Petrography and Stable Isotope Geochemistry of the Cretaceous El

JOURNAL GEOLOGICAL SOCIETY OF INDIA
Vol.77, April 2011, pp.349-359
Petrography and Stable Isotope Geochemistry of the Cretaceous El
Abra Limestones (Actopan), Mexico: Implication on Diagenesis
JOHN S. ARMSTRONG-ALTRIN1, J. MADHAVARAJU2, ALCIDES N. SIAL3, JUAN J. KASPER-ZUBILLAGA1,
R. NAGARAJAN4, K. FLORES-CASTRO5 and JANET LUNA RODRÍGUEZ5
1
Instituto de Ciencias del Mar y Limnología, Unidad de Geología Marina y Ambiental, Universidad Nacional
Autónoma de México, Circuito Exterior s/n, 04510, Mexico D.F., Mexico. Email: john_arms@yahoo.com
2
Estación Regional del Noroeste, Instituto de Geología, Universidad Nacional Autónoma de México, Hermosillo
83000, Sonora, Mexico. Email: jmadhavaraju@yahoo.com; mj@geologia.unam.mx
3
Nucleo de Estudos Geoquímicos e Laboratório de Isótopos Estáveis (NEG - LABISE), Departmento de Geologia,
Universidade Federal de Pernambuco, Caixa Posta 7852, 50670-000 Recife, PE, Brazil.
4
Department of Applied Geology, School of Engineering and Science, Curtin University of Technology, CDT 250,
98009, Miri, Sarawak, Malaysia. Email: nagarajan@curtin.edu.my
5
Universidad Autónoma del Estado de Hidalgo, Centro de Investigaciones en Ciencias de la Tierra, Ciudad
Universitaria, Carretera Pachuca-Tulancingo Km. 4.5, 42184 Pachuca, Hidalgo, Mexico.
Abstract: Petrography and stable isotopes (carbon and oxygen) geochemistry of limestones from the El Abra Formation,
Actopan, were studied to identify their digenetic environments. The major petrographic types identified are mudstone,
wackestone, grainstone, and boundstone. Most of the studied samples show positive δ13C values, except two samples
(2 and 28), which are slightly negative values (-0.27‰ and -0.02‰). The organic remains identified in foraminiferal
wackestone type can be responsible for the negative δ13C values. The δ18O values range from -12.41‰ to -4.02‰ and
indicate meteoric diagenesis.
Keywords: Carbonate Rocks, Carbon and Oxygen Isotopes, Diagenesis, Hidalgo State, Mexico.
INTRODUCTION
The Cretaceous carbon cycle was concerned by a series
of oceanic anoxic events and contemporaneous phases of
platform destruction, which are marked by the carbon
isotopic signatures in carbonate rocks (Föllmi et al. 1994;
Weissert et al. 1998; Veizer et al. 1999). Similarly, carbon
isotope record of Lower Cretaceous is scattered by highamplitude positive δ13C excursions (Weissert and Erba,
2004). The studies reported in Lower Cretaceous shallow
marine carbonates have shown evidences for global scale
tectonics (Gröcke et al. 2005; Maheshwari et al. 2005;
Amodio et al. 2008), paleooceanographic processes (Kumar
et al. 2002; Madhavaraju et al. 2004), climatic and biotic
changes (Deshpande et al. 2003; Mishra et al. 2010; Préat
et al. 2010; Tewari et al. 2010). The negative oxygen isotope
values reveal either to increased temperature or introduction
of meteoric water during diagenesis, while the carbon
fluctuations relate to presence of organic matter or CO2
produced by various organic reactions (Armstrong-Altrin
et al. 2009).
The studies of modern carbonate soils (Cerling, 1984;
Cerling et al. 1989; Quade et al. 1989) show that there is a
direct relationship between the carbon isotopic value of
coexisting organic matter and carbonate where respiration
rates are high. Also, the isotopic composition of carbon in
the shells of modern marine and non-marine mollusks is
similar to that of carbonate rocks (Keith et al. 1964). The
water-rock ratio is a factor making the isotopic compositions
of the mineral phase shifting to water phase (Jenseuis et al.
1988). The oxygen isotope ratio of minerals is mainly
controlled by temperature of the minerals and origin of
the fluids and carbon isotopes may reflect various sources
of carbon including bacterial sulphate reduction,
fermentation, and dissolution of carbonate minerals
(Morad et al. 1990; Yoshioka et al. 2003; Ader et al. 2009;
Chakraborty et al. 2010). Similarly, oxygen and carbon
isotope values are negative in fresh water carbonates
than in marine carbonates (Gokdag, 1974; Mirsal and
Zankl, 1979; Madhavaraju et al. 2004; Santos et al. 2004;
Nagarajan et al. 2008). In this work, the diagenetic
0016-7622/2011-77-4-349/$ 1.00 © GEOL. SOC. INDIA
350
JOHN S. ARMSTRONG-ALTRIN AND OTHERS
environmental signatures of the El Abra limestones have
been studied through microfacies analysis and carbon and
oxygen isotope values.
STUDY AREA
The Actopan platform is located at the south-east end of
the great platform of Valles San Luis Potosí (hereafter
referred to as VSLP) and the platform sedimentary rocks
are exposed from Zimapan (southern part) to Actopan
(eastern part) (Fig. 1). The 24 m thick carbonate sequence
of El Abra Formation is exposed in the quarry located near
Actopan (Fig. 2 and Figs. 3a and b). Based on the lithological
variations, it has been divided into five distinct litho units,
97°52´
100°23´
City
deposition of the shallow marine sedimentation was
controlled by normal fault. The continuous subsidence of
the platform resulted in a thick sequence (about 1800 m) of
shallow marine carbonate facies (Aguayo-Camargo, 1998).
The El Abra Formation has been divided into two facies,
viz. Taninul and El Abra (Fig. 4). The Taninul facies include
platform margin reefal carbonate rocks (Bonet, 1952;
Aguayo-Camargo, 1978) whereas the El Abra facies is
considered as a lagoonal or back-reef deposit (Johnson et
al. 1988). The dominant lithology of the Taninul facies is
rudist-fragment lime packstone and grainstone. The presence
of radiolitids seen in this facies is grouped as clusters in
growth position while the caprinids are disoriented
(Alencaster and Garcia-Barrera, 2008). The El Abra facies
includes muddy carbonate with miliolid foraminifera,
mollusks, ostracodes, and calcareous algae (AguayoCamargo, 1998).
Study Area
Scale
23°44´
Matchuala
Ciudad Victoria
100 Km
0 10 2030 40 50 70
23°44´
N
Valles-San Luis Potosí
Platform (VSLP)
Tampico
22°15´
Gulf
of
Mexico
22°15´
San Luis Potosí
Ciudad Valles
Toliman
Reef
Zimapan
Basin
El Doctor
Platform
20°35´
Querétaro
Metztitlán
Actopan
Platform
20°35´ N
Ixmiquilpan
Actopan
Pachuca
100°23´
97°52´ W
Fig.1. Mid-Cretaceous paleogeography of east-central Mexico,
showing Valles- San Luis Potosí platform (modified after
Carrillo-Bravo, 1971).
viz., (i) clastic limestone, (ii) shell limestone with calcite
veins, (iii) shell limestone, (iv) algal limestone and
(v) calcrete. The Cretaceous El Abra limestones of VSLP
platform are shallow, protected back-reef to reef facies in
an interval of 7 to 8 km (Carrillo-Bravo, 1971; LópezDoncel, 2003; Carrasco-Velázquez et al. 2004) and
developed on several shallow water platforms in eastern
Mexico (Carrillo-Bravo, 1971; Enos, 1974). El Abra
Formation was deposited on an upfaulted block, a
transitional fore-reef to pelagic deposit that inter-fingered
with the basinal, carbonate, pelagic sediments of the
Tamaulipas Superior (Aguayo-Camargo, 1998). The
METHODOLOGY
Fifty thin sections were prepared from the laboratory
“El Pyroxeno” and studied in the petrography laboratory at
Estación Regional del Noroeste, Universidad Nacional
Autónoma de México, Hermosillo, Mexico. The
thin-sections were subjected to Alizarin Red-S stain to
distinguish the carbonate minerals. Friedman (1959) organic
stain specific for calcite and Katz and Friedman (1965)
combined organic and inorganic stain specific for iron rich
calcite have been adopted to identify the mineralogical
variations.
The carbon and oxygen isotopes have been analyzed for
23 samples of the El Abra Formation. Carbon and oxygen
isotope analyses were carried out at the Stable Isotope
Laboratory (LABISE) of the Federal University of
Pernambuco, Brazil. For carbon and oxygen isotopic
determinations, CO 2 was extracted from powdered
carbonates in a high vacuum line after reaction with H3PO4
at 25°C, and cryogenically cleaned according to the method
described by Craig (1957). CO2 gas released by this method
was analyzed for carbon and oxygen isotopes in a double
inlet, triple collector SIRA II mass spectrometer, using the
reference gas BSC (Borborema Skarn Calcite) calibrated
against NBS-18, NBS-19, and NBS-20, has a value of 11.28 ‰PDB for δ18O and -8.58‰PDB for δ13C. The results
are reported as per mil (‰) δ18O and δ13C values relative to
Pee Dee belemnite (PDB international standard). The
conversion of SMOW values to PDB standard have
been attempted by using the following formula δ18Ocalcite
(SMOW) = 1.03086 δ18Ocalcite (PDB) + 30.86 (Friedman
and O’Neil, 1977).
JOUR.GEOL.SOC.INDIA, VOL.77, APRIL 2011
PETROGRAPHY AND STABLE ISOTOPE GEOCHEMISTRY OF THE CRETACEOUS EL ABRA LIMESTONES, MEXICO
351
98°22´
99°13´
20 km
0
Ayotuxtla
20°29´
Ixmiquilpan
20°29´
120º
90º
N
Santiago de Anaya
Chilcuautla
Cerro Colorado
USA
30º
Study Area
Mexico Gulf of Mexico
Gulf of
California
Actopan
Tepatepec
Study Area
10º
Cuba
Central
America
Pacific Ocean
Huasca
Tezontepec
San Agustín
Tlaxiaca
Mineral del Monte
La Lagunilla
Pachuquilla
Tulancingo
N
Zapotlàn de Juàrez
19°58´
19°58´ N
Tecocomulco
98°22´ W
99°13´
Extrusive
Intermediate igneous rocks (Lower Tertiary)
Soil (Quaternary)
Extrusive
basic igneous rocks (Quaternary)
Extrusive
Intermediate igneous rocks (Tertiary)
Limestone, Shale (Upper Cretaceous)
Sandstone, Conglomerate (Tertiary)
Limestone (Lower Cretaceous)
Extrusive acid igneous rocks (Tertiary)
Limestone, Shale (Upper Jurassic)
Extrusive basic igneous rocks (Tertiary)
Shale, Sandstone (Lower Jurassic)
Sandstone, Tuff (Upper Tertiary)
Intrusive acid igneous rocks (Mesozoic)
Shale, Sandstone (Palaeocene)
Sandstone, Conglomerate (Triassic)
Limestone (Cretaceous)
Fig.2. Simplified geological map of the study area
RESULTS
Petrography
A petrographic description of El Abra carbonate rock
types has been documented based on carbonate
classifications of Folk (1959) and Dunham (1962). Four
petrographic types have been identified i.e., mudstone (Figs.
5a and b), wackestone (Figs. 5c-e), grainstone (Figs. 5f-h),
and boundstone (Figs. 5i and j).
Mudstone
Fenestral mudstone (Figs. 5a and b) exhibits fenestral
JOUR.GEOL.SOC.INDIA, VOL.77, APRIL 2011
fabrics (millimeter to centimeter size voids). The fenestrae
are originated from the decomposition of algae and
cyanobacteria. Based on the growth pattern of algal elements
the voids developed either as irregular roofs (stromatactis)
or as isolated spar filled voids (bird’s eyes). The thin calcite
coating is observed within the fenestral structure. Matrix
mainly consists of non-laminated or poorly structured
microbialites.
Wackestone
Pelletal wackestone is named after the presence of
numerous fecal pellets (Fig. 5c). These fecal pellets are
352
JOHN S. ARMSTRONG-ALTRIN AND OTHERS
Sample
number Lithology
m
Top
Maastrichtian
24
TAMUÍN-MEM
Campanian
28
27
26
25
24
23
22
Algal limestone
Lower Cretaceous
21
20
19
18
17
Shell limestone
16
15
12
14
13
12
11
Shell limestone
with calcite veins
10
C
R
E
T
A
C
E
O
U
S
U
P
P
E
R
7
6
5
4
Santonian
SAN
FELIPE
SAN
FELIPE
Coniacian
AGUA
NUEVA
Turonian
Cenomanian
L
O
W
E
R
9
8
6
MÉNDEZ
SHALE
Calcrete
18
FORMATION
STAGE
EL
ABRA
(El Abra memberTaninul member)
TAMABRA
AND
UPPER
TAMAULIPAS
Albian
(Interdigited)
OTATES
Aptian
LOWER
TAMAULIPAS
Barremian
Clastic limestone
3
Fig.4. Generalized Cretaceous stratigraphic column of the
easternmost Valles-San Luis Potosí Platform and Tampico
Emabyment (after Aguayo-Camargo, 1998).
2
0
1
Bottom
Fig. 3a
3b
m
6
4
2
0
Fig.3. (a) Lithostratigraphy of the quarry section showing
sample locations. (b) Field photograph showing the quarry
section
composed of micrite and are lacking recognizable internal
structure. In cross section, the pellets are generally rounded,
elongated or rod shape. The size of the pellets is ranging
from < 100 µm to several millimeters. The surfaces of these
pellets are smooth and dull in appearance. Most of the pellets
are homogenous in nature, whereas some pellets contain
silt-sized inclusions (quartz or skeletal debris). The dark
color of the grains is due to the high content of organic matter
or iron sulphides and these pellets are poorly sorted in nature.
The sorting of the fecal pellets provides a first approximation
of the hydrodynamic condition of these grains (Wanless et
al. 1981). The pore spaces are filled with microsparite and
sparry calcite cement. The foraminiferal wackestone (Figs.
5d and e) has >10% of organic remains, which are floating
on the micritic matrix. The organic remains include miliolids
and textularia, and both of which exhibit micritised bivalve
shells. Thin film of isopachous microcrystalline calcite
cement is formed around the bioclasts. The internal chambers
of the miliolid and textularia are partly or wholly filled with
microsparite calcite cement. The limestone exhibits
numerous interparticle pore spaces. The smaller pore spaces
are filled with microsparite whereas the larger pore spaces
are filled with sparry calcite cement. The limestone shows
minor veins, which are also filled with microsparite calcite
cement.
Grainstone
Ooid grainstone encloses assorted size of ooids with
concentric layers (Figs. 5f and g). They are subrounded
(spherical) to oblate (spheroidal) in shape. Some ooid like
lumps are seen, which exhibit concentric or with out
JOUR.GEOL.SOC.INDIA, VOL.77, APRIL 2011
PETROGRAPHY AND STABLE ISOTOPE GEOCHEMISTRY OF THE CRETACEOUS EL ABRA LIMESTONES, MEXICO
5a
5b
5c
5d
5e
5f
353
Fig.5. (a) Mudstone: Thin section photomicrograph showing mud supported rock, filled with micro sparite cement (scale bar = 0.5 mm).
(b) Fenestral mudstone: photomicrograph exhibits millimeter to centimeter size voids. Also showing distinct bird’s eye view,
which are mainly originated from the decomposition of algae and cyanobacteria (scale bar = 0.5mm). (c) Pelletal wackestone:
photomicrograph showing the presence of numerous fecal pellets and the pore spaces are filled with microsparite and sparry
calcite cement (scale bar = 0.5 mm). (d and e) Foraminiferal wackestone: thin section photomicrograph consists of more than
10% of organic remains (miliolid and textularia), which are floating on the micritic matrix. The pore spaces are filled with
microsparite and sparry calcite cements (scale bar = 0.5 mm). (f ) Ooid grainstone: photomicrograph showing concentric ooids of
different sizes. These ooids were undergone compaction effects and the concentric layers were removed or destroyed during
compaction (scale bar = 0.5 mm).
JOUR.GEOL.SOC.INDIA, VOL.77, APRIL 2011
354
JOHN S. ARMSTRONG-ALTRIN AND OTHERS
5g
5h
5i
5j
Fig.5. (g) Ooid grainstone: photomicrograph showing concentric ooids of different sizes. These ooids were undergone compaction
effects and the concentric layers were removed or destroyed during compaction (scale bar = 0.5mm). (h) Pelloid grainstone:
exhibits small to large size pelloids. Most of the pelloids are sub-rounded in shape and are devoid of internal structure (scale bar
= 0.5 mm). (i and j) Laminated bindstone: illustrates three distinct layers 1) microbial crust layer, 2) sparry calcite cement layer,
and 3) pelloid rich layer. The size, shape and position of these pelloids represent different mode of origin.
concentric layers. The incompletion or removal of concentric
layers is due to attrition prior to re-deposition. These ooids
were undergone compaction effects; indicated by plastic
bending of ooid grains, flattened grains and parallel grain
contacts. The layers in the ooids were removed during
compaction. The nuclei of most of the ooids are dissolved
or removed. The leaching of nuclei has created a secondary
intra-particle porosity filled with cement and leads to
oomoulds. The ooid grainstone exhibits mouldic porosity
formed by selective dissolution of ooids. Thin line of calcite
cementation is seen below the deformed ooids, which suggest
that the precipitation of calcite cement in the meteoricvadose zone subsequent to shallow burial compaction.
Pelloid grainstone (Fig. 5h) exhibits small to large size
pelloids. Most of the pelloids are sub-rounded in shape and
are devoid of internal structure. These pelloids are mainly
embedded in the microsparite calcite cement. The pelloid
grainstone exhibits thick fracture, which is filled with sparry
calcite cement.
Bindstone
Laminated bindstone (Figs. 5i and j) exhibits microbial
crust layer, sparry calcite cement layer, and pelloid rich layer.
The sparry calcite cement separates the laminated pelloid
rich layer (microbial crust layer) and non-laminated pelloid
rich layer. The microbial crust layer consists of thinner
micritic and thicker pelloid rich layers and showing
alternative light and dark color layers. The crust mainly
consists of strongly undulated spongiostromate micritic layer
separated by sparry calcite rich layer. The spongiostromate
micritic crust showing variable thickness of the micritic
laminae and these crusts are centimeters in thick. The
JOUR.GEOL.SOC.INDIA, VOL.77, APRIL 2011
PETROGRAPHY AND STABLE ISOTOPE GEOCHEMISTRY OF THE CRETACEOUS EL ABRA LIMESTONES, MEXICO
Carbon and Oxygen Isotopes
The δ13C values vary from -0.27‰ to 2.67‰ PDB
(Table 1) and these δ13C values are comparable to the
modern carbonate sediments (range from 0 to +4‰ PDB,
McKenzie, 1981). The δ18O values range from -12.41‰ to
-4.02‰ PDB (Table 1). Significant negative δ18O values
are observed in the samples 28 and 23 (-12.41‰ and
-11.65‰, respectively).
Table 1. Carbon and oxygen isotopic data for the El Abra limestones
Sample
1
2
3
4
5
6
7
11
12
13
14
15
16
17
18
19
20
21
22
23
26
27
28
Lithology
Clastic limestone
Clastic limestone
Clastic limestone
Clastic limestone
Clastic limestone
Clastic limestone
Clastic limestone
Shell limestone
Shell limestone
Shell limestone
Shell limestone
Shell limestone
Shell limestone
Shell limestone
Shell limestone
Shell limestone
Algal limestone
Algal limestone
Algal limestone
Algal limestone
Algal limestone
Algal limestone
Algal limestone
δ13C
(‰ PDB)
δ18O
(‰ PDB)
δ18O
(SMOW)1
1.52
-0.27
0.87
1.39
1.14
1.17
0.59
2.67
2.18
2.28
1.77
1.65
1.25
2.07
2.49
2.15
1.35
0.65
1.70
1.05
1.44
1.97
-0.02
-4.61
-5.93
-6.29
-5.57
-5.88
-5.70
-6.21
-4.09
-5.81
-5.69
-7.36
-6.42
-8.99
-8.39
-4.02
-4.50
-8.99
-6.05
-5.12
-11.65
-6.81
-6.25
-12.41
26.10
24.75
24.37
25.12
24.80
24.98
24.46
26.65
24.87
24.99
23.27
24.25
21.59
22.21
26.72
26.22
21.59
24.63
25.59
18.86
23.84
24.42
18.06
1
δ18Ocalcite (SMOW) = 1.03086 δ18Ocalcite (PDB) + 30.86 (Friedman and
O’Neil, 1977)
marginal marine environment. The pelloids in grainstone
were subjected to compaction effect prior to stylolitization.
The size, shape, and position of these pelloids represent
different modes of origin. Similarly, the presence of pellets
and pelloids infers the protected shallow water environment.
The type of ooids and its contacts between the grains is
probably due to the result of early solution. The alteration
of partially dissolved ooid grains indicates that the
compaction effect took place during shallow burial.
Carbon and Oxygen Isotopic Variations
The δ13C values (-0.27‰ to 2.67‰ PDB; Table 1; Fig.
6) of the El Abra limestones indicate the re-equilibration
between rock components with isotopically light waters
(fresh waters), and presence of marine signals (unaltered or
less altered). Isotopically light carbon may be derived from
decay of organic materials in soils and incorporated into
soil gas as in the vadose zone. Variations in δ13C seawater
composition have been documented in pelagic and
hemiplegic carbonates from different locations and time
periods (Weissert, 1989; Föllmi et al. 1994; Grötsch et al.
1998; Wendler et al. 2009). Short term variations in the δ13C
signature of shallow water carbonates are widely used to
interpret the primary variations in the oceanic δ13C signal
Sample
number
Top
The presence of neomorphic fibrous calcite, drusy sparry
calcite, vadose silt and blocky sparry calcite cement types
in the El Abra limestones indicates the emergence and
submergence of the Cretaceous platform. Micritic cement
in laminated bindstone may indicates the depositional
environment probably be a sheltered lagoon or shallow
JOUR.GEOL.SOC.INDIA, VOL.77, APRIL 2011
18
d13C‰
0
d O‰
5
-15
-10
-5
0
28
27
26
25
24
23
22
18
21
20
19
18
17
16
15
12
14
13
12
11
DISCUSSION
Diagenesis
-5
24
Lower Cretaceous
pelloids (Fig. 5j) are sub-rounded to elongate in shape. The
non-laminated pelloid rich layer mainly consist of poorly
sorted pelloid grains. These pelloids were subjected to
lesser compaction effects than the pelloids present in the
laminated layers.
355
10
9
8
6
7
6
5
4
3
2
1
0m
Bottom
Fig.6. Stratigraphic section and carbon and oxygen isotopes profile
for the El Abra quarry.
JOHN S. ARMSTRONG-ALTRIN AND OTHERS
d18O‰ (PDB)
–10
+20
–8
–6
–4
–0
–2
+15
+4
+2
+6
1 Fermentation cements
1
2 Marine dolomites
+10
3 Evaporative dolomites
+5
9
10
11
+0
5
4 6
7
4 Ooids
2
8
5 Marine cements
3
6 Warm-water carbonate sediments
δ13C ‰ (PDB)
of the Early Cretaceous (Jenkyns, 1995; Vahrenkamp, 1996;
Grötsch et al. 1998). The high algal population and
photosynthetic activity in the shallow marginal marine
environment can give positive δ13C values (Milliman and
Muller, 1977; Nelson, 1988). Considering the El Abra
limestones, the presence of organic remains identified
petrographically in the foraminiferal wackestone can be
responsible for the negative δ13C values in two samples
(S.Nos. 2 and 28).
The δ18O values are scattered (-12.41‰ to -4.02‰ PDB)
than the δ13C values (-0.27‰ to 2.67‰ PDB; Table 1;
Fig. 6). The δ18O value in the clastic limestone is about 4.61‰ whereas it decreases to -12.41‰ at the top of the
calcrete section. These negative δ 18O values indicate
meteoric water diagenesis (Veizer and Demovic, 1973).
Furthermore, wide variations in δ18O values (Table 1) of
the El Abra limestones may relate to regressive sea-level
cycles and sub-aerial exposure (Carpenter et al. 1988;
Longstaffe et al. 1992; Ludvigson et al. 1994). Fluctuating
relative sea-level controlled the position of the coastal
aquifer, with sea-level rise resulting in more marine pore
fluids, and lowering of relative sea-level resulted in seaward
shifts of the coastal mixing zone and consequent freshening
of pore fluids. Carbonate cement precipitated during this
active hydrological history recorded the isotopic
characteristics of the parent fluids (Coniglio et al. 2000).
However, oxygen isotopes are more susceptible to diagenesis
than the carbon isotopes (Morse and Mackenzie, 1990). This
is partly due to the temperature-related fractionation
observed in oxygen isotopes. Diagenesis often results more
negative δ18O values in marine carbonates (Land, 1970;
Allan and Matthews, 1977). Because cementation and recrystallization often takes place in fluids depleted in δ18O
with respect to sea water (e.g. meteoric water) or at elevated
temperatures (burial conditions). Hence, the observed spread
in negative δ18O values of the El Abra limestones indicate
that they were altered by diagenesis.
Hudson (1977) proposed the δ18O versus δ13C bivariate
diagram with generalized isotopic fields for carbonate
components, sediments, limestones, cements, dolomites, and
concretions. Later, Nelson and Smith (1996) modified and
distinguished a number of characteristic isotope fields for
carbonates of different origins. In this diagram (Fig. 7) most
of the El Abra limestones plot in the marine limestone and
burial cement fields (Field nos. 10 and 11), which also
reveals the alteration during diagenesis.
The intensity of diagenetic alteration in limestones is
estimated by plotting δ13C and δ18O values (Fig. 8). The
δ13C values show statistically positive correlation with δ18O
values (r = 0.46, n = 23; critical t value for 99% confidence
–5
13
–10
14
12
)
7 Warm-water skeletons
12
15
8 Oozes
9 Burial dolomites
–15
10 Burial cements
15
11 Marine limestones
–20
16
12 Meteoric cements
–25
13 Freshwater limestones
14 Soil calcites
–30
15 Mixing-zone dolomites
–35
16 Early concretions
17
17 Methane-derived cements
–40
–45
Fig.7. Reference δ18O and δ13C diagram showing isotope fields
for carbonate components, sediments, limestones,
dolomites, and concretions. The samples 23 and 28 are not
included in this diagram because of its depleted negative
oxygen isotope values (Table 1).
level is 0.487; Verma, 2005), such positive relationship
between δ13C and δ18O indicates that the El Abra limestones
were altered by diagenesis (Marshall, 1992; Buonocunto
et al. 2002).
Water/rock Interaction
The δ18O value is a sensitive parameter for evaluating
water-rock ratios, during multiple interactions of meteoric
water with the limestones in an open diagenetic system. The
oxygen isotopic composition of the rock can achieve isotopic
equilibrium with the water at a relatively low water/rock
ratio because water (H2O) forms a very large reservoir of
-4
r = 0.46; n = 23
-6
G 18O ‰ (PDB)
356
-8
-10
-12
-14
-1
0
1
2
3
G 13C ‰ (PDB)
Fig.8. δ13C-δ18O bivariate plot for the El Abra limestones. n =
number of samples.
Fig. 8
JOUR.GEOL.SOC.INDIA, VOL.77, APRIL 2011
PETROGRAPHY AND STABLE ISOTOPE GEOCHEMISTRY OF THE CRETACEOUS EL ABRA LIMESTONES, MEXICO
oxygen. The reservoir for carbon in water is much smaller
than for oxygen and much higher water/rock ratios are
needed to lower the δ13C composition of the limestones
significantly (Hudson, 1977; Armstrong-Altrin et al. 2009).
This water/rock ratio-dependent change is reflected by
the spread in the δ18O (-12.41‰ to -4.02‰ PDB) and
δ 13C (-0.27‰ to 2.67‰ PDB) values of the El Abra
limestones. The variations in δ13C values are characterized
by a change in the diagenetic environments (from vadose
to phreatic) and reveal that the diagenetic system was
relatively open. Similarly, the photosynthetic activities of
algal population (identified in laminated bindstone) in
shallow marine environment may result a change in carbon
isotopic composition of the El Abra limestones (Gobron
et al. 2006).
CONCLUSIONS
The petrographic types; mudstone, wackestone,
357
grainstone, and boundstone indicate a slow rate of
sedimentation and shallow water environment in a carbonate
ramp. Fecal pellets identified in pelletal wackstone are
devoid of internal structure and poorly sorted, which
indicates the hydrodynamic condition of the pelletal grains.
The positive δ13C isotope signature seems to be related to
the sub aerial exposure of the El Abra limestones. The
observed spread in negative δ18O values of the El Abra
limestones indicates that they were altered by diagenesis.
Acknowledgements: We are grateful to Eumir Everest
Herrera Gutiérrez, Norma Liliana Cruz Ortíz, and Díaz
Cerón Verónica for their assistance during field work. JSA
wishes to express his gratefulness to Instituto de Ciencias
del Mar y Limnología (project no. 616) and SEP-PROMEP
(UAEHGO-PTC-280) for financial assistance. The authors
thank Dr. Surendra P. Verma for suggestions on the earlier
version of the manuscript. We are grateful to the reviewer
and the editor for their constructive comments.
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