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Precambrian Research 103 (2000) 147–173
Tectonics and sedimentation in a paleo/mesoproterozoic
rift-sag basin (Espinhaço basin, southeastern Brazil)
Marcelo A. Martins-Neto
Departamento 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 (Espinhaço Megasequence, southeastern Brazil) indicates deposition in a rift-sag basin. Four
basin evolution stages are recognized (prerift, rift, transitional and flexural). The four stages can be represented by six
unconformity-bounded tectonosequences. The unconformities are recognized in the field and mappable even on a
regional scale. The prerift and rift stages of the Espinhaço basin were filled by products of continental depositional
systems. The prerift stage probably represents the first product of the rifting process, before the development of the
half-grabens that characterize the rift stage. During the rift stage, mechanical subsidence due to lithospheric stretching
was predominant and led to episodic rising of the depositional base level. As a result, the basin fill is characterized
by coarsening-upward intervals. Paleocurrent patterns indicate that block tilting and half-graben subsidence/uplift
controlled sediment dispersion. The first marine incursion within the Espinhaço basin marks the change in the
subsidence regime of the basin. The evolution of the transitional and flexural stages was probably controlled by
thermal subsidence due to thermal contraction of the lithosphere during cooling. The transitional stage was
characterized by relatively low subsidence rates. Higher subsidence rates and a consequent sea-level rise characterize
the flexural stage of the Espinhaço basin, in which three second-order transgressive-progradational sequences can be
recognized. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: Brazil; Proterozoic; Tectonostratigraphic units; Basin analysis; Tectonics; Sedimentation
www.elsevier.com/locate/precamres
1. Introduction
Rift-sag basins represent the tectonic product of
aborted passive-margin development (Allen and
Allen, 1990). These basins, commonly referred to
as aulacogens or failed rifts, display a ‘steer’s
head’ geometry due to early rifting and sub-
sequent thermally-driven downwarping (White
and McKenzie, 1988). The stratigraphic frame-
work, structural style, and tectonic evolution of
rift-sag basins have been interpreted mainly
through 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: neto@degeo.ufop.br (M.A. Martins-Neto).
0301-9268/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S0301-9268(00)00080-2
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 have
been published.
Deposits of the paleo/mesoproterozoic Espin-
haço basin outcrop in the central and western parts
of the Serra do Espinhaço, southeastern Brazil
(Fig. 1). Because of inversion during the neo-
proterozoic (Brasiliano/Pan African orogeny), and
because of a deep erosional level, the Espinhaço
basin provides an opportunity to evaluate the
paleogeographic and tectonic evolution of a rift-sag
basin through outcrop data. Of particular
importance, deposits recording initial rifting are
exposed. These constitute a valuable source of data
to characterize the three-dimensional architecture
of an early half-graben, and to document the
relationships between tectonics and sedimentation
that might not be available through geophysical
studies alone.
This paper presents a model for the
tectono-sedimentary evolution of the Espinhaço
rift-sag basin, based on geological mapping and
integrated sedimentologic, paleogeographic,
stratigraphic, structural and tectonic studies,
carried out in the central and western parts of the
southern Serra do Espinhaço, Brazil (Fig. 1). Early
workers (e.g. Pflug, 1965) described the Espinhaço
supergroup as a miogeosynclinal sequence.
Subsequently, a rift to passive-margin setting was
proposed (Pflug et al., 1980), and Martins-Neto
(1993), Schobbenhaus (1993), Dussin and Dussin
(1995) suggested, based on the ensialic setting and
stratigraphic framework, deposition in an
intracratonic basin, with an initial rift phase and a
subsequent flexural phase. The present paper
provides detail documenting the prerift, rift,
transitional and flexural stages of basin evolution.
2. Regional setting
The southern Serra do Espinhaço belongs to the
external zone of the Araçuaı́ fold-thrust belt,
Fig. 2. Lithostratigraphy of the southern Serra do Espinhaço
after Pflug (1968), Fogaça et al. (1984), Dossin et al. (1984),
Almeida Abreu and Pflug (1992) (modified after Martins-
Neto, 1993). Geochronological ages are from (1) Machado
et 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 Espinhaço. Inset 1,
central part of the southern Serra do Espinhaço; inset 2, western part of the southern Serra do Espinhaço and Serra Mineira; inset
3, eastern domain of the Serra do Cabral; inset 4, Serra da Água Fria. Location of Fig. 5 shown inside inset 1. Regional map of
the São Francisco craton simplified from Alkmim and Marshak (1998). Geology of the southern Serra do Espinhaço simplified from
Pedrosa-Soares et al. (1994).
M.A. Martins-Neto / Precambrian Research 103 (2000) 147–173150
Fig. 3. Stratigraphic framework and tectono-depositional features of the paleo/meso and neoproterozoic cover sequences of the São
Francisco craton and external zone of the Araçuaı́ fold belt.
along the southeastern margin of the neo-
proterozoic São Francisco craton, southeastern
Brazil (Fig. 1; Brito Neves and Cordani, 1991;
Trompette et al., 1992; Schobbenhaus, 1993;
Alkmim et al., 1993; Martins-Neto, 1998a). The
paleo/mesoproterozoic Espinhaço supergroup (Fig.
2) is the predominant unit in the southern Serra do
Espinhaço, overlying the archean basement
complex and supracrustal rocks of the archean/
paleoproterozoic Rio Paraúna supergroup uncon-
formably (Pflug, 1965). It is in turn unconformably
overlain by the neoproterozoic São Francisco
supergroup (Fig. 2; Pflug and Renger, 1973;
Dupont, 1996; Martins-Neto et al., 1997a,b, 1999a).
The maximum age of the Espinhaço supergroup
is limited by U/Pb dating of metamorphic zircon
in pre-Espinhaço units (1844915 Ma; Machado et
al., 1989), and depositional ages from magmatic
zircons in volcanic-bearing units in the lower parts
of the Espinhaço 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). The
minimum age of the Espinhaço supergroup is
less constrained, and is currently defined by basic
intrusions that cut the entire column (ca. 1.1–
0.9 Ga; Brito Neves et al., 1979; Machado et al.,
1989).
The southern Serra do Espinhaço was deformed
and metamorphosed in the Brasiliano/Pan African
event (650–500 Ma, Brito Neves et al., 1979;
Marshak and Alkmim, 1989; Schobbenhaus, 1993) ,
a major collisional episode that generated a
fold-trust terrane throughout the southern Serra do
Espinhaço (Herrgesell and Pflug, 1986).
M.A. Martins-Neto / Precambrian Research 103 (2000) 147–173 151
Fig. 4. Stratigraphic chart for the Espinhaço Megasequence showing the main characteristics of the tectono-sedimentary units
(modified from Martins-Neto, 1995a).
3. Stratigraphic framework
The Espinhaço supergroupis a thick (�4000
m) succession of siliciclastic metasedimentary
rocks containing subordinate volcanic intervals
(Fig. 2). These have been grouped lithostratigraph-
ically into nine formations (Pflug, 1968; Almeida
Abreu and Pflug, 1992). Herein, adopting a genetic
stratigraphic approach, the Espinhaço supergroup
is referred to as the ‘Espinhaço 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 (prerift
stage), the Natureza, São João da Chapada and
Sopa-Brumadinho tectonosequences (rift stage);
the Galho do Miguel tectonosequence (transitional
stage) and the Conselheiro Mata tectonosequence
(flexural stage). Each tectonosequence records
linked depositional systems accumulated in a spe-
cific tectonic phase of the basin, and is defined by
major regional bounding unconformities (Fig. 4;
Da Silva, 1993; Martins-Neto, 1995a,b). These
unconformities represent periods of significant tec-
tonically induced paleogeographic reorganization.
During the rift stage, mechanical subsidence due to
lithospheric 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 adopted
herein is based on the definition of a major uncon-
formity-bounded megasequence, allowing recogni-
tion and analysis of a true basin entity, even
though its life span is not well constrained. Each
tectonosequence represents the record of an evolu-
tionary stage of the basin, with its own accommo-
dation history. Contacts are recognized in the field
and mappable even on a regional scale (Fig. 5).
4. Prerift stage: Olaria tectonosequence
The prerift stage of the Espinhaço basin is
represented by the poorly-exposed Olaria tec-
tonosequence. The unit is ca. 150 m thick. It
consists mainly of sheet-like bodies of immature
fine-grained, parallel-stratified sandstones (Fig. 6)
that contain local current ripples and cross-stratifi-
cation, and are locally capped by thin layers of
M.A. Martins-Neto / Precambrian Research 103 (2000) 147–173152
Fig. 5. Geologic map of tectono-stratigraphic units of the central part of the southern Espinhaço range (see Fig. 1 for location),
southeastern Brazil (modified from Reis, 1999; Euzébio, 1999; Martins-Neto et al., 1999b). Inset shows location of Fig. 7.
M.A. Martins-Neto / Precambrian Research 103 (2000) 147–173 153
Fig. 6. Photograph showing sheet-flood sandstones of the
Olaria braidplain deposits. Note hammer (circled) for scale.
terpretations are limited by poor outcrop and
high strain, the lack of volcanic rocks and of
evidence of synsedimentary faults (in contrast to
overlying sequences) suggest that the Olaria tec-
tonosequence was deposited in an initial sag that
was the product of ductile stretching, before the
upper crust had reached its elastic limit. How-
ever, clastic dykes filled by conglomeratic sand-
stones occur at the top of the Olaria
tectonosequence. These are truncated at the an-
gular unconformity with the overlying Natureza
tectonosequence, and are likely precursors of ac-
tive shallow-level faulting.
5. Rift stage: Natureza; São João da Chapada;
Sopa-Brumadinho tectonosequences
The rift stage of the Espinhaço basin is charac-
terized by three phases all of which are bounded
by angular unconformities (Figs. 4, 5 and 7),
synrift 1 (Natureza tectonosequence); synrift 2
(São João da Chapada tectonosequence); and
synrift 3 (Sopa-Brumadinho tectonosequence).
5.1. Synrift 1 phase (Natureza tectonosequence)
The Natureza tectonosequence is up to 200 m
thick (cf. Reis, 1999). Massive and inversely
graded clast-supported conglomerates interpreted
as debris-flow deposits (Fig. 8a) occur above the
mudstones (sericitic phyllites). The deposits are
likely high-energy ephemeral flood deposits simi-
lar to those described by Tunbridge (1981, 1984).
Some lenticular bodies composed of fine-grained
sandstones with high textural maturity occur in
the middle portion of the unit (Reis, 1999). These
display large-scale cross-stratification (sets up to
3 m thick) with high-angle (30–35°) truncations
may be suggestive of an eolian origin. Thick
layers of sericitic phyllites, which may represent
deposition in ponds, occur locally.
The available data (Table 1) suggest that the
Olaria tectonosequence was probably deposited
in alluvial braidplains associated with restricted
lacustrine and eolian environments. Although in-
Table 1
Summary of the depositional characteristics of the Olaria tectonosequence
Depositional Sedimentary featuresKey lithologies Depositional process
setting
Sheet floods under upper flowPoorly-sorted, medium toAlluvial Parallel-stratification, sheet like
braidplain 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 migration
Floodplain depositsPelites Thin mud drapes
Vertical accretionPelites Metric thick bodiesLacustrine
Eolian dune migrationMetric thick cross-beds withEolian Hypermature fine-grained
high-angle (30–35°) truncationssandstones
M.A. Martins-Neto / Precambrian Research 103 (2000) 147–173154
basal angular unconformity. The debris-flow
conglomerates are associated with clast- and
matrix-supported stratified conglomerates con-
sidered as traction deposits (Fig. 8b) and are
interpreted as having been deposited in an alluvial
fan and braided stream depositional system. The
conglomeratic deposits are covered by a sandy
braided fluvial succession (Fig. 8c) interbedded
with eolian sandstones (Fig. 8d, Table 2) (Silva,
1995; Reis, 1999).
Conglomerate sections and the angular
character of the unconformity at the base of the
Natureza tectonosequence are suggestive of
tectonic influence such as block tilting or selective
uplift and/or subsidence. This signifies rupture of
the upper crust and the development of the first
half-grabens in the Espinhaço basin. The limited
areal extent of the Natureza tectonosequence
suggests a restricted character of the basin at this
time. The absence of volcanic rocks infers a
stretching factor b (White and McKenzie, 1988)
of less than 2 for this phase.
5.2. Synrift 2 phase: (São João da Chapada
tectonosequence)
The São João da Chapada tectonosequence is
up to 300 m thick. Locally, above the angular
unconformity (Fig. 9), conglomerates and breccias
contain abundant sandstone clasts derived from
the underlying Natureza tectonosequence. These
have been interpreted as fault-scarp talus deposits
formed by cohesionless mass-flows, and are strong
indicators 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 the
Espinhaço basin is marked by basic rocks near the
base of the São João da Chapada tectonose-
quence (Hoppe and Otto, 1982; Uhlein, 1991;
Schobbenhaus, 1993; Dussin, 1994; Dussin and
Dussin, 1995). Layers of hematitic phyllites within
these units have been interpreted as volcanic beds
altered by Paleoproterozoic subaerial weathering
(lateritization) and subsequent metamorphism
(Knauer and Schrank, 1993).
The São João da Chapada tectonosequence
defines an overall coarsening-upward succession
from lacustrine to deltaic to fluvial deposits (Fig.
10). Fine-grained sandstones and pelites at the
base, probably deposited in a storm-influenced
lacustrine environment, represent rapid subsi-
dence and abrupt basin deepening. These are
overlain by sandstone bodies of likely deltaic
origin that display a sigmoidal geometry and are
arranged in coarsening- and thickening-upward
sequences.Prograding over the deltaic/lacustrine
deposits are extensive braidplain sandstones that
Fig. 7. Geologic map showing the angular character, denoted
by strike and dip of the unconformity separating the synrift 1
(Natureza) and synrift 2 (São João da Chapada) tectonose-
quences (see Fig. 5 for location). Note that the eolian deposits
of the Natureza tectonosequence were totally eroded in the
southern part of the map (modified from Reis, 1999; Euzébio,
1999; Martins-Neto et al., 1999b). The horizontal bar shows
location of Fig. 10.
M.A. Martins-Neto / Precambrian Research 103 (2000) 147–173 155
Fig. 8. Photographs showing deposits of the synrift 1 Natureza tectonosequence, (a) Debris-flow conglomerates. Note inverse
grading in the body comprising the lower half of the outcrop; (b) stratified, sheet-flood conglomerates. Note hammer (circled) for
scale; (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 São João da Chapada
tectonosequence. These signify relatively reduced
rates of subsidence (Martins-Neto, 1993, 1994).
The São João da Chapada braidplain deposits
(Table 3, Fig. 11a) are characterized by sheet-like
bodies of coarse and poorly-sorted sandstones
that display unidirectional paleocurrents with re-
markably little dispersion throughout the study
area (Fig. 11b). The sandstones generally define
fining- and thinning-upward sequences and are
invariably bounded by low-relief erosional sur-
faces (Martins-Neto, 1994). Typical sequences
start with parallel-stratified and low-angle cross-
stratified sandstones (Sh/Sl) and are capped by
trough cross-stratified sandstones (St) and locally,
mudstone layers, characteristic of high-energy
ephemeral streams (e.g. Miall and Gibling, 1978;
Tunbridge, 1981, 1984; Lawrence and Williams,
1987; Muñoz et al., 1992). As detailed elsewhere
(Martins-Neto, 1994), the superposition of such
waning-flood sheet sandstones built up a braid-
plain that extended ca. 35 km transverse to the
regional paleoslope (Fig. 12).
5.3. Synrift 3 phase (Sopa-Brumadinho
tectonosequence)
The Sopa-Brumadinho tectonosequence records
the peak of extensional tectonics in the Espinhaço
basin. Deposits of this phase define fault-block
related depocenters (see Fig. 5 and also cross-sec-
tions in Fig. 16), and consist of siliciclastic
metasediments 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 at
its base by a prominent angular unconformity and
at its top by a marine transgressive surface. As
M.A. Martins-Neto / Precambrian Research 103 (2000) 147–173156
Table 2
Facies of the Natureza tectonosequence and interpretation
Facies Interpretation
Massive to inversely Debris flows
graded, clast-supported
conglomerates
Parallel-stratified, Traction deposits
matrix-supported
conglomerates
Parallel-stratified, Sheet floods under upper
flow regimepoorly-sorted sandstones
Trough cross-stratified, Fluvial 3D-dune migration
(sensu Ashley, 1990) underpoorly sorted sandstones
lower flow regime
Planar cross-stratified, Fluvial 2D-dune migration
poorly sorted sandstones (sensu Ashley, 1990) under
lower flow regime
Ripple cross-laminated Current-ripple migration
sandstones
Floodplain depositsPelites
Eolian dune migrationHypermature fine-grained,
metric thick coss-bedded
sandstones
Fig. 9. Photographs showing (a) view of the angular unconfor-
mity separating the Natureza and São João da Chapada
tectonosequences and (b) detail emphasizing the erosional
nature of this unconformity.
discussed in the following sub-headings, lacus-
trine, fan-delta and fluvial sedimentation took
place in half-grabens that were compartmental-
ized by north-trending normal faults and east-
trending transfer faults. Subsidence was episodic
and controlled by block tilting in asymmetric
grabens.
5.3.1. Lacustrine, fan-delta and flu6ial deposits
The facies of the Sopa-Brumadinho tectonose-
quence are arranged in 40–80 m thick sequences
that coarsen and then fine upward (Fig. 13a and
b). At the base of each sequence are laminated
pelites (facies F). These are followed by, graded
and 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 fine
upward to various sandstone facies (Fig. 13a).
Some sequences lack the lower turbiditic graded
sandstone component, and laminated mudstones
pass directly to stacked streamflood sandstones
(e.g. Fig. 13b). Mudlumps, formed by the rapid
loading of debris-flows onto water-saturated
muds (Dailly, 1976; Lewis, 1997) are locally devel-
oped. An approximately 200 m thick section of
parallel-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 facies
and the sedimentary processes that generated
them can be found in Martins-Neto (1995d,
1996b).
These rocks were likely deposited in a lacustrine
fan-delta to fluvial system (cf. Nemec and Steel,
1988). The coarsening-upward and fining-upward
M.A. Martins-Neto / Precambrian Research 103 (2000) 147–173 157
Fig. 10. (a) Outcrop section (see Fig. 7 for location) showing the overall coarsening-upward arrangement from lacustrine to deltaic
to fluvial deposits of the São João da Chapada tectonosequence; (b) detail showing the thickening-upward arrangement of the initial
deltaic progradation.
Table 3
Facies of the São João da Chapada braidplain deposits (codes modified after Miall, 1978)
Code Characteristics Interpretation
Massive to parallel-stratified conglomeratesGm Migration of longitudinal bars
Sh/Sl Sheet floods under upper flow regimeParallel-stratified to low-angle (B10°) cross-stratified
sandstones
Trough cross-stratified sandstonesSt 3D-dune migration (sensu Ashley, 1990) under lower
flow regime
Planar cross-stratified sandstonesSp 2D-dune migration (sensu Ashley, 1990) under lower
flow regime
Current-ripple migrationSr Ripple cross-laminated sandstones
Floodplain depositsF Pelites
M.A. Martins-Neto / Precambrian Research 103 (2000) 147–173158
Fig. 11. (a) Measured sedimentological sections of the São João da Chapada braidplain deposits. Geological map modified from
Schöll and Fogaça (1981), Chaves et al. (1985). (b) Paleocurrent data from São João da Chapada braidplain (316 measurements
from cross-stratified sandstones). Modified after Martins-Neto (1993).
sequences are interpreted to represent repeated
episodes of rapid flooding (basal laminated mud
units) followed by progradation of fan lobes
which evolved from distal (subaqueous) to proxi-
mal (subaerial) environments. The fining-upward
parts represent the abandonment phases of the
depositional lobes. Discriminating between lacus-
trine and shallow marine deposits is commonly
difficult in Precambrian strata (e.g. Eriksson et
al., 1998). A lacustrine origin is favored in the
present example because pelitic rocks of facies F
define a number of isolated N–S elongate lenses.
5.3.2. Tectonically-dri6en cyclicity
The lacustrine, fan-delta and fluvial coarsening-
upward and fining-upward sequences are consid-
ered to represent repeated tectonically-driven
pulses (see e.g. Blair and Bilodeau, 1988). Accord-
ingly, flood surfaces and pelitic rocks at the base
of each sequence are the immediate response to
fault-induced subsidence of the basin floor, and
subsequent fan-delta progradation records a de-
layed response, reflecting erosional stripping of
relatively uplifted source areas. Fining-upward
trends at the top of the sequencessignify a reduc-
tion in source-area relief during continued tec-
tonic quiescence.
Those successions where fan-delta progradation
initiated with sandy turbidity currents (e.g. Fig.
13a), may indicate deeper lakes formed by greater
subsidence, whereas those in which laminated
pelites pass directly to streamflow sandstones (e.g.
M.A. Martins-Neto / Precambrian Research 103 (2000) 147–173 159
Fig. 13b) may imply shallower lakes. Smaller-
scale pulses may be recorded by thin pelite layers
interbedded with some of the sandstones near the
base of the sequences (Fig. 13a and b and Fig.
14a). Although post-depositional processes have
strongly modified the original thicknesses of these
layers, thickness variation suggests varying subsi-
dence pulse magnitudes.
5.3.3. Half-graben geometry
In 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). The
younger lobe (Lavrinha mine) forms a wedge
positioned basinward of an older fan segment
(Diamante Vermelho mine). Correction for tec-
tonic tilt using marine deposits of the overlying
Galho 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 the
younger fan. These results are compatible with
published data of recent fan slopes (cf. Wells,
1984; Blair and McPherson, 1994). Basinward
stepping and increasing angle of tilt with age
(reflecting progressive rotation due to faulting) is
characteristic of many fans, modern and ancient
(e.g. Hooke, 1972; Steel et al., 1977; Heward,
1978a,b; Kleinspehn et al., 1984; Blair, 1987). In
contrast to these basinward stepping lobes, to the
northeast, at the Brumadinho mine (Fig. 14b),
clast-supported talus breccias form vertically
stacked linear bodies in direct fault contact with
older rocks. The two types of fans (Fig. 15) as
well as the geologic map and the cross-section
A–A% in Fig. 16 indicate an asymmetric-graben
geometry, with the Diamante Vermelho/Lavrinha
system representing hanging-wall sourced fans at
the unfaulted ramping margin, and the Brumad-
inho fans marking footwall sourced fans at the
faulted border (cf. Hooke, 1972; Gawthorpe and
Colella, 1990).
5.3.4. Synsedimentary faults
East-trending transfer faults can be recognized
(Figs. 5 and 16). Paleocurrent patterns illustrate
the 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 São João da Chapada braidplain. L, longitudinal; T, transverse (sensu
Miall, 1981). Not to scale (modified after Martins-Neto, 1993).
M.A. Martins-Neto / Precambrian Research 103 (2000) 147–173160
Fig. 13. Measured sedimentological sections of the Sopa-Brumadinho tectonosequence (a and b). Section 1 (a) was measured in the
Diamante Vermelho and Lavrinha diamond mines (see also Fig. 14a) and section 2 (b) was measured in the Sopa mine (see Fig. 1
for location of the localities). (c) Paleocurrent data from the Sopa-Brumadinho tectonosequence (rose diagram summarizes 319
measurements from cross-stratified sandstones from the whole unit in the studied area). Modified after Martins-Neto (1993).
M.A. Martins-Neto / Precambrian Research 103 (2000) 147–173 161
Table 4
Facies of the Sopa-Brumadinho tectonosequence (conglomer-
ate codes after Martins-Neto, 1993, 1996b; sandstone codes
after Miall, 1978)
Code InterpretationCharacteristics
UngradedUG-CS Cohesionless debria
clast-supported flows
conglomerates
Inversely graded Cohesionless debrisIG-CS
clast-supported flows
conglomerates
Normally gradedNG-CS Cohesionless debris
flowsclast-supporte
conglomerates
Crudely stratified HyperconcentratedCS-CS
clast-supported flows
conglomerates
TB-CS Fault-adjacent talusTalus-breccia
clast-supported breccias
conglomerates
Sand matrix-supportedS-MS Cohesionless debris
flowsconglomerates
Mud matrix-supported Cohesive debris flowsM-MS
conglomerates
Sm Massive sandstones Hyperconcentrated
flows
Parallel-stratifiedSh Sheet floods under
upper flow regimesandstones
Trough cross-stratifiedSt 3D-dune migration
sandstones (sensu Ashley, 1990)
under lower flow
regime
Sp 2D-dune migrationPlanar cross-stratified
sandstones (sensu Ashley, 1990)
under lower flow
regime
Graded-stratifiedSg Turbidity currents
sandstones
F Vertically accretedPelites
lacustrine deposits
Chapada, Natureza, and Olaria tectonosequences
(section A–A%, Fig. 16).
Several minor north-trending normal faults are
preserved in the Sopa-Brumadinho deposits. These
structures do not cut overlying beds, demonstrat-
ing that they formed as synsedimentary growth
faults (Fig. 17). East-trending ‘release faults’ (De-
stro, 1995) are normal faults that terminate
against, and accommodate differential displace-
ment along, the north-trending structures (Fig. 18).
5.3.5. Bimodal 6olcanic rocks
Mafic and felsic volcanic rocks form local in-
terbeds in the Sopa-Brumadinho tectonosequence.
The rhyolites and their intrusive counterparts
(Borrachudos Suite) display a metaluminous to
subalkalic character and are enriched in K, Fe, Nb,
Y, Zr, Ga and light REE (Dussin, 1994; Dussin
and Dussin, 1995). According to Dussin and
Dussin (1995), the geochemical data and isotopic
compositions 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 Miguel
tectonosequence
The first marine incursion within the Espinhaço
Basin, represented by deposits of the Galho do
Miguel tectonosequence, marks the beginning of
the transitional stage (Martins-Neto, 1993). A
regionally mappable transgressive surface sepa-
rates these deposits from the underlying rift succes-
sion (Figs. 5, 14, 16 and 19). The Galho do Miguel
tectonosequence contains basal shallow-marine de-
posits that onlapped underlying units from the
east, and extensive eolian sand sheets that pro-
graded from the west (Table 5).
Wave and storm-dominated siliciclastic shelf
deposits define a lower transgressive succession
and an upper progradational succession (Table 5,
Fig. 20a). Paleocurrent data (Fig. 20b) and facies
distribution suggest a north-trending shoreline
and east-dipping paleoslope. In the transgressive
succession, shoreface, transition-zone and off-
shore deposits can be recognized. In the prograda-
south of Sopa, paleocurrents are easterly, roughly
corresponding to the regional paleocurrent trend
of the Sopa-Brumadinho tectonosequence (Fig.
13c). South of this fault (locality ‘C’, Fig. 16)
paleocurrents are dispersed and display a vector
mean 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 the
transfer fault and to the west and east by fault-
bounded paleohighs of the underlying São João da
M.A. Martins-Neto / Precambrian Research 103 (2000) 147–173162
tional succession, a complete offshore to coastal
suite of facies is developed (offshore, transition
zone, lower shoreface, upper shoreface, beach,
eolian). In contrast to underlying units, overall
rates of sediment supply were low relative to rates
of subsidence and/or sea-level rise. The initial
transgressive succession probably reflects an inad-
equate sediment supply compared with the rate of
relative sea-level rise, whereas the progradational
succession probably resulted from sediment sup-
ply exceeding relative sea-level rise.
The paleogeography of the Galho do Miguel
tectonosequence was strongly controlled by the
final disposition of earlier rift stages. This is illus-
trated by the distribution of eolian deposits (Figs.
5 and 21). Above synrift 3 (Sopa-Brumadinho
tectonosequence) depocenters, the dune fields pro-
graded eastward, whereas above structural highs
eolian deposits are thin or absent, and eolian
dunes did not reach the easternparts of the
Espinhaço range.
The Galho do Miguel tectonosequence marks
the change from predominantly mechanical, lo-
cally compensated subsidence to thermal, region-
ally compensated subsidence. This transitional
stage was characterized by relatively low subsi-
dence rates by comparison to the previous and
subsequent stages. The resultant decrease in ac-
commodation space caused enlargement of the
basin, as the sediment supply was maintained. In
addition, increased flexural rigidity due to litho-
spheric cooling likely favored basin widening.
7. Flexural stage: Conselheiro Mata
tectonosequence
The deposits of the flexural stage of the Espin-
haço 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).
M.A. Martins-Neto / Precambrian Research 103 (2000) 147–173 163
Fig. 15. Sketch showing the spatial relationship between depositional lobes in half-grabens, (a) progradational relationship between
hanging-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 from
Heward, 1978b).
heiro Mata tectonosequence. The base of this unit
is defined by a maximum flooding surface, which
marks the greatest expansion of the Espinhaço 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 depositional
sequences with a transgressive base and a progra-
dational top can be recognized in the Conselheiro
Mata tectonosequence (Fig. 23). The first se-
quence (250–400 m thick) contains transgressive
barred nearshore deposits and progradational
beach to shallow-marine deposits (Dupont, 1995;
Espinoza, 1996). The second sequence (250–350
m thick) comprises transgressive shelf deposits
overlain by progradational alluvial plain to
coastal successions (Dupont, 1995). The third se-
quence (200–300 m thick) is defined by transgres-
sive mixed siliciclastic–carbonate shelf deposits
overlain by coastal to fluvial sediments (Batista et
al., 1986; Dupont, 1995). The top of the third
depositional sequence marks the final filling up of
the Espinhaço basin. The thicknesses (ca. 300 m)
and the estimated time (some 10 million years) for
deposition of each sequence suggest that they may
represent tectonically controlled second-order
cycles. These second-order cycles could be the
M.A. Martins-Neto / Precambrian Research 103 (2000) 147–173164
Fig. 16. Geological map of the Sopa-Guinda area (simplified from Reis, 1999; Martins-Neto et al., 1999b) showing paleocurrent
diagrams for different localities (see text for explanation), as well as schematic, not-scaled cross-sections. Numerals adjacent to the
rose diagrams represent number of measurements. Symbols in the sections are consistent with the map, except the gray fill, which
represents lacustrine pelites of the Sopa-Brumadinho tectonosequence.
M.A. Martins-Neto / Precambrian Research 103 (2000) 147–173 165
product of variations in the in-plane stress field
(Martins-Neto, 1998a).
8. Discussion
Some aspects regarding the evolution of the
Espinhaço basin remain uncertain. One impor-
tant question concerns the larger-scale structure
of the rift stage. The dimensions of the half-
grabens described above indicate that they are
relatively minor structures, and evidence of link-
age to a master fault is lacking. However, the
predominance of eastward-directed paleocurrents
in deposits related to lateral-transport systems
(Leeder and Gawthorpe, 1987) may indicate a
master fault located to the east, outside the study
area (see Martins-Neto, 1994, 1996b). Thick con-
glomeratic successions in the eastern domains of
the Espinhaço range may be related to such a
Fig. 18. East-trending synsedimentary ‘release faults’ (Destro,
1995), which terminate against the north-trending normal fault
of Fig. 17.
Fig. 17. Photograph and sketch showing minor north-trending
synsedimentary normal fault preserved in the Sopa-Brumad-
inho deposits. The fault controlled the site of conglomerate
deposition. Photograph taken in the Diamante Vermelho dia-
mond mine (see Fig. 14 for location of the mine).
master fault, but full documentation is inhibited
by 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 estimate
for the inferred basin stretching factor of 2–3,
predicting a thicker synrift fill. The relatively thin
observed deposits may be partly due to syndepo-
sitional cannibalization, as illustrated by abun-
dant intraformational clasts at the base of the
São João da Chapada tectonosequence. In addi-
tion, only the external and consequently thinner
portion of the synrift wedge, close to the rift
flexural border, occurs in the studied area (Fig.
24). The postulated rift master fault located in
the eastern domains of the Espinhaço range may
also account for the thin synrift stratigraphic
M.A. Martins-Neto / Precambrian Research 103 (2000) 147–173166
Fig. 19. Outcrop section and interpretation sketch showing transgressive surface separating lacustrine pelites and a small deltaic lobe
of the rift stage from shallow-marine transgressive deposits of the Galho do Miguel tectonosequence (transitional stage) (modified
after Martins-Neto, 1993).
Table 5
Summary of the depositional characteristics of the Galho do Miguel tectonosequence
Depositional processDepositional Key lithologies Sedimentary features
setting
Eolian Metric to decametric thickHypermature fine-grained Eolian dune migration by
sandstones combination of sand-flow andcross-beds; ripple marks; bimodal
lamination sand-fall
Beach Medium-grained sandstones with Parallel-lamination or stratification Beach lamination (Clifton, 1979),
locally with internal inverse grain segregation withinheavy-minerals levels
grading; low-angel truncations; high-energy flows during wave
backwashripple marks
Upper 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-induced
processesplanar cross-stratificationwith heavy-minerals levels
Amalgamated well-sorted,Lower shoreface Parallel to slightly undulated Amalgamated storm events
lamination; hummockyfine-grained sandstones
cross-stratification; wave ripples
Transition zone Discrete storm beds (Dott andParallel stratification; hummockyFine-grained sandstones
Bourgeois, 1982; Walker et al.,cross-stratification; wave ripplesinterbedded with pelites
1983) interbedded with fair-weather
muds
Sheet-like layers with normal Vertical accretion of mud bellowSericitic phylites (metamorphosedOffshore
storm wave base with interveninggrading from sandstones tomudstones) with centimetric
turbidity currentssandstones/siltstones layers siltstones
M.A. Martins-Neto / Precambrian Research 103 (2000) 147–173 167
Fig. 20. (a) Measured sedimentological sections of the base of the Galho do Miguel tectonosequence. Transgressive section measured
15 km north from Gouveia and progradational section measured 2 km south from Guinda. (b) Paleocurrent diagrams, where the
measurements from cross-stratifications and asymmetrical wave-current ripples represent directions of bedform migration and
measurements from symmetrical wave ripples represent crest directions.
M.A. Martins-Neto / Precambrian Research 103 (2000) 147–173168
Fig. 21. Cross-stratified sandstone of the Galho do Miguel
eolian deposits.
tonosequences (in ascending order), Olaria,
Natureza, São João da Chapada, Sopa-Brumad-
inho, Galho do Miguel, Conselheiro Mata. The
Olaria tectonosequence records the prerift stage,
the Natureza, São João da Chapada and Sopa-
Brumadinho tectonosequencesrecord the rift
stage, the Galho do Miguel tectonosequence the
transitional stage, and the Conselheiro Mata tec-
tonosequence the flexural stage. Each tectonose-
Fig. 22. Sedimentological section measured along the Pedreira
creek at the eastern border of the Serra do Cabral (see Fig. 1
for location), showing the coarsening-upward rhythms which
characterize the marine transgression at the transition between
the Galho do Miguel and Conselheiro Mata tectonosequences
(modified after Espinoza, 1996).
thickness measured in the central part of the
Espinhaço range.
The full life span of the Espinhaço basin re-
mains unknown and further geochronologic data
are required. Available geochronologic data indi-
cate rifting at ca. 1720 Ma. Using maximum
intervals derived from the literature of ca. 50
million year for rifting and ca. 200 million year for
thermal subsidence (Brunet and Le Pichon, 1982;
Allen and Allen, 1990; Perrodon and Zabec, 1990;
Bond et al., 1995; Klein, 1995), final filling of the
Espinhaço basin may have taken place at ca. 1500
Ma, giving a spread of ca. 200–250 million year
for the basin life span, which is compatible with
published 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 tectonosequences
described herein and to calibrate potential future
studies directed towards quantifying rates of sedi-
mentation and subsidence.
9. Conclusions
Field-based sedimentologic, paleogeographic,
stratigraphic, structural and tectonic studies in the
paleo/mesoproterozoic Espinhaço basin, south-
eastern Brazil, indicate deposition in a rift-sag
basin. The basin evolved in four stages (prerift,
rift, transitional and flexural), as represented by
rocks of the Espinhaço Megasequence. The Espin-
haço Megasequence can be divided into six tec-
M.A. Martins-Neto / Precambrian Research 103 (2000) 147–173 169
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 depositional
systems, and is bounded by regional unconformi-
ties. These unconformities mark times when tec-
tonically controlled reorganization of basin
paleogeography took place.
The prerift and rift stages of the Espinhaço basin
are recorded by continental deposits in alluvial
braidplain, 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 flexural
stages was probably controlled by thermal subsi-
dence from contraction of the lithosphere due to
cooling. The transitional stage was characterized
by relatively low subsidence rates. This induced a
decrease of the accommodation space and, as the
sediment supply was maintained, caused an en-
largement of the basin. The basal and top portions
of 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 consequent
sea-level rise characterize the flexural stage of the
Espinhaço basin. Shallow-marine shelf deposits
predominate 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 subsidence
rates, whereas the progradational tops reflect the
Fig. 24. Cartoon showing probable location (inset at bottom
left) of the studied rift deposits in the overall rift architecture.
Original drawing, kindly furnished by N. Destro, was made
based on the structural elements of the Tucano rift after
Aragão and Peraro (1994). Not to scale.
M.A. Martins-Neto / Precambrian Research 103 (2000) 147–173170
relatively slow subsidence rates and overfill of the
accommodation space. These sequences probably
correspond to tectonically controlled second-or-
der cycles.
Acknowledgements
I would like to thank Fernando Alkmim, João
Hippertt and P. Southgate for useful commen-
taries on an earlier draft. Dave Nelson and espe-
cially Larry Aspler are thanked for careful reviews
of the manuscript. Wulf Mueller and Pat Eriksson
are also thanked for their criticism and construc-
tive reviews of the paper. The research, which
resulted in this paper was supported by the
FAPEMIG-Research Support Foundation of Mi-
nas Gerais State, Brazil (contract no. CEX 895/
95) and by the CNPq-Brazilian National Research
Council through a researcher fellowship to the
author (contract no. 300404/94-8).
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