Proterozoic First-Order Sedimentary Sequences of the Sao Francisco

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, eastern Brazil

Fernando F. Alkmima,*, Marcelo A. Martins-Netob 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 article info abstract

Article history: The São Francisco craton in eastern Brazil hosts sedimentary sequences deposited between the Paleo- Received 4 May 2010 archean (w3300 Ma) and Late Neoproterozoic (w580 Ma). Proterozoic successions occurring in this Received in revised form region comprise ﬁve 1st-order sedimentary sequences, which besides episodes of global signiﬁcance 15 August 2011 record major basin-forming events. The ca. 8000 m-thick Minas-Itacolomi 1st-order sequence, exposed Accepted 18 August 2011 in the Brazilian mining district of the Quadrilátero Ferrífero and containing as marker bed the Lake Available online 5 September 2011 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- Keywords: First-order sequences forming events, which affected the host continent of the São Francisco craton at around 1.75 Ga and Proterozoic 1.57 Ga. The Macaúbas sequence and its correlatives in the extracratonic domains witness the individ- São Francisco craton ualization of a São Francisco-Congo plate in synchronicity with the break-up of Rodinia in the Cryogenian Brazil 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. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction accretionary material (Brito Neves et al., 1999; Almeida et al., 2000; Alkmim et al., 2001). The best preserved and most intensively studied Precambrian We selected for discussion in this paper ﬁve Proterozoic 1st- sedimentary record of South America is exposed in a relatively order sequences exposed in the São Francisco craton region. The small area of eastern Brazil, which corresponds to the São Francisco sequences chosen for discussion record not only the Proterozoic craton and its margins (Fig. 1). The São Francisco craton (Almeida history of the São Francisco craton, but also tectonic and climatic et al., 2000) hosts sedimentary successions accumulated in the events of global signiﬁcance. Archean sedimentary successions, time interval between the Neoarchean (w2800 Ma) and Late although present in many sectors of the craton, are tectonically Neoproterozoic (w580 Ma). dismembered and demand further detailed studies. For this reason The Precambrian nucleus of the South American continent, they will be not addressed in this paper. consisting essentially of Archean to Paleoproterozoic rocks with As a consequence of its previous residence in Gondwana, the São minor additions of Meso- and Neoproterozoic juvenile material, Francisco craton has an African counterpart (Fig. 1), which is rep- comprises cratons and Neoproterozoic orogenic belts (Fig. 1). The resented by the Congo craton. The link between the São Francisco São Francisco together with three other cratons of South America and Congo cratons is made by a crustal bridge that connects the corresponds to the internal portions of the plates involved in the Paleoproterozoic Eastern Bahia orogenic domain in Brazil to the assembly of West Gondwana by the end of the Proterozoic Era. The Paleoproterozoic Francevillian or West Central African belt in Neoproterozoic or Brasiliano orogenic belts, on the other hand, Gabon (Fig. 1). This piece of crust remained intact from the middle encompass the margin of those plates and the intervening 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., * Corresponding author. Tel.: þ55 31 3559 1849; fax: þ55 31 3559 1600. 1992, 2001). For this reason, correlatives of the focused Brazilian E-mail addresses: [email protected], [email protected] (F.F. Alkmim). 1st-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: 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).

Special emphasis is given to the correlations between the Neo- Proterozoic and Phanerozoic sedimentary units. The ca. 2.1 Ga proterozoic successions exposed on both cratons. Mineiro belt (Teixeira and Figueiredo, 1991; Teixeira et al., 2000) The concept of 1st-order sequence adopted in the present paper that bounds the Archean block to the south involves the Archean was postulated by Martins-Neto et al. (2001) and Catuneanu et al. basement, Paleoproterozoic passive margin to foreland basin units, (2005). Since Precambrian basins are often characterized by poor and voluminous granitoids emplaced between 2.25 and 2.02 Ga stratal preservation and by a general lack of time control, an (Machado et al., 1996; Teixeira et al., 2000; Noce et al., 2000; Ávila approach based on the stratigraphic hierarchy is considered the et al., 2010). The famous Brazilian mining district known as the best way to interpret the stratigraphic framework. Stratigraphic Quadrilátero Ferrífero (“Iron Quadrangle”)(Dorr, 1969) is located in hierarchy involves the separation of different orders of sequences the foreland domain of the Mineiro belt and corresponds to the and bounding surfaces based on their relative importance. In this type area of the Paleoproterozoic Minas-Itacolomi sequence, dis- context, changes in the tectonic setting provide the key criterion for cussed in the next section. the basic subdivision of the rock record. Thus, a first-order The Proterozoic cover units of the São Francisco craton occur in sequence corresponds to an entire sedimentary basin-fill succes- two distinct tectonic domains: the São Francisco basin and the sion, regardless of the origin and life span of each particular basin. Paramirim aulacogen (Fig. 2). The poly-historic São Francisco basin This first-order basin-fill succession can be in turn subdivided into extends over ca. 800 km along the NS-trending lobe of the craton second- and lower-order sequences, based on the steps involved in and is bounded by emergent thrusts of the adjacent Neoproterozoic the tectonic evolution of the basin, irrespective of the time span orogenic belts, except to the south and northeast (Dominguez, between two same-order consecutive events. 1993; Alkmim and Martins-Neto, 2001; Zalán and Romeiro-Silva, 2007). The southern limit is erosional. To the northeast the basin 2. The São Francisco craton is bounded by the Paramirim corridor, a Neoproterozoic intracratonic deformation zone, which affects the neighboring Para- In the continent interior, the São Francisco craton is bounded by mirim aulacogen (Alkmim and Martins-Neto, 2001; Cruz and the Neoproterozoic Brasília, Araçuaí, Rio Preto, Riacho do Pontal Alkmim, 2006). The Precambrian basin-fill units comprise the and Sergipano orogenic belts (Fig. 2)(Almeida et al., 1981). Along Espinhaço and São Francisco supergroups, of Paleo/Mesoproter- the eastern Brazilian coast, the craton boundary is made up by ozoic and Neoproterozoic ages, respectively. The Phanerozoic Cretaceous passive margin basins, developed on the cratonic bridge section contains the glacial sediments of the Permo-carboniferous that once linked the São Francisco and Congo blocks. Santa Fé Group (Campos and Dardenne, 1994), as well as the The basement of the São Francisco craton consists of an Archean Cretaceous rift-sag succession of the Areado, Urucuia, and Mata da nucleus and two segments of a Paleoproterozoic orogen (Alkmim Corda Groups (Sgarbi et al., 2001). and Marshak, 1998; Teixeira et al., 2000; Barbosa and Sabaté, The Paramirim aulacogen (sensu Cruz and Alkmim, 2006) 2004)(Fig. 1). The Archean block, which corresponds to the NS- encompasses two morphotectonic domains of the São Francisco elongated segment of the craton, is almost entirely covered by craton: the northern Espinhaço range and a large plateau referred F.F. Alkmim, M.A. Martins-Neto / Marine and Petroleum Geology 33 (2012) 127e139 129

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 of Fig. 5; the dotted line the location of the seismic section shown on Fig. 9.

to as the Chapada Diamantina (Fig. 2). Developed around 1.75 Ga, West Congolian Supergroup discussed further in this paper, occurs in the Paramirim aulacogen experienced a long subsidence history in the foreland domain of the West Congolian belt, as well as in the the course of the Proterozoic, which is also recorded by the Espin- Sangha Aulacogen or Comba basin (Trompette, 1994; Alvarez and haço and São Francisco supergroups (Schobbenhaus, 1996; Maurin, 1991), a partially inverted, NE-trending rift (Fig. 3). Dardenne, 2000; Danderfer et al., 2009). During the Neo- The 1st-order sequences addressed in the next sections are proterozoic Brasiliano event, the Paramirim aulacogen underwent (Fig. 4): (i) the Paleoproterozoic Minas-Itacolomi sequence; (ii) the partial inversion. A system of double-verging and NNW-trending Paleo/Mesoproterozoic Espinhaço sequences; (iii) the Neo- faults and folds developed in the aulacogen trough and became proterozoic Macaúbas sequence, and (iv) the Neoproterozoic its most prominent fabric elements (Cruz and Alkmim, 2006). Bambuí sequence. For each sequence we describe the lithologic The African counterpart of the São Francisco, the Congo craton, content, boundaries, sedimentologic aspects, and correlative units, extends over the large area of the Congo, Cubango, Zambezi, and discussing in addition their ages and tectonic signiﬁcance. Cunene river basins in Central Africa. Fringed by the Oubanguides, Kibaran, Damara and West Congolian belts (Fig. 3), the Congo craton 3. The Minas-Itacolomi sequence consists of a large Archean nucleus bounded to northwest by the Paleoproterozoic West Central African belt (Trompette, 1994; Lying unconformably on top of an Archean greenstone belt, Feybesse et al., 1998). Approximately two thirds of the cratonic the Minas Supergroup together with the Itacolomi Group (Dorr, domain is covered by Phanerozoic and Precambrian sedimentary 1969) represents a ca. 8.000 m-thick 1st-order sequence, which units. The Archean basement is exposed in the shield areas of Angola, tracks the operation of a Wilson cycle in the early Paleoproterozoic, Cameroon-Gabon-Congo, Zaire and Kasai (Fig. 3)(Trompette, 1994). between 2.5 and 2.0 Ga (Alkmim and Marshak, 1998). Minas and The Precambrian cover section, which includes the Neoproterozoic Itacolomi strata are deformed and metamorphosed to lower 130 F.F. Alkmim, M.A. Martins-Neto / Marine and Petroleum Geology 33 (2012) 127e139

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) (after Trompette, 1994; Alvarez, 1995). greenschist to lower amphibolites facies conditions in their type The ca. 400 m-thick package of marine sediments that includes area, the mining district of the Quadrilátero Ferrífero (QF) (Herz, banded iron formations and carbonates (Itabira Group) (Dorr, 1978)(Fig. 5). The succession comprises, as follows, three 2nd- 1969) that record the full development the passive margin order sequences (Fig. 6): basin along the southern edge of the ancient craton (Alkmim and Marshak, 1998); - (1) A ca. 1450 m-thick package of continental to marine sedi- The ca. 450 m-thick pile of shallow marine to deep water ments (Dorr, 1969; Renger et al., 1995) that represents the sediments (Piracicaba Group) (Dorr, 1969; Renger et al., 1995), development stage of the passive margin basin; consisting mainly of siliciclastics with minor carbonates. - (2) an up to 3500 m-thick package of turbiditic pelites, wackes, lithic conglomerates, and diamictites (Sabará Group), repre- The regional 2nd-order unconformities that separate the three senting syn-orogenic sediments (Dorr, 1969; Renger et al., 2nd-order sequences mentioned above were recognized and 1995; Reis et al., 2002) shed from a colliding magmatic arc mapped already in the pioneering work by Dorr (1969). and spread over an evolving foreland basin onto the craton The maximum age of the basal sandstones of the Minas- margin around 2.1 Ga; Itacolomi sequence, given by detrital zircons, is 2584 þ 10 Ma - (3) an up to 1800 m-thick section of alluvial sandstones, (Hartmann et al., 2006). Limestones from the middle portion of conglomerates and minor pelites (Itacolomi Group) (Dorr, the Gandarela Formation (Fig. 6) yielded a PbePb depositional age 1969) deposited in intramontane grabens, during the collapse of 2420 þ 19 Ma (Babinski et al., 1995), whereas the ages obtained phase of the Paleoproterozoic orogen developed along the from detrital zircons extracted from the Sabará and Itacolomi margin of the Archean nucleus of the São Francisco craton groups were 2125 þ 4 Ma and 2059 þ 58 Ma, respectively (Alkmim and Marshak, 1998). (Machado et al., 1996). According to these data, the development of the Minas passive margin, as recorded by the two oldest 3rd-order The basal 2nd-order succession can be subdivided into three sequences, took place in the time interval between 2.6 and 2.4 Ga 3rd-order sequences, which are (Fig. 6): and progressed afterward with deposition of another third 3rd- order sequence (Piracicaba Group, Fig. 6), whose depositional age is A ca. 600 m-thick package, consisting of alluvial to marine not constrained. The inversion of the Minas passive margin sandstones, conglomerates and subordinate pelites, comprising occurred only ca. 300 Ma later, as registered by the deposition of the Tamanduá and Caraça groups (Dorr, 1969), which represent the syn-orogenic Sabará turbidites (Machado et al., 1996; Reis et al., the rift and transitional phases of the passive margin devel- 2002), which also mark a major change in the source of Minas opment (Renger et al., 1995; Alkmim and Marshak, 1998); sediments. Paleogeographic studies carried out in the QF (Dorr, F.F. Alkmim, M.A. Martins-Neto / Marine and Petroleum Geology 33 (2012) 127e139 131

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).

1969; Renger et al., 1995; Machado et al., 1996) indicate Archean values in the range of 0.3 and 0.4 permil for the dolomites imme- sources located to the north for the rift to passive margin 2nd- diately beneath and above the breccia layer. Early Paleoproterozoic order sequence. A distinct and non-cratonic source located to glacial events, some of them of global signiﬁcance, took place in the south and southeast is required to explain the presence of 2.1 Ga time interval of 2.4e2.2 Ga (e.g., Hambrey and Harlam, 1981; Evans old granitoid clasts and zircons in the Sabará and Itacolomi et al., 1997; Kirschvink et al., 2000). The Gandarela breccias are, in conglomerates (Dorr, 1969; Machado et al., 1996; Alkmim and our view, candidates for representing one of these events, a matter Marshak, 1998; Reis et al., 2002). that demands further investigation. The previously mentioned geochronologic data also imply that Correlatives of the Minas-Itacolomi sequence are: 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 - The Jacobina Group consisting of quartzites, conglomerates and estimation is in agreement with the depositional age interval of the subordinate pelites that occurs in the foreland domain of the Siderian banded iron formations worldwide (Klein, 2005; Clout and Paleoproterozoic Eastern Bahia orogenic zone in the northern Simonson, 2005). According to several authors, the Siderian banded São Francisco craton (Ledru et al., 1994, 1997). iron formations record the onset of the atmospheric oxygenation - The Ogooué, Nyong, Ayna, and Francevillian units, which process (e.g., Cloud, 1973; Konhauser et al., 2002; Guo et al., 2009). comprise a 2.5 to 2.0 Ga succession exposed in the internal and Also noteworthy, is a 20 cm to 50 m-thick conglomerate and foreland domains of the Paleoproterozoic West Central African breccia layer, consisting of carbonate, chert, banded iron formation, belt in Gabon (Feybesse et al., 1998). 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 4. The Espinhaço sequences performed in the type section of the Gandarela Formation by Bekker et al. (2003) yielded d13C positive values between 0.2 and The quartz-arenite dominated package of the Paleo/Meso- 0.4 permil for the dolomites below the breccia layer and negative proterozoic Espinhaço Supergroup was deposited in a rift-sag 132 F.F. Alkmim, M.A. Martins-Neto / Marine and Petroleum Geology 33 (2012) 127e139

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.

successor basin, nucleated in the continental mass that hosted the which reflect the evolution of two superimposed rift-sag basins present-day São Francisco craton and its margins in the Staterian in the time intervals of 1.75e1.57 Ga and 1.57e0.9 Ga (Figs. 4 and period, around 1.75 Ga (Uhlein et al., 1998; Brito Neves et al., 1996; 8). Martins-Neto, 2000). The Espinhaço Supergroup is the main fill unit The Espinhaço I sequence occurs in the western Chapada Dia- of the Paramirim aulacogen (Fig. 7), occurring also in the core of mantina and along the eastern escarpment of the northern Espin- some antiformal structures of the São Francisco basin. Its type area, haço range (Fig. 7), regions that correspond to the ancient trough of the Southern Espinhaço range, however lies outside of the São the Paramirim aulacogen. A ca. 600 m-thick pile of rhyolites, dacites Francisco craton. The Southern Espinhaço range corresponds to the and volcaniclastics (Novo Horizonte and São Simão formations, NS-trending segment of the Neoproterozoic Araçuaí fold-thrust Fig. 8) that grade upward into an 850 m-thick package of alluvial to belt that fringes the craton to the east (Fig. 7). lacustrine sediments forms the oldest 2nd-order rift sequence. Studies conducted by Martins-Neto (2000, 2007, 2009) in the Fluvial and lacustrine deposits (550e650 m) cover the volcanic southern Espinhaço range led to the conclusion that the Espinhaço succession, which is locally underlain by alluvial/aeolian sand- Supergroup comprises a single 1st-order sequence, encompassing stones and conglomerates, representing pre-rift deposits (Algodão pre-rift, rift, transitional and sag evolutionary stages of an intra- and Serra da Gameleira formations, Fig. 8)(Danderfer Filho and continental basin. However, the first synthesis of the geology of the Dardenne, 2002; Guimarães et al., 2008; Loureiro et al., 2008). northern São Franciso craton (Inda and Barbosa, 1978) already Separated from the previous sequences by an erosional unconfor- suggested a more complex stratigraphic development for the mity and marking a considerable expansion of the basin area, the Espinhaço Supergoup in the Paramirim aulacogen. In fact, sedi- 1000e2500-thick 2nd-order sag sequence of the Paraguaçu Group mentological, stratigraphical and geochronological studies pub- (Fig. 8) includes aeolian and marine deposits (Guimarães et al., lished in the last few years (Danderfer Filho and Dardenne, 2002; 2008; Loureiro et al., 2008). Guimarães et al., 2008; Danderfer et al., 2009), indicate that the The nucleation of the Espinhaço rift took place at about 1.75 Ga, Espinhaço package in the Paramirim aulacogen comprises at least as indicated by the age of the basal volcanic rocks (Schobbenhaus two unconformity bounded 1st-order sequences, here referred to as et al., 1994; Brito Neves et al., 1996; Babinski et al., 1994; Espinhaço I and II sequences (Fig. 8). Furthermore, a geochrono- Danderfer et al., 2009) of the Novo Horizonte and São Simão logical investigation recently performed in the southern Espinhaço formations (Fig. 8). The sag stage did not persist after 1.57 Ga, the range by Chemale et al. (2010) revealed that one of the unconfor- age of a second episode of basic magmatism (Babinski et al., 1999; mities identified by Martins-Neto (2000) in the lower portion of the Battilani et al., 2007; Loureiro et al., 2008; Danderfer et al., 2009) supergroup corresponds to an hiatus of at least 500 Ma. recorded by the basal unit of the Espinhaço II sequence that marks The Espinhaço I and II 1st-order sequences can be in turn the onset of renewed extensional tectonism in the Paramirim subdivided into five 2nd-order unconformity bounded sequences, aulacogen. F.F. Alkmim, M.A. Martins-Neto / Marine and Petroleum Geology 33 (2012) 127e139 133

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).

Alluvial sandstones, diamond-bearing conglomerates and basic Formation and Conselheiro Mata Group (corresponding to the volcanics cut by dikes and sills of the same nature form the base of homonymous 2nd-order sequences of Martins-Neto, 2000, 2009) the Espinhaço II 1st-order sequence in the Chapada Diamantina suggests that these units form a single rift-sag sequence, whose (Dominguez, 1993; Battilani et al., 2007; Guimarães et al., 2008) maximum age is 1.2 Ga (Chemale et al., 2010). Thus, these two (Figs. 7 and 8). This ca. 400 m-thick succession grades upward into groups of units represent the Espinhaço I and II 1st-order aeolian deposits, which are in turn covered by marine pelites and sequences, respectively. sandstones containing intercalations of carbonates and minor The Araí and Rio Preto Groups (Brito Neves et al., 1996) exposed evaporites. Fluvial to estuarine deposits lying on an erosional in the Brasília and Rio Preto belts on the northwestern margin of unconformity on top of the marine pelites (Dominguez, 1993) the São Francisco craton are correlatives to the Espinhaço I represent the youngest Espinhaço 2nd-order sequence in the sequence. Chapada Diamantina (Morro do Chapéu Formation, Fig. 8). Acid to In the Congo craton region, correlatives of the Espinhaço intermediate volcanics (Bomba Formation, Danderfer et al., 2009) sequences are: (Fig. 8), covered by alluvial/aeolian sandstones, form the base of the Espinhaço II 1st-order sequence in the northern Espinhaço range. - The Chela Group, consisting of a ca. 600 m-thick succession of These units are in turn overlain by a ca. 700 m-thick package of marine sandstones interbedded with acid volcanic, pelites, shallow marine and deltaic deposits. conglomerates and subordinate carbonates that cover Archean Zircons extracted from the volcanics of the Bomba Formation basement of the Angola block in the southwestern portion of yield UePb SHRIMP ages of 1582 þ 8 Ma and 1569 þ 14 Ma the craton (Torquato and Fogaça, 1981; Pedreira and De Waele, (Danderfer et al., 2009). According to these authors the dates 2008)(Figs. 3 and 4). Basal acid volcanics and volcaniclastics of obtained constrain the timing of a second phase of rifting acting the middle portion of the sequence have been dated at upon the pre-existing Espinhaço basin at around 1570 Ma. 1790 þ 17 Ma and 1718 þ 12 Ma, respectively (Mccourt et al., According to the geochronological data recently obtained by 2004). Chemale et al. (2010) in the type area of the Espinhaço Supergroup, - The Kibaran Supergroup exposed along the eastern margin the basal Bandeirinha and São João da Chapada formations (the of the craton (Fig. 3), which is made up of a very thick package syn-rift Olaria, Natureza and São João da Chapada 2nd-order of pelites that grade upward into sandstones. The maximum sequences of Martins-Neto, 2000, 2009) were deposited around age of the middle portion of the supergroup is estimated 1.75 Ga. The age spectrum obtained from detrital zircons extracted at ca. 1.38 Ga (Kokonyangi et al., 2004; Pedreira and De Waele, from the overlaying Sopa-Brumadinho Formation, Galho do Miguel 2008). 134 F.F. Alkmim, M.A. Martins-Neto / Marine and Petroleum Geology 33 (2012) 127e139

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 from Inda 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 after Uhlein et al., 1998).

5. The Macaúbas sequence 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 According to Martins-Neto (2007, 2009) and Martins-Neto and sandstones, pelites, diamictites, carbonates, basic volcanics, and Hercos (2002), the Macaúbas 1st-order sequence is composed of banded iron formations metamorphosed under greenschist to continental glaciogenic rift to siliciclastic-carbonatic passive amphibolite facies conditions (Uhlein et al., 1999; Pedrosa-Soares margin deposits in the São Francisco craton and its margins (Figs. 4, et al., 2001, 2008; Martins-Neto et al., 2001; Tack et al., 2001). In 9 and 10). this context, the Macaúbas sequence represents pre-rift, rift, tran- The Macaúbas sequence comprises the oldest known Neo- sitional and passive margin stages of the development of a Red Sea proterozoic sedimentary units of both the São Francisco basin and type basin, the Macaúbas basin, formed between the São Francisco Paramirim aulacogen. In the exposures of the São Francisco basin the peninsula and the Congo continent in the course of the Cryogenian sequence is represented by the Jequitaí Formation, a relatively thin Period. Closure of the Macaúbas basin around 580 Ma led to the package (ca. 180 m) of glaciogenic deposits, including diamictites, development of the Araçuaí-West Congo orogen (Fig. 2)(Uhlein sandstones and pelites (Dardenne, 1978; Montes et al., 1985; et al., 1999; Pedrosa-Soares et al., 2001, 2008; Tack et al., 2001; Karfunkel and Hoppe, 1988; Uhlein et al., 1999; Martins-Neto and Alkmim et al., 2006). Hercos, 2002). In the Chapada Diamantina (eastern Paramirim The onset of the Macaúbas rifting is associated with a ca. 850 Ma aulacogen) the sequence also consists of a relatively thin succession rift-related magmatic event documented in the craton interior and (ca. 230 m) of glaciomarine and lacustrine sediments (Bebedouro in the Araçuaí-West Congo Orogen (Pedrosa-Soares et al., 2008; Formation) (Montes et al.,1985; Guimarães et al., 2008), whereas the Silva et al., 2008; Danderfer et al., 2009). Furthermore, glacio- correlative unit in the northern Espinhaço range (western Para- genic sediments of the Macaúbas sequence contain detrital zircons mirim aulacogen), the Santo Onofre Group, is made up of ca. 1500 m dated at ca. 864 and 880 Ma by Pedrosa-Soares et al. (2000) and of siliciclastic turbidites (Danderfer Filho and Dardenne, 2002). Rodrigues (2008), respectively. However, the full development of the Macaúbas sequence can The Vazante and Canastra Groups as well as the Cubatão Dia- be observed outside the craton, in the Araçuaí-West Congo orogen. mictite (basal unit of Ibiá Group) (Figs. 4 and 7)(Dardenne, 2000; F.F. Alkmim, M.A. Martins-Neto / Marine and Petroleum Geology 33 (2012) 127e139 135

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 on Guimarães et al., 2008; Loureiro et al., 2008; Danderfer et al., 2009).

Azmy et al., 2006, 2008) exposed along the western boundary of sandstone intercalations toward the top, characterizing an overall the São Francisco basin and in the adjacent Brasília belt, are transgressive sequence (Dardenne, 2000). Provenance studies potential correlatives of the Macaúbas 1st-order sequence. The conducted in the Canastra Group indicated Mesoproterozoic rocks Vazante Group is composed of intercalated carbonates and pelites as its main source (Valeriano et al., 2004) and a maximum depo- containing sandstones and diamictites at the base. Available sitional age of 1040 Ma (Rodrigues, 2008; Pimentel et al., 2011). The geochronologic and provenance studies (Pimentel et al., 2001, basal diamictites of the Ibiá Group, interpreted as glacial sediments 2011; Coelho et al., 2008; Rodrigues, 2008) suggest that the (Pereira et al., 1994), were deposited between 850 and 640 Ma, as Vazante sediments, shed from a cratonic source, accumulated in the indicated by the ages of detrital zircons obtained by Rodrigues time between 925 and 750 Ma. The Canastra Group, exposed in (2008) and Pimentel et al. (2011). a relatively large area in the central and southern Brasilía belt Seismic data collected in the São Francisco basin indicate that consists of a ca. 800 m succession of greenschist facies pelites with the Macaúbas 1st-order sequence together with its correlatives carbonate lenses at the base, followed by black shales and pelite/ occur regionally, covering pre-Neoproterozoic rocks (Fig. 9). These

Figure 9. Interpreted reﬂection seismic section across the São Francisco craton (see Fig. 2 for location) showing the recognized 1st-order sequences, as well as lower-order sequences of the Macaúbas correlative units (modiﬁed after Martins-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). 136 F.F. Alkmim, M.A. Martins-Neto / Marine and Petroleum Geology 33 (2012) 127e139

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). data also allow the subdivision of the 1st-order sequence into two sequence that marks the downwarping behavior of a substantial 2nd-order sequences: rift and passive margin. The passive margin part of the São Francisco craton during a high sea-level event in the 2nd-order sequence can be further subdivided into three 3rd-order time after 630 Ma. As a consequence of the development of the sequences. The unit displays an overall wedge shape, with Brasília belt, the craton domain was then converted into a foreland maximum thickness reaching ca. 6000 m in the westernmost part basin (Castro and Dardenne, 2000; Alkmim and Martins-Neto, of the section, close to the Brasilia belt. The outcropping black 2001; Martins-Neto, 2007, 2009). shales/marls with intervals showing high TOC in the Canastra and The Bambuí sequence can be subdivided into three 2nd-oder Vazante groups comprise one of the highest potential hydrocarbon coarsening upward and progradational sequences (Fig. 10), whose source rocks of the São Francisco basin (Martins-Neto, 2005, 2007, facies architecture are distinct in the western and eastern sectors of 2009). São Francisco basin. The western portion, acting as a foredeep to the The rift-related sediments of the Sansikwa Subgroup together Brasília orogenic front, was filled mainly by siliciclastics, whilst the with the glaciogenic Lower Mixtite Formation and the passive eastern sector, representing the flexural ramp, received fine- margin succession of the Haut Schiloango Subgroup (Tack et al., grained siliciclastics interbedded with shallow water carbonates 2001; Frimmel et al., 2006) are the correlatives of the Macaúbas (Martins-Neto, 2007, 2009). The succession exposed in the Para- sequence in the West Congolian belt (Pedrosa-Soares et al., 2008). mirim aulacogen is also made up of 2nd-order coarsening upward Detrital zircons extracted from the sandstones of the Sansikwa sequences of pelites and shallow water carbonates (Misi and Veizer, Subgroup yield a maximum depositional age of 923 þ 43 Ma 1998; Misi et al., 2007). (Frimmel et al., 2006). Discontinuous beds of conglomerates, breccias and pelites The Macaúbas glaciogenic sediments, deposited between (Carrancas Formation, Fig. 10) overlain by limestones showing 864 Ma (age of the youngest detrital zircon so far found in the unit, pseudomorphs of aragonite fans (Sete Lagoas Formation, Fig. 10) Pedrosa-Soares et al., 2000) and 740 Ma (age of the basal carbon- mark the base of the Bambuí Group in some areas along the ates of the overlying Bambuí sequence, Babinski et al., 2007), could southern border of the São Francisco basin, where the Macaúbas be viewed as a manifestation of the Sturtian global glaciation, sequence is absent. Sedimentological, geochemical and geochro- analogous to the way Frimmel et al. (2006) interpreted the Lower nological studies carried out in the last few years on these units led Mixtite Formation of the West Congolian Supergroup. to the following conclusions:

6. The Bambuí sequence - The Carrancas conglomerates, with a maximum depositional age of 1431 þ 68 Ma (Rodrigues, 2008) represent fluvial The Bambuí Group, composed of alternating siliciclastic and deposits filling valleys carved in the cratonic basement (Vieira carbonate deposits, is the main fill unit of the São Francisco basin. et al., 2007). Also occurring within large synformal structures of the Paramirim - The limestones containing aragonite fan pseudomorphs at the aulacogen (Fig. 7), the Bambuí Group comprises a 1st-order base 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 content Francisco-Congo in the periphery of the supercontinent or as an and negative d13C values (Misi et al., 2007). They have been isolated piece (e.g. Hoffman, 1991; Weil et al., 1998; Pisarevsky interpreted as a Sturtian cap carbonate (Vieira et al., 2007). et al., 2003). - The middle and upper portions of the Sete Lagoas Formation Regardless of its relationship to Rodinia, São Francisco-Congo constitute a distinct transgressive to regressive sequence, became an individual continent surrounded by passive margins characterized by higher organic matter content and positive sometime during the Cryogenian period, as indicated by the sedi- d13C values (Martins and Lemos, 2007; Vieira et al., 2007). mentary record of the Macaúbas sequence and its correlatives. The Furthermore, detrital zircons extracted from these intervals onset of the Macaúbas rifting in the São Francisco craton has been yield a maximum depositional age of 610 Ma (Rodrigues, 2008; related to a ca. 850 Ma bimodal magmatic event documented in the Pimentel et al., 2011). São Francisco craton and its margins (Pedrosa-Soares et al., 2008). In the western margin of the Congo craton, this event was preceded Geochronological determinations on detrital zircons extracted by two magmatic episodes, the basic and acid volcanism of the from the middle and upper portions of the Bambuí Group resulted Zadianian and Mayumbian groups, respectively, whose ages fall in in maximum ages of 650 and 616 Ma, respectively (Rodrigues, the interval of 1000e920 Ma (Tack et al., 2001). The evolution of the 2008; Pimentel et al., 2011). Macaúbas rifting during the Cryogenian period resulted in the Correlatives of the Bambuí 1st-order sequence on the Congo development of a large gulf, partially ﬂoored by oceanic crust that craton and its western margin correspond to the upper section of separated the São Francisco peninsula from the Congo continent the West Congolian Group that includes the glaciogenic Upper (Pedrosa-Soares et al., 2008). Mixtite Formation, the shallow marine carbonates of the Schisto- The São Francisco-Congo continent and various fragments Calcaire Subgroup and the molasse deposits of the Mpioka derived from Rodinia started to converge by the end of the Cry- Subgroup (Tack et al., 2001; Frimmel et al., 2006; Pedrosa-Soares ogenian period. Their reassembly resulted in the formation of et al., 2008). The Upper Mixtite Formation, which has no equiva- Gondwana during the Ediacaran period. In this process the margins lents in the Bambuí sequence, was tentatively correlated to the of the São Francisco-Congo were diachronically converted into the Marinoan global glaciation deposits by Frimmel et al. (2006). Brasiliano/PanAfrican orogenic belts; its interior, affected by a general marine transgression in the time after 610 Ma eventually 7. Tectonic signiﬁcance of the São Francisco craton 1st-order behaved as a foreland basin, receiving the sediments of the Bambuí sequences sequence and its correlatives.

As described in the previous sections, the Proterozoic 1st-order 8. Final remarks sequences exposed in the São Francisco craton and its margins contain the record of basin-forming events that took place in The study of the Proterozoic sedimentary record of the São distinct scenarios in the time interval between the beginning of the Francisco craton performed by several authors in the last few years w Paleoproterozoic ( 2.5 Ga) and the end of the Ediacaran period of allows the synthesis presented in the previous sections. However, w the Neoproterozoic Era ( 580 Ma). many questions related to the geological history and trajectory of The oldest event, recorded by the lower portion of the Minas- the São Francisco-Congo craton during the Proterozoic remain Itacolomi sequence and its correlatives, corresponds to the devel- open, demanding further investigations. Some of these questions opment of passive margin basins along the borders of the Archean are: (i) does the breccia/conglomerate layer of the Gandarela nuclei of the present-day São Francisco and Congo cratons, Formation of the lower Minas Supergroup record the Paleoproter- between 2.5 Ga and 2.1 Ga. The convergence and collision of these ozoic global glaciation event?; (ii) what is the age and tectonic nuclei in the course of the ca. 2.1 Ga Transamazonian and Eburnean significance of the unconformity bounded 2nd-order sequence (Fig. 4) events of South America and Africa, respectively, are rep- represented by the Piracicaba Group of the middle Minas-Itacolomi resented by the upper 2nd-order sequences of the Minas-Itacolomi sequence?; (iii) what is the age and tectonic significance of the fl sequence and its correlatives. These events seem to re ect the upper marine portion of the Espinhaço II 1st-order sequence?; (iv) assembly of a supercontinent during the Orosirian period of the do the glacial sediments of the Macaúbas sequence and its correl- Paleoproterozoic Era, the Atlantica supercontinent (Rogers, 1996), atives represent the global Sturtian glacial event? In other words, followed by Columbia (Rogers and Santosh, 2004; Zhao et al., are the cap carbonates found at the base of the Bambuí group 2004), whose reconstructions, though not fully accomplished, related to the Macaúbas glacial sediments? have progressed significantly in the last few years. A system of rift basins, among them the Paramirim aulacogen, Acknowledgments nucleated in the continental plate that hosted the São Francisco and Congo cratons around 1.75 Ga. Currently referred to as the Espin- This paper resulted from studies conducted by the authors in the haço rifting or Staterian taphrogenesis (Brito Neves et al., 1996)in São Francisco craton region in the last 20 years. F.F.Alkmim (Grant the Brazilian literature, this extensional episode is associated with #307531/2009-0) and M.Martins-Neto have been supported by the the extrusion of bimodal volcanics and the deposition of the lower CNPq (Brazilian Council for Scientific and Technological Develop- Espinhaço I sequence in the São Francisco craton and its margins. ment) through these years. R Mazumder and an anonymous Renewed rifting around 1.57 Ga (Danderfer et al., 2009) led to reviewer are thanked for their comments and suggestions. The reactivation of pre-existing extensional fabrics and deposition of authors thank specially P. Eriksson for the invitation to submit this the lower Espinhaço II sequence in the Paramirim aulacogen. paper to the Marine and Petroleum Geology special issue and also The 1.57 Ga rifting event was followed by sag basin development for his review that considerably improved the original manuscript. 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 References Francisco-Congo craton and its relation to the Rodinia superconti- Alkmim, F.F., Marshak, S., 1998. Transamazonian Orogeny in the Southern São nent, assembled by the end of the era, are not well understood. Francisco Craton, Minas Gerais, Brazil: Evidence for Paleoproterozoic collision Rodinia reconstructions available in the literature show the São and collapse in the Quadrilátero Ferrífero. Precambrian Research 90, 29e58. 138 F.F. Alkmim, M.A. Martins-Neto / Marine and Petroleum Geology 33 (2012) 127e139

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