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Sequence stratigraphic framework of Proterozoic successions in eastern Brazil
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Marine and Petroleum Geology 26 (2009) 163–176 www.elsevier.com/locate/marpetgeo
Sequence stratigraphic framework of Proterozoic successions in eastern Brazil Marcelo A. Martins-Neto Geology Department, School of Mines, Federal University of Ouro Preto, NUPETRO—Nucleus of Petroleum Geology of the Gorceix Foundation, Caixa Postal 173, 35 400-000 Ouro Preto, MG, Brazil Received 9 August 2007; received in revised form 28 September 2007; accepted 4 October 2007
Abstract First-order regional unconformities separate Proterozoic supracrustal rocks of the gas-bearing Sa˜o Francisco craton and surrounding thrust-and-fold belts (eastern Brazil) into four genetic stratigraphic units, which constitute 1st-order basin-fill sequences (Espinhac- o, Canastra–Paranoa´–Vazante, Macau´bas and Bambuı´ sequences). The Espinhac- o 1st-order sequence (ca. 1.73–1.50 Ga) records a stage of aborted lithospheric stretching of the Sa˜o Francisco-Congo continental mass. The Espinhac- o basin displays a ‘‘steer’s-head’’ geometry, where four 2nd-order sequences are recognized (prerift, rift, transitional and flexural). Third-order rift-propagation unconformities allowed the definition of three 3rd-order synrift sequences within the rift 2nd-order sequence. The 2nd-order flexural sequence of the Espinhac- o basin can be subdivided into three 3rd-order transgressive–regressive sequences. The Canastra–Paranoa´–Vazante and Macau´bas 1st-order sequences (ca. 1200–630 Ma) comprise rift to drift successions deposited during the breakup of the supercontinent Rodinia and the opening of the Brazilide and Adamastor oceans, respectively, to the west and east of the Sa˜o Francisco paleocontinent. According to seismic data, the passive-margin Canastra–Paranoa´–Vazante 1st-order sequence can be divided into two 2nd-order sequences: rift and passive margin. The passive-margin 2nd-order sequence can be further subdivided into three 3rd-order sequences. The outcropping Canastra–Paranoa´–Vazante sequence consists mainly of shallow-marine deposits, which contain black shales/marls with intervals showing high total organic carbon (TOC) contents, comprising one of the highest potential hydrocarbon source rocks of the Sa˜o Francisco basin. The Bambuı´ foreland 1st-order sequence (ca. 750–550 Ma) formed as a consequence of thrust loading related to shortening in the Brası´ lia fold belt on the western flank of the Sa˜o Francisco craton, during the closing of the Brazilide ocean and the assembly of the West Gondwana supercontinent. The Bambuı´ sequence shows two distinct successions, which reflect the main basin paleogeographic settings: foredeep and flexural ramp. Three transgressive–progradational 2nd-order sequences can be recognized in the ramp succession of the Bambuı´ basin. r 2007 Published by Elsevier Ltd. Keywords: Precambrian; Proterozoic; Sequence stratigraphy; Eastern Brazil; Sa˜o Francisco craton; Sa˜o Francisco basin
1. Introduction The record of the Late Paleoproterozoic to Late Neoproterozoic geodynamic history of eastern Brazil (Sa˜o Francisco craton as well as Brası´ lia and Arac- uaı´ fold Tel.: +55 31 3559 7164; fax: +55 31 3559 7174.
E-mail addresses: [email protected], [email protected] (M.A. Martins-Neto). 0264-8172/$ - see front matter r 2007 Published by Elsevier Ltd. doi:10.1016/j.marpetgeo.2007.10.001
belts, Fig. 1) is preserved by the sedimentary successions of the Espinhac- o, Canastra–Paranoa´–Vazante, Macau´bas and Bambuı´ basins, the last three being considered together as the gas-bearing Sa˜o Francisco basin (Martins-Neto et al., 2001; Martins-Neto and Alkmim, 2001; MartinsNeto, 2005, 2007). Each of these major sedimentary successions represents a 1st-order sequence (or megasequence), which is the record of an unconformity-bounded, single basin-fill cycle (Martins-Neto et al., 2001; Catuneanu
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Fig. 1. Simplified geologic map of eastern Brazil (modified after Alkmim and Martins-Neto, 2001). Note location of seismic profile of Fig. 6.
et al., 2005). Collectively, these basins track major plate reorganizations that affected the Sa˜o Francisco craton through a time interval greater than 1 Gy. The purpose of this paper is to present a synthesis of the sequence stratigraphic framework of the Proterozoic successions in eastern Brazil, describing the major sedimentary 1st-order sequences preserved in each of these basins and their geodynamic significance in a plate tectonic scenario. The sequence stratigraphic approach adopted herein (following Martins-Neto et al., 2001; Catuneanu et al., 2005; Catuneanu, 2006) is summarized in the first part of the paper. Since the gas-bearing Sa˜o Francisco basin is an exploration frontier area with poor data coverage, a wellconstrained geologic model is a fundamental step in modeling its petroleum system. 2. Precambrian sequence stratigraphy Sequence stratigraphy studies the change in depositional trends (stacking patterns) in response to the interplay of accommodation and sediment supply, regardless of scale and age of the strata under analysis (e.g., Vail et al., 1977;
Posamentier and Vail, 1988; Posamentier et al., 1988; Weimer and Posamentier, 1993; Emery and Myers, 1996; Posamentier and Allen, 1999; Catuneanu, 2006). Therefore, the method of sequence stratigraphy can be applied from individual depositional systems to entire sedimentary basin-fills, as well as in Precambrian to Phanerozoic successions. In a recent paper, Catuneanu et al. (2005) discuss the principles of sequence stratigraphy and its applicability to Precambrian sedimentary successions. Since Precambrian basins are often characterized by poor stratal preservation and by a general lack of time control, an approach based on the stratigraphic hierarchy is the best way to interpret the stratigraphic framework and understand basin evolution. Sequence hierarchy involves the separation of different orders of stratigraphic sequences and bounding surfaces based on their relative importance. Within a hierarchical system, the most important sequence is recognized as of ‘‘first-order’’ and may be subdivided into two or more ‘‘second-order’’ sequences. In turn, a 2nd-order sequence may be subdivided into two or more ‘‘third-order’’ sequences, and so on. The more important sequences are designated as ‘‘high-order’’ (at the top of the hierarchy pyramid), and usually occur with a low frequency in the stratigraphic record. The less important sequences are of lower-order, and occur more frequently in the rock record (Catuneanu et al., 2005). In this context, time is largely irrelevant as a parameter in the classification of Precambrian stratigraphic sequences, and it is rather the change in the tectonic setting that provides the key criterion for the basic subdivision of the rock record into basin-fill successions separated by 1storder sequence boundaries (Catuneanu et al., 2005). Similarly, Martins-Neto et al. (2001) proposed a hierarchic ordering based on the steps involved in the tectonic evolution of the basin. According to these authors, a 1storder sequence (or megasequence) is bounded by 1st-order unconformities and encompasses assemblages evolved under the influence of a single-coherent tectonic regime, such as a complete passive-margin succession. A 2nd-order sequence, bounded by 2nd-order unconformities, represents a package related to different stages/mechanisms of subsidence, but genetically linked to a common tectonic regime, like a rift-stage succession of a passive-margin basin. A 3rd-order sequence is bounded by 3rd-order unconformities and includes assemblages representing systems tracts genetically linked to a common subsidence mechanism stage, such as deposits related to synrift pulses of a rift-stage succession or depositional sequences related to major eustatic cycles in a passive-margin, 2nd-order succession. First-order sequences correspond therefore to entire sedimentary basin-fills, regardless of the origin and life span of each particular basin. These 1st-order basin-fill successions are in turn subdivided into 2nd- and lowerorder sequences as a function of the shifts in the balance
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Fig. 2. Chronostratigraphic chart of southeastern Brazil (southern Sa˜o Francisco craton, Brası´ lia and Arac- uai fold belts) showing the 1st-order basin-fill cycles (modified after Martins-Neto and Alkmim, 2001). Gray areas represent main stratigraphic hiatuses. The Phanerozoic sequences (Santa Fe´ and Areado/Mata da Corda/Urucuia) are not emphasized in this paper.
between accommodation and sedimentation at various scales of observation, irrespective of the time span between two same-order consecutive events. Sequences identified in any particular basin are not expected to correlate to other 1st- and lower-order sequences of other basins, which may have a different timing and duration (Catuneanu et al., 2005).
covered by Proterozoic (the subject of this paper) and Phanerozoic successions (Fig. 2). The Late Paleoproterozoic to Early Mesoproterozoic Espinhac- o 1st-order sequence comprises the record of a stage of aborted lithospheric stretching, whereas the Late Mesoproterozoic to Late Neoproterozoic gas-bearing Sa˜o Francisco basin records a complete cycle of supercontinent breakup and assembly.
3. Stratigraphic framework of eastern Brazil 3.1. Espinhac- o 1st-order sequence The Basement of eastern Brazil, represented by the Sa˜o Francisco-Congo continental mass that had amalgamated during the Transamazonian-Eburnian orogeny (ca. 2.2– 2.0 Ga; Trompette, 1994; Alkmim and Marshak, 1998), is
The Paleo/Mesoproterozoic (ca. 1.73–1.50 Ga) metasedimentary Espinhac- o 1st-order sequence occurs in the northern Sa˜o Francisco craton (Paramirim Aulacogen,
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Cruz and Alkmim, 2006) and in the Arac- uaı´ fold-andthrust belt, eastern Brazil (Martins-Neto, 2000). Sedimentologic, paleogeographic, stratigraphic, structural and tectonic studies on the Espinhac- o 1st-order sequence in the Arac- uaı´ belt indicate deposition in a rift-sag basin (Martins-Neto, 2000). The basin displays a ‘‘steer’s-head’’ geometry where four basin evolution stages are recognized (prerift, rift, transitional and flexural). These four stages form the basic subdivision of the 1st-order basin-fill sequence, and therefore can be equated with unconformity-bounded 2nd-order sequences. The unconformities are recognized in the field and mapped on a regional scale. The prerift and rift 2nd-order sequences of the Espinhac- o basin consist of nonmarine depositional systems, deposited during continental lithospheric stretching. The recognition of 3rd-order, rift-propagation unconformities allowed the
definition and mapping of three 3rd-order synrift sequences within the 2nd-order rift sequence (Fig. 3). The first marine incursion within the Espinhac- o basin marks the change in the subsidence regime of the basin from extensional to thermally driven, due to thermal contraction of the lithosphere during cooling, during which the transitional and flexural stages of the Espinhac- o riftsag basin evolved (for details see Martins-Neto, 2000). The transitional stage was characterized by relatively low subsidence rates. Higher subsidence rates and a consequent base level rise characterized the flexural stage of the Espinhac- o basin. At an increasing level of detail, the ca. 900-m-thick 2nd-order flexural (‘‘Conselheiro Mata’’) sequence of the Espinhac- o basin can be subdivided into three 3rd-order transgressive–regressive sequences (Fig. 4). These 3rd-order
Fig. 3. Sequence stratigraphy of rift deposits of the Espinhac- o basin, showing three syn-rift 3rd-order sequences within the 2nd-order rift sequence. The photograph shows part of the lower sequences (Natureza and Sa˜o Joa˜o da Chapada), emphasizing the 3rd-order bounding surface in the field.
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Fig. 4. Schematic stratigraphic cross-section showing the three 3rd-order transgressive–regressive sequences of the Conselheiro Mata 2nd-order sequence of the Espinhac- o 1st-order sequence (modified after Dupont 1995). The eastern section is about 900 m thick.
sequences were first recognized by Dupont (1995), each of them with a transgressive base and a progradational top. The first sequence (250–400 m thick) contains transgressive barred nearshore deposits and progradational beach to shallow-marine deposits. Its maximum flooding surface marks the greatest expansion of the Espinhac- o sea (Martins-Neto, 2000). The second sequence (250–350 m thick) comprises transgressive shelf deposits overlain by progradational alluvial plain to coastal successions. The third sequence (200–300 m thick) is defined by transgressive mixed siliciclastic-carbonate shelf deposits overlain by coastal to fluvial sediments. The top of the third sequence marks the final filling of the Espinhac- o basin, and therefore corresponds to a 1st-order sequence boundary. Each of these 3rd-order sequences has an estimated duration in the range of tens of millions of years, and they are considered to be the product of in-plane stress variations (Martins-Neto, 2000). 3.2. Rodı´nia–Gondwana 1st-order sequences The Late Mesoproterozoic to Early Cambrian Rodinia– Gondwana succession of the Sa˜o Francisco craton as well as of the adjacent Brası´ lia and Arac- uaı´ orogenic belts (Fig. 1) comprises the record of successive 1st-order basin-fill cycles, which have been combined in order to define the larger Sa˜o Francisco basin (Martins-Neto, 2005, 2007; Martins-Neto et al., 2001). Integrated seismic, well and outcrop studies in this package attest to sedimentation having started with the development of a full passivemargin basin setting on each side of the Sa˜o Francisco paleocontinent (Canastra–Paranoa´–Vazante basin to the west and Macau´bas basin to the east) and having ended with a convergent cycle related to the assembly of West Gondwana (arc-related and foreland basins) (Fig. 5). The total thickness of the package in the western Sa˜o Francisco craton area can reach ca. 10 km. The approximately W–E
regional seismic section across the Sa˜o Francisco craton (Fig. 6) shows the main stratigraphic and structural features of its cover sedimentary successions. 3.2.1. Canastra–Paranoa´–Vazante 1st-order sequence According to seismic data, the Late Mesoproterozoic to Early Neoproterozoic passive-margin Canastra–Paranoa´– Vazante Megasequence can be divided into two 2nd-order sequences (Fig. 6): rift and passive margin (Martins-Neto, 2005, 2007). The passive-margin 2nd-order sequence can be further subdivided into three 3rd-order sequences (lower, intermediate and upper sequences). The unit has an overall wedge shape, with maximum thickness reaching ca. 6000 m in the westernmost part of the area (Fig. 6). The rift deposits occur in the western domains of the Sa˜o Francisco craton in normal-fault controlled depocenters (Fig. 6). The lower passive-margin 3rd-order sequence is characterized by a stratified seismofacies, also occurring in the western part of the Sa˜o Francisco craton. The intermediate passive-margin 3rd-order sequence shows a widespread distribution over the Sa˜o Francisco craton, reaching its eastern domains (Fig. 6), suggesting that this unit represents the record of the maximum expansion of the Canastra–Paranoa´–Vazante sea. The upper part of the passive-margin 2nd-order sequence outcrops in the surrounds of the town of Cristalina, Goia´s State (Fig. 1), where it can be directly related to seismic data. The unit in this area is composed of shelf siliciclastic mudstones/siltstones (rhythmites), which grade upward to shallow-marine well-sorted quartz-sandstones (Fig. 7a). Other outcrops of the Canastra–Paranoa´–Vazante sequence occur only within thrust sheets of the Brası´ lia belt. Shallow-marine sandstones predominate in the Canastra Range area (Fig. 1). In the Vazante area, further to the north, the unit consists mainly of shallow-marine, peliticdolomitic deposits, which contains black shales/marls with
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Fig. 5. Stratigraphic column of the Sa˜o Francisco basin showing its Late Mesoproterozoic to Late Neoproterozoic 1st-order sequences (modified after Martins-Neto and Alkmim, 2001). Dashed lines separate the tectonic domains.
Fig. 6. Interpreted reflection seismic profile across the Sa˜o Francisco craton (see Fig. 1 for location) showing the recognized 1st-order sequences, as well as lower-order sequences of the Canastra–Paranoa´–Vazante passive-margin 1st-order sequence (modified after Martins-Neto, 2005). See also the tectonic overprint of the western Brası´ lia and eastern Arac- uaı´ thrust-and-fold belts, both verging to the craton interior. Vertical scale in two-way travel time.
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Fig. 7. (a) Shallow-marine well-sorted quartz-sandstones of the upper part of the Canastra–Paranoa´–Vazante passive-margin 1st-order sequence. (b) Conophyton and columnar (c) stromatolites of the Canastra–Paranoa´–Vazante passive-margin 1st-order sequence, Vazante area. (d) Black shales of the Canastra–Paranoa´–Vazante sequence in outcrop and drill core (e). Note in (e) association with inter-tidal carbonates with bird’s eye structures (examples arrowed), indicating a coastal, shallow-water environment for their deposition. (f) Turbidites and debris-flow diamictites of the rift-stage (lower 2nd-order sequence) of the Macau´bas passive-margin 1st-order sequence. Note person for scale (circled).
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intervals showing high total organic carbon (TOC) contents (average 3–4% and maximum value of 15.6%), comprising one of the most potential hydrocarbon source rocks of the Sa˜o Francisco basin. Locally, sandstones, diamictites and stromatolite beds (Fig. 7b, c) can also be observed. The black shales (Fig. 7d) occur intercalated with inter-tidal carbonates exhibiting bird’s eye structures (Fig. 7e), indicating a coastal, shallow-water environment for their deposition. Further to the north, in the Brası´ lia area (Fig. 1), algal-mat and stromatolite-bearing carbonates occur associated with the shallow-marine siliciclastics (Dardenne, 2000). 3.2.2. Macau´bas 1st-order sequence The early Neoproterozoic Macau´bas basin (Figs. 5 and 6) comprises two 2nd-order sequences: rift and passive margin (Martins-Neto et al., 2001; Pedrosa-Soares et al., 2008). The rift 2nd-order sequence includes glacio-continental and glacio-marine deposits. Continental glaciogenic rift deposits comprise ice-proximal alluvial-fan, fluvial and lacustrine facies (Karfunkel and Hoppe, 1988; Martins-Neto and Hercos, 2002). In response to glacioeustatic sea-level fluctuations, these interfinger distally with shallow-marine deposits, characterized by sandstones, siltstones and pelites, with parallel-lamination, wave ripple marks and hummocky cross-stratification (Karfunkel and Hoppe, 1988; Martins-Neto and Hercos, 2002). Further to the east in the Arac- uaı´ fold belt (Fig. 1) the unit is composed of marine diamictites, conglomerates, sandstones (turbidites), wackestones and pelites (Fig. 7f) deposited by sediment gravity flows (Uhlein et al., 1999; Martins-Neto and Hercos, 2002). The passive-margin 2nd-order sequence includes wackestones, sandstones and pelites, that accumulated in a shelf-slope-deep-sea depositional system, as well as a volcanic-exhalative unit (associated with ultramafic rocks, Ribeira˜o da Folha ophiolites) inferred to be the remnant of the northernmost branch of the Adamastor ocean (Pedrosa-Soares et al., 1998, 2008). 3.2.3. Bambuı´ 1st-order sequence Carbonate to siliciclastic deposits of the Bambuı´ 1storder sequence (Mid to Late Neoproterozoic) overlie much of the Sa˜o Francisco craton (Figs. 5 and 6). Seismic data indicate a classic wedge shape for the Bambuı´ foreland deposits, with thicknesses of ca. 4000 m to the west, at the depocenter, and of ca. 800 m to the east, close to the flexural border (Fig. 6). Bambuı´ rocks are also exposed in the Brasiliano/Pan African orogenic belts surrounding the Sa˜o Francisco craton, mainly in the Brası´ lia belt on its western flank. The stratigraphic and structural frameworks of the Bambuı´ 1st-order sequence indicate deposition in a foreland basin generated during thrusting and crustal loading in the Brası´ lia fold belt along the western margin of the Sa˜o Francisco paleocontinent (Martins-Neto and Alkmim, 2001; Martins-Neto et al., 2001; Martins-Neto, 2005,
2007), related to ca. 800–550 Ma arc–continent to continent–continent collision (Pimentel et al., 2000). A foreland setting is supported by (i) the overall geometry of the basin as indicated by regional reflection seismic profiles, in which deposits define an eastward-tapering wedge, ca. 4 km thick adjacent to the Brası´ lia fold belt but only a few hundred meters thick close to the Arac- uaı´ fold belt (Fig. 6); (ii) the tectonic imbrication between Brası´ lia fold-belt slices and locally derived Bambuı´ deposits (Sambura´ Formation); (iii) the regional distribution of depositional systems; and (iv) by provenance studies that indicate mixed (magmatic arc, plutonic and metamorphic) sources and derivation from erosion of the hinterland during thrusting (Guimara˜es and Dardenne, 1998; Castro and Dardenne, 2000; Coelho, 2007). The Bambuı´ 1st-order foreland basin sequence shows two distinct successions, which reflect the main basin paleogeographic settings. The first is located in the western domains of the preserved basin and related to the main basin depocenter (foredeep), where deep-water, mainly siliciclastic, turbidite-bearing deposits predominate. These deposits are totally involved in the thin-skinned imbricated fan, which characterizes the western part of the basin. A second succession is recognized in the central and eastern domains, related to the flexural ramp of the basin, and shows the classic stratigraphy of the Bambuı´ 1st-order sequence, where fine-grained siliciclastic rocks are interbedded with carbonates (Fig. 8). The well 1-RF-1-MG (Fig. 9; Fugita and Clark, 2001) is located in this domain, showing the Bambuı´ sequence with approximately 1750 m
Fig. 8. Stratigraphic column of the Bambuı´ foreland 1st-order sequence (modified after Martins-Neto and Alkmim, 2001).
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Fig. 9. Simplified composite log of well 1-RF-1-MG (modified after Fugita and Clark, 2001), showing the lithostratigraphic units and 2ndorder sequences of the 1st-order Bambuı´ sequence.
preserved thickness, as the base of the well is very close to the Bambuı´ basal unconformity. Three 2nd-order sequences (transgressive–progradational ‘‘megacycles’’, Dardenne, 1981) can be recognized in the ramp succession of the Bambuı´ basin (Figs. 8, 9). The basal cycle consists of thin (up to 80 m) conglomerates and diamictites (Carrancas Formation, Fig. 10a) covered by mudstones/siltstones interbedded with limestones (locally with stromatolites, Fig. 10b), dolostones and argillaceous limestones (Sete Lagoas Formation). The mixed siliciclastic-carbonate ramp prograded westward over the deeper-water succession, producing the first transgressive– progradational 2nd-order sequence (Fig. 8). The Carrancas diamictites occur regionally as thin and discontinuous bodies and have been interpreted as the record of the Sturtian glaciation on the Sa˜o Francisco craton (e.g., Martins-Neto and Alkmim, 2001). A Sete Lagoas Formation cap carbonate has been dated (Pb–Pb whole-rock isochron, Babinski and Kaufman, 2003), yielding an
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age of 740722 Ma, supporting the Sturtian age of the glacial event. The second transgressive–progradational 2nd-order sequence (Fig. 8) begins with the shales, siltstones and argillaceous limestones of the Serra de Santa Helena Formation, over which limestones, calcarenites, oolitic limestones and siltstones (deposited in a storm influenced ramp, Lagoa do Jacare´ Formation, Fig. 10e, f) prograded. The uppermost transgressive–progradational 2nd-order sequence consists at its base of the marine shales and siltstones of the Serra da Saudade Formation, which is covered by the siltstones, arkoses quartz sandstones and conglomerates of the Treˆs Marias Formation, deposited in a storm dominated shallow-marine system, passing upwards to alluvial environments (Fig. 8). Seismic interpretation tied to the well 1-RF-1-MG, and surface geology allowed the recognition and mapping of the flexural ramp Bambuı´ succession (Fig. 11). Adjacent to the Brasilia fold belt, proximal foreland basin deposits of the Bambuı´ 1st-order sequence (foredeep domain), invariably involved in the Brası´ lia belt thrusting and folding (see Fig. 6), consist of fan-delta conglomerates and sandstones (Sambura´ Formation, Fig. 8), as well as marine shales, which were deposited in the main depocenter of the basin. Gravity flow deposits (turbidite-bearing submarine fans) can be found interbedded with the deepmarine shales (Fig. 12a, b), mainly deformed into the thinskinned belt (see Fig. 6). The paleogeography of the Bambuı´ foreland basin can be reconstructed (i) by the provenance of the clastic deposits and (ii) by the distribution of the carbonate facies. Paleocurrent data of the topmost Treˆs Marias Formation (e.g., Chiavegatto, 1992) indicate sourcing from W and NW and overall sediment dispersal in a N–S direction. The regional distribution of carbonate facies shows shallowwater, stromatolitic carbonates to the east (Fig. 10b–d), locally slumped, slope carbonates in the central region of the basin, and thin, condensed calcilutites to the west, the last interbedded with deeper-water turbidite-shale facies. The data above indicate an approximately NS-elongated basin, with the main depocenter to the west, close to the orogenic front, and a ramp setting located to the east, close to the basin flexural border, with the basin bathymetry increasing from east to west. 4. Tectonic evolution The 1st-order sequences described above as belonging to the Sa˜o Francisco basin evolved during Rodinia breakup and West Gondwana assembly, while the Espinhac- o 1storder sequence represents an older tectonic cycle (Fig. 13). The fragmentation process of the paleocontinent Rodinia, which was made up by an Archean to Paleoproterozoic basement crosscut by a network of Late Paleoproterozoic to Early Mesoproterozoic failed rifts (including the Espinhac- o 1st-order sequence, Fig. 13a), probably started at the end of the Mesoproterozoic, as attested by the ages
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Fig. 10. (a) Diamictites of the Carrancas Formation (probable record of the Sturtian glaciation), at the base of the Bambuı´ sequence (see Fig. 8), overlain by Sete Lagoas Formation cap carbonates. Note lens cap for scale. (b) Bambuı´ 1st-order sequence (top of Sete Lagoas Formation, see Fig. 8) stromatolite mats. (c) Detail of stromatolite mats showing deflection by current to the left of the photo. Note diagenetic overgrowth of aragonite needles (example arrowed). Note also thinning-upward character of the mats inside a cycle. (d) Columnar stromatolites Sete Lagoas Formation, Bambuı´ Group. (e) Lagoa do Jacare´ Formation, Bambuı´ sequence (see Fig. 8), high-frequency, prograding succession (parasequence), showing a transition from storm-influenced rhythmites at the base to coastal oolitic calcarenites at the top. Scale is 10 cm long. (f) Carbonate storm beds, Lagoa do Jacare´ Formation, Bambuı´ sequence. Note preserved hummockies at quarry floor.
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Fig. 11. Seismic profile in the cratonic area of the Sa˜o Francisco basin showing the seismic expression of 1st-order sequences and sequence boundaries. The Bambuı´ 1st-order sequence is composed of three 2nd-order sequences (see also Figs. 8 and 9). The Canastra–Paranoa´–Vazante 1st-order sequence represents passive-margin deposits of the western flank of the Sa˜o Francisco craton, whose closure generated the Brası´ lia fold belt (modified after Catuneanu et al., 2005). Vertical scale in two-way travel time.
of volcano-sedimentary successions in the Brasilia belt (Dardenne, 2000; Pimentel et al., 2000), as well as by ages of detrital zircons in Neoproterozoic sediments (Valeriano et al., 2004; Coelho, 2007). An initial rift system evolved to a classic passive margin, defining the western margin of the Sa˜o Francisco-Congo paleocontinent. This stage is represented by the successions of the Canastra–Paranoa´– Vazante 1st-order sequence (Figs. 6, 13b). Rifting processes started on the eastern Sa˜o Francisco paleocontinent at about 900 Ma (Martins-Neto et al., 2001; Pedrosa-Soares et al., 2008), leading to breakup and establishment of a passive margin with a narrow oceanic basin. The Macau´bas 1st-order sequence represents the record of the rift to passive-margin basin (Fig. 13c, d). The onset of westward subduction processes and the development of magmatic arcs also started at about 900 Ma (Pimentel et al., 1997, 1999) as a consequence of consumption of oceanic lithosphere of the Brazilide ocean beneath the Amazon craton. On the other side of the vast Brazilide ocean, at its junction with the Sa˜o Francisco paleocontinent, the passive margin continued to evolve up to about 750 Ma (Fig. 13c), when accretionary processes probably started, marking the end of the passive-margin setting on the western border of the Sa˜o Francisco paleocontinent (Fig. 13d). After 750 Ma, westward subduction and the accretion of magmatic arcs and a microcontinent (Goia´s terrane) led to the development of the Brası´ lia fold belt (accretionary orogen stage, Fig. 13d), culminating in two continent– continent collisions (collisional orogen stage, Fig. 13e): Parana´-Sa˜o Francisco (southern Brası´ lia belt) and Amazon-Sa˜o Francisco (northern Brası´ lia belt) (Valeriano et al., 2004). During the time interval of the orogenic processes (750–550 Ma), the cratonic portion of the Sa˜o Francisco paleocontinent behaved as a flexural basin in
response to the tectonic load of the encroaching Brası´ lia fold belt (Fig. 13d, e), where the Bambuı´ 1st-order sequence was deposited. Eastward subduction and magmatic-arc construction initiated at the eastern part of the Sa˜o Francisco paleocontinent at ca. 630 Ma, and the Salinas succession (Lima et al., 2002; Pedrosa-Soares et al., 2008) was deposited in an arc-related setting (Fig. 13e). This scenario evolved up to 585 Ma (Pedrosa-Soares et al., 2001, 2008), when collisional processes closed the ocean and erected the Arac- uaı´ /West Congo orogen (Fig. 13f), deforming the eastern domains of the Bambuı´ deposits. 5. Conclusions Sequence stratigraphy was applied to organize the Proterozoic supracrustal rocks of the gas-bearing Sa˜o Francisco craton and surrounding thrust-and-fold belts (eastern Brazil), using the hierarchy approach described by Catuneanu et al. (2005). According to these authors, a sequence hierarchy involves the separation of different orders of stratigraphic sequences and bounding surfaces based on their relative importance. This approach has also contributed to the better understanding of the basin evolution through time. Four genetic stratigraphic units have been identified, which constitute 1st-order basinfill sequences (i.e., the Espinhac- o, Canastra–Paranoa´– Vazante, Macau´bas and Bambuı´ sequences). The Espinhac- o 1st-order sequence (ca. 1.73–1.50 Ga) records a stage of aborted lithospheric stretching of the Sa˜o Francisco-Congo continental mass. The Espinhac- o basin displays a ‘‘steer’s-head’’ geometry, where four 2nd-order sequences are recognized (prerift, rift, transitional and flexural). Third-order rift-propagation unconformities allowed the definition of three 3rd-order synrift sequences
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Fig. 12. Turbidites of the Bambuı´ foreland 1st-order sequence: road-cut outcrop in (a) and detail photograph in (b).
within the rift 2nd-order sequence. The 2nd-order flexural sequence of the Espinhac- o basin can be subdivided into three 3rd-order transgressive–regressive sequences. Integrated seismic, well and outcrop studies in the Late Mesoproterozoic to Early Cambrian Rodinia-Gondwana succession attest to sedimentation having started with the development of a full passive-margin basin setting on each side of the Sa˜o Francisco paleocontinent (Canastra– Paranoa´–Vazante basin to the west and Macau´bas basin to the east) and having ended with a convergent cycle related to the assembly of West Gondwana (arc-related and foreland basins). The passive-margin Canastra–Paranoa´–Vazante 1st-order sequence can be divided into two 2nd-order sequences: rift and passive margin. The passive-margin
2nd-order sequence can be further subdivided into three 3rd-order sequences. The outcropping Canastra–Paranoa´– Vazante sequence consists mainly of shallow-marine deposits, which contain black shales/marls with intervals showing high TOC contents, comprising one of the highest potential hydrocarbon source rocks of the Sa˜o Francisco basin. The Bambuı´ foreland 1st-order sequence (ca. 750– 550 Ma) formed as a consequence of thrust loading related to shortening in the Brası´ lia fold belt on the western flank of the Sa˜o Francisco craton, during the closing of the Brazilide ocean and the assembly of the West Gondwana supercontinent. Three transgressive–progradational 2ndorder sequences can be recognized in the ramp succession of the Bambuı´ basin.
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Fig. 13. Cartoon showing the geologic evolution of the southern Sa˜o Francisco basin (modified after Martins-Neto and Alkmim, 2001). Ages of each tectonic phase represent maximum and minimum values.
Acknowledgments This work is a synthesis of the results of several research projects carried out in the last 20 years and I acknowledge several colleagues and graduate students for numerous discussions and valuable contributions. Support for this work over the years has been provided by the FAPEMIGResearch Support Foundation of Minas Gerais State, Brazil, CNPq-Brazilian National Research Council, and by the Petrobras-Petro´leo Brasileiro S.A. I also thank the ANP-Brazilian National Petroleum Agency for permission to use seismic data for academic purposes. Comments and suggestions by the MPG reviewers Jorge Magalhaes and Patricia Corcoran, as well as by Editor-in-Chief Octavian Catuneanu, were very helpful in improving the quality of this paper. References Alkmim, F.F., Marshak, S., 1998. Transamazonian orogeny in the southern Sa˜o Francisco craton region, Minas Gerais, Brazil: evidence for paleoproterozoic collision and collapse in the Quadrila´tero Ferrı´ fero. Precambrian Research 90, 29–58.
Alkmim, F.F., Martins-Neto, M.A., 2001. A bacia intracratoˆnica do Sa˜o Francisco: arcabouc- o estrutural e cena´rios evolutivos. In: Pinto, C.P., Martins-Neto, M. A., (Eds.), Bacia do Sa˜o Francisco: Geologia e Recursos Naturais. Belo Horizonte (SBG/MG), pp. 9–30. Babinski, M., Kaufman, A.J., 2003. First direct dating of a Neoproterozoic post-glacial cap carbonate. In: Fourth South American Symposium on Isotope Geology, Salvador, Brazil, Short Papers, pp. 321–323. Castro, P.T.A., Dardenne, M.A., 2000. The sedimentology, stratigraphy and tectonic context of the Sa˜o Francisco supergroup at the southern boundary of the Sa˜o Francisco craton, Brazil. Revista Brasileira de Geocieˆncias 30, 345–437. Catuneanu, O., 2006. Principles of Sequence Stratigraphy. Elsevier, Amsterdam, 375pp. Catuneanu, O., Martins-Neto, M.A., Eriksson, P.G., 2005. Precambrian sequence stratigraphy. Sedimentary Geology 176, 67–95. Chiavegatto, J.R.S., 1992. Ana´lise estratigra´fica das sequeˆncias tempestı´ ticas da Fm. Treˆs Marias (Proterozo´ico Sup.), na porc- a˜o meridional da bacia do Sa˜o Francisco. M.Sc. Thesis, Geology Departament, Federal University of Ouro Preto, 196pp. Coelho, J.C.C., 2007. Caracterizac- a˜o estratigra´fica e estrutural da porc- a˜o oeste da Bacia Neoproterozo´ica do Sa˜o Francisco na zona transicional da Faixa Brası´ lia e Cra´ton do Sa˜o Francisco, com base em dados de campo e sı´ smica de reflexa˜o. M.Sc. Thesis, Geology Departament, Federal University of Ouro Preto, Contribuic- o˜es a`s Cieˆncias da Terra, vol. 42, 114pp. Cruz, S.C.P., Alkmim, F.F., 2006. The tectonic interaction between the Parmirin Aulacogen and the Arac- uaı´ Belt, Sa˜o Francisco craton
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Precambrian Basin Evolution: Change Over Time. Sedimentary Geology Special Issue, vol. 141, pp. 343–370. Pedrosa-Soares, A.C., Vidal, P., Leonardos, O.H., Brito Neves, B.B., 1998. Neoproterozoic oceanic remnants in eastern Brazil: further evidence and refutation of an exclusively ensialic evolution for the Arac- uaı´ -West Congo orogen. Geology 26, 519–522. Pedrosa-Soares, A.C., Noce, C.M., Wiedemann, C.M., Pinto, C.P., 2001. The Arac- uaı´ –West Congo orogen in Brazil: an overview of a confined orogen formed during Gondwanland assembly. Precambrian Research 110, 307–323. Pedrosa-Soares, A.C., Alkmim, F.F., Tack, L., Noce, C.M., Babinski, M., Silva, L.C., Martins-Neto, M.A., 2008. Similarities and differences between the Brazilian and African counterparts of the Neoproterozoic Arac- uaı´ –West Congo orogen. Gondwana Res., in press. Pimentel, M.M., Whitehouse, M.J., Viana, M.G., Fuck, R.A., Machado, N., 1997. The Mara Rosa arc in Tocantins province: further evidence for Neoproterozoic crustal accretion in Central Brazil. Precambrian Research 81, 299–310. Pimentel, M.M., Fuck, R.A., Botelho, N.F., 1999. Granites and the geodynamic history of the Neoproterozoic Brasilia belt, central Brazil: a review. Lithos 46, 463–483. Pimentel, M.M., Fuck, R.A., Jost, H., Ferreira, C.F., Arau´jo, S.M., 2000. The basement of the Brası´ lia fold belt and the Goia´s magmatic arc. In: Cordani, U. G., Thomaz Filho, A., Campos, D.A., (Eds.), Tectonic evolution of South America. Rio de Janeiro 31st International Geological Congress, pp. 195–229. Posamentier, H.W., Vail, G.P., 1988. Eustatic controls on clastic deposition II—sequence and systems tract models. In: Wilgus, C.K., Hastings, B.S., Kendall, C.G., Posamentier, H.W., Ross, C.A., Van Wagoner, J.C. (Eds.), Sea Level Changes—An Integrated Approach, vol. 42. SEPM Spec. Publ., pp. 125–154. Posamentier, H.W., Allen, G.P., 1999. Siliciclastic sequence stratigraphy: concepts and aplications. SEPM Concepts in Sedimentology and Paleontology 7, 210. Posamentier, H.W., Jervey, M.T., Vail, P.R., 1988. Eustatic controls on clastic deposition I—conceptual framework. In: Wilgus, C.K., Hastings, B.S., Kendall, C.G., Posamentier, H.W., Ross, C.A., Van Wagoner, J.C. (Eds.), Sea Level Changes—An Integrated Approach, vol. 42. SEPM Spec. Publ., pp. 109–124. Trompette, R., 1994. Geology of Western Gondwana (2000–500 Ma): Pan-African-Brasiliano Aggregation of South America and Africa. Balkema, Rotterdam, 350pp. Uhlein, A., Trompette, R., Alvarenga, C.J.S., 1999. Neoproterozoic glacial and gravitational sedimentation on a continental rifted margin: the Jequitaı´ –Macau´bas sequence (Minas Gerais, Brazil). Journal of South American Earth Sciences 12, 435–451. Vail, P.R., Mitchum Jr., R.M. Todd, R.G., Widmier, J.M., Thompson III, S., Sangree, J.B., Bubb, J.N., Hatlelid, W.G., 1977. Seismic stratigraphy and global changes of sea-level. In: Payton, C.E., (Ed.), Seismic Stratigraphy—Applications to Hydrocarbon Exploration. AAPG Memoir, vol. 26, pp. 49–212. Valeriano, C.M., Machado, N., Simonetti, A., Valladares, C.S., Seer, H.J., Simo˜es, L.S.A., 2004. U-Pb geochronology of the southern Brası´ lia belt (SE-Brazil): sedimentary provenance, Neoproterozoic orogeny and assembly of West Gondwana. Precambrian Research 130, 27–55. Weimer, P., Posamentier, H.W., (Eds.), 1993. Siliciclastic Sequence Stratigraphy. AAPG Memoir, vol. 58, 492pp.
Marine and Petroleum Geology 26 (2009) 163–176 www.elsevier.com/locate/marpetgeo
Sequence stratigraphic framework of Proterozoic successions in eastern Brazil Marcelo A. Martins-Neto Geology Department, School of Mines, Federal University of Ouro Preto, NUPETRO—Nucleus of Petroleum Geology of the Gorceix Foundation, Caixa Postal 173, 35 400-000 Ouro Preto, MG, Brazil Received 9 August 2007; received in revised form 28 September 2007; accepted 4 October 2007
Abstract First-order regional unconformities separate Proterozoic supracrustal rocks of the gas-bearing Sa˜o Francisco craton and surrounding thrust-and-fold belts (eastern Brazil) into four genetic stratigraphic units, which constitute 1st-order basin-fill sequences (Espinhac- o, Canastra–Paranoa´–Vazante, Macau´bas and Bambuı´ sequences). The Espinhac- o 1st-order sequence (ca. 1.73–1.50 Ga) records a stage of aborted lithospheric stretching of the Sa˜o Francisco-Congo continental mass. The Espinhac- o basin displays a ‘‘steer’s-head’’ geometry, where four 2nd-order sequences are recognized (prerift, rift, transitional and flexural). Third-order rift-propagation unconformities allowed the definition of three 3rd-order synrift sequences within the rift 2nd-order sequence. The 2nd-order flexural sequence of the Espinhac- o basin can be subdivided into three 3rd-order transgressive–regressive sequences. The Canastra–Paranoa´–Vazante and Macau´bas 1st-order sequences (ca. 1200–630 Ma) comprise rift to drift successions deposited during the breakup of the supercontinent Rodinia and the opening of the Brazilide and Adamastor oceans, respectively, to the west and east of the Sa˜o Francisco paleocontinent. According to seismic data, the passive-margin Canastra–Paranoa´–Vazante 1st-order sequence can be divided into two 2nd-order sequences: rift and passive margin. The passive-margin 2nd-order sequence can be further subdivided into three 3rd-order sequences. The outcropping Canastra–Paranoa´–Vazante sequence consists mainly of shallow-marine deposits, which contain black shales/marls with intervals showing high total organic carbon (TOC) contents, comprising one of the highest potential hydrocarbon source rocks of the Sa˜o Francisco basin. The Bambuı´ foreland 1st-order sequence (ca. 750–550 Ma) formed as a consequence of thrust loading related to shortening in the Brası´ lia fold belt on the western flank of the Sa˜o Francisco craton, during the closing of the Brazilide ocean and the assembly of the West Gondwana supercontinent. The Bambuı´ sequence shows two distinct successions, which reflect the main basin paleogeographic settings: foredeep and flexural ramp. Three transgressive–progradational 2nd-order sequences can be recognized in the ramp succession of the Bambuı´ basin. r 2007 Published by Elsevier Ltd. Keywords: Precambrian; Proterozoic; Sequence stratigraphy; Eastern Brazil; Sa˜o Francisco craton; Sa˜o Francisco basin
1. Introduction The record of the Late Paleoproterozoic to Late Neoproterozoic geodynamic history of eastern Brazil (Sa˜o Francisco craton as well as Brası´ lia and Arac- uaı´ fold Tel.: +55 31 3559 7164; fax: +55 31 3559 7174.
E-mail addresses: [email protected], [email protected] (M.A. Martins-Neto). 0264-8172/$ - see front matter r 2007 Published by Elsevier Ltd. doi:10.1016/j.marpetgeo.2007.10.001
belts, Fig. 1) is preserved by the sedimentary successions of the Espinhac- o, Canastra–Paranoa´–Vazante, Macau´bas and Bambuı´ basins, the last three being considered together as the gas-bearing Sa˜o Francisco basin (Martins-Neto et al., 2001; Martins-Neto and Alkmim, 2001; MartinsNeto, 2005, 2007). Each of these major sedimentary successions represents a 1st-order sequence (or megasequence), which is the record of an unconformity-bounded, single basin-fill cycle (Martins-Neto et al., 2001; Catuneanu
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Fig. 1. Simplified geologic map of eastern Brazil (modified after Alkmim and Martins-Neto, 2001). Note location of seismic profile of Fig. 6.
et al., 2005). Collectively, these basins track major plate reorganizations that affected the Sa˜o Francisco craton through a time interval greater than 1 Gy. The purpose of this paper is to present a synthesis of the sequence stratigraphic framework of the Proterozoic successions in eastern Brazil, describing the major sedimentary 1st-order sequences preserved in each of these basins and their geodynamic significance in a plate tectonic scenario. The sequence stratigraphic approach adopted herein (following Martins-Neto et al., 2001; Catuneanu et al., 2005; Catuneanu, 2006) is summarized in the first part of the paper. Since the gas-bearing Sa˜o Francisco basin is an exploration frontier area with poor data coverage, a wellconstrained geologic model is a fundamental step in modeling its petroleum system. 2. Precambrian sequence stratigraphy Sequence stratigraphy studies the change in depositional trends (stacking patterns) in response to the interplay of accommodation and sediment supply, regardless of scale and age of the strata under analysis (e.g., Vail et al., 1977;
Posamentier and Vail, 1988; Posamentier et al., 1988; Weimer and Posamentier, 1993; Emery and Myers, 1996; Posamentier and Allen, 1999; Catuneanu, 2006). Therefore, the method of sequence stratigraphy can be applied from individual depositional systems to entire sedimentary basin-fills, as well as in Precambrian to Phanerozoic successions. In a recent paper, Catuneanu et al. (2005) discuss the principles of sequence stratigraphy and its applicability to Precambrian sedimentary successions. Since Precambrian basins are often characterized by poor stratal preservation and by a general lack of time control, an approach based on the stratigraphic hierarchy is the best way to interpret the stratigraphic framework and understand basin evolution. Sequence hierarchy involves the separation of different orders of stratigraphic sequences and bounding surfaces based on their relative importance. Within a hierarchical system, the most important sequence is recognized as of ‘‘first-order’’ and may be subdivided into two or more ‘‘second-order’’ sequences. In turn, a 2nd-order sequence may be subdivided into two or more ‘‘third-order’’ sequences, and so on. The more important sequences are designated as ‘‘high-order’’ (at the top of the hierarchy pyramid), and usually occur with a low frequency in the stratigraphic record. The less important sequences are of lower-order, and occur more frequently in the rock record (Catuneanu et al., 2005). In this context, time is largely irrelevant as a parameter in the classification of Precambrian stratigraphic sequences, and it is rather the change in the tectonic setting that provides the key criterion for the basic subdivision of the rock record into basin-fill successions separated by 1storder sequence boundaries (Catuneanu et al., 2005). Similarly, Martins-Neto et al. (2001) proposed a hierarchic ordering based on the steps involved in the tectonic evolution of the basin. According to these authors, a 1storder sequence (or megasequence) is bounded by 1st-order unconformities and encompasses assemblages evolved under the influence of a single-coherent tectonic regime, such as a complete passive-margin succession. A 2nd-order sequence, bounded by 2nd-order unconformities, represents a package related to different stages/mechanisms of subsidence, but genetically linked to a common tectonic regime, like a rift-stage succession of a passive-margin basin. A 3rd-order sequence is bounded by 3rd-order unconformities and includes assemblages representing systems tracts genetically linked to a common subsidence mechanism stage, such as deposits related to synrift pulses of a rift-stage succession or depositional sequences related to major eustatic cycles in a passive-margin, 2nd-order succession. First-order sequences correspond therefore to entire sedimentary basin-fills, regardless of the origin and life span of each particular basin. These 1st-order basin-fill successions are in turn subdivided into 2nd- and lowerorder sequences as a function of the shifts in the balance
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Fig. 2. Chronostratigraphic chart of southeastern Brazil (southern Sa˜o Francisco craton, Brası´ lia and Arac- uai fold belts) showing the 1st-order basin-fill cycles (modified after Martins-Neto and Alkmim, 2001). Gray areas represent main stratigraphic hiatuses. The Phanerozoic sequences (Santa Fe´ and Areado/Mata da Corda/Urucuia) are not emphasized in this paper.
between accommodation and sedimentation at various scales of observation, irrespective of the time span between two same-order consecutive events. Sequences identified in any particular basin are not expected to correlate to other 1st- and lower-order sequences of other basins, which may have a different timing and duration (Catuneanu et al., 2005).
covered by Proterozoic (the subject of this paper) and Phanerozoic successions (Fig. 2). The Late Paleoproterozoic to Early Mesoproterozoic Espinhac- o 1st-order sequence comprises the record of a stage of aborted lithospheric stretching, whereas the Late Mesoproterozoic to Late Neoproterozoic gas-bearing Sa˜o Francisco basin records a complete cycle of supercontinent breakup and assembly.
3. Stratigraphic framework of eastern Brazil 3.1. Espinhac- o 1st-order sequence The Basement of eastern Brazil, represented by the Sa˜o Francisco-Congo continental mass that had amalgamated during the Transamazonian-Eburnian orogeny (ca. 2.2– 2.0 Ga; Trompette, 1994; Alkmim and Marshak, 1998), is
The Paleo/Mesoproterozoic (ca. 1.73–1.50 Ga) metasedimentary Espinhac- o 1st-order sequence occurs in the northern Sa˜o Francisco craton (Paramirim Aulacogen,
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Cruz and Alkmim, 2006) and in the Arac- uaı´ fold-andthrust belt, eastern Brazil (Martins-Neto, 2000). Sedimentologic, paleogeographic, stratigraphic, structural and tectonic studies on the Espinhac- o 1st-order sequence in the Arac- uaı´ belt indicate deposition in a rift-sag basin (Martins-Neto, 2000). The basin displays a ‘‘steer’s-head’’ geometry where four basin evolution stages are recognized (prerift, rift, transitional and flexural). These four stages form the basic subdivision of the 1st-order basin-fill sequence, and therefore can be equated with unconformity-bounded 2nd-order sequences. The unconformities are recognized in the field and mapped on a regional scale. The prerift and rift 2nd-order sequences of the Espinhac- o basin consist of nonmarine depositional systems, deposited during continental lithospheric stretching. The recognition of 3rd-order, rift-propagation unconformities allowed the
definition and mapping of three 3rd-order synrift sequences within the 2nd-order rift sequence (Fig. 3). The first marine incursion within the Espinhac- o basin marks the change in the subsidence regime of the basin from extensional to thermally driven, due to thermal contraction of the lithosphere during cooling, during which the transitional and flexural stages of the Espinhac- o riftsag basin evolved (for details see Martins-Neto, 2000). The transitional stage was characterized by relatively low subsidence rates. Higher subsidence rates and a consequent base level rise characterized the flexural stage of the Espinhac- o basin. At an increasing level of detail, the ca. 900-m-thick 2nd-order flexural (‘‘Conselheiro Mata’’) sequence of the Espinhac- o basin can be subdivided into three 3rd-order transgressive–regressive sequences (Fig. 4). These 3rd-order
Fig. 3. Sequence stratigraphy of rift deposits of the Espinhac- o basin, showing three syn-rift 3rd-order sequences within the 2nd-order rift sequence. The photograph shows part of the lower sequences (Natureza and Sa˜o Joa˜o da Chapada), emphasizing the 3rd-order bounding surface in the field.
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Fig. 4. Schematic stratigraphic cross-section showing the three 3rd-order transgressive–regressive sequences of the Conselheiro Mata 2nd-order sequence of the Espinhac- o 1st-order sequence (modified after Dupont 1995). The eastern section is about 900 m thick.
sequences were first recognized by Dupont (1995), each of them with a transgressive base and a progradational top. The first sequence (250–400 m thick) contains transgressive barred nearshore deposits and progradational beach to shallow-marine deposits. Its maximum flooding surface marks the greatest expansion of the Espinhac- o sea (Martins-Neto, 2000). The second sequence (250–350 m thick) comprises transgressive shelf deposits overlain by progradational alluvial plain to coastal successions. The third sequence (200–300 m thick) is defined by transgressive mixed siliciclastic-carbonate shelf deposits overlain by coastal to fluvial sediments. The top of the third sequence marks the final filling of the Espinhac- o basin, and therefore corresponds to a 1st-order sequence boundary. Each of these 3rd-order sequences has an estimated duration in the range of tens of millions of years, and they are considered to be the product of in-plane stress variations (Martins-Neto, 2000). 3.2. Rodı´nia–Gondwana 1st-order sequences The Late Mesoproterozoic to Early Cambrian Rodinia– Gondwana succession of the Sa˜o Francisco craton as well as of the adjacent Brası´ lia and Arac- uaı´ orogenic belts (Fig. 1) comprises the record of successive 1st-order basin-fill cycles, which have been combined in order to define the larger Sa˜o Francisco basin (Martins-Neto, 2005, 2007; Martins-Neto et al., 2001). Integrated seismic, well and outcrop studies in this package attest to sedimentation having started with the development of a full passivemargin basin setting on each side of the Sa˜o Francisco paleocontinent (Canastra–Paranoa´–Vazante basin to the west and Macau´bas basin to the east) and having ended with a convergent cycle related to the assembly of West Gondwana (arc-related and foreland basins) (Fig. 5). The total thickness of the package in the western Sa˜o Francisco craton area can reach ca. 10 km. The approximately W–E
regional seismic section across the Sa˜o Francisco craton (Fig. 6) shows the main stratigraphic and structural features of its cover sedimentary successions. 3.2.1. Canastra–Paranoa´–Vazante 1st-order sequence According to seismic data, the Late Mesoproterozoic to Early Neoproterozoic passive-margin Canastra–Paranoa´– Vazante Megasequence can be divided into two 2nd-order sequences (Fig. 6): rift and passive margin (Martins-Neto, 2005, 2007). The passive-margin 2nd-order sequence can be further subdivided into three 3rd-order sequences (lower, intermediate and upper sequences). The unit has an overall wedge shape, with maximum thickness reaching ca. 6000 m in the westernmost part of the area (Fig. 6). The rift deposits occur in the western domains of the Sa˜o Francisco craton in normal-fault controlled depocenters (Fig. 6). The lower passive-margin 3rd-order sequence is characterized by a stratified seismofacies, also occurring in the western part of the Sa˜o Francisco craton. The intermediate passive-margin 3rd-order sequence shows a widespread distribution over the Sa˜o Francisco craton, reaching its eastern domains (Fig. 6), suggesting that this unit represents the record of the maximum expansion of the Canastra–Paranoa´–Vazante sea. The upper part of the passive-margin 2nd-order sequence outcrops in the surrounds of the town of Cristalina, Goia´s State (Fig. 1), where it can be directly related to seismic data. The unit in this area is composed of shelf siliciclastic mudstones/siltstones (rhythmites), which grade upward to shallow-marine well-sorted quartz-sandstones (Fig. 7a). Other outcrops of the Canastra–Paranoa´–Vazante sequence occur only within thrust sheets of the Brası´ lia belt. Shallow-marine sandstones predominate in the Canastra Range area (Fig. 1). In the Vazante area, further to the north, the unit consists mainly of shallow-marine, peliticdolomitic deposits, which contains black shales/marls with
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Fig. 5. Stratigraphic column of the Sa˜o Francisco basin showing its Late Mesoproterozoic to Late Neoproterozoic 1st-order sequences (modified after Martins-Neto and Alkmim, 2001). Dashed lines separate the tectonic domains.
Fig. 6. Interpreted reflection seismic profile across the Sa˜o Francisco craton (see Fig. 1 for location) showing the recognized 1st-order sequences, as well as lower-order sequences of the Canastra–Paranoa´–Vazante passive-margin 1st-order sequence (modified after Martins-Neto, 2005). See also the tectonic overprint of the western Brası´ lia and eastern Arac- uaı´ thrust-and-fold belts, both verging to the craton interior. Vertical scale in two-way travel time.
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Fig. 7. (a) Shallow-marine well-sorted quartz-sandstones of the upper part of the Canastra–Paranoa´–Vazante passive-margin 1st-order sequence. (b) Conophyton and columnar (c) stromatolites of the Canastra–Paranoa´–Vazante passive-margin 1st-order sequence, Vazante area. (d) Black shales of the Canastra–Paranoa´–Vazante sequence in outcrop and drill core (e). Note in (e) association with inter-tidal carbonates with bird’s eye structures (examples arrowed), indicating a coastal, shallow-water environment for their deposition. (f) Turbidites and debris-flow diamictites of the rift-stage (lower 2nd-order sequence) of the Macau´bas passive-margin 1st-order sequence. Note person for scale (circled).
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intervals showing high total organic carbon (TOC) contents (average 3–4% and maximum value of 15.6%), comprising one of the most potential hydrocarbon source rocks of the Sa˜o Francisco basin. Locally, sandstones, diamictites and stromatolite beds (Fig. 7b, c) can also be observed. The black shales (Fig. 7d) occur intercalated with inter-tidal carbonates exhibiting bird’s eye structures (Fig. 7e), indicating a coastal, shallow-water environment for their deposition. Further to the north, in the Brası´ lia area (Fig. 1), algal-mat and stromatolite-bearing carbonates occur associated with the shallow-marine siliciclastics (Dardenne, 2000). 3.2.2. Macau´bas 1st-order sequence The early Neoproterozoic Macau´bas basin (Figs. 5 and 6) comprises two 2nd-order sequences: rift and passive margin (Martins-Neto et al., 2001; Pedrosa-Soares et al., 2008). The rift 2nd-order sequence includes glacio-continental and glacio-marine deposits. Continental glaciogenic rift deposits comprise ice-proximal alluvial-fan, fluvial and lacustrine facies (Karfunkel and Hoppe, 1988; Martins-Neto and Hercos, 2002). In response to glacioeustatic sea-level fluctuations, these interfinger distally with shallow-marine deposits, characterized by sandstones, siltstones and pelites, with parallel-lamination, wave ripple marks and hummocky cross-stratification (Karfunkel and Hoppe, 1988; Martins-Neto and Hercos, 2002). Further to the east in the Arac- uaı´ fold belt (Fig. 1) the unit is composed of marine diamictites, conglomerates, sandstones (turbidites), wackestones and pelites (Fig. 7f) deposited by sediment gravity flows (Uhlein et al., 1999; Martins-Neto and Hercos, 2002). The passive-margin 2nd-order sequence includes wackestones, sandstones and pelites, that accumulated in a shelf-slope-deep-sea depositional system, as well as a volcanic-exhalative unit (associated with ultramafic rocks, Ribeira˜o da Folha ophiolites) inferred to be the remnant of the northernmost branch of the Adamastor ocean (Pedrosa-Soares et al., 1998, 2008). 3.2.3. Bambuı´ 1st-order sequence Carbonate to siliciclastic deposits of the Bambuı´ 1storder sequence (Mid to Late Neoproterozoic) overlie much of the Sa˜o Francisco craton (Figs. 5 and 6). Seismic data indicate a classic wedge shape for the Bambuı´ foreland deposits, with thicknesses of ca. 4000 m to the west, at the depocenter, and of ca. 800 m to the east, close to the flexural border (Fig. 6). Bambuı´ rocks are also exposed in the Brasiliano/Pan African orogenic belts surrounding the Sa˜o Francisco craton, mainly in the Brası´ lia belt on its western flank. The stratigraphic and structural frameworks of the Bambuı´ 1st-order sequence indicate deposition in a foreland basin generated during thrusting and crustal loading in the Brası´ lia fold belt along the western margin of the Sa˜o Francisco paleocontinent (Martins-Neto and Alkmim, 2001; Martins-Neto et al., 2001; Martins-Neto, 2005,
2007), related to ca. 800–550 Ma arc–continent to continent–continent collision (Pimentel et al., 2000). A foreland setting is supported by (i) the overall geometry of the basin as indicated by regional reflection seismic profiles, in which deposits define an eastward-tapering wedge, ca. 4 km thick adjacent to the Brası´ lia fold belt but only a few hundred meters thick close to the Arac- uaı´ fold belt (Fig. 6); (ii) the tectonic imbrication between Brası´ lia fold-belt slices and locally derived Bambuı´ deposits (Sambura´ Formation); (iii) the regional distribution of depositional systems; and (iv) by provenance studies that indicate mixed (magmatic arc, plutonic and metamorphic) sources and derivation from erosion of the hinterland during thrusting (Guimara˜es and Dardenne, 1998; Castro and Dardenne, 2000; Coelho, 2007). The Bambuı´ 1st-order foreland basin sequence shows two distinct successions, which reflect the main basin paleogeographic settings. The first is located in the western domains of the preserved basin and related to the main basin depocenter (foredeep), where deep-water, mainly siliciclastic, turbidite-bearing deposits predominate. These deposits are totally involved in the thin-skinned imbricated fan, which characterizes the western part of the basin. A second succession is recognized in the central and eastern domains, related to the flexural ramp of the basin, and shows the classic stratigraphy of the Bambuı´ 1st-order sequence, where fine-grained siliciclastic rocks are interbedded with carbonates (Fig. 8). The well 1-RF-1-MG (Fig. 9; Fugita and Clark, 2001) is located in this domain, showing the Bambuı´ sequence with approximately 1750 m
Fig. 8. Stratigraphic column of the Bambuı´ foreland 1st-order sequence (modified after Martins-Neto and Alkmim, 2001).
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Fig. 9. Simplified composite log of well 1-RF-1-MG (modified after Fugita and Clark, 2001), showing the lithostratigraphic units and 2ndorder sequences of the 1st-order Bambuı´ sequence.
preserved thickness, as the base of the well is very close to the Bambuı´ basal unconformity. Three 2nd-order sequences (transgressive–progradational ‘‘megacycles’’, Dardenne, 1981) can be recognized in the ramp succession of the Bambuı´ basin (Figs. 8, 9). The basal cycle consists of thin (up to 80 m) conglomerates and diamictites (Carrancas Formation, Fig. 10a) covered by mudstones/siltstones interbedded with limestones (locally with stromatolites, Fig. 10b), dolostones and argillaceous limestones (Sete Lagoas Formation). The mixed siliciclastic-carbonate ramp prograded westward over the deeper-water succession, producing the first transgressive– progradational 2nd-order sequence (Fig. 8). The Carrancas diamictites occur regionally as thin and discontinuous bodies and have been interpreted as the record of the Sturtian glaciation on the Sa˜o Francisco craton (e.g., Martins-Neto and Alkmim, 2001). A Sete Lagoas Formation cap carbonate has been dated (Pb–Pb whole-rock isochron, Babinski and Kaufman, 2003), yielding an
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age of 740722 Ma, supporting the Sturtian age of the glacial event. The second transgressive–progradational 2nd-order sequence (Fig. 8) begins with the shales, siltstones and argillaceous limestones of the Serra de Santa Helena Formation, over which limestones, calcarenites, oolitic limestones and siltstones (deposited in a storm influenced ramp, Lagoa do Jacare´ Formation, Fig. 10e, f) prograded. The uppermost transgressive–progradational 2nd-order sequence consists at its base of the marine shales and siltstones of the Serra da Saudade Formation, which is covered by the siltstones, arkoses quartz sandstones and conglomerates of the Treˆs Marias Formation, deposited in a storm dominated shallow-marine system, passing upwards to alluvial environments (Fig. 8). Seismic interpretation tied to the well 1-RF-1-MG, and surface geology allowed the recognition and mapping of the flexural ramp Bambuı´ succession (Fig. 11). Adjacent to the Brasilia fold belt, proximal foreland basin deposits of the Bambuı´ 1st-order sequence (foredeep domain), invariably involved in the Brası´ lia belt thrusting and folding (see Fig. 6), consist of fan-delta conglomerates and sandstones (Sambura´ Formation, Fig. 8), as well as marine shales, which were deposited in the main depocenter of the basin. Gravity flow deposits (turbidite-bearing submarine fans) can be found interbedded with the deepmarine shales (Fig. 12a, b), mainly deformed into the thinskinned belt (see Fig. 6). The paleogeography of the Bambuı´ foreland basin can be reconstructed (i) by the provenance of the clastic deposits and (ii) by the distribution of the carbonate facies. Paleocurrent data of the topmost Treˆs Marias Formation (e.g., Chiavegatto, 1992) indicate sourcing from W and NW and overall sediment dispersal in a N–S direction. The regional distribution of carbonate facies shows shallowwater, stromatolitic carbonates to the east (Fig. 10b–d), locally slumped, slope carbonates in the central region of the basin, and thin, condensed calcilutites to the west, the last interbedded with deeper-water turbidite-shale facies. The data above indicate an approximately NS-elongated basin, with the main depocenter to the west, close to the orogenic front, and a ramp setting located to the east, close to the basin flexural border, with the basin bathymetry increasing from east to west. 4. Tectonic evolution The 1st-order sequences described above as belonging to the Sa˜o Francisco basin evolved during Rodinia breakup and West Gondwana assembly, while the Espinhac- o 1storder sequence represents an older tectonic cycle (Fig. 13). The fragmentation process of the paleocontinent Rodinia, which was made up by an Archean to Paleoproterozoic basement crosscut by a network of Late Paleoproterozoic to Early Mesoproterozoic failed rifts (including the Espinhac- o 1st-order sequence, Fig. 13a), probably started at the end of the Mesoproterozoic, as attested by the ages
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Fig. 10. (a) Diamictites of the Carrancas Formation (probable record of the Sturtian glaciation), at the base of the Bambuı´ sequence (see Fig. 8), overlain by Sete Lagoas Formation cap carbonates. Note lens cap for scale. (b) Bambuı´ 1st-order sequence (top of Sete Lagoas Formation, see Fig. 8) stromatolite mats. (c) Detail of stromatolite mats showing deflection by current to the left of the photo. Note diagenetic overgrowth of aragonite needles (example arrowed). Note also thinning-upward character of the mats inside a cycle. (d) Columnar stromatolites Sete Lagoas Formation, Bambuı´ Group. (e) Lagoa do Jacare´ Formation, Bambuı´ sequence (see Fig. 8), high-frequency, prograding succession (parasequence), showing a transition from storm-influenced rhythmites at the base to coastal oolitic calcarenites at the top. Scale is 10 cm long. (f) Carbonate storm beds, Lagoa do Jacare´ Formation, Bambuı´ sequence. Note preserved hummockies at quarry floor.
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Fig. 11. Seismic profile in the cratonic area of the Sa˜o Francisco basin showing the seismic expression of 1st-order sequences and sequence boundaries. The Bambuı´ 1st-order sequence is composed of three 2nd-order sequences (see also Figs. 8 and 9). The Canastra–Paranoa´–Vazante 1st-order sequence represents passive-margin deposits of the western flank of the Sa˜o Francisco craton, whose closure generated the Brası´ lia fold belt (modified after Catuneanu et al., 2005). Vertical scale in two-way travel time.
of volcano-sedimentary successions in the Brasilia belt (Dardenne, 2000; Pimentel et al., 2000), as well as by ages of detrital zircons in Neoproterozoic sediments (Valeriano et al., 2004; Coelho, 2007). An initial rift system evolved to a classic passive margin, defining the western margin of the Sa˜o Francisco-Congo paleocontinent. This stage is represented by the successions of the Canastra–Paranoa´– Vazante 1st-order sequence (Figs. 6, 13b). Rifting processes started on the eastern Sa˜o Francisco paleocontinent at about 900 Ma (Martins-Neto et al., 2001; Pedrosa-Soares et al., 2008), leading to breakup and establishment of a passive margin with a narrow oceanic basin. The Macau´bas 1st-order sequence represents the record of the rift to passive-margin basin (Fig. 13c, d). The onset of westward subduction processes and the development of magmatic arcs also started at about 900 Ma (Pimentel et al., 1997, 1999) as a consequence of consumption of oceanic lithosphere of the Brazilide ocean beneath the Amazon craton. On the other side of the vast Brazilide ocean, at its junction with the Sa˜o Francisco paleocontinent, the passive margin continued to evolve up to about 750 Ma (Fig. 13c), when accretionary processes probably started, marking the end of the passive-margin setting on the western border of the Sa˜o Francisco paleocontinent (Fig. 13d). After 750 Ma, westward subduction and the accretion of magmatic arcs and a microcontinent (Goia´s terrane) led to the development of the Brası´ lia fold belt (accretionary orogen stage, Fig. 13d), culminating in two continent– continent collisions (collisional orogen stage, Fig. 13e): Parana´-Sa˜o Francisco (southern Brası´ lia belt) and Amazon-Sa˜o Francisco (northern Brası´ lia belt) (Valeriano et al., 2004). During the time interval of the orogenic processes (750–550 Ma), the cratonic portion of the Sa˜o Francisco paleocontinent behaved as a flexural basin in
response to the tectonic load of the encroaching Brası´ lia fold belt (Fig. 13d, e), where the Bambuı´ 1st-order sequence was deposited. Eastward subduction and magmatic-arc construction initiated at the eastern part of the Sa˜o Francisco paleocontinent at ca. 630 Ma, and the Salinas succession (Lima et al., 2002; Pedrosa-Soares et al., 2008) was deposited in an arc-related setting (Fig. 13e). This scenario evolved up to 585 Ma (Pedrosa-Soares et al., 2001, 2008), when collisional processes closed the ocean and erected the Arac- uaı´ /West Congo orogen (Fig. 13f), deforming the eastern domains of the Bambuı´ deposits. 5. Conclusions Sequence stratigraphy was applied to organize the Proterozoic supracrustal rocks of the gas-bearing Sa˜o Francisco craton and surrounding thrust-and-fold belts (eastern Brazil), using the hierarchy approach described by Catuneanu et al. (2005). According to these authors, a sequence hierarchy involves the separation of different orders of stratigraphic sequences and bounding surfaces based on their relative importance. This approach has also contributed to the better understanding of the basin evolution through time. Four genetic stratigraphic units have been identified, which constitute 1st-order basinfill sequences (i.e., the Espinhac- o, Canastra–Paranoa´– Vazante, Macau´bas and Bambuı´ sequences). The Espinhac- o 1st-order sequence (ca. 1.73–1.50 Ga) records a stage of aborted lithospheric stretching of the Sa˜o Francisco-Congo continental mass. The Espinhac- o basin displays a ‘‘steer’s-head’’ geometry, where four 2nd-order sequences are recognized (prerift, rift, transitional and flexural). Third-order rift-propagation unconformities allowed the definition of three 3rd-order synrift sequences
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Fig. 12. Turbidites of the Bambuı´ foreland 1st-order sequence: road-cut outcrop in (a) and detail photograph in (b).
within the rift 2nd-order sequence. The 2nd-order flexural sequence of the Espinhac- o basin can be subdivided into three 3rd-order transgressive–regressive sequences. Integrated seismic, well and outcrop studies in the Late Mesoproterozoic to Early Cambrian Rodinia-Gondwana succession attest to sedimentation having started with the development of a full passive-margin basin setting on each side of the Sa˜o Francisco paleocontinent (Canastra– Paranoa´–Vazante basin to the west and Macau´bas basin to the east) and having ended with a convergent cycle related to the assembly of West Gondwana (arc-related and foreland basins). The passive-margin Canastra–Paranoa´–Vazante 1st-order sequence can be divided into two 2nd-order sequences: rift and passive margin. The passive-margin
2nd-order sequence can be further subdivided into three 3rd-order sequences. The outcropping Canastra–Paranoa´– Vazante sequence consists mainly of shallow-marine deposits, which contain black shales/marls with intervals showing high TOC contents, comprising one of the highest potential hydrocarbon source rocks of the Sa˜o Francisco basin. The Bambuı´ foreland 1st-order sequence (ca. 750– 550 Ma) formed as a consequence of thrust loading related to shortening in the Brası´ lia fold belt on the western flank of the Sa˜o Francisco craton, during the closing of the Brazilide ocean and the assembly of the West Gondwana supercontinent. Three transgressive–progradational 2ndorder sequences can be recognized in the ramp succession of the Bambuı´ basin.
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Fig. 13. Cartoon showing the geologic evolution of the southern Sa˜o Francisco basin (modified after Martins-Neto and Alkmim, 2001). Ages of each tectonic phase represent maximum and minimum values.
Acknowledgments This work is a synthesis of the results of several research projects carried out in the last 20 years and I acknowledge several colleagues and graduate students for numerous discussions and valuable contributions. Support for this work over the years has been provided by the FAPEMIGResearch Support Foundation of Minas Gerais State, Brazil, CNPq-Brazilian National Research Council, and by the Petrobras-Petro´leo Brasileiro S.A. I also thank the ANP-Brazilian National Petroleum Agency for permission to use seismic data for academic purposes. Comments and suggestions by the MPG reviewers Jorge Magalhaes and Patricia Corcoran, as well as by Editor-in-Chief Octavian Catuneanu, were very helpful in improving the quality of this paper. References Alkmim, F.F., Marshak, S., 1998. Transamazonian orogeny in the southern Sa˜o Francisco craton region, Minas Gerais, Brazil: evidence for paleoproterozoic collision and collapse in the Quadrila´tero Ferrı´ fero. Precambrian Research 90, 29–58.
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