uhlein et al., 1999; Pedrosa-Soareset al., 2001, 2008; Martins-Neto et al., 2001; Tack et al., 2001). Inthis context, the Macaúbas sequence represents pre-rift, rift, tran-sitional and passive margin stages of the development of a Red Seatype basin, the Macaúbas basin, formed between the São Franciscopeninsula and the Congo continent in the course of the CryogenianPeriod. Closure of the Macaúbas basin around 580 Ma led to thedevelopment of the Araçuaí-West Congo orogen (Fig. 2) (Uhleinet al., 1999; Pedrosa-Soares et al., 2001, 2008; Tack et al., 2001;Alkmim et al., 2006).

The onset of theMacaúbas rifting is associated with a ca. 850Marift-related magmatic event documented in the craton interior andin 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 zirconsdated at ca. 864 and 880 Ma by Pedrosa-Soares et al. (2000) andRodrigues (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 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 markedwith a star denote ages of the youngest detrital zircons found in the unit (see the text for explanations and references) (based on Guimarães et al., 2008; Loureiro et al., 2008;Danderfer et al., 2009).

F.F. Alkmim, M.A. Martins-Neto / Marine and Petroleum Geology 33 (2012) 127e139 135

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

Figure 9. Interpreted reflection seismic section across the São Francisco craton (see Fig.sequences of the Macaúbas correlative units (modified after Martins-Neto, 2005, 2009). See averging to the craton interior (Vertical scale in two-way-travel time).

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

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

2 for location) showing the recognized 1st-order sequences, as well as lower-orderlso the tectonic overprint of western Brasília and eastern Araçuaí fold-thrust belts, both

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; numbersmarked with a star denote ages of the youngest detrital zircons found in the unit. See the text for explanations and references).

F.F. Alkmim, M.A. Martins-Neto / Marine and Petroleum Geology 33 (2012) 127e139136

data also allow the subdivision of the 1st-order sequence into two2nd-order sequences: rift and passive margin. The passive margin2nd-order sequence can be further subdivided into three 3rd-ordersequences. The unit displays an overall wedge shape, withmaximum thickness reaching ca. 6000 m in the westernmost partof the section, close to the Brasilia belt. The outcropping blackshales/marls with intervals showing high TOC in the Canastra andVazante groups comprise one of the highest potential hydrocarbonsource rocks of the São Francisco basin (Martins-Neto, 2005, 2007,2009).

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

The Macaúbas glaciogenic sediments, deposited between864 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), couldbe viewed as a manifestation of the Sturtian global glaciation,analogous to the way Frimmel et al. (2006) interpreted the LowerMixtite Formation of the West Congolian Supergroup.

6. The Bambuí sequence

The Bambuí Group, composed of alternating siliciclastic andcarbonate deposits, is the main fill unit of the São Francisco basin.Also occurring within large synformal structures of the Paramirimaulacogen (Fig. 7), the Bambuí Group comprises a 1st-order

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

The Bambuí sequence can be subdivided into three 2nd-odercoarsening upward and progradational sequences (Fig. 10), whosefacies architecture are distinct in the western and eastern sectors ofSão Francisco basin. Thewestern portion, acting as a foredeep to theBrasília orogenic front, was filled mainly by siliciclastics, whilst theeastern 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 upwardsequences of pelites and shallowwater carbonates (Misi and Veizer,1998; Misi et al., 2007).

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

- The Carrancas conglomerates, with a maximum depositionalage of 1431 þ 68 Ma (Rodrigues, 2008) represent fluvialdeposits filling valleys carved in the cratonic basement (Vieiraet al., 2007).

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

F.F. Alkmim, M.A. Martins-Neto / Marine and Petroleum Geology 33 (2012) 127e139 137

(Babinski et al., 2007), show very low organic matter contentand negative d13C values (Misi et al., 2007). They have beeninterpreted as a Sturtian cap carbonate (Vieira et al., 2007).

- The middle and upper portions of the Sete Lagoas Formationconstitute a distinct transgressive to regressive sequence,characterized by higher organic matter content and positived13C values (Martins and Lemos, 2007; Vieira et al., 2007).Furthermore, detrital zircons extracted from these intervalsyield a maximum depositional age of 610 Ma (Rodrigues, 2008;Pimentel et al., 2011).

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

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

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

As described in the previous sections, the Proterozoic 1st-ordersequences exposed in the São Francisco craton and its marginscontain the record of basin-forming events that took place indistinct scenarios in the time interval between the beginning of thePaleoproterozoic (w2.5 Ga) and the end of the Ediacaran period ofthe 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 Archeannuclei of the present-day São Francisco and Congo cratons,between 2.5 Ga and 2.1 Ga. The convergence and collision of thesenuclei 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-Itacolomisequence and its correlatives. These events seem to reflect theassembly of a supercontinent during the Orosirian period of thePaleoproterozoic 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 andCongo cratons around 1.75 Ga. Currently referred to as the Espin-haço rifting or Staterian taphrogenesis (Brito Neves et al., 1996) inthe Brazilian literature, this extensional episode is associated withthe extrusion of bimodal volcanics and the deposition of the lowerEspinhaço I sequence in the São Francisco craton and its margins.Renewed rifting around 1.57 Ga (Danderfer et al., 2009) led toreactivation of pre-existing extensional fabrics and deposition ofthe lower Espinhaço II sequence in the Paramirim aulacogen.

The 1.57 Ga rifting eventwas followed by sag basin developmentand marine incursions in the regions represented by the SãoFrancisco craton and its margins, as recorded by the upper Espin-haço II sequence. However, the Mesoproterozoic history of the SãoFrancisco-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 anisolated piece (e.g. Hoffman, 1991; Weil et al., 1998; Pisarevskyet al., 2003).

Regardless of its relationship to Rodinia, São Francisco-Congobecame an individual continent surrounded by passive marginssometime during the Cryogenian period, as indicated by the sedi-mentary record of the Macaúbas sequence and its correlatives. Theonset of the Macaúbas rifting in the São Francisco craton has beenrelated to a ca. 850Ma bimodal magmatic event documented in theSão Francisco craton and its margins (Pedrosa-Soares et al., 2008).In the western margin of the Congo craton, this event was precededby two magmatic episodes, the basic and acid volcanism of theZadianian and Mayumbian groups, respectively, whose ages fall inthe interval of 1000e920Ma (Tack et al., 2001). The evolution of theMacaúbas rifting during the Cryogenian period resulted in thedevelopment of a large gulf, partially floored by oceanic crust thatseparated the São Francisco peninsula from the Congo continent(Pedrosa-Soares et al., 2008).

The São Francisco-Congo continent and various fragmentsderived from Rodinia started to converge by the end of the Cry-ogenian period. Their reassembly resulted in the formation ofGondwana during the Ediacaran period. In this process the marginsof the São Francisco-Congo were diachronically converted into theBrasiliano/PanAfrican orogenic belts; its interior, affected bya general marine transgression in the time after 610 Ma eventuallybehaved as a foreland basin, receiving the sediments of the Bambuísequence and its correlatives.

8. Final remarks

The study of the Proterozoic sedimentary record of the SãoFrancisco craton performed by several authors in the last few yearsallows the synthesis presented in the previous sections. However,many questions related to the geological history and trajectory ofthe São Francisco-Congo craton during the Proterozoic remainopen, demanding further investigations. Some of these questionsare: (i) does the breccia/conglomerate layer of the GandarelaFormation of the lower Minas Supergroup record the Paleoproter-ozoic global glaciation event?; (ii) what is the age and tectonicsignificance of the unconformity bounded 2nd-order sequencerepresented by the Piracicaba Group of the middle Minas-Itacolomisequence?; (iii) what is the age and tectonic significance of theupper 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 grouprelated to the Macaúbas glacial sediments?

Acknowledgments

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

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