05 - Universidade de Lisboa

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111, B06204, doi:10.1029/2005JB004106, 2006
Remagnetization in bituminous limestones of the Neoproterozoic
Araras Group (Amazon craton): Hydrocarbon maturation, burial
diagenesis, or both?
E. Font,1,2 R. I. F. Trindade,1 and A. Nédélec2
Received 27 September 2005; revised 13 January 2006; accepted 9 March 2006; published 16 June 2006.
[1] Neoproterozoic carbonates of the Araras Group exhibit two distinct magnetic
components across the same carbonate succession in a cross-section between the Amazon
craton and the Paraguay fold belt. Pink dolostones of the Mirassol d’Oeste Formation
carry a dual polarity, primary component, whereas black bituminous limestones of the
Guia Formation yield a secondary postfolding component. Magnetic signatures of the
Guia limestones, such as high anhysteretic remanence magnetization/saturation isothermal
remanence magnetization ratios, high-frequency-dependent magnetic susceptibility and
contradictory Lowrie-Fuller and Cisowski tests, are typical of remagnetized carbonates.
Unblocking temperatures suggest that the stable high-temperature remanence is carried by
both pyrrhotite and magnetite for which an authigenic origin is suggested by scanning
electron microscope observations. The different magnetic properties noted between
dolostones with or without bitumen and between dolostones and limestones in the same
metamorphic conditions lead to the hypothesis that the amount of hydrocarbon as well
as the lithology influence nucleation of authigenic magnetic minerals in these rocks.
Presence of magnetite pseudoframboids and euhedral iron sulphide crystals occurring in
fracture and voids are in favor of a chemical remanence (CRM). The presence of
pyrrhotite as one of the main carriers of CRM in these rocks, and its association with
bitumen in fractures is probably related to an epigenetic enrichment of sulfur due to
hydrocarbon seepage. However, hydrocarbon maturation solely could not explain the
differences of the magnetic mineralogy observed in the craton and the fold belt. Enhanced
magnetite formation in the thrust and fold belt is interpreted to be the result of higher
temperatures leading to stronger diagenesis of clay minerals.
Citation: Font, E., R. I. F. Trindade, and A. Nédélec (2006), Remagnetization in bituminous limestones of the Neoproterozoic Araras
Group (Amazon craton): Hydrocarbon maturation, burial diagenesis, or both?, J. Geophys. Res., 111, B06204,
1. Introduction
[2] Carbonate rocks often show very stable magnetization
but of secondary origin. Large-scale remagnetization in
such rocks has been vastly documented for the Phanerozoic
[e.g., McCabe and Elmore, 1989; Huang and Opdyke,
1996; Xu et al., 1998] and at a minor extent for the
Neoproterozoic [D’Agrella-Filho et al., 2000; Trindade et
al., 2003, 2004]. Remagnetized carbonates are recognized
by typical magnetic properties [e.g., Jackson, 1990; Jackson
et al., 1992, 1993; Channell and McCabe, 1994], and the
nature of the remanence carriers themselves has been
extensively documented by scanning electron microscopy
studies [e.g., Suk et al., 1990, 1993; Sun and Jackson, 1994;
Departamento de Geofı́sica, Instituto de Astronomia, Geofı́sica e
Ciências Atmosféricas, Universidade de São Paulo, São Paulo, Brazil.
Laboratoire des Mécanismes et Transferts en Géologie, UMR 5563,
Observatoire Midi-Pyrenees, Université Paul-Sabatier, Toulouse, France.
Copyright 2006 by the American Geophysical Union.
Weil and Van der Voo, 2002]. Even though some earliest
studies have suggested a thermoviscous resetting of the
natural remanent magnetization (NRM) to explain the
carbonate remagnetization [Horton et al., 1984; Kent,
1985], a chemical remanence magnetization (CRM) acquisition is presently recognized as the most suitable mechanism [e.g., McCabe and Elmore, 1989]. We note, however,
that the cause of the CRM is still debatable. Questions also
remain in respect to the source of the iron and to the role of
external/internal fluids in the formation of the new magnetic
[3] The most commonly invoked fluid-related agent of
chemical remagnetization in carbonates is the orogenrelated fluids, believed to be capable of causing magnetite
authigenesis along vast regions [Oliver, 1986; McCabe
and Elmore, 1989]. Alternatively, magnetite formation
may be linked to hydrocarbon migration and/or organic
matter maturation [Elmore et al., 1987; McCabe and
Elmore, 1989; Brothers et al., 1996; Banerjee et al.,
1997; Blumstein et al., 2004]. Brothers et al. [1996], for
example, have experimentally demonstrated that organic
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Figure 1. (a) Structural map of the Amazon craton and the northern Paraguay thrust and fold belt with
localization of the Terconi quarry and outcrops from Cáceres region (squares), Mato Grosso state, Brazil.
(b) Geological section A-B along a NW-SE transect from the Terconi quarry to Cuiabá city passing
through the Cáceres region. (c) Illite crystallinity indices along the same section from Alvarenga [1990].
acids produced by maturation of organic matter can be
responsible for magnetite formation and consequently for
remagnetization. On the other hand, authigenic magnetite
could also be linked to conversion of smectite to illite
[Elmore et al., 1993; Hirt et al., 1993; Katz et al., 2000;
Gill et al., 2002]. This reaction would furnish the iron
needed for the new magnetic minerals. Recently, Moreau et
al. [2005] have shown that sedimentary rocks can be
remagnetized during burial diagenesis with fluids being
produced in situ during burial.
[4] In the Amazon craton, carbonates of the Neoproterozoic Araras Group exhibit two distinct magnetic components across the same carbonate succession [Trindade et al.,
2003]. At the base of the Araras Group (Figure 1), just
above diamictites of the Puga Formation, pink dolostones of
the Mirassol d’Oeste Formation carry a dual polarity primary component, whose remanence is carried by detrital
specular hematite [Font et al., 2005]. Upsection, grey to
black bituminous limestones of the Guia Formation are
found both in the cratonic area, above the Mirassol d’Oeste
dolostones, and inside the Paraguay belt, in the Cáceres
region. These limestones yield a secondary component
confirmed by a negative fold test. Stratigraphy and sedimentology of these rocks are well established by Nogueira
et al. [2003] and illite crystallinity index for both the craton
and the fold belt are provided by Alvarenga [1990].
[5] The aim of this paper is to establish the nature of the
magnetic carriers of the Guia limestones remanent magnetization and investigate the role of lithotype, temperature
and presence of organic matter in influencing remagnetization. The contrasting lithology found between the Mirassol
d’Oeste dolostones and the Guia limestones, the presence of
bitumen in part of the stratigraphic section, and the temperature gradient toward the fold belt make this region an
excellent natural laboratory to study remagnetization processes. In this way, we have conducted a detailed rock
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Table 1. Site-Mean Paleomagnetic Directions of TR and TE Samplesa
Bedding Attitudes
Mean NRM, mA/m
D, deg
I, deg
a95, deg
Mean direction
Mean values of the natural remanence magnetization (NRM), declination (D), inclination (I), statistical value of a95, number of samples giving results
(n) and total number of samples analyzed (N), dispersion parameter (k).
Rejected for mean direction.
magnetic survey on Guia samples, at the craton and the fold
belt, including a series of magnetic tests complemented by
scanning electron microscope (SEM) observations and
energy dispersive spectra (EDS) analyses. On the basis of
these results, the mechanisms of remanence acquisition and
the source of iron for authigenic magnetic minerals in these
rocks are discussed.
2. Geological Setting and Sampling
[6] The Neoproterozoic cap carbonates of the Araras
Group are exposed on the southeast of the Amazon craton,
where they overlie diamictites of the Puga Formation
without significant hiatus [Nogueira et al., 2003]. The base
of the Araras Group succession is represented by the dolostones of the Mirassol d’Oeste Formation and the limestones of the Guia Formation which are the focus of this
work (Figure 1a).
[7] The Mirassol d’Oeste Formation is represented by
laminated pinkish dolomudstone principally constituted of
planar to undulated microbialites with fenestral porosity and
occurrence of tube-like structures in the intermediate part of
the sequence. The top of the sequence changes to grey
dolostones, fulfilled by hydrocarbon, with calcite cements
and megaripples. Upsection, the Guia Formation is entirely
constituted by bituminous limestones with terrigenous
grains. In both grey dolostones and limestones, the hydrocarbon fulfills fractures, styloliths, primary and secondary
porosity and dissolution cavities. In the underlying dolostones, bitumen is absent except for the immediately adjacent megaripple level suggesting that the Guia Formation is
the source rock for the hydrocarbon.
[8] Both Mirassol d’Oeste and Guia Formations crop out
at the border of the Amazon craton, in the Terconi quarry, in
nondeformed subhorizontal layers. Close to Cáceres, the
Guia Formation crop out inside the deformed zone of the
Paraguay belt (Figure 1b). At Terconi, the basal section of
the Guia Formation crops out above the Mirassol d’Oeste
dolostones, whereas in Cáceres, we found the top of the
same unit, covered by the dolostones of the Serra do
Quilombo Formation. This stratigraphic stacking is corroborated by facies analysis and chemostratigraphy [Nogueira
et al., 2003, 2006].
[9] The Paraguay belt is a 1200 km long thrust and fold
belt bordering the Amazon craton and the Rio de la Plata
block (Figure 1). Normal to its strike, the belt comprises an
undeformed sector at the border of the Amazon craton, and
the external and internal zones [Alvarenga, 1990]. From the
craton to the internal zone, deformation increases altogether
with the schistosity and metamorphism. Accordingly,
illite crystallinity indices provided by Alvarenga [1990]
(Figure 1c) indicate an increase in maximum temperatures
on the same direction.
[10] Seventeen paleomagnetic sites were collected along
more than 50 m of the carbonate succession. In the Terconi
quarry, samples (T13 – T19) were collected approximately
on meter-scale spacing, whereas in Cáceres, samples (T20 –
T28) are spaced of a distance of 5 – 10 m. First paleomagnetic results from dolostones showed a dual-polarity magnetization with northward (southward) and negative
(positive) magnetic directions that provided a 22 paleolatitude [Trindade et al., 2003]. Results obtained from the
Guia limestones showed a northeast, positive (single polarity) component that failed a fold test [Trindade et al., 2003].
This component gives a paleomagnetic pole that overlaps
those obtained for remagnetized carbonates from the São
Francisco craton, whose magnetization acquisition was
dated at around 520 Ma [D’Agrella-Filho et al., 2000;
Trindade et al., 2004].
3. Methods
[11] A total of 135 oriented samples from both the
bituminous upper dolostones of Mirassol d’Oeste Formation
(24 samples) and the limestones of the Guia Formation (111
samples) were analyzed after stepwise thermal cleaning
(Table 1). In order to reduce viscous overprints on the
characteristic magnetization, samples were first stored in a
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Figure 2. Paleomagnetic direction (declination Dec, inclination Inc) and magnetic properties (NRM,
Kfd, SIRM, and DP) of Mirassol d’Oeste cap dolostones and bituminous Guia limestones [after Trindade
et al., 2003; Font et al., 2005]. Saturation-induced remanent magnetization (SIRM) and the dispersion
parameter (DP) are obtained by the LAP-GAP-SAP analysis of Kruiver et al. [2001]. Kfd is the frequency
dependence susceptibility characterizing the superparamagnetic particles contribution [Jackson et al.,
1992]. Shaded area indicates bituminous dolostones.
low-field chamber for 3 months. Paleomagnetic data were
obtained after alternating field (AF) and thermal treatments
using 16 to 20 progressive steps. Measurements were
performed in an automatic three-axis 2G-cryogenic magnetometer, housed in a magnetically shielded room (ambient
field <1000 nT). The demagnetization data were plotted by
Zijderveld [1967] diagrams, and principal component analysis [Kirschvink, 1980] was used to determine magnetic
directions. Mean directions were calculated using Fisherian
statistics [Fisher, 1953].
[12] The rock magnetic measurements consisted of isothermal remanence magnetization (IRM) and anhysteretic
remanence magnetization (ARM) acquisition and demagnetization curves (Lowrie-Fuller and Cisowski tests [Johnson
et al., 1975; Cisowski, 1981]), demagnetization of triorthogonal IRM acquisition [Lowrie, 1990] and measurements of frequency dependence on the magnetic
susceptibility. The IRMs were induced using a SI-4 magnetizer system (Sapphire Instruments), while demagnetization and remanence measurements were performed with the
2G-cryogenic magnetometer coupled to a triorthogonal AF
demagnetizer. For the Lowrie-Fuller and Cisowski tests,
samples were first AF demagnetized at 200 mT and then
acquired an ARM with a peak AF field of 160 mT and a
biasing DC field of 0.1 mT. After AF demagnetization,
samples acquired an IRM up to 160 mT and were subsequently stepwise demagnetized at fields up to 160 mT. The
Lowrie test was performed after IRM acquisition at 1.4, 0.4
and 0.12 T fields along the z, y, and x axis, respectively.
Samples were then thermally demagnetized up to 700C.
Thermal demagnetization was performed using a Magnetic
Measurements oven (peak temperature within ±5C). The
frequency ependence of the magnetic susceptibility was
obtained after three series of measurements with a Bartington MS2 dual-frequency (470 and 4700 Hz) alternating
current bridge on whole rock specimens.
[13] SEM observations and EDS analyses were performed on both carbon-coated and noncoated polished
sections and rock fragments cut from the paleomagnetic
samples, with SEMs Jeol JSM-T330A and Jeol JSM6360LV both coupled to EDS analyzers.
4. Paleomagnetism
[14] The samples carry a natural remanent magnetization
(NRM) ranging from 0.07 to 0.16 mA/m in the Terconi
quarry and from 0.12 to 1.62 mA/m in Cáceres (Table 1 and
Figure 2). Over the 111 samples collected in the bituminous
limestones of the Guia Formation, 93 samples had given
reliable magnetic directions (83% of the totality) after
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Figure 3
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Figure 4. Stereographic projections of posttilting samplebased mean magnetic directions in bituminous Guia limestones from Terconi and Cáceres. The stereographic
projection of site-mean directions and the mean directions
of the correspondent paleomagnetic pole are given on the
thermal demagnetization (Table 1). Maximum angular deviation (MAD) values for principal components varies from
1.3 to 42.9 (in sample TE18.D3) with a mean MAD value of
10.88. No reliable data were obtained from the 24 samples
collected in the bituminous dolostones of the Mirassol
d’Oeste Formation where NRM intensities are the weakest
(Table 1 and Figure 2).
[15] Previous results obtained from the basal pink cap
dolostones indicate a primary magnetization carried by
detrital hematite [Trindade et al., 2003; Font et al., 2005]
(Figure 2). Orthogonal plots show a univectorial component
with unblocking temperatures between 600 and 670C for
both polarities (Figure 3a). At the upper, bituminous level of
dolostones, remanence rapidly reaches the noise level of our
cryogenic magnetometer giving rise to chaotic Zijderveld
diagrams (Figure 3b). Conversely, bituminous limestones
found upsection show well-defined single- and multivectorial orthogonal plots after heating (Figure 3c). In most
samples, a low-temperature component was isolated between 0 and 250C and is interpreted to be a viscous
remanent magnetization (VRM). In diagrams of normalized
intensity versus increasing demagnetization temperature,
samples from Terconi quarry were completely demagnetized at 320 – 360C indicating pyrrhotite as the unique
carrier of remanent magnetization. The large range of
unblocking temperatures of 40C around 340C found in
our samples suggest that iron sulphide did not behave
systematically as ideal pyrrhotite during thermal treatment
[Rochette et al., 1990; Zegers et al., 2003]. In contrast to
Terconi samples, samples from the deformed zone of
Cáceres have shown a second step at around 500C,
suggesting mixture of pyrrhotite and magnetite (Figure 3c).
Presence of magnetite in addition to pyrrhotite is probably
responsible for the higher NRM intensities observed in
these samples (Figure 2). Both pyrrhotite and magnetite
carry the same magnetic component in Cáceres (Table 1 and
Figure 4).
[16] More than six samples of bituminous limestones
were used to calculate the magnetic component of each
paleomagnetic site. Magnetic directions (declination, inclination) and statistic values (a95 and k) for each site are
given in Table 1. Site with a95 > 17 were dismissed. The
stereographic projections of the sample-based mean magnetic directions for each site are shown in Figure 4. In all
samples, a high-temperature component is isolated after the
250C heating step and cluster into the positive northeast
sector of the stereograph (Figure 4). In sites where NRM
intensities are weaker (Terconi quarry), mean values of a95
are approximately 15, whereas most sites from Cáceres,
where NRM is higher, have a95 less than 8 (Table 1).
Finally, based on site mean directions, a paleomagnetic pole
is defined at +332.6/+29.9 with a95 = 4.3 and k = 92.2
(Table 1).
5. Rock Magnetism
5.1. IRM Acquisition Curves
[ 17 ] Isothermal remanent magnetization acquisition
curves were obtained at fields up to 160 mT for 8 representative samples of the Guia Formation (Figure 5a). IRM
acquisition curves are smooth with concave shape and reach
90% of saturation at 160 mT which typifies low- to
medium-coercivity remanence carriers.
[18] IRM curves were then analyzed by cumulative logGauss functions (CLG) using the software developed by
Kruiver et al. [2001] (Figure 5b). The maximum saturation
field used here (160 mT) is much lower than the 1T field
Figure 3. Examples of orthogonal and demagnetization intensity plots of samples from (a) Mirassol d’Oeste cap
dolostones, (b) intermediary bituminous dolostones, and (c) bituminous Guia limestones (both Terconi and Cáceres region).
Units of orthogonal plots are in mA/m.
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Figure 5. (a) IRM acquisition curves of Guia limestones samples from Terconi and Cáceres, and (b)
example of LAP-GAP-SAP analysis of sample TE20.B2 obtained using the software developed by
Kruiver et al. [2001]. (c) Representation of the IRM parameters for limestones samples from Terconi
(circle) and from Cáceres (triangle). SIRM is the calculated saturation isothermal remanent
magnetization; DP is a dispersion parameter.
originally used by these authors. However, hysteresis curves
reveal that our samples saturate at fields below 200 mT (see
below) thus justifying the use of these low inducing fields.
The CLG analysis is based on the assumption that the IRM
of each magnetic mineral conforms to a cumulative log
Gaussian function of the magnetizing field and that the
different curves combine linearly [Robertson and France,
1994]. Each magnetic carrier has characteristic values of
saturation IRM (SIRM), B1=2 and dispersion parameter
(DP). The B1=2 parameter (Hcr) represents the field at
which half of the SIRM is reached and the DP parameter
reflects the dispersion of the logarithmic distribution around
B1=2 (corresponding to one standard deviation independent
of concentration). For all samples, the CLG models were
fitted with a single component for each rock-type suggesting only one magnetic carrier. Log B1=2 and calculated
SIRM values are comparable for all samples and are
bracketed between 48 and 90 mT and 20 and 200 mA/m,
respectively, thus indicating low- to medium-coercivity
magnetic carriers for these rocks. DP and SIRM values
vary significantly and are higher in samples from the
Cáceres region than Terconi samples (Figure 5c). SIRM
values depend principally on the nature of the magnetic
carriers, while DP variations could be due to several factors
such as the homogeneity of the grain size distribution and/or
to the crystallinity of the magnetic minerals into the sediments [e.g., Kruiver et al., 2003]. In Guia limestones, the
different pattern of SIRMs observed between Terconi and
Cáceres samples could be due to a different nature and/or a
mixture of magnetic carriers.
5.2. Thermal Demagnetization of Triaxial IRM
[19] The Lowrie test allows one to identify the magnetic
minerals by simultaneously analyzing their unblocking
temperatures and remanent coercivities [Lowrie, 1990]. This
test was performed for the same samples of Guia limestones
already used in the IRM analyses. The results of triaxial
IRM demagnetization curves are represented together with
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Figure 6
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the total remanence demagnetization curves (the sum of
the x, y, and z components) to highlight the unblocking
temperatures of the remanence carriers (Figure 6).
Coercivity intervals of 1.4– 0.4 T, 0.4 – 0.12 T and <0.12 T
are referred to as hard (z), medium (y) and soft (x) fractions,
[20] Thermal demagnetization of the induced IRMs gives
rise to smooth and sigmoidal curves. The dominant carriers
are distributed mainly in the soft and medium fraction for all
samples while the hard fraction gives low readings sometimes close to the noise level of our cryogenic magnetometer (Figure 6). In all samples, medium and soft fractions
demagnetized at temperatures of 280– 320C and at 580 –
600C indicating pyrrhotite and magnetite, respectively,
as carriers of the isothermal remanent magnetization.
Pyrrhotite is depicted in all samples in the soft and medium-coercivity fractions while magnetite is evident in the
low-coercivity fractions for most samples (see TE14.B1).
Furthermore, all the samples have a significant medium
component persisting after heating to 400C which could
also be attributed to magnetite. The total remanence curves
(Figure 6) show that both pyrrhotite and magnetite are
predominant in samples collected in the Terconi quarry
while samples from the Cáceres region show magnetite as
the principal carrier of the isothermal remanent magnetization. We note that the contribution of magnetite to the stable
high-temperature magnetization observed on these rocks is
also weak (Figures 3c and 6).
5.3. Lowrie-Fuller and Cisowski Tests
[21] The modified Lowrie-Fuller test [Johnson et al.,
1975] is based on the stability of IRM and ARM to the
AF demagnetization. It was originally designed to distinguish between single-domain (SD) and multidomain (MD)
magnetite grains, the ARM being harder than the IRM in
SD grains. However, later measurements have shown that
even large hydrothermal MD grains could show an SD-like
behavior [e.g., Heider et al., 1992; Halgedahl, 1998]. At
any rate, the test has been extensively used as a diagnostic
of remagnetization in carbonate rocks [Channell and
McCabe, 1994; Huang and Opdyke, 1996; D’Agrella-Filho
et al., 2000]. IRM has shown to be harder than ARM in
remagnetized carbonates, whereas in nonremagnetized ones
both remanences have shown comparable stabilities against
AF demagnetization. Remagnetized samples are also characterized by symmetrical IRM acquisition and demagnetisation curves in the Cisowski [1981] plot, suggesting a low
degree of interaction of the fine magnetic carriers [Channell
and McCabe, 1994; Huang and Opdyke, 1996]. These tests
were applied to 11 samples of the Guia limestones of which
8 are given in Figure 7.
[22] Differences in shape of ARM and IRM demagnetization curves are noted between the Terconi and Cáceres
results. Samples collected in the Terconi quarry show either
SD-like behavior (TE14.B1, TE16.A1 and TE18.C3) or
comparable stabilities (TE13.E3). Samples collected in the
Cáceres region systematically show the MD-like LowrieFuller behavior that typifies remagnetized carbonates
(20.B2, 23.F2, 24.C2, 26A2). We suggest that the opposite
results obtained in Terconi and Cáceres may reflect variations in the bimodal distribution of coercivities due to
different proportions in the mixture of two magnetic carriers
(namely pyrrhotite and magnetite).
[23 ] Results of the Cisowski test give an R value
(Figure 7) between 0.36 and 0.41 indicating low to moderate
interactions of fine ferromagnetic particles. Approximation
of the remanence coercive force (Hcr) given by the projection
of R on the abscise axis, is comprised between 30 to 40 mT
confirming that low- to intermediate-coercivity minerals are
the dominant magnetic carriers. Values of R and Hcr are
slightly superior in the Cáceres samples (R < 0.40) than in
Terconi samples (R 0.40), confirming the different magnetic mineralogies of the Terconi and Cáceres localities
(Figure 7).
5.4. Kfd and ARM/SIRM Ratio
[24] Remagnetized carbonates have remarkable magnetic
properties which can be used as proxies. For these rocks,
characteristic values of frequency dependence of susceptibility and ARM/SIRM ratios have been reported: Kfd 5%
(Kfd = [Khf-Klf]/Klf where Khf is high-frequency susceptibility and Klf is low-frequency susceptibility) and ARM/
SIRM > 10% [Jackson et al., 1992, 1993]. These values are
linked to a very significant contribution of ultrafine particles
to the bulk magnetic properties. Concerning the frequencydependent susceptibility, at high frequency (4700 Hz) the
measurement time is short enough for these particles to
behave as stable SD grains, while at low frequency (470 Hz)
the measurement time is longer than the relaxation time and
particles behave as superparamagnetic (SP). In our samples,
Kfd values vary between 0.8 and 20% and are intimately
related to the facies and the locality. Values of Guia limestones lie above the limits defined for remagnetized carbonates [Jackson et al., 1993], whereas pink dolostone values
indicate absence of SP particles (Figure 8). In contrast to the
basal dolostones, Terconi pink dolostones containing bitumen, at the top of the Mirassol d’Oeste Formation, show a
strong contribution of SP particles. Also, the Guia limestones shows different results depending on the location:
Cáceres samples have globally higher Kfd values than
Terconi samples indicating a major contribution of SP
particles at the fold belt.
6. Microscopy
6.1. Optical Microscopy
[25] Samples are constituted by alternation of thin limemudstone and calcite laminations (Figures 9a and 9b).
Bitumen is essentially localized in the intercrystalline porosity, in contact to calcite layers (Figure 9b). Bitumen is
also present in stylolith, primary porosity (fenestral) and
Figure 6. Examples of thermal demagnetization of triorthogonal IRM (soft 0.12T, medium 0.4T, hard 1.2T) in the Guia
limestones from the Terconi quarry and Cáceres region. Samples contain predominantly pyrrhotite and magnetite. Their
distinctive unblocking temperatures are better illustrated by the thermal demagnetization curves of the normalized resultant
magnetization (IRM/SIRM) shown on the right.
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Figure 7. Results of the Lowrie-Fuller and Cisowski test for the Guia limestones of Terconi quarry and
Cáceres region. Parameter R is the crossing point of IRM acquisition and demagnetization curves. Hcr is
obtained projecting R on the abscissa.
voids (Figure 9b) and frequently associated with euhedral
secondary dolomite [Nogueira et al., 2003].
6.2. Scanning Electronic Microscopy
[26] A SEM study was performed in order to determine
the nature of the ferromagnetic minerals present in the
rocks, and to infer their origin within the sediment. Such
method has already proved its efficiency in linking the
magnetic properties of the rock to the ferromagnetic minerals observed in the scanning electron microscope [Suk et
al., 1990; Sun and Jackson, 1994; Xu et al., 1998].
[27] The bituminous dolostones are constituted by dolomicrite and dolosparite (Figure 9c, EDS 1) associated to zones
rich in organic matter (Figure 9c, EDS 2). The latter contains
abundant dolomite rhombohedra (Figure 9c, EDS 3). In
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Figure 8. Frequency dependence susceptibility (Kfd)
versus ARM/IRMmax ratio (%) for the Mirassol d’Oeste
cap dolostones, the bituminous Guia limestones of Terconi
and Cáceres, and for one sample from the intermediary
bituminous dolostone layer of Terconi section. Typical
values of Kfd and ARM/SIRM for remagnetized carbonates
are noted [Jackson et al., 1992, 1993].
sample TE13, iron sulphides were depicted by EDS analysis
of the bituminous dolomicrite (Figure 9c, EDS 4).
[28] The Guia bituminous limestones from Terconi are
constituted by a calcic and dolomitic matrix (Figure 10a).
Iron-bearing minerals are essentially represented by iron
sulphides, essentially pyrite and, at a minor content, pyrrhotite (Figure 10a, EDS 1). Iron sulphides are localized in
the matrix or in contact with pockets containing organic
matter (Figure 10a, EDS 2). In sample TE18, numerous iron
sulphides are localized along a stratification plane where
clay minerals dominate (Figure 10b, EDS 3). In this zone,
Cu and Zn are also present and could be the result of
mineralization. In the analysis of the matrix of sample TE18
(EDS 4), calcite and aluminosilicates coexist with barytine
(Figure 10b). The distribution of iron-bearing minerals
(principally iron sulfides, Figures 10c – 10f, EDS 5 – 6) into
the matrix is not homogeneous. Instead, it appears to be
concentrated in the organic matter-rich zones and in voids
(Figure 10c). In sample TE18, a crust essentially composed
of iron with less than 20 mm in thickness and 50 to 100 mm
in length was observed (Figure 10d). The fragment appears
to be internally structured with laminations and is very
similar to those described by Belkaakoul and Aı̈ssaoui
[1997] in Jurassic shallow water carbonates of the Paris
Basin, also affected by extensive remagnetization. In the
same sample, an octahedral coarse-grained (4 mm) magnetite was encountered (Figure 10e). The mineral seems to be
included into the matrix and its habit suggests an authigenic
origin. This kind of magnetite has already been observed in
the Paleozoic remagnetized carbonates of North America
[Sun and Jackson, 1994]. Iron sulphides are ubiquitous in
samples from Terconi (and in minor extent in Cáceres) and
are responsible for a major contribution of S in EDS
analyses. They present a wide range of sizes, from less
than 1 mm to more than 10 mm in diameter. The coarser
population presents various morphologies, from euhedral to
xenomorphic, whereas the smaller grains, generally associated with magnetite, are represented by cubic pyrite crystals.
Distinction based on chemical composition (relative proportion of Fe and S) between pyrrhotite, greigite or pyrite,
could not be checked with needed accuracy by EDS
analyses. At any rate, since some amounts of magnetic
pyrrhotite have been depicted in the rocks by the magnetic
data, some of the sulphide grains observed in the thin
section should correspond to this iron sulphide. For instance, some iron sulphides show the typical monoclinic
structure of pyrrhotite (Figures 10f and 10g, EDS 6).
[29] The Guia limestones from Cáceres are constituted by
a (calcic and) dolomitic matrix associated to calcic concretions (Figure 11a, EDS 1) and organic matter-rich zones
(pockets) (Figure 11a, EDS 2). The matrix of the rocks
also contains organic matter and abundant iron oxides
(Figure 11a, EDS 3). A detail of the calcic concretions
shown in Figure 11a is illustrated by Figure 11b. Iron oxides
(Figure 11b, EDS 4) are identified both in the concretions
and in matrix. Dolomite (grey zones) and calcite (bright
zones) coexist in the concretion (Figure 11b, EDS 5–6). In
sample TE22, framboidal structures are observed in voids
(Figure 11c) and show, after analysis by EDS, a major
contribution of sulphur with minor counts of iron. Individual
euhedral crystals are less than 1 mm in diameter and dominantly octahedral in shape. Framboids are well documented in
remagnetized carbonates [Suk et al., 1990; Sun and Jackson,
1994], where SD to PSD magnetites were described.
[30] All samples present some amounts of terrigenous
elements as Mg, Si and Al and nonmagnetic minerals were
also observed. EDS analyses has also identified the presence
of rutile, titanite and pyrite and locally barytine.
7. Discussion
[31] The Guia limestones have shown a negative fold test
for their paleomagnetic directions. Accordingly, these rocks
show typical magnetic signatures of remagnetized carbonates such as high ARM/SIRM ratio, high-frequency dependency of magnetic susceptibility and contradictory
Lowrie-Fuller and Cisowski tests. Unblocking temperatures
suggest that secondary remanence is carried sometimes by
pyrrhotite and more frequently by a mixture of magnetite/
pyrrhotite, whose authigenic origin is suggested by SEM
observations. Although some authors have proposed a
thermoviscous origin for remagnetization in carbonates
[e.g., Kent, 1985], the low burial temperatures (below
250C) and high unblocking temperatures obtained for the
studied rocks preclude such possibility. A chemical remanent magnetization (CRM) is more suitable with the presence of magnetite framboids, iron sulphide crystals with
well preserved faces and edges and their occurrence preferentially in voids or microcracks. However, different
magnetic mineralogies are observed along the sedimentary
succession (Figure 2) suggesting that several factors control
the magnetic carrier production and the CRM remagnetization process.
[32] Recently, Moreau et al. [2005] have shown that
calcite content, organic matter and smectite to illite transformation could explain a CRM acquisition in marly sediments. Their experimental study reveals that magnetite is
produced at temperatures as low as 150C, after heating in
controlled atmosphere of inert gas. Hence the fluids generated by moderate heating would suffice for magnetite
synthesis. Their experiments reinforce the idea already put
forward by several authors that sedimentary rocks can be
11 of 17
Figure 9. Petrographic photographs of thin sections from bituminous limestones and dolostones: (a)
bitumen fulfilling intercrystalline porosity (sample TE22) and (b) deformed bituminous sheet cutting
original petrographic structures (sample TE24). Scanning electron microscopy photographs and EDS of
iron-bearing minerals in rock fragment of (c) the bituminous dolostones (TE13, Terconi) showing
bituminous pockets, euhedral dolomite, and numerous iron sulphides.
remagnetized during burial diagenesis with fluids produced
during burial and that both organic matter maturation and
clay transformation appear to be key elements for such a
remagnetization. The fact that a different magnetic mineralogy is observed in the studied localities of Terconi and
Cáceres, where physical-chemical conditions were different,
suggest that temperature, fluids and deformation did play an
important role in producing the authigenic magnetic mineralogy of the Guia limestones and give us an insight into the
role of these factors in the CRM acquisition mechanism in
carbonate rocks. Below we discuss each factor separately.
7.1. Hydrocarbon Maturation
[33] First, a strongly distinct magnetic mineralogy is
observed between pink (Mirassol d’Oeste) dolostones carrying a primary dual-polarity magnetization and the bituminous dolostones devoid of remanent magnetization.
Detrital hematite/magnetite are identified as carriers of the
primary magnetization of the Mirassol d’Oeste dolostones
[Font et al., 2005]. Detrital hematite was not depicted by
magnetic tests nor observed in SEM in dolostones containing bitumen. Instead, iron sulphides are present in some
amounts (compare SEM observations) as well as superparamagnetic particles, not observed but indicated by high
Kfd values. The fact that both pink and bituminous dolostones are stratigraphically continuous and both crop out in
the same nonmetamorphic context suggest that the presence
of oil was the main factor contributing for the destruction of
the original magnetic minerals (i.e., detrital hematite and
magnetite). However, since no magnetic directions were
retrieved by these rocks, this factor alone was not sufficient
to produce a stable secondary magnetization.
[34] In the bituminous Guia limestones higher NRM
intensities are noted (Figure 2) and a stable magnetization
is isolated. This secondary remanent magnetization is carried by magnetite and a mixture of pyrrhotite/magnetite.
The great amount of iron sulphides and their association
with bitumen in fractures and microcracks are in favor of a
chemical precipitation probably related to an epigenetic
enrichment of sulfur due to hydrocarbon maturation [Weaver
et al., 2002; Otofuji et al., 2003; Zwing et al., 2005]. This
suggests that when bitumen is associated to the limestone
facies (as bituminous pockets or impregnating the carbonatic
12 of 17
Figure 10
13 of 17
matrix), it is responsible directly or indirectly for a CRM
acquisition by magnetic carrier production.
[35] An indirect evidence for a relationship between
hydrocarbons and magnetic minerals in some carbonates
is given by the presence of spherical magnetite supposed to
be a product of microbial degradation [Elmore et al., 1987;
Elmore and Leach, 1990]. Bacteria are remarkable in their
metabolic diversity due to their ability to oxidize and reduce
metals, which lead to the precipitation, transformation, or
dissolution of magnetic minerals. Magnetotactic bacteria,
which live in shallow to deep marine, form intracellular
chains of small single domain magnetite grains called
‘‘magnetosomes’’ [e.g., Kirschvink and Chang, 1984;
Chang et al., 1987]. Although there are no reports of
magnetotactic bacteria living in hydrocarbon-related environments, at least some of the anaerobic varieties, for which
the distribution and importance in natural environments are
still unknown, could possibly live in hydrocarbon-related
settings [Machel and Burton, 1991]. Lovely et al. [1987]
discovered an organism (GS-15) that produced important
quantities of ultrafine extracellular magnetite via the oxidation of acetate and concomitant reduction of Fe3+ and Fe2+
under laboratory conditions. Such organisms may thus
produce free iron via oxidation of certain hydrocarbons in
anaerobic diagenetic environments. Oxidation reaction of
acetate is provided by Croal et al. [2004]:
CH3 COO þ 8FeðOHÞ3 þ15Hþ ! 2HCO
3 þ 8Fe 20H2 O ð1Þ
[36] Release of Fe2+ ions by microbial oxidation (equation (1)) is done from soluble iron-bearing minerals. Poorly
soluble iron-bearing minerals such as magnetite or pyrite are
not oxidized by these organisms, whereas the more soluble
siderite and ferrous sulphides are. This gives a possible
explanation for the correlation between authigenic magnetic
mineral content and percentage of calcite (siderite?), noted
in the present study and in the work of Moreau et al. [2005].
To oxide siderite and/or iron sulfides, reduced sulphur (as
H2S and HS) must be present [Goldhaber and Reynolds,
1991]. In an organic-matter-rich environment, H2S could
form by bacterial SO2
4 reduction, thermal release of H2S
from organic matter and thermochemical SO2
4 reduction.
Also, bacteria have the particularity to acidify the microenvironment around cell aggregates, facilitating the dissolution of ferrous sulfide (FeS) releasing available dissolved
HS [Kappler and Newman, 2004], as described in the
following equation [Schippers and Joergensen, 2002]:
FeS þ Hþ $ Fe2þ þ HS
[37] On the other hand, once dissolved Fe2+ is available, it reacts with the aqueous sulphur species to form
diagenetic iron sulfides (greigite, pyrite, pyrrhotite and
[38] Mineral precipitation generally is inorganic and occurred where the sulphide ion meet divalent ferrous ions,
close to the surface of the organisms, or after migration of
the ions. The best example of mineralization close to the
surface of the bacteria are clusters of framboidal pyrite, such
as those observed in the Guia limestone (Figure 10f), which
can be considered as pyritized bacteria [Machel and Burton,
1991]. This would indicate an anaerobic environment influenced by microbial activity in these rocks and suggest
that the iron and sulphur ions forming the magnetic carriers
of the Guia limestone remanence could possibility be
originated by hydrocarbon maturation via microbial activity,
and precipitated authigenic minerals in the voids and microcraks of the rock.
7.2. Burial Diagenesis
[39] In the Guia limestones, a different magnetic mineralogy is observed between sites from the Terconi quarry,
where geological strata are nonmetamorphozed, and sites
from the metamorphic thrust and fold belt region of
Cáceres. Samples from Terconi quarry exhibit unblocking
temperatures between 250 and 350C suggesting solely
pyrrhotite as the remanence carrier. Samples from Cáceres
exhibit a second step in NRM decay curves at around 550C
suggesting a mixture of pyrrhotite and magnetite. Alternatively, this mixture could be present in both localities but the
concentration of magnetic carriers must be higher in
Cáceres. Both, pyrrhotite and magnetite carry the same
remanence in these samples. Each group of samples is
characterized by different responses in the Lowrie-Fuller
test and distinct ranges of Kfd, DP, and SIRM values
(Figures 2, 6, and 8). It suggests that, in addition to
hydrocarbon maturation described above, another factor
must be invoked to explain the different magnetic mineralogy observed in the Terconi quarry and in the Cáceres
region. The transformation of clay minerals during burial
diagenesis, in particular from smectite to illite, could be one
of these potential factors [e.g., Hirt et al., 1993; Katz et al.,
2000; Gill et al., 2002; Blumstein et al., 2004; Moreau et
al., 2005]. As illustrated by the Figure 1, the two study
sections (Terconi and Cáceres) are located in distinct
structural zones where deformation, and consequently burial
diagenesis, increase from the Terconi quarry, on the border
of the Amazon craton, to the thrust and fold zone of the
Paraguay belt, near Cáceres. Illite crystallinity data from
this transect clearly show that both tectonic deformation and
metamorphic temperature increase from the border of the
craton to the internal zone of the fold belt (Figure 1). The
dependence of the magnetic properties of the Guia limestones on the illite crystallinity indirectly suggest that part of
the magnetite contributing to the stable high-temperature
Figure 10. Scanning electron microscopy photographs and EDS spectra of iron-bearing minerals in rock fragment of the
Guia limestones from Terconi. (a) Iron sulphides (i.e., pyrite and pyrrhotite) are disseminated in the matrix (Mg calcite) and
sometimes associated to bituminous pockets (sample TE15). (b) Numerous pyrites are localized in clay-rich layers, and
barytine is sometimes encountered in the calcic matrix (sample TE18). (c) Iron sulphides and oxides are localized in
microcracks and fractures (sample TE18, Terconi). (d) Crust of pure iron stick on the matrix (sample TE18, Terconi). (e)
Octahedral magnetite in the calcite matrix (sample TE18). (f) and (g) Monoclinic sulphur-bearing mineral (pyrrhotite?)
(sample TE18). In EDS 5 and 6, O and C are not depicted because they range out of the limit of the Jeol JSM-T330A
14 of 17
Figure 11. Scanning electron microscopy photographs and EDS spectra of iron-bearing minerals in
rock fragment of the Guia limestones from Cáceres. (a) Calcic concretions and bituminous pockets
(sample TE26). (b) Detail of the calcic concretions shown in Figure 11a illustrating calcite (bright zones)
containing dolomite crystals (grey zones) and an isolated iron oxide. (c) Typical framboids with presence
of pyrite and magnetite (sample TE22, Cáceres).
remanence of these rocks may come from clay minerals
8. Conclusions
[40] The Guia limestones of the Araras Group (Mato
Grosso, Brazil) show typical magnetic signatures of
remagnetized carbonates such as high ARM/SIRM ratios,
high-frequency-dependent magnetic susceptibility and
contradictory Lowrie-Fuller and Cisowski tests. Unblocking
temperatures suggest that the stable high-temperature rem-
anence is carried by both pyrrhotite and a mixture of
magnetite/pyrrhotite, whose authigenic origin in relation
to hydrocarbon is indicated by SEM observations. The
presence of magnetite framboids and euhedral iron sulphide
crystals often located near or into fractures are in favor of a
chemical remanent magnetization (CRM). Different magnetic properties between pink dolostones carrying a primary
detrital remanent magnetization and bituminous dolostones
devoid of remanent magnetization suggest that hydrocarbons are responsible for the attenuation of the NRM. In
contrast, the Guia limestones have higher NRM intensities
15 of 17
and pyrrhotite±magnetite is the principal carrier of the
magnetization. Sulfides associated with bitumen are probably related to an epigenetic enrichment of sulfur due to
hydrocarbon seepage. Mixture of water and organic molecules with iron or sulphur bearing minerals could produce
redox conditions favorable to the formation of iron sulphide
and magnetite. However, hydrocarbon maturation cannot
solely explain the difference in the magnetic mineralogy
observed in the Terconi and Cáceres region and another
factor is evocated to explain such differences. A higher
amount of magnetite in the metamorphic fold belt (Cáceres)
is interpreted to be the result of higher temperature conditions inducing clay minerals transformation and providing
additional iron for authigenic magnetic minerals to form.
[41] Acknowledgments. This research is part of the Ph.D. thesis of
the senior author. It was supported by the Brazilian FAPESP (02/02762-0
and 05/53521-1), the French CNRS program ECLIPSE and the CAPESCOFECUB cooperation program (442/4). We thank M. D’Agrella Filho,
A.C.R. Nogueira, and C. Ricomini, for their help in discussions and field
work, and H. Sampaio, T. Aigouy, and P. de Parceval for assistance in
microscopic observations. We appreciate the editorial comments by Randolph J. Enkin and the insightful reviews by Mike Jackson and Richard D.
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E. Font and R. I. F. Trindade, Departamento de Geofı́sica, Instituto de
Astronomia, Geofı́sica e Ciências Atmosféricas, Universidade de São
Paulo, Rua do Matão, 1226, 05508-090, São Paulo, Brazil. ([email protected]
A. Nédélec, UMR 5563, LMTG, Université Paul-Sabatier, 14 avenue
Edouard Belin, F-31400 Toulouse, France. ([email protected])
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