1 Journal of South American Earth Sciences 12 (1999) 453±470 Basin in lling in the southern-central part of the Sergipano Belt (NE Brazil) and implications for the evolution of Pan-African/ Brasiliano cratons and Neoproterozoic sedimentary cover Luiz J.H. D'el-Rey Silva* Universidade de BrasõÂlia-Instituto de GeocieÃncias, Campus UniversitaÂrio Darcy Ribeiro, Asa Norte, CEP , BrasõÂlia-DF, Brazil Abstract The Neoproterozoic sedimentary cover deposited across the interface of several Pan-African/Brasiliano fold-thrust belts with their respective cratons is strongly similar and has been widely correlated throughout Gondwana. In particular, the upper part of the cratonic cover of the SaÄ o Francisco Craton has been interpreted as a ring of foreland basin sediments. However, detailed studies carried out around the southern-central part of the Sergipano Belt (NE Brazil) and its interface with the northern margin of the SaÄ o Francisco Craton demonstrate that: (1) sedimentation records the evolution of a passive continental margin and is divided into two cycles (I and II), each one comprising a basal siliciclastic megasequence overlain by a carbonate megasequence; (2) the cratonic cover comprises cycle I and part of the basal megasequence of cycle II; (3) all of these rocks spread continuously across the craton margin into the Sergipano Belt, where they occur around basement domes and are overlain by a metadiamictite formation and a metacarbonate formation that complete cycle II; and (4) basement and cover underwent the same Brasiliano (670±600 Ma) compressive deformation under sub-greenschist metamorphic conditions. These data deny the foreland basin model for the cratonic sediments to the south of the Sergipano Belt and, coupled with recent data on the evolution of other margins of the craton, indicate that the Neoproterozoic sedimentary cover derives from highs existing close to the centre of the ancient SaÄ o Francisco Plate. This sedimentary cover was also in uenced by highs of an Andean-type margin that evolved ca 900±640 Ma along the western side of the plate. Such evolution also applies to the Neoproterozoic cover of other cratons of the Pan-African/Brasiliano orogeny. # 1999 Elsevier Science Ltd. All rights reserved. Resumo A cobertura sedimentar NeoproterozoÂ ica da interface entre as vaâ rias faixas dobradas Pan-Africanas/Brasilianas e seus respectivos craâ tons guarda similaridades muito fortes e tem sido amplamente correlacionada e, particularmente a parte superior da cobertura que circunda o CraÂ ton SaÄ o Francisco tem sido suposta ou interpretada como detritos oriundos da erosaä o das faixas dobradas, depositados em bacias tipo foreland. Contudo, estudos realizados na parte centro-sul da Faixa Sergipana (NE do Brasil) e sua interface com o CraÂ ton SaÄ o Francisco demonstram que: (1) A sedimentac aä o registra a evoluc aä o de margem continental passiva e eâ dividida em dois ciclos (I e II) cada um formado por megasequeã ncia basal siliciclaâ stica sobreposta por megasequeã ncia carbonaâ tica; (2) A cobertura cratoã nica compreende os sedimentos do Ciclo I e a megasequeã ncia siliciclaâ stica do ciclo II; (3) Todas essas rochas espalham-se continuamente do craâ ton para a faixa, onde ocorrem em torno de domos do embasamento e saä o sotopostas a metadiamictitos e metacarbonatos que completam o Ciclo II; e (4) Embasamento e cobertura registram a mesma deformac aä o compressiva Brasiliana, sob metamor smo maâ ximo do faâ cies xisto verde (670±600 Ma atraâ s). Esses dados negam o modelo foreland para a cobertura cratoã nica a sul da Faixa Sergipana e, combinados com outros recentemente publicados para outras margens do craâ ton, indicam que a sedimentac aä o da cobertura NeoproterozoÂ ica como um todo foi controlada por altos proâ ximos ao centro da antiga placa SaÄ o Francisco e, particularmente ao longo da borda oeste da * Fax: or address: (L.J.H. D'el-Rey Silva) /99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S (99)
2 454 L.J.H. D'el-Rey Silva / Journal of South American Earth Sciences 12 (1999) 453±470 placa, tambeâ m por altos da margem continental andina ali desenvolvida entre 900 e 640 Ma atraâ s. Tal evoluc aä o aplica-se tambeâ m aá cobertura NeoproterozoÂ ica de outros craâ tons do sistema orogeã nico Pan-Africano/Brasiliano. # 1999 Elsevier Science Ltd. All rights reserved. 1. Introduction The Sergipano Belt of NE Brazil (Fig. 1) is a triangle-shaped, ESE±WNW trending orogenic belt lying between the SaÄ o Francisco Craton to the south, and the Pernambuco-Alagoas Massif, which is part of the Borborema Province, to the north (Fig. 1; Almeida et al., 1981; Santos and Brito Neves, 1984). The belt occupies the central part of the e2000 km long megaorogen (Fig. 2) connecting the Oubanguides Belt (or North Equatorial Belt) of Africa, with the Riacho do Pontal and Rio Preto belts of Brazil (Davison and Santos, 1989; Jardim de SaÂ et al., 1992; Jardim de SaÂ, 1994; Trompette, 1994). Because the SaÄ o Francisco Craton is surrounded by several Pan-African/ Brasiliano deformation belts (Fig. 2) and because the Sergipano Belt contains basement domes mantled by metasediments with well preserved sedimentary features, lying less than 5 km away from the craton margin (the Itaporanga Fault, as will be seen ahead), understanding the tectonic evolution of the craton and Fig. 1. Major tectonic elements of northeastern Brazil: in (a) the SaÄ o Francisco Craton (SFC) and the Borborema Province (BP), and in (b) the Riacho do Pontal (RP) and Sergipano (S) belts, the latter one lying to the south of the Pernambuco-Alagoas Massif (Pe-Al). The SaÄ o Luiz Craton is also shown. PaSZ = Patos Shear Zone, PeSZ = Pernambuco Shear Zone. Based on Almeida et al. (1981) and Mascarenhas et al. (1984). See text. Fig. 2. Simpli ed map showing the main lithotectonic units around the SaÄ o Francisco-Congo and Kalahari cratons, based on Trompette (1994) and Germs (1995). Lithostratigraphy legend: (cratons) 1 = Basement, 2 = Paleo-Mesoproterozoic cover rocks, 3 = Neoproterozoic cover rocks, 4 = Late Neoproterozoic-Cambrian cover rocks; (belts) 5 = Polycyclic basement, 6 = Paleo- Mesoproterozoic metasediments, 7 = Neoproterozoic metasediments with ma c-ultrama c intercalations and indication of thrusting movement. B = BrasõÂ lia, C = Arac uaõâ, D = West Congo, E = Kaoko, F = Damara, G = Gariep, H = Malmesbury (or Vanrhynsdorp) belts. See text.
3 L.J.H. D'el-Rey Silva / Journal of South American Earth Sciences 12 (1999) 453± Fig. 3. Simpli ed geological map of the Sergipano Belt. Modi ed from D'el-Rey Silva (1992). The Belo Monte-Jeremoabo fault (BMJF) stretches up to the border of the Sergipe-Alagoas basin, and separates the internal zone (to the north) from the intermediate, external and cratonic zones (to the south). The internal zone stands for a Meso-Neoproterozoic Andean-type margin. The other zones stand for a deformed passive continental margin. See text. the belt may provide answers to questions concerning the evolution of Proterozoic supercontinents. The interface between the fold-thrust belts and the SaÄ o Francisco-Congo and Kalahari cratons (Fig. 2) is covered by generally at-lying continental to shallow marine siliciclastic and carbonate Neoproterozoic sediments which are su ciently similar to encourage several authors (e.g. Teixeira and Figueredo, 1991; Trompette, 1994, and many references therein) to postulate their stratigraphic correlation across the cratons and into the external units of the marginal belts. The upper section of the Neoproterozoic cover around the SaÄ o Francisco Craton has been interpreted as foreland basin deposits recording the erosion of the belts themselves, implying a late-post tectonic radial sedimentary in ux towards the centre of the craton (e.g. Dominguez, 1993). However, based on detailed studies carried out in the area surrounding the Itabaiana and SimaÄ o Dias domes (Fig. 3), coupled with the results of detailed sedimentology studies performed in a locality about 50 km to the south, within the SaÄ o Francisco Craton (Saes, 1984), this model has been ruled out in the southern part of the Sergipano Belt and an alternative explanation has been required for the cratonic sediments (D'el-Rey Silva, 1992, 1995a,b; D'el-Rey Silva and McClay, 1995). To reach this alternative, the results of these studies (summarised here) are combined with other data published more recently for other margins of the SaÄ o Francisco Craton, allowing to discuss the correlation of the Neoproterozoic cratonic cover and its origin considering the evidence for mantle plume-controlled uplift of the ancient SaÄ o Francisco Plate simultaneously with two oceanic margins, one active and the other passive, that existed respectively along its western and northern edges, between 11.0 and 0.65 Ga, as discussed below. Such events are coeval with the initial and nal break-up of the Rodinia supercontinent (11000±700 Ma, as in Unrug, 1997) and are applicable to other cratons and belts of western Gondwana as well.
4 456 L.J.H. D'el-Rey Silva / Journal of South American Earth Sciences 12 (1999) 453±470 Fig. 4. Summary stratigraphy of the Itabaiana-Carira dome area, based on D'el-Rey Silva (1992, 1995b), D'el-Rey Silva and McClay (1995). Sedimentation lasted from E1.0 Ga (U±Pb zircon data in metavolcanics of the MarancoÂ domain (Brito Neves et al., 1993)) to possibly 0.65 Ga (see text). New U±Pb data from zircons in volcaniclastics indicate an older age of 810 Ma for the RibeiroÂ polis Formation (personal communication Van Schmus, February 1999). 2. The Sergipano belt: An overview 2.1. Structural/metamorphic zones and lithotectonic domains The Sergipano Belt (Fig. 3) is cross-cut by the Phanerozoic Sergipe-Alagoas and Tucano-JatobaÂ sedimentary basins and, according to its structural and metamorphic features, may be divided longitudinally, from north to south, into an internal, an intermediate, an external and a cratonic zone, whereas its lithotypes have been grouped by Santos et al. (1988), Davison and Santos (1989) and Silva Filho (1998) in seven lithotectonic domains named Sul-Alagoas, CanindeÂ, Poc o Redondo, MarancoÂ, MacurureÂ, Vaza Barris and EstaÃ ncia. These domains are all bounded by highangle thrusts associated with sinistral and dextral sense strike-parallel movement, respectively, to the east and west of the Tucano±JatobaÂ basin (Jardim de SaÂ et al., 1986; D'el-Rey Silva, 1992, 1995a). The crystalline basement crops out in the Itabaiana, SimaÄ o Dias and Jirau do Ponciano gneiss domes, within the belt, or in the craton itself (Fig. 3). Near the belt, the cratonic basement consists of Archean-Paleoproterozoic granulite-gneiss terrains, and Paleoproterozoic greenstone sequences (Mascarenhas et al., 1984; Teixeira and Figueiredo, 1991; Barbosa, 1996; Silva, 1996). The Jirau do Ponciano and SimaÄ o Dias domes display gneisses with Rb±Sr whole rock and isochron ages around 2500 Ma (Amorim et al., 1993; Humphrey and Allard, 1969). The internal zone comprises the domains to the north of the Belo Monte±Jeremoabo fault (BMJF). It exhibits a Neoproterozoic polyphase compressive deformation with north/northeast verging F 2 folds/ thrusts, fold axis parallel to a northwest-trending stretching lineation (Amorim, 1995), amphibolite-granulite metamorphism (greenschist is retrogressive), and
5 L.J.H. D'el-Rey Silva / Journal of South American Earth Sciences 12 (1999) 453± Fig. 5. Simpli ed geological map of the Itabaiana-Carira area. Combining data in D'el-Rey Silva (1992, 1995a,b) and in Santos et al. (1988). The line section AB (see Fig. 6) and the rectangle area of Fig. 7 are both indicated. See text. kyanite-bearing assemblages of intermediate pressure in areas close to the basement dome (Jardim de SaÂ et al., 1981; Silva, 1993). The intermediate, external and cratonic zones comprise respectively the MacurureÂ, Vaza Barris and EstaÃ ncia domains and are separated by the SaÄ o Miguel do Aleixo and the Itaporanga faults (Fig. 3). The rst two zones display a D 1 ±D 3 polyphase deformation, with F 2 folds/thrusts verging mostly to the south/southwest, fold axis parallel to a west/northwest trending and shallowly plunging stretching lineation, and a north±south amphibolite to greenschist facies range of metamorphism. In contrast,
6 458 L.J.H. D'el-Rey Silva / Journal of South American Earth Sciences 12 (1999) 453±470 the cratonic zone displays much less deformation and almost no metamorphism, so the sediments are generally at-lying (D'el-Rey Silva 1992, 1995a). Based on age determinations in Brito Neves et al. (1993) and Van Schmus et al. (1995), and on his own eld and geochemical studies, mainly REE, Silva Filho (1998) states: (1) the MarancoÂ and CanindeÂ domains are intraoceanic arcs consisting respectively of metasediments and metavolcanics displaying zircon U±Pb ages of Ma and Ma, along with a 1940 Ma suite of juvenile gabbros and associated metasediments/metavolcanics. Both domains exhibit deformed S-type granites with 1715 Ma U±Pb zircon ages; (2) the Poc o Redondo domain consists of tonalitic migmatites and paragneisses; and (3) The Sul- Alagoas domain comprises lithotypes typical of an accretionary prism, as well as the Jirau do Ponciano basement dome that is overlain by a sillimanite-bearing quartzite formation (not shown in Fig. 3; Silva Filho et al., 1978a; Amorim, 1995), all intruded by tonalitic granitoids displaying Rb±Sr age of Ma. As detailed by D'el-Rey Silva (1992, 1995b) the EstaÃ ncia, Vaza Barris and MacurureÂ domains form a craton-platform-basinal continuous wedge developed on a continental margin (Fig. 4). The MacurureÂ Domain is a <13 km-thick wedge comprising the MacurureÂ Group (siliciclastic and carbonate metasediments and metavolcanics of eugeoclinal a nities), a suite of syn- to post-tectonic calc-alkaline granites (only partially displayed in Fig. 3), also found in the internal zone, as well as in the Pernambuco-Alagoas Massif (Giuliani and Santos, 1988; Fujimori, 1989; Silva Filho et al., 1992), and the JuaÂ Formation: undeformed psephites derived from rocks of the MarancoÂ domain and in lling a small graben (Silva Filho et al., 1978a; Santos et al., 1988). The Vaza Barris domain is a 1 to e4 km-thick wedge of platformal (miogeoclinal), shallow marine siliciclastic and carbonate metasediments and minor volcanic rocks occurring around basement domes and divided into the Miaba, SimaÄ o Dias, and Vaza Barris groups. The EstaÃ ncia domain is a 1±3 km-thick blanket of continental to shallow marine siliciclastic and carbonate sediments resting unconformably on the SaÄ o Francisco Craton and divided into the EstaÃ ncia and SimaÄ o Dias groups Summary tectonic evolution D'el-Rey Silva (1992, 1995a) interpreted the Sergipano Belt in terms of an oblique collision of the Pernambuco-Alagoas Massif and the ancient SaÄ o Francisco-Congo Plate, after subduction of an ocean to the north, being the MarancoÂ, Poc o Redondo and CanindeÂ domains allochtonous terrains or not. Rotation of kinematics vectors due to the northwest trend of the continental margin accounts for the dextral slip of the faults in the western side of the belt (Fig. 3) and the JuaÂ Formation graben records late- to post-tectonic belt-parallel stretching. Silva Filho (1998) shows that the subduction zone existed since around 1045 Ma, with formation of intraoceanic arcs up to 1940 Ma, and accretion around 715 Ma to an Andean-type margin developed along the southern border of the Pernambuco-Alagoas Massif. The collision of the SaÄ o Francisco-Congo plate resulted in the polyphase deformation and regional metamorphism dated 1670±600 Ma, according to Rb±Sr data on phyllites of the external zone (Brito Neves and Cordani, 1973) and in syn- to late-tectonic granites within the intermediate and internal zones (Chaves, 1991; McReath et al., 1998; Silva Filho, 1998). Such evolution implies a suture zone that includes the MacurureÂ fault and the BMJF (Fig. 3), close to which metamorphism attained the highest pressures in the belt (Silva Filho, 1998). The Riacho do Pontal Belt (the western continuity of the Sergipano Belt, Fig. 2) displays very similar features, including the continuity of the suture line (Jardim de SaÂ et al., 1992; Jardim de SaÂ, 1994; Torres et al., 1994), so Angelim (1998) has suggested the same tectonic evolution as portrayed here. 3. Basin evolution in the southern-central part of the belt 3.1. Main geological features A large area stretching over the EstaÃ ncia, Vaza Barris and MacurureÂ domains (the Itabaiana-Carira area, Fig. 5) has been studied in detail. The Itaporanga fault separates the EstaÃ ncia domain (to the south) from gneiss domes and the (meta)sedimentary/ volcanic cover of the two other domains. The Itabaiana and SimaÄ o Dias domes are granite-granodiorite, amphibolite-grade gneiss bodies intruded by basic-ultrabasic rocks and granites, all partially retrometamorphosed to the greenschist facies. The crystalline basement for the MacurureÂ domain has been proven by geophysical studies and by small slices of gneisses that occur in the hangingwall of thrust faults (D'el-Rey Silva, 1992, 1995b). The basement-cover contact dips at low-angles along the northern, eastern, and western margins of the Itabaiana dome, as well as along the western side of the SimaÄ o Dias dome. Elsewhere, the contact dips at high-angles. Rocks on both sides of the contact are generally sheared, except in one part of the eastern margin of the Itabaiana dome and one part of the western margin of the SimaÄ o Dias dome, where undeformed conglomerate rests upon an erosive unconformity on well banded and mylonitic gneiss.
7 L.J.H. D'el-Rey Silva / Journal of South American Earth Sciences 12 (1999) 453± Fig. 6. Simpli ed geological cross-section along line AB shown in Fig. 5. From D'el-Rey Silva (1992, 1995a). Details in the text. The basement's metamorphic banding (S n ) and the cover's original layering (S o ) record similar D 1 ±D 3 deformational evolution, during which mappable structures developed in the area. These include the Itaporanga, Escarpa, Pelada, Mocambo, RibeiroÂ polis, SaÄ o Miguel do Aleixo and Dores faults, which are sub-vertical, WNW±ESE trending thrusts (interpreted as inverted normal faults Ð see ahead) associated with sinistral strike-slip and with F 2 folds. The larger ones are seen in the western sides of both basement domes. Beyond S n and S o, these structures also a ect layerparallel foliation (S 1 ), small- to mesoscopic-scale recumbent F 1 folds, and a stratigraphic inversion mapped in the southwestern side of the area (Fig. 5) and ascribed to D 1 nappes. The structural pattern in vertical cross-sections (Fig. 6) is that of a generally at lying cratonic zone, where deformation is mostly characterised by gentle-open F 2 folds that a ect S o, but a gradual increase in the intensity of the axial planar foliation (S 2 ) and in the aspect ratio of these folds is observed close to and across the Itaporanga fault, towards the north. Within the external and intermediate zones, small- to large-scale F 1 folds and nappes are a ected by F 2 folds to form a classical type 3 interference pattern (Ramsay, 1967) subsequently disrupted and uplifted by the high angle thrust/strike-slip faults, interpreted as such because they merge at the same basal extensional detachment of the basin opening (Fig. 6; D'el-Rey Silva, 1992, 1995a). The intense D 2 attening imprinted numerous mesoscopic, tight F 2 folds with a penetrative WNW± ESE trending S 2 foliation that is sub-parallel to S n,s o, and S 1, except in the F 2 hinges, and that dips steeply to NNE. The F 2 fold axes are parallel to a penetrative stretching lineation well documented by several strain markers in most of the rock types. Strike-slip faults, such as the Vaza Barris fault, the Mocambo shear, and others along the eastern margin of the Itabaiana dome (Fig. 5), as well as D 3 kinks and folds (smallscale and transversal to D 1 ±D 2 structures) developed late in this evolution. Many eld data collected during a detailed investigation carried out by the author (October/1998) in most of the cratonic zone far to the south, demonstrate that deformation is because the rocks slipped down to the north, accompanying the continental underthrust of basement and cover to the north, during collision Megasequence stratigraphy The basin in lling is divided into two sedimentary cycles, each one formed by two megasequences (Fig. 4): Cycle I holds a lower siliciclastic megasequence comprising the JueteÃ, Itabaiana and RibeiroÂ polis for-
8 460 L.J.H. D'el-Rey Silva / Journal of South American Earth Sciences 12 (1999) 453±470 Fig. 7. Summary of the 1:50,000 scale original geological map by D'el-Rey Silva (1992) showing the occurrence of the SimaÄ o Dias Group around the locality of Ac ude do AnaÄ o, along the western side of the SimaÄ o Dias dome, across the Sergipe/Bahia State boundary (a), above the Miaba Group and below the Vaza Barris Group, and (b) in the core of the Paripiranga antiform. D 1 structural data were omitted for simplicity. Legend as in Fig. 6. mations, and a lower carbonate megasequence comprising the AcauaÄ and Jacoca formations. Cycle II holds an upper siliciclastic megasequence comprising the SimaÄ o Dias Group and the Palestina Formation, and an upper carbonate megasequence (the Olhos D'aÂ gua Formation). The lateral correlation between the formations of each megasequence in Cycle I is constrained by their same stratigraphic position, by the strong similarity of the rock types, and by the fact that the SimaÄ o Dias Group forms a blanket of siliciclastics that is continuous across the craton-fold belt boundary (D'el-Rey Silva, 1992, 1995b; D'el-Rey Silva and McClay, 1995) The lower siliciclastic megasequence The JueteÃ Formation does not crop out in the study area proper, but occurs to the south, in the EstaÃ ncia domain (Fig. 5). It unconformably overlies the basement and consists of arkosic sandstone with conglomerate lenses of basement-derived clasts, argillite, and
9 L.J.H. D'el-Rey Silva / Journal of South American Earth Sciences 12 (1999) 453± diamictites. The Itabaiana Formation is 1400±700 m- thick in the southern, eastern and western sides of the Itabaiana dome, and consists of (local) trough crossbedded basal conglomerate, planar cross-bedded feldspathic quartzite, metasiltite and meta-argillite containing lenses of matrix-supported conglomerate and conglomeratic sandstone with basement-derived clasts, particularly in the eastern and western sides of the Itabaiana dome. Elsewhere this formation is E30 m- thick, and consists of mica quartzite and/or arkosic sandstone intercalated with red brown phyllite. The RibeiroÂ polis Formation is 0±100 m-thick but may reach as much as 500 m, and consists of silty phyllite with intercalations of metagreywacke, pebbly phyllite, argillite and acid/intermediary to basic metavolcanics. It also encloses thin bodies of coarsening-upward, matrix-supported conglomerate with clasts of the underlying rocks (western margin of the Itabaiana dome). Gravity-driven diamictites observed along the trace of the Mocambo fault (northeastern margin of the Itabaiana dome) contain meter-size clasts of gneiss (Humphrey and Allard, 1969) The lower carbonate megasequence The AcauaÄ Formation conformably overlies the JueteÃ Formation and unconformably overlies the basement gneiss, and consists of basal limestone, dolomite, calci-argillite with lenses of limestone, and layers of limestone and dolomite with thin intercalations of negrained, cross-bedded, red sandstone and green argillite (Saes, 1984; Silva Filho, 1982). The Jacoca Formation is E300 m-thick around the Itabaiana dome and consists of light gray metadolomite passing upwards into thinly intercalated layers of metadolomite, metachert and calci-phyllite, and into E1 m-thick layers of more massive metadolomite. Elsewhere it consists of thin metadolomite layers intercalated with phyllite (SimaÄ o Dias dome) or ne-grained metalimestone layers intercalated with variegated metapelite (north of the Mocambo fault; Fig. 5) The upper siliciclastic megasequence To the south, within the cratonic EstaÃ ncia domain, the 1700±2500 m-thick Lagarto-Palmares Formation is a coarsening-upward sequence of mudstone, siltstone, ne- and medium-grained calcite-cemented sandstone, and lithic wacke. Saes (1984) showed that the sandstone layers have detrital carbonate grains, and that the normally gradual passage from sandstone to the upper lithic wackes is locally marked, however, by lens-shaped bodies of breccia with boulders of carbonate rocks and gneiss, and conglomerate with reworked and much smaller clasts of carbonate rocks, basementderived high- and low-grade metamorphic rocks, and serpentinite. The Lagarto-Palmares Formation spreads onto the southern part of the map area and inter ngers with the JacareÂ Formation, which in turn inter ngers with the Frei Paulo Formation. The entire SimaÄ o Dias Group rests above the Miaba Group and is overlain by the Vaza Barris Group, in the core of the Paripiranga basement-cored antiform (Fig. 7), as well as to the east of the Itabaiana dome. The JacareÂ Formation consists of light brown to variegated metasiltite with lenses of ne- to medium-grained metasandstone. The Frei Paulo Formation is 1100 m up to e500 m-thick and consists of light gray to brown or variegated silty phyllite with intercalations of metasandstone and wacke, in areas to the south of the Escarpa fault and to the north of the Mocambo fault. In the latter area it encloses mappable lenses of calcitecemented lithic wacke (Fig. 5), generally termed lithofacies FP1, which are petrographically similar to those of the Lagarto-Palmares sandstone. In the area between the two faults, and also to the east of the Itabaiana dome, the dominant lithofacies are quartzsericite-chlorite phyllite, thinly interbedded metagreywacke and metalimestone, metarhythmite, minor metasandstone, together with local lenses of highly weathered metavolcanics. The entire sequence is named lithofacies FP3. The formation also occurs to the west of the SimaÄ o Dias dome (Fig. 5), and consists of sandy phyllite with intercalations of metasandstone. The Palestina Formation unconformably overlies the SimaÄ o Dias Group and consists of metadiamictite and (pebbly) phyllite, and E10 m-thick lenses of coarsegrained, iron-cemented quartzite. The metadiamictite contains clasts of gneiss, granite, quartz vein, quartzite, phyllite, and metacarbonate, all supported in a greenish gray, extremely foliated quartz-sericite matrix. The thickest sections (e500 m) are found in the area bounded by the Escarpa and Pelada faults, a 1350 km-long trench termed Vaza Barris trough (D'el-Rey Silva, 1992). In this trough, the diamictite matrix is ner and the >10 cm-size clasts are abundant. Around the SimaÄ o Dias dome, the Palestina Formation is 110±100 m-thick, the matrix may be siltier and e10 cm-size clasts are generally rare, despite a 4 m-long block of gneiss found in a large outcrop near the western end of the trace of the SimaÄ o Dias fault. Outcrops of Palestina metadiamictite to east of the Itabaiana dome contain boulders of Itabaiana quartzite and Lagarto-Palmares metasandstone The upper carbonate megasequence The Olhos D'aÂ gua Formation is 1200 m up to e1300 m-thick and occurs around the SimaÄ o Dias dome, as well as to the east of the Itabaiana dome and to the north of the Pelada fault (Fig. 4). The typical section (from the SimaÄ o Dias dome to NNW) comprises a basal unit of 100's meters of thinly interbedded, light gray to black, ne-grained metalimestone with minor intercalations of metasiltite, meta-argillite
10 462 L.J.H. D'el-Rey Silva / Journal of South American Earth Sciences 12 (1999) 453±470 Fig. 8. a±e. Cartoons to illustrate the evolution of sedimentation in the Neoproterozoic continental margin, respectively across the areas precursor of the Itabaiana (a±c) and SimaÄ o Dias domes (d,e). Based on D'el-Rey Silva (1992, 1995b). See text. and metachert. This unit passes to 100's m of interbedded, oolitic, pelloidal, black metalimestone with intercalations of brown metapelite and meter-thick layers of gray, cross-bedded, wave-reworked metadolarenite. Approaching the Escarpa fault, this intermediary unit passes gradually into 100's of meters of negrained, black metalimestone intercalated with layers of red-brown pyrite-bearing metapelite. The MacurureÂ Group consists of biotite- and staurolite-garnet schist, quartzite, chlorite-quartz schist, metacarbonate, phyllite, metasiltite, metagreywacke, and intermediary metavolcanics, all intruded by granite-granodiorite bodies (Silva Filho et al., 1978a,b; Santos et al., 1988; Davison and Santos, 1989) Age and tectonic control of sedimentation An upper limit for the age of sedimentation of Ma may be deduced from U±Pb zircon data in metavolcanics within schists of the MarancoÂ domain (Brito Neves et al., 1993) and on the existence of Ma island-arcs (Silva Filho, 1998). These data imply the existence of a e1000 Ma continental margin along the northern border of the ancient SaÄ o Francisco-Congo plate. Van Schmus (personal communication, 1999) reports that detritral zircons from volcaniclastics of the RibeiroÂ polis Formation display U±Pb ages of Ma, whereas newly formed zircons have ages of 1850 to 1810 Ma. Van Schmuss (personal communication, 1999) interprets the younger ages as the maximum age of sedimentation (Fig. 4). Basin in lling lasted up to 1650 Ma. Lithofacies and thickness distribution of the metasediments around the gneiss domes and across the SimaÄ o Dias, Pelada, Escarpa and Mocambo faults, coupled with data from the RibeiroÂ polis and Palestina diamictites, as well as data from the literature, allowed D'el-Rey Silva (1992) and D'el-Rey Silva and McClay
11 L.J.H. D'el-Rey Silva / Journal of South American Earth Sciences 12 (1999) 453± (1995) to conclude that: (1) sedimentation was tectonically controlled; (2) the diamictites record tectonic instability across the basin margin and interior; (3) the basin was asymmetric, deepened northwards and eastwards, and contained basement paleohighs (precursors of the Itabaiana and SimaÄ o Dias domes); (4) the Vaza Barris trough was a long-lived feature in the basin, as it received relatively deeper-water sediments since Jacoca and Frei Paulo time, and continued during Palestina Formation time; and (5) most of the regional thrust faults in the area may be interpreted as inverted normal faults Continental margin extensional model A simple shear style of crustal extension (cf. Lister and Davis, 1989) may be invoked for the opening of a precursor basin above a linked listric detachment (cf. Gibbs, 1984), whereas the west±east asymmetry of the basin and its sedimentary in lling may result from an oblique extension of the northern margin of the SaÄ o Francisco Craton (cf. Gibbs, 1987). In such a model, extension of the upper crust would take place above a ramp- at listric basal detachment, and the deformation of the hangingwall may have been accommodated by synthetic and antithetic, planar and listric normal faults (Fig. 8a±e). The listric fault style is clearly favored because: (a) the basin must have been asymmetric; (b) thickness generally increases to the north, away from the SaÄ o Francisco Craton; (c) extensional fault activity is indicated by large polylithic clasts of both basement and cover in the diamicties found adjacent to the SimaÄ o Dias and Mocambo faults, pointing to uplift-erosion of footwall and possibly hangingwall blocks twice in the evolution of the basin; and (d) the lack of multiple wedge shaped conglomeratic sequences. These are much expected in basins dominantly formed by domino style faults (e.g. Davison, 1989) or even in a exural-cantilever model (cf. Kusznir et al., 1991). Finally, (e) the Vaza Barris trough ts well in a progressive extension of the basement above a listric detachment, and may be interpreted as a crestal collapse graben structure related to the evolution of the two basement domes (Fig. 8c±e; D'el-Rey Silva, 1992, 1995b) Cycle I of basin in lling Deposition of the lower siliciclastic and carbonate megasequences is better understood across the area of the precursor to the Itabaiana dome (Fig. 8a±c). Cycle I started with deposition of alluvial/ uvial sediments in a continental environment (trough cross-bedded basal conglomerates, conglomeratic sandstones, diamictites, arkoses and argillites of the JueteÃ -Itabaiana formations) and continued with the progradation of the bulk of E1 m-size planar cross-bedded, relatively wellsorted and pure Itabaiana quartzites, most likely in a site that evolved to a 700 m-deep depression destined to become the Itabaiana dome (Fig. 8a). This site evolved into a roll-over syncline, the preferred depocentre of the RibeiroÂ polis phyllite, and was invaded at times by volcanics and also received diamictites deposited around an incipient paleohigh (Fig. 8b). The AcauaÄ -Jacoca carbonates record the ooding of the cratonic area. True carbonate lithofacies around the Itabaiana dome indicate uplift of basement highs adjacent to an incipient trough (Fig. 8c) Cycle II of basin in lling Deposition of the upper siliciclastic and carbonate megasequences is better understood in sections across the areas of the precursor to the SimaÄ o Dias dome and Vaza Barris trough (Fig. 8d,e). The upper siliciclastic megasequence records a major marine regression and the increasing supply of detritus shed onto the AcauaÄ -Jacoca carbonate platform. Muds, silts and sands of the basal section were succeeded upwards by increasingly coarser sands and lithic wackes within the Lagarto-Palmares Formation, which passed laterally into more distal siltites and phyllites deposited northwards. Extensional tectonics and uplift of basement blocks contemporaneous with the sedimentation of this megasequence is recorded by the carbonate breccia at the bottom of the SimaÄ o Dias Group, in the EstaÃ ncia domain, and by the Palestina diamictites in the Vaza Barris trough. Afterwards, the basin was roofed by the thick Olhos D'aÂ gua carbonate platform (upper carbonate megasequence). The vertical succession of primary structures of the Lagarto-Palmares Formation, described in detail by D'el-Rey Silva (1992) and D'el-Rey Silva and McClay (1995), strongly suggests increasing energy in the sedimentary environment, and compares with those expected in thick sandstones sequences of shallow marine platforms (Tillman, 1985; Walker, 1985), allowing us to conclude that the formation is a coarseningupwards siliciclastic sequence lying across the cratonbasin interface. The unequivocal stratigraphic position and the sedimentological characteristics of the SimaÄ o Dias Group, either in the craton or around the domes, plus the fact that domes and sedimentary cover display the same deformational/metamorphic evolution, rule out previously suggested thrust-fold belt/foreland basin models (e.g. Silva Filho et al., 1978a,b; Dominguez, 1993) and argue for another explanation for the evolution of the precursor basin. As the upper sandstones and lithic wackes must have come from southern sources, their lithic fraction may be explained by ero-
12 464 L.J.H. D'el-Rey Silva / Journal of South American Earth Sciences 12 (1999) 453±470 Frei Paulo phyllites record these siliciclastics in a more distal position; that is why the relatively deep-water and ner fractions of the Frei Paulo Formation are in the Vaza Barris trough (Fig. 8d). Deposition of the Vaza Barris Group (Fig. 8e) is discussed below. Continued uplift of the craton would lead to progressively faster extension, exposure, rotation and erosion of all rocks in the basin and deposition of the Palestina diamictites in the Vaza Barris trough (Fig. 8e). 4. Discussion 4.1. Correlation of the Neoproterozoic cover Fig. 9. Cartoon to illustrate the uplift of the centre of the ancient plate which was precursor of the SaÄ o Francisco Craton. The BambuõÂ Group and equivalent sediments that in lled the rim of sedimentary basins were derived from erosion of the central highs and, to the west, from erosion of an Andean-type margin that is now incorporated into the central zone of the BrasõÂ lia Belt. This Andean-type margin originally included a belt of arc-type juvenile material accreted between 1900±640 Ma, and thrusts/folds probably developed between 790±640 Ma, before the continental collision that built the Tocantins Province. Sediment inputs for siliciclastic detrital rocks of the Bambui Group are from (1) Saes (1984) and (2) Castro and Dardenne (1998). Details in the text. sion of greenstone sequences found in the craton, nearby the belt (D'el-Rey Silva, 1992). In addition, the origin of the whole SimaÄ o Dias Group and Palestina diamictites (the upper siliciclastic megasequence) may be assigned to a progressively faster uplift of the source area far to the south, possibly located somewhere close to the centre of the ancient plate (Fig. 9). Erosion of such a dynamic source would provide basal mud and silt and an increasing supply of coarsening-upwards sands with carbonate detritus (the Lagarto-Palmares Formation). To date, oil wells have cored AcauaÄ carbonates underneath the Phanerozoic Tucano basin, much farther to the south, near Salvador (Silva Filho, 1982). A braided delta system (sensu McPherson et al., 1987) accounts for transport of these sediments to the northeast, onto the platform, where they underwent wave reworking in a shallow marine environment. The JacareÂ metasiltites and the Noting that the various formations within the Neoproterozoic rocks covering the SaÄ o Francisco Craton (Fig. 10) show strongly similar petrography and stratigraphic position, D'el-Rey Silva (1992) correlated the cover adjacent to the Sergipano Belt with the cover adjacent to the Rio Preto, BrasõÂ lia, and Arac uaõâ belts, as well as inside the Lenc oâ is basin (in the northern half of the craton), all of which have already been correlated with the BambuõÂ Group of Dardenne (1978), by Silva et al. (1989) and Uhlein et al. (1990), although adopting di erent names from area to area. For the purposes in here, it is important to say that the same correlation has been adopted for the Neoproterozoic cover of the Pan-African/Brasiliano cratons (e.g., Teixeira and Figueredo, 1991; Dominguez, 1993; Trompette, 1994; and others therein). It is also important to highlight two remarkable facts that add strong support to these correlations (Fig. 11): (1) It has been increasingly recognised that the age of initial sedimentation in the Neoproterozoic basins precursor of the Pan-African-Brasiliano belts is Ma (Unrug, 1997); and (2) The age 1650 Ma marks the onset of Brasiliano Cycle deformation/regional metamorphism in many of these belts, as in the Sergipano, BrasõÂ lia, Arac uaõâ belts in Brazil (respectively in Brito Neves and Cordani, 1973; Pimentel et al., 1996; and Pedrosa-Soares et al. 1998) and in African belts (Germs, 1995). Obviously, this bracket of ages strongly supports the correlation of formations, even if those with equivalent stratigraphic positions have slightly di erent ages (note that the E810 Ma and Ma ages in Fig. 11 are essentially the same within analytical error), and regardless of small variations in their sedimentary environments. The fact that the megasequence stratigraphy of the southern part of the Sergipano Belt also applies to the lithostratigraphic record that Henry et al. (1990) described for the Damara Belt (Fig. 11) implies a similar tectonic evolution for basement and respective basins. This is
13 L.J.H. D'el-Rey Silva / Journal of South American Earth Sciences 12 (1999) 453± Fig. 10. Lithostratigraphic correlation across the Neoproterozoic cratonic cover of the SaÄ o Francisco Craton. Modi ed from D'el-Rey Silva (1992). Details in the text. made more evident by noting the correlation between the Palestina and Chuos diamictites, and that the latter ones are continuous across the Damara, Gariep, and Malmesbury (or Vanrhynsdorp) belts of southern Africa (Fig. 2; Germs, 1995), all in similar stratigraphic position of the Upper Tilloid Formation of the West Congo Belt, according to lithostratigraphic columns in Schermerhorn (1981), in Fig of Trompette (1994), or in Tack (1995). True foreland basin deposits related to the Damara Belt are not older than 650 Ma, whereas the Chuos Formation is 1700 Ma (Fig. 11). This upper limit is well constrained by zircon U±Pb ages of 1728 Ma and 1717 Ma for volcanic rocks underlying, respectively, the Chuos diamictites and their equivalent in the Gariep Belt (Germs, 1995). Thus, it is realistic to apply the same 700±650 Ma age interval to the Vaza Barris Group, because these are the younger pre-deformation sediments in the Sergipano Belt and correlate to the Tsumeb Subgroup in the Damara Belt (Fig. 11). If so, the SimaÄ o Dias Group (Sergipano Belt) is older than 1700 Ma Tectonic implications The data available on the southern-central part of the Sergipano Belt imply that: (1) either the Neoproterozoic sedimentary cover of the SaÄ o Francisco Craton does not correlate at all; or (2) the correlation is valid, as has been widely believed in the literature. All evidence discussed in this paper clearly supports the correlation, because the E1000 to E700 Ma age interval for the cratonic cover adjacent to the Sergipano Belt (Fig. 10) agrees with the 1940 Ma to 1700 Ma age interval for the BambuõÂ Group (U±Pb data from volcanic rocks underlying it: Machado et al., 1989, and Pb/Pb data for BambuõÂ carbonates: Babinski et al., 1993). Moreover, the new data for a e1000 Ma continental margin along the northern edge of the SaÄ o Francisco plate (Fig. 9; Silva Filho, 1998) implies that the very basal siliciclastics of the lowest megasequence in the Sergipano Belt (formed by the base of the JueteÃ Formation, the Itabaiana Formation and its lateral continuity into schists and quartzites of the MacurureÂ Group; Fig. 4) should correlate with sediments equivalent to the Meso-Neoproterozoic ParanoaÂ Group that underlies the BambuõÂ Group in the BrasõÂ lia and other belts (Fig. 10). This interpretation matches an earlier prediction by Silva Filho et al. (1978a) that the JueteÃ and Itabaiana quartzite formations correlate to the quartzite deposited around the Jirau do Ponciano dome. In the Sul-Alagoas domain they record the earlier sedimentation on both continental margins that collided to form the Sergipano Belt. Thus, the uppermost diamictites in the JueteÃ and RibeiroÂ polis formations (Fig. 4) should correlate to the JequitaõÂ and Canabravinha formations at the base of the BambuõÂ Group (Fig. 10). This correlation is expected because the Canabravinha Formation is in the Rio Preto Belt and this shares the same megaorogen with the Sergipano Belt (Fig. 2).
14 466 L.J.H. D'el-Rey Silva / Journal of South American Earth Sciences 12 (1999) 453±470 Fig. 11. Lithostratigraphic correlation across the Neoproterozoic cover of the cratonic areas adjacent to the Sergipano and Damara Belts. Modi ed from D'el-Rey Silva (1992) with addition of U±Pb ages for the Sergipano Belt (the bold numbers in brackets), and for the Damara (Germs, 1995). Details in the text Mantle plume-controlled uplift and coeval continental margins In order to understand the origin of the Neoproterozoic cover and to be faithful to the geologic data, one must take the following into consideration: 1. Rb±Sr and K±Ar isotopic data from di erent parts of the craton indicate that the ancient SaÄ o Francisco plate experienced uplift since the Mesoproterozoic/Neoproterozoic transition (Teixeira and Silva, 1993), probably controlled by a Neoproterozoic mantle plume existing underneath the ancient plate, as indicated by swarms of 1000± 900 Ma mantle-derived ma c dikes along the margins of the SaÄ o Francisco and Congo cratons (Correa-Gomes and Oliveira, 1997), and also in the southern to central parts of the SaÄ o Francisco Craton (Silva et al., 1995). 2. The Andean-type continental margin along the western edge of the ancient SaÄ o Francisco Plate (Fig. 9), as documented by a composite magmatic arc formed by accretion of island (and continental) arclike juvenile material to the western side of the GoiaÂ s Block (Fig. 2) during the 900±640 Ma age interval (Pimentel and Fuck, 1992; Viana et al., 1995; Pimentel et al., 1996, 1997, 1998; Ribeiro and Pimentel, 1998). 3. East-vergent folds/thrusts must have a ected the upper crust in the easternmost part of the Andeantype margin, in the 800±640 Ma age interval, during the rst stages of uplift of three ultrama c/ma c granulite facies layered intrusions in the eastern side of the GoiaÂ s Block, requiring the underthrust of the ancient SaÄ o Francisco Plate to the west (D'el-Rey Silva et al., 1996, 1997). 4. The passive continental margins along the northern and southern edges of the plate (Silva Filho, 1998; Pedrosa-Soares et al., 1998).
15 L.J.H. D'el-Rey Silva / Journal of South American Earth Sciences 12 (1999) 453± Origin of the cratonic cover Thus, the most probable scenario for the origin of the Neoproterozoic cover (=BambuõÂ Group and equivalents) should combine the e ects of plate uplift with the e ect of a marginal subduction zone (Fig. 9), similar to that envisaged by Burgess et al. (1997) to explain the cratonic sediments of central-western North America. Initial uplift, just before onset of subduction, led to radial spreading of JequitaõÂ -type diamictites (a glacial-tectonic control is envisaged by Karfunkel and Hoppe, 1988). Subduction started and a magmatic arc began to develop along the western margin; deposition of carbonate sequences took place in peripheral seas surrounding the central highs and bounded to the west by the arc. Further and faster uplift of the centre supplied detritus for increasingly coarser sandstones and wackes of the upper BambuõÂ Group (equivalent to the Lagarto-Palmares Formation, Fig. 10) in the evolving margins to the north/northwest and southeast (Fig. 9), but deposition of these sediments on the western half of the plate was in uenced by the evolving Andean-type continental margin as well. Detailed studies carried out in the BrasõÂ lia Belt and its cratonic area indicate that: (1) these sandstones and wackes derive from arc-related (meta)volcano/sedimentary and sedimentary rocks (GuimaraÄ es, 1997; GuimaraÄ es and Dardenne, 1998); (2) conglomerate bodies of the BambuõÂ Group derived from the west (Fig. 9) and from eroded thrusts/folds formed in the southern part of the belt (Castro and Dardenne, 1998); and (3) the psamo-pelitic-carbonatic Vazante Group (Dardenne et al., 1998) was derived largely from igneous and metamorphic rocks associated with 800 Ma old tectonic events. Whatever the original direction of the paleocurrents, the sediments were reworked in their environment of nal deposition (almost always shallow marine), so the existence of few paleocurrent determinations (Fig. 9) is not critical. The strong similarity within the lithostratigraphic record in Brazil and Africa points out to a similar tectonic evolution for the other cratons and surrounding basins of the Neoproterozoic orogeny, and the depositional events portrayed here relate to the initial and nal break-up of the Rodinia supercontinent, respectively around 1000 and 700 Ma (combining Brito Neves et al., 1993; Van Schmus et al., 1993, 1995; and Unrug, 1997). Thus, deposition of the Palestina diamictites, and African equivalents, records further extension in the passive margin around 700 Ma (Fig. 8e), probably associated with nal uplift of the central highs, whereas the Olhos D'aÂ gua carbonates and equivalents (Fig. 11) record nal quiescence by 1650 Ma (the onset of continental collisions). The western side of the ancient SaÄ o Francisco Plate must have received sediments (for example, the uppermost layers of BambuõÂ sediments, the Vazante Group) at this time as well. 5. Conclusions Detailed studies of the southern-central part of the Sergipano Belt, at the northeastern edge of the SaÄ o Francisco Craton, NE Brazil, allow us to construct a craton-platform-basin continuous lithostratigraphy comprising four megasequences, all similar to the 11.0±0.65 Ga lithostratigraphic record of other basins surrounding the SaÄ o Francisco Craton, and similar basins in Africa, pointing to a common tectonic evolution for those parts of Rodinia destined to become the cratons forming a large part of the Pan-African/ Brasiliano Orogeny. This analysis and an inventory of geologic data in the literature indicate that sedimentation was controlled by the mantle plume-derived uplift of the ancient plate, and by the evolution of an Andean-type continental margin along its western border. Basal siliciclastics of the Neoproterozoic sediments record erosional products of a central high which were deposited across the continental margins, before the onset of lithospheric subduction underneath the western margin. The overlying carbonate formations were deposited in shallow seas that developed around the central high, and were open to deep ocean basins except to the west, where an Andean-type magmatic arc was growing. The uppermost sandstones and wackes record further and faster uplift of the central areas, and also the erosion of fold/thrust highs developed along the Andean-type margin, in the 800±650 Ma interval. However, in the basin precursor of the Sergipano Belt, as well as across African basins, the end of this interval (700±650 Ma) marks the deposition of distinct diamictite and carbonate formation (Palestina- and Olhos D'aÂ gua-type sediments) and re ects further continental extension followed by tectonic quiescence across the northeastern edge of the SaÄ o-francisco plate and across other margins of the Congo plate. The last episodes preceded formation of the surrounding belts of deformation and metamorphism by continental collision (1650±600 Ma ago). Acknowledgements This paper combines my PhD studies (RHUL, UK, 1989±1992) with further research around the SaÄ o Francisco Craton and in the pertinent literature. I thank very much CNPq, Brazil (Grants /93-9 and /95-9), as well as Professor K. R. McClay (my PhD supervisor), Professor R.A. Fuck, Professor M.A. Dardenne and Dr. M.M. Pimentel (UnB, Brazil)
16 468 L.J.H. D'el-Rey Silva / Journal of South American Earth Sciences 12 (1999) 453±470 and two anonymous referees, for their review of an earlier version of this paper, and Dr. Luc Tack (Belgium) for further discussion about the West Congo Belt. Professor R. Trompette and another anonymous referee are thanked for their useful comments on the nal version. References Almeida de, F.F.M., Hasui, Y., Brito de, Neves B.B., Fuck, R.A., Brazilian Structural Provinces: an introduction. Earth Sciences Review 17, 1±29 Special Issue. Amorim, J.L Programa Levantamentos GeoloÂ gicos BaÂ sicos do Brasil. Arapiraca, Folha SC.24-X-D-V. MME-SME-CPRM, 100 pp. Amorim, J.L., Torres, H.H.F., da Silva Filho, M.A., O Complexo de embasamento da Faixa Sergipana na regiaä o de Jirau do Ponciano (Al): Estratigra a, evoluc aä o tectonometamoâ r- ca e potencialidade metalogeneâ tica. In: SBG SimpoÂ sio de Geologia do Nordeste 15, NatalBoletim de Resumos, pp. 240± 242. 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