Tectonics and sedimentation in a paleo mesoproterozoic ...directory.umm.ac.id/Data Elmu/jurnal/P/Precambrian Research/Vol103... · Tectonics and sedimentation in a paleo:mesoproterozoic

Precambrian Research 103 (2000) 147–173

Tectonics and sedimentation in a paleo/mesoproterozoicrift-sag basin (Espinhaco basin, southeastern Brazil)

Marcelo A. Martins-NetoDepartamento de Geologia, Escola de Minas, Uni6ersidade Federal de Ouro Preto, Caixa Postal 173,

Campus Morro do Cruzeiro s/n, 35400-000 Ouro Preto, MG, Brazil

Received 2 June 1999; accepted 20 March 2000

Abstract

Sedimentologic, paleogeographic, stratigraphic, structural and tectonic studies in a paleo/mesoproterozoic metased-imentary succession (Espinhaco Megasequence, southeastern Brazil) indicates deposition in a rift-sag basin. Fourbasin evolution stages are recognized (prerift, rift, transitional and flexural). The four stages can be represented by sixunconformity-bounded tectonosequences. The unconformities are recognized in the field and mappable even on aregional scale. The prerift and rift stages of the Espinhaco basin were filled by products of continental depositionalsystems. The prerift stage probably represents the first product of the rifting process, before the development of thehalf-grabens that characterize the rift stage. During the rift stage, mechanical subsidence due to lithospheric stretchingwas predominant and led to episodic rising of the depositional base level. As a result, the basin fill is characterizedby coarsening-upward intervals. Paleocurrent patterns indicate that block tilting and half-graben subsidence/upliftcontrolled sediment dispersion. The first marine incursion within the Espinhaco basin marks the change in thesubsidence regime of the basin. The evolution of the transitional and flexural stages was probably controlled bythermal subsidence due to thermal contraction of the lithosphere during cooling. The transitional stage wascharacterized by relatively low subsidence rates. Higher subsidence rates and a consequent sea-level rise characterizethe flexural stage of the Espinhaco basin, in which three second-order transgressive-progradational sequences can berecognized. © 2000 Elsevier Science B.V. All rights reserved.

Keywords: Brazil; Proterozoic; Tectonostratigraphic units; Basin analysis; Tectonics; Sedimentation

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1. Introduction

Rift-sag basins represent the tectonic product ofaborted passive-margin development (Allen andAllen, 1990). These basins, commonly referred toas aulacogens or failed rifts, display a ‘steer’s

head’ geometry due to early rifting and sub-sequent thermally-driven downwarping (Whiteand McKenzie, 1988). The stratigraphic frame-work, structural style, and tectonic evolution ofrift-sag basins have been interpreted mainlythrough seismic reflection profiles (e.g. Meyerhoff,1982; Schlee and Hinz, 1987; Badley et al., 1988).With exceptions (e.g. Eriksson et al., 1993), fewE-mail address: [email protected] (M.A. Martins-Neto).

0301-9268/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.

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M.A. Martins-Neto / Precambrian Research 103 (2000) 147–173148

Fig. 1.

M.A. Martins-Neto / Precambrian Research 103 (2000) 147–173 149

examples based on direct field observations havebeen published.

Deposits of the paleo/mesoproterozoic Espin-haco basin outcrop in the central and western partsof the Serra do Espinhaco, southeastern Brazil(Fig. 1). Because of inversion during the neo-proterozoic (Brasiliano/Pan African orogeny), andbecause of a deep erosional level, the Espinhacobasin provides an opportunity to evaluate thepaleogeographic and tectonic evolution of a rift-sagbasin through outcrop data. Of particularimportance, deposits recording initial rifting areexposed. These constitute a valuable source of datato characterize the three-dimensional architectureof an early half-graben, and to document therelationships between tectonics and sedimentationthat might not be available through geophysicalstudies alone.

This paper presents a model for thetectono-sedimentary evolution of the Espinhacorift-sag basin, based on geological mapping andintegrated sedimentologic, paleogeographic,stratigraphic, structural and tectonic studies,carried out in the central and western parts of thesouthern Serra do Espinhaco, Brazil (Fig. 1). Earlyworkers (e.g. Pflug, 1965) described the Espinhacosupergroup as a miogeosynclinal sequence.Subsequently, a rift to passive-margin setting wasproposed (Pflug et al., 1980), and Martins-Neto(1993), Schobbenhaus (1993), Dussin and Dussin(1995) suggested, based on the ensialic setting andstratigraphic framework, deposition in anintracratonic basin, with an initial rift phase and asubsequent flexural phase. The present paperprovides detail documenting the prerift, rift,transitional and flexural stages of basin evolution.

2. Regional setting

The southern Serra do Espinhaco belongs to theexternal zone of the Aracuaı fold-thrust belt,

Fig. 2. Lithostratigraphy of the southern Serra do Espinhacoafter Pflug (1968), Fogaca et al. (1984), Dossin et al. (1984),Almeida Abreu and Pflug (1992) (modified after Martins-Neto, 1993). Geochronological ages are from (1) Machadoet al. (1989), (2) Dussin and Dussin (1995) and (3) Buch-waldt et al. (1999). MRBU, major regional bounding uncon-formity.

Fig. 1. Location of the studied areas (numbered insets) and localities cited in the text in the Southern Serra do Espinhaco. Inset 1,central part of the southern Serra do Espinhaco; inset 2, western part of the southern Serra do Espinhaco and Serra Mineira; inset3, eastern domain of the Serra do Cabral; inset 4, Serra da Agua Fria. Location of Fig. 5 shown inside inset 1. Regional map ofthe Sao Francisco craton simplified from Alkmim and Marshak (1998). Geology of the southern Serra do Espinhaco simplified fromPedrosa-Soares et al. (1994).


Fig. 3. Stratigraphic framework and tectono-depositional features of the paleo/meso and neoproterozoic cover sequences of the SaoFrancisco craton and external zone of the Aracuaı fold belt.

along the southeastern margin of the neo-proterozoic Sao Francisco craton, southeasternBrazil (Fig. 1; Brito Neves and Cordani, 1991;Trompette et al., 1992; Schobbenhaus, 1993;Alkmim et al., 1993; Martins-Neto, 1998a). Thepaleo/mesoproterozoic Espinhaco supergroup (Fig.2) is the predominant unit in the southern Serra doEspinhaco, overlying the archean basementcomplex and supracrustal rocks of the archean/paleoproterozoic Rio Parauna supergroup uncon-formably (Pflug, 1965). It is in turn unconformablyoverlain by the neoproterozoic Sao Franciscosupergroup (Fig. 2; Pflug and Renger, 1973;Dupont, 1996; Martins-Neto et al., 1997a,b, 1999a).

The maximum age of the Espinhaco supergroupis limited by U/Pb dating of metamorphic zirconin pre-Espinhaco units (1844915 Ma; Machado etal., 1989), and depositional ages from magmatic

zircons in volcanic-bearing units in the lower partsof the Espinhaco supergroup include, (1) 1711 Ma(U/Pb; N. Machado oral commun., 1993;in Schobbenhaus, 1993); (2) 1710912 Ma(207Pb/206Pb; Dussin and Dussin, 1995); and(3) 171592 Ma (U/Pb; Machado et al., 1989). Theminimum age of the Espinhaco supergroup isless constrained, and is currently defined by basicintrusions that cut the entire column (ca. 1.1–0.9 Ga; Brito Neves et al., 1979; Machado et al.,1989).

The southern Serra do Espinhaco was deformedand metamorphosed in the Brasiliano/Pan Africanevent (650–500 Ma, Brito Neves et al., 1979;Marshak and Alkmim, 1989; Schobbenhaus, 1993) ,a major collisional episode that generated afold-trust terrane throughout the southern Serra doEspinhaco (Herrgesell and Pflug, 1986).


Fig. 4. Stratigraphic chart for the Espinhaco Megasequence showing the main characteristics of the tectono-sedimentary units(modified from Martins-Neto, 1995a).

3. Stratigraphic framework

The Espinhaco supergroup is a thick (�4000m) succession of siliciclastic metasedimentaryrocks containing subordinate volcanic intervals(Fig. 2). These have been grouped lithostratigraph-ically into nine formations (Pflug, 1968; AlmeidaAbreu and Pflug, 1992). Herein, adopting a geneticstratigraphic approach, the Espinhaco supergroupis referred to as the ‘Espinhaco Megasequence’,which is the record of an unconformity-bounded,single basin-fill cycle. The basin-fill cycle is com-posed of six tectonosequences, from bottom to top(Figs. 3 and 4), the Olaria tectonosequence (preriftstage), the Natureza, Sao Joao da Chapada andSopa-Brumadinho tectonosequences (rift stage);the Galho do Miguel tectonosequence (transitionalstage) and the Conselheiro Mata tectonosequence(flexural stage). Each tectonosequence recordslinked depositional systems accumulated in a spe-cific tectonic phase of the basin, and is defined bymajor regional bounding unconformities (Fig. 4;Da Silva, 1993; Martins-Neto, 1995a,b). Theseunconformities represent periods of significant tec-tonically induced paleogeographic reorganization.

During the rift stage, mechanical subsidence due tolithospheric stretching controlled basin evolution.The transitional and flexural stages were con-trolled by thermal subsidence arising from con-traction of the lithosphere due to cooling.

The genetic stratigraphic approach adoptedherein is based on the definition of a major uncon-formity-bounded megasequence, allowing recogni-tion and analysis of a true basin entity, eventhough its life span is not well constrained. Eachtectonosequence represents the record of an evolu-tionary stage of the basin, with its own accommo-dation history. Contacts are recognized in the fieldand mappable even on a regional scale (Fig. 5).

4. Prerift stage: Olaria tectonosequence

The prerift stage of the Espinhaco basin isrepresented by the poorly-exposed Olaria tec-tonosequence. The unit is ca. 150 m thick. Itconsists mainly of sheet-like bodies of immaturefine-grained, parallel-stratified sandstones (Fig. 6)that contain local current ripples and cross-stratifi-cation, and are locally capped by thin layers of


Fig. 5. Geologic map of tectono-stratigraphic units of the central part of the southern Espinhaco range (see Fig. 1 for location),southeastern Brazil (modified from Reis, 1999; Euzebio, 1999; Martins-Neto et al., 1999b). Inset shows location of Fig. 7.


Fig. 6. Photograph showing sheet-flood sandstones of theOlaria braidplain deposits. Note hammer (circled) for scale.

terpretations are limited by poor outcrop andhigh strain, the lack of volcanic rocks and ofevidence of synsedimentary faults (in contrast tooverlying sequences) suggest that the Olaria tec-tonosequence was deposited in an initial sag thatwas the product of ductile stretching, before theupper crust had reached its elastic limit. How-ever, clastic dykes filled by conglomeratic sand-stones occur at the top of the Olariatectonosequence. These are truncated at the an-gular unconformity with the overlying Naturezatectonosequence, and are likely precursors of ac-tive shallow-level faulting.

5. Rift stage: Natureza; Sao Joao da Chapada;Sopa-Brumadinho tectonosequences

The rift stage of the Espinhaco basin is charac-terized by three phases all of which are boundedby angular unconformities (Figs. 4, 5 and 7),synrift 1 (Natureza tectonosequence); synrift 2(Sao Joao da Chapada tectonosequence); andsynrift 3 (Sopa-Brumadinho tectonosequence).

5.1. Synrift 1 phase (Natureza tectonosequence)

The Natureza tectonosequence is up to 200 mthick (cf. Reis, 1999). Massive and inverselygraded clast-supported conglomerates interpretedas debris-flow deposits (Fig. 8a) occur above the

mudstones (sericitic phyllites). The deposits arelikely high-energy ephemeral flood deposits simi-lar to those described by Tunbridge (1981, 1984).Some lenticular bodies composed of fine-grainedsandstones with high textural maturity occur inthe middle portion of the unit (Reis, 1999). Thesedisplay large-scale cross-stratification (sets up to3 m thick) with high-angle (30–35°) truncationsmay be suggestive of an eolian origin. Thicklayers of sericitic phyllites, which may representdeposition in ponds, occur locally.

The available data (Table 1) suggest that theOlaria tectonosequence was probably depositedin alluvial braidplains associated with restrictedlacustrine and eolian environments. Although in-

Table 1Summary of the depositional characteristics of the Olaria tectonosequence

Depositional Sedimentary featuresKey lithologies Depositional processsetting

Sheet floods under upper flowPoorly-sorted, medium toAlluvial Parallel-stratification, sheet likebraidplain fine-grained sandstones geometry regime

Poorly-sorted, medium to Trough cross-stratification 3D-dune migration (sensu Ashley,fine-grained sandstones 1990) under lower flow regime

Planar cross-stratificationPoorly-sorted, medium to 2D-dune migration (sansu Ashley,1990) under lower flow regimefine-grained sandstones

Fine-grained sandstones Ripple cross-lamination Current-ripple migrationFloodplain depositsPelites Thin mud drapesVertical accretionPelites Metric thick bodiesLacustrineEolian dune migrationMetric thick cross-beds withEolian Hypermature fine-grained

high-angle (30–35°) truncationssandstones


basal angular unconformity. The debris-flowconglomerates are associated with clast- andmatrix-supported stratified conglomerates con-sidered as traction deposits (Fig. 8b) and areinterpreted as having been deposited in an alluvialfan and braided stream depositional system. Theconglomeratic deposits are covered by a sandybraided fluvial succession (Fig. 8c) interbeddedwith eolian sandstones (Fig. 8d, Table 2) (Silva,1995; Reis, 1999).

Conglomerate sections and the angularcharacter of the unconformity at the base of theNatureza tectonosequence are suggestive oftectonic influence such as block tilting or selectiveuplift and/or subsidence. This signifies rupture ofthe upper crust and the development of the firsthalf-grabens in the Espinhaco basin. The limitedareal extent of the Natureza tectonosequencesuggests a restricted character of the basin at thistime. The absence of volcanic rocks infers astretching factor b (White and McKenzie, 1988)of less than 2 for this phase.

5.2. Synrift 2 phase: (Sao Joao da Chapadatectonosequence)

The Sao Joao da Chapada tectonosequence isup to 300 m thick. Locally, above the angularunconformity (Fig. 9), conglomerates and brecciascontain abundant sandstone clasts derived fromthe underlying Natureza tectonosequence. Thesehave been interpreted as fault-scarp talus depositsformed by cohesionless mass-flows, and are strongindicators of repeated local fault uplift and canni-balization as part of the rifting process (Martins-Neto, 1993).

The onset of volcanism in the evolution of theEspinhaco basin is marked by basic rocks near thebase of the Sao Joao da Chapada tectonose-quence (Hoppe and Otto, 1982; Uhlein, 1991;Schobbenhaus, 1993; Dussin, 1994; Dussin andDussin, 1995). Layers of hematitic phyllites withinthese units have been interpreted as volcanic bedsaltered by Paleoproterozoic subaerial weathering(lateritization) and subsequent metamorphism(Knauer and Schrank, 1993).

The Sao Joao da Chapada tectonosequencedefines an overall coarsening-upward successionfrom lacustrine to deltaic to fluvial deposits (Fig.10). Fine-grained sandstones and pelites at thebase, probably deposited in a storm-influencedlacustrine environment, represent rapid subsi-dence and abrupt basin deepening. These areoverlain by sandstone bodies of likely deltaicorigin that display a sigmoidal geometry and arearranged in coarsening- and thickening-upwardsequences. Prograding over the deltaic/lacustrinedeposits are extensive braidplain sandstones that

Fig. 7. Geologic map showing the angular character, denotedby strike and dip of the unconformity separating the synrift 1(Natureza) and synrift 2 (Sao Joao da Chapada) tectonose-quences (see Fig. 5 for location). Note that the eolian depositsof the Natureza tectonosequence were totally eroded in thesouthern part of the map (modified from Reis, 1999; Euzebio,1999; Martins-Neto et al., 1999b). The horizontal bar showslocation of Fig. 10.


Fig. 8. Photographs showing deposits of the synrift 1 Natureza tectonosequence, (a) Debris-flow conglomerates. Note inversegrading in the body comprising the lower half of the outcrop; (b) stratified, sheet-flood conglomerates. Note hammer (circled) forscale; (c) horizontally stratified (Sh) and trough cross-stratified sandstones (St) of braided fluvial origin; (d) metric-scale cross-stratified sandstones of eolian origin.

constitute ca. 80% of the Sao Joao da Chapadatectonosequence. These signify relatively reducedrates of subsidence (Martins-Neto, 1993, 1994).

The Sao Joao da Chapada braidplain deposits(Table 3, Fig. 11a) are characterized by sheet-likebodies of coarse and poorly-sorted sandstonesthat display unidirectional paleocurrents with re-markably little dispersion throughout the studyarea (Fig. 11b). The sandstones generally definefining- and thinning-upward sequences and areinvariably bounded by low-relief erosional sur-faces (Martins-Neto, 1994). Typical sequencesstart with parallel-stratified and low-angle cross-stratified sandstones (Sh/Sl) and are capped bytrough cross-stratified sandstones (St) and locally,mudstone layers, characteristic of high-energyephemeral streams (e.g. Miall and Gibling, 1978;Tunbridge, 1981, 1984; Lawrence and Williams,1987; Munoz et al., 1992). As detailed elsewhere

(Martins-Neto, 1994), the superposition of suchwaning-flood sheet sandstones built up a braid-plain that extended ca. 35 km transverse to theregional paleoslope (Fig. 12).

5.3. Synrift 3 phase (Sopa-Brumadinhotectonosequence)

The Sopa-Brumadinho tectonosequence recordsthe peak of extensional tectonics in the Espinhacobasin. Deposits of this phase define fault-blockrelated depocenters (see Fig. 5 and also cross-sec-tions in Fig. 16), and consist of siliciclasticmetasediments and bimodal metavolcanic rocks(greenstones, hematitic phyllites and rhyolites),reaching thicknesses up to 800 m (Reis, 1999;Martins-Neto et al., 1999b). The unit is limited atits base by a prominent angular unconformity andat its top by a marine transgressive surface. As


Table 2Facies of the Natureza tectonosequence and interpretation

Facies Interpretation

Massive to inversely Debris flowsgraded, clast-supportedconglomerates

Parallel-stratified, Traction depositsmatrix-supportedconglomerates

Parallel-stratified, Sheet floods under upperflow regimepoorly-sorted sandstones

Trough cross-stratified, Fluvial 3D-dune migration(sensu Ashley, 1990) underpoorly sorted sandstoneslower flow regime

Planar cross-stratified, Fluvial 2D-dune migrationpoorly sorted sandstones (sensu Ashley, 1990) under

lower flow regimeRipple cross-laminated Current-ripple migration

sandstonesFloodplain depositsPelitesEolian dune migrationHypermature fine-grained,

metric thick coss-beddedsandstones

Fig. 9. Photographs showing (a) view of the angular unconfor-mity separating the Natureza and Sao Joao da Chapadatectonosequences and (b) detail emphasizing the erosionalnature of this unconformity.

discussed in the following sub-headings, lacus-trine, fan-delta and fluvial sedimentation tookplace in half-grabens that were compartmental-ized by north-trending normal faults and east-trending transfer faults. Subsidence was episodicand controlled by block tilting in asymmetricgrabens.

5.3.1. Lacustrine, fan-delta and flu6ial depositsThe facies of the Sopa-Brumadinho tectonose-

quence are arranged in 40–80 m thick sequencesthat coarsen and then fine upward (Fig. 13a andb). At the base of each sequence are laminatedpelites (facies F). These are followed by, gradedand stratified sandstone beds (facies Sg); massive,parallel-stratified, trough cross-stratified, and pla-nar cross-stratified sandstones (facies Sm, Sh, St,and Sp); and predominantly debris flow conglom-eratic units (Table 4). The conglomerates then fineupward to various sandstone facies (Fig. 13a).Some sequences lack the lower turbiditic gradedsandstone component, and laminated mudstonespass directly to stacked streamflood sandstones(e.g. Fig. 13b). Mudlumps, formed by the rapid

loading of debris-flows onto water-saturatedmuds (Dailly, 1976; Lewis, 1997) are locally devel-oped. An approximately 200 m thick section ofparallel-stratified, trough cross-stratified and pla-nar cross-stratified sandstones (9pelites) inter-venes within the predominantly cyclic succession(Reis, 1999; Martins-Neto et al., 1999b).

Table 4 summarizes the 13 facies of the Sopa-Brumadinho tectonosequence and their interpre-tation. More detailed descriptions of these faciesand the sedimentary processes that generatedthem can be found in Martins-Neto (1995d,1996b).

These rocks were likely deposited in a lacustrinefan-delta to fluvial system (cf. Nemec and Steel,1988). The coarsening-upward and fining-upward


Fig. 10. (a) Outcrop section (see Fig. 7 for location) showing the overall coarsening-upward arrangement from lacustrine to deltaicto fluvial deposits of the Sao Joao da Chapada tectonosequence; (b) detail showing the thickening-upward arrangement of the initialdeltaic progradation.

Table 3Facies of the Sao Joao da Chapada braidplain deposits (codes modified after Miall, 1978)

Code Characteristics Interpretation

Massive to parallel-stratified conglomeratesGm Migration of longitudinal barsSh/Sl Sheet floods under upper flow regimeParallel-stratified to low-angle (B10°) cross-stratified

sandstonesTrough cross-stratified sandstonesSt 3D-dune migration (sensu Ashley, 1990) under lower

flow regimePlanar cross-stratified sandstonesSp 2D-dune migration (sensu Ashley, 1990) under lower

flow regimeCurrent-ripple migrationSr Ripple cross-laminated sandstonesFloodplain depositsF Pelites


Fig. 11. (a) Measured sedimentological sections of the Sao Joao da Chapada braidplain deposits. Geological map modified fromScholl and Fogaca (1981), Chaves et al. (1985). (b) Paleocurrent data from Sao Joao da Chapada braidplain (316 measurementsfrom cross-stratified sandstones). Modified after Martins-Neto (1993).

sequences are interpreted to represent repeatedepisodes of rapid flooding (basal laminated mudunits) followed by progradation of fan lobeswhich evolved from distal (subaqueous) to proxi-mal (subaerial) environments. The fining-upwardparts represent the abandonment phases of thedepositional lobes. Discriminating between lacus-trine and shallow marine deposits is commonlydifficult in Precambrian strata (e.g. Eriksson etal., 1998). A lacustrine origin is favored in thepresent example because pelitic rocks of facies Fdefine a number of isolated N–S elongate lenses.

5.3.2. Tectonically-dri6en cyclicityThe lacustrine, fan-delta and fluvial coarsening-

upward and fining-upward sequences are consid-

ered to represent repeated tectonically-drivenpulses (see e.g. Blair and Bilodeau, 1988). Accord-ingly, flood surfaces and pelitic rocks at the baseof each sequence are the immediate response tofault-induced subsidence of the basin floor, andsubsequent fan-delta progradation records a de-layed response, reflecting erosional stripping ofrelatively uplifted source areas. Fining-upwardtrends at the top of the sequences signify a reduc-tion in source-area relief during continued tec-tonic quiescence.

Those successions where fan-delta progradationinitiated with sandy turbidity currents (e.g. Fig.13a), may indicate deeper lakes formed by greatersubsidence, whereas those in which laminatedpelites pass directly to streamflow sandstones (e.g.


Fig. 13b) may imply shallower lakes. Smaller-scale pulses may be recorded by thin pelite layersinterbedded with some of the sandstones near thebase of the sequences (Fig. 13a and b and Fig.14a). Although post-depositional processes havestrongly modified the original thicknesses of theselayers, thickness variation suggests varying subsi-dence pulse magnitudes.

5.3.3. Half-graben geometryIn regional cross section, fan development is

asymmetric. In the west, diamond mining opera-tions have exposed the spatial and angular rela-tionships between two fan-deltas (Fig. 14a). Theyounger lobe (Lavrinha mine) forms a wedgepositioned basinward of an older fan segment(Diamante Vermelho mine). Correction for tec-tonic tilt using marine deposits of the overlyingGalho do Miguel tectonosequence as a paleohori-zontal datum, yields a primary dip of ca.10–15°for the older fan segment and ca. 5–10° for theyounger fan. These results are compatible withpublished data of recent fan slopes (cf. Wells,1984; Blair and McPherson, 1994). Basinwardstepping and increasing angle of tilt with age

(reflecting progressive rotation due to faulting) ischaracteristic of many fans, modern and ancient(e.g. Hooke, 1972; Steel et al., 1977; Heward,1978a,b; Kleinspehn et al., 1984; Blair, 1987). Incontrast to these basinward stepping lobes, to thenortheast, at the Brumadinho mine (Fig. 14b),clast-supported talus breccias form verticallystacked linear bodies in direct fault contact witholder rocks. The two types of fans (Fig. 15) aswell as the geologic map and the cross-sectionA–A% in Fig. 16 indicate an asymmetric-grabengeometry, with the Diamante Vermelho/Lavrinhasystem representing hanging-wall sourced fans atthe unfaulted ramping margin, and the Brumad-inho fans marking footwall sourced fans at thefaulted border (cf. Hooke, 1972; Gawthorpe andColella, 1990).

5.3.4. Synsedimentary faultsEast-trending transfer faults can be recognized

(Figs. 5 and 16). Paleocurrent patterns illustratethe control exerted by transfer faults and fault-bounded blocks on sediment dispersal (Fig. 16).North of an east-trending transfer fault located

Fig. 12. Schematic paleogeographical reconstruction of the Sao Joao da Chapada braidplain. L, longitudinal; T, transverse (sensuMiall, 1981). Not to scale (modified after Martins-Neto, 1993).


Fig. 13. Measured sedimentological sections of the Sopa-Brumadinho tectonosequence (a and b). Section 1 (a) was measured in theDiamante Vermelho and Lavrinha diamond mines (see also Fig. 14a) and section 2 (b) was measured in the Sopa mine (see Fig. 1for location of the localities). (c) Paleocurrent data from the Sopa-Brumadinho tectonosequence (rose diagram summarizes 319measurements from cross-stratified sandstones from the whole unit in the studied area). Modified after Martins-Neto (1993).


Table 4Facies of the Sopa-Brumadinho tectonosequence (conglomer-ate codes after Martins-Neto, 1993, 1996b; sandstone codesafter Miall, 1978)

Code InterpretationCharacteristics

UngradedUG-CS Cohesionless debriaclast-supported flowsconglomeratesInversely graded Cohesionless debrisIG-CSclast-supported flowsconglomeratesNormally gradedNG-CS Cohesionless debris

flowsclast-supporteconglomeratesCrudely stratified HyperconcentratedCS-CSclast-supported flowsconglomerates

TB-CS Fault-adjacent talusTalus-brecciaclast-supported brecciasconglomeratesSand matrix-supportedS-MS Cohesionless debris

flowsconglomeratesMud matrix-supported Cohesive debris flowsM-MSconglomerates

Sm Massive sandstones Hyperconcentratedflows

Parallel-stratifiedSh Sheet floods underupper flow regimesandstones

Trough cross-stratifiedSt 3D-dune migrationsandstones (sensu Ashley, 1990)

under lower flowregime

Sp 2D-dune migrationPlanar cross-stratifiedsandstones (sensu Ashley, 1990)

under lower flowregime

Graded-stratifiedSg Turbidity currentssandstones

F Vertically accretedPeliteslacustrine deposits

Chapada, Natureza, and Olaria tectonosequences(section A–A%, Fig. 16).

Several minor north-trending normal faults arepreserved in the Sopa-Brumadinho deposits. Thesestructures do not cut overlying beds, demonstrat-ing that they formed as synsedimentary growthfaults (Fig. 17). East-trending ‘release faults’ (De-stro, 1995) are normal faults that terminateagainst, and accommodate differential displace-ment along, the north-trending structures (Fig. 18).

5.3.5. Bimodal 6olcanic rocksMafic and felsic volcanic rocks form local in-

terbeds in the Sopa-Brumadinho tectonosequence.The rhyolites and their intrusive counterparts(Borrachudos Suite) display a metaluminous tosubalkalic character and are enriched in K, Fe, Nb,Y, Zr, Ga and light REE (Dussin, 1994; Dussinand Dussin, 1995). According to Dussin andDussin (1995), the geochemical data and isotopiccompositions of Nd (oNd1730 Ma between −10.1 and−6.2) and of Sr (87Sr/86Sri=0.7057) indicate par-tial melting of crustal sources, typical of continen-tal rift settings.

6. Transitional stage: Galho do Migueltectonosequence

The first marine incursion within the EspinhacoBasin, represented by deposits of the Galho doMiguel tectonosequence, marks the beginning ofthe transitional stage (Martins-Neto, 1993). Aregionally mappable transgressive surface sepa-rates these deposits from the underlying rift succes-sion (Figs. 5, 14, 16 and 19). The Galho do Migueltectonosequence contains basal shallow-marine de-posits that onlapped underlying units from theeast, and extensive eolian sand sheets that pro-graded from the west (Table 5).

Wave and storm-dominated siliciclastic shelfdeposits define a lower transgressive successionand an upper progradational succession (Table 5,Fig. 20a). Paleocurrent data (Fig. 20b) and faciesdistribution suggest a north-trending shorelineand east-dipping paleoslope. In the transgressivesuccession, shoreface, transition-zone and off-shore deposits can be recognized. In the prograda-

south of Sopa, paleocurrents are easterly, roughlycorresponding to the regional paleocurrent trendof the Sopa-Brumadinho tectonosequence (Fig.13c). South of this fault (locality ‘C’, Fig. 16)paleocurrents are dispersed and display a vectormean toward the south–southeast. At this site,alluvial-fan/fan-delta sediments fill a small north-trending graben that is cut to the north by thetransfer fault and to the west and east by fault-bounded paleohighs of the underlying Sao Joao da


tional succession, a complete offshore to coastalsuite of facies is developed (offshore, transitionzone, lower shoreface, upper shoreface, beach,eolian). In contrast to underlying units, overallrates of sediment supply were low relative to ratesof subsidence and/or sea-level rise. The initialtransgressive succession probably reflects an inad-equate sediment supply compared with the rate ofrelative sea-level rise, whereas the progradationalsuccession probably resulted from sediment sup-ply exceeding relative sea-level rise.

The paleogeography of the Galho do Migueltectonosequence was strongly controlled by thefinal disposition of earlier rift stages. This is illus-trated by the distribution of eolian deposits (Figs.5 and 21). Above synrift 3 (Sopa-Brumadinhotectonosequence) depocenters, the dune fields pro-graded eastward, whereas above structural highseolian deposits are thin or absent, and eoliandunes did not reach the eastern parts of the

Espinhaco range.The Galho do Miguel tectonosequence marks

the change from predominantly mechanical, lo-cally compensated subsidence to thermal, region-ally compensated subsidence. This transitionalstage was characterized by relatively low subsi-dence rates by comparison to the previous andsubsequent stages. The resultant decrease in ac-commodation space caused enlargement of thebasin, as the sediment supply was maintained. Inaddition, increased flexural rigidity due to litho-spheric cooling likely favored basin widening.

7. Flexural stage: Conselheiro Matatectonosequence

The deposits of the flexural stage of the Espin-haco basin belong to the ca. 900 m thick Consel-

Fig. 14. Cross-sections in outcrops of the Sopa-Brumadinho tectonosequence (modified after Martins-Neto, 1993).


Fig. 15. Sketch showing the spatial relationship between depositional lobes in half-grabens, (a) progradational relationship betweenhanging-wall-derived fans at the flexural border (see Fig. 14a and section AA% on Fig. 16) and (b) aggradational relationship(vertically stacked) between footwall-derived fans at the fault border (see Fig. 14b and section AA% on Fig. 16) (modified fromHeward, 1978b).

heiro Mata tectonosequence. The base of this unitis defined by a maximum flooding surface, whichmarks the greatest expansion of the Espinhaco sea(Martins-Neto, 1995b; Espinoza, 1996). Locally,the lower boundary is defined by multiple shoal-ing-upward sequences 10–20 m thick (Fig. 22;Espinoza, 1996).

According to Dupont (1995), three depositionalsequences with a transgressive base and a progra-dational top can be recognized in the ConselheiroMata tectonosequence (Fig. 23). The first se-quence (250–400 m thick) contains transgressivebarred nearshore deposits and progradationalbeach to shallow-marine deposits (Dupont, 1995;

Espinoza, 1996). The second sequence (250–350m thick) comprises transgressive shelf depositsoverlain by progradational alluvial plain tocoastal successions (Dupont, 1995). The third se-quence (200–300 m thick) is defined by transgres-sive mixed siliciclastic–carbonate shelf depositsoverlain by coastal to fluvial sediments (Batista etal., 1986; Dupont, 1995). The top of the thirddepositional sequence marks the final filling up ofthe Espinhaco basin. The thicknesses (ca. 300 m)and the estimated time (some 10 million years) fordeposition of each sequence suggest that they mayrepresent tectonically controlled second-ordercycles. These second-order cycles could be the


Fig. 16. Geological map of the Sopa-Guinda area (simplified from Reis, 1999; Martins-Neto et al., 1999b) showing paleocurrentdiagrams for different localities (see text for explanation), as well as schematic, not-scaled cross-sections. Numerals adjacent to therose diagrams represent number of measurements. Symbols in the sections are consistent with the map, except the gray fill, whichrepresents lacustrine pelites of the Sopa-Brumadinho tectonosequence.


product of variations in the in-plane stress field(Martins-Neto, 1998a).

8. Discussion

Some aspects regarding the evolution of theEspinhaco basin remain uncertain. One impor-tant question concerns the larger-scale structureof the rift stage. The dimensions of the half-grabens described above indicate that they arerelatively minor structures, and evidence of link-age to a master fault is lacking. However, thepredominance of eastward-directed paleocurrentsin deposits related to lateral-transport systems(Leeder and Gawthorpe, 1987) may indicate amaster fault located to the east, outside the studyarea (see Martins-Neto, 1994, 1996b). Thick con-glomeratic successions in the eastern domains ofthe Espinhaco range may be related to such a

Fig. 18. East-trending synsedimentary ‘release faults’ (Destro,1995), which terminate against the north-trending normal faultof Fig. 17.

Fig. 17. Photograph and sketch showing minor north-trendingsynsedimentary normal fault preserved in the Sopa-Brumad-inho deposits. The fault controlled the site of conglomeratedeposition. Photograph taken in the Diamante Vermelho dia-mond mine (see Fig. 14 for location of the mine).

master fault, but full documentation is inhibitedby intense Brasiliano/Pan African deformation.

A second question concerns the apparent lim-ited thickness (ca. 1300 m) of synrift deposits.Synrift volcanic rocks provide a rough estimatefor the inferred basin stretching factor of 2–3,predicting a thicker synrift fill. The relatively thinobserved deposits may be partly due to syndepo-sitional cannibalization, as illustrated by abun-dant intraformational clasts at the base of theSao Joao da Chapada tectonosequence. In addi-tion, only the external and consequently thinnerportion of the synrift wedge, close to the riftflexural border, occurs in the studied area (Fig.24). The postulated rift master fault located inthe eastern domains of the Espinhaco range mayalso account for the thin synrift stratigraphic


Fig. 19. Outcrop section and interpretation sketch showing transgressive surface separating lacustrine pelites and a small deltaic lobeof the rift stage from shallow-marine transgressive deposits of the Galho do Miguel tectonosequence (transitional stage) (modifiedafter Martins-Neto, 1993).

Table 5Summary of the depositional characteristics of the Galho do Miguel tectonosequence

Depositional processDepositional Key lithologies Sedimentary featuressetting

Eolian Metric to decametric thickHypermature fine-grained Eolian dune migration bysandstones combination of sand-flow andcross-beds; ripple marks; bimodal

lamination sand-fallBeach Medium-grained sandstones with Parallel-lamination or stratification Beach lamination (Clifton, 1979),

locally with internal inverse grain segregation withinheavy-minerals levelsgrading; low-angel truncations; high-energy flows during wave

backwashripple marksUpper shoreface Amalgamated well-sorted, fine- to Parallel lamination; wave ripples; Intercalation of storm and

medium-grained sandstone beds locally small-scale, trough and fair-weather wave-inducedprocessesplanar cross-stratificationwith heavy-minerals levels

Amalgamated well-sorted,Lower shoreface Parallel to slightly undulated Amalgamated storm eventslamination; hummockyfine-grained sandstonescross-stratification; wave ripples

Transition zone Discrete storm beds (Dott andParallel stratification; hummockyFine-grained sandstonesBourgeois, 1982; Walker et al.,cross-stratification; wave ripplesinterbedded with pelites1983) interbedded with fair-weathermuds

Sheet-like layers with normal Vertical accretion of mud bellowSericitic phylites (metamorphosedOffshorestorm wave base with interveninggrading from sandstones tomudstones) with centimetricturbidity currentssandstones/siltstones layers siltstones


Fig. 20. (a) Measured sedimentological sections of the base of the Galho do Miguel tectonosequence. Transgressive section measured15 km north from Gouveia and progradational section measured 2 km south from Guinda. (b) Paleocurrent diagrams, where themeasurements from cross-stratifications and asymmetrical wave-current ripples represent directions of bedform migration andmeasurements from symmetrical wave ripples represent crest directions.


Fig. 21. Cross-stratified sandstone of the Galho do Migueleolian deposits.

tonosequences (in ascending order), Olaria,Natureza, Sao Joao da Chapada, Sopa-Brumad-inho, Galho do Miguel, Conselheiro Mata. TheOlaria tectonosequence records the prerift stage,the Natureza, Sao Joao da Chapada and Sopa-Brumadinho tectonosequences record the riftstage, the Galho do Miguel tectonosequence thetransitional stage, and the Conselheiro Mata tec-tonosequence the flexural stage. Each tectonose-

Fig. 22. Sedimentological section measured along the Pedreiracreek at the eastern border of the Serra do Cabral (see Fig. 1for location), showing the coarsening-upward rhythms whichcharacterize the marine transgression at the transition betweenthe Galho do Miguel and Conselheiro Mata tectonosequences(modified after Espinoza, 1996).

thickness measured in the central part of theEspinhaco range.

The full life span of the Espinhaco basin re-mains unknown and further geochronologic dataare required. Available geochronologic data indi-cate rifting at ca. 1720 Ma. Using maximumintervals derived from the literature of ca. 50million year for rifting and ca. 200 million year forthermal subsidence (Brunet and Le Pichon, 1982;Allen and Allen, 1990; Perrodon and Zabec, 1990;Bond et al., 1995; Klein, 1995), final filling of theEspinhaco basin may have taken place at ca. 1500Ma, giving a spread of ca. 200–250 million yearfor the basin life span, which is compatible withpublished data on average duration for first-order,basin-fill cycles (e.g. Krapez, 1993; Miall, 1997).High resolution geochronologic data are also re-quired to test the validity of the tectonosequencesdescribed herein and to calibrate potential futurestudies directed towards quantifying rates of sedi-mentation and subsidence.

9. Conclusions

Field-based sedimentologic, paleogeographic,stratigraphic, structural and tectonic studies in thepaleo/mesoproterozoic Espinhaco basin, south-eastern Brazil, indicate deposition in a rift-sagbasin. The basin evolved in four stages (prerift,rift, transitional and flexural), as represented byrocks of the Espinhaco Megasequence. The Espin-haco Megasequence can be divided into six tec-


Fig. 23. Schematic stratigraphic sections showing the three depositional sequences of the Conselheiro Mata tectonosequence(modified after Dupont, 1995). Section at Conselheiro Mata area is about 900 m thick.

quence includes the record of linked depositionalsystems, and is bounded by regional unconformi-

ties. These unconformities mark times when tec-tonically controlled reorganization of basinpaleogeography took place.

The prerift and rift stages of the Espinhaco basinare recorded by continental deposits in alluvialbraidplain, lake and alluvial fans/fan-delta envi-ronments. During the rift stage, mechanical subsi-dence due to lithospheric stretching predominated.

The evolution of the transitional and flexuralstages was probably controlled by thermal subsi-dence from contraction of the lithosphere due tocooling. The transitional stage was characterizedby relatively low subsidence rates. This induced adecrease of the accommodation space and, as thesediment supply was maintained, caused an en-largement of the basin. The basal and top portionsof the transitional stage are represented by shal-low-marine deposits, whereas the middle part con-tains thick and extensive eolian deposits.

Higher subsidence rates and the consequentsea-level rise characterize the flexural stage of theEspinhaco basin. Shallow-marine shelf depositspredominate in this stage. Three depositional se-quences with a transgressive base and a prograda-tional top can be recognized. The basal trans-gressive parts represent initially rapid subsidencerates, whereas the progradational tops reflect the

Fig. 24. Cartoon showing probable location (inset at bottomleft) of the studied rift deposits in the overall rift architecture.Original drawing, kindly furnished by N. Destro, was madebased on the structural elements of the Tucano rift afterAragao and Peraro (1994). Not to scale.


relatively slow subsidence rates and overfill of theaccommodation space. These sequences probablycorrespond to tectonically controlled second-or-der cycles.

Acknowledgements

I would like to thank Fernando Alkmim, JoaoHippertt and P. Southgate for useful commen-taries on an earlier draft. Dave Nelson and espe-cially Larry Aspler are thanked for careful reviewsof the manuscript. Wulf Mueller and Pat Erikssonare also thanked for their criticism and construc-tive reviews of the paper. The research, whichresulted in this paper was supported by theFAPEMIG-Research Support Foundation of Mi-nas Gerais State, Brazil (contract no. CEX 895/95) and by the CNPq-Brazilian National ResearchCouncil through a researcher fellowship to theauthor (contract no. 300404/94-8).

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