Geochemical Journal, Vol. 47, pp. 537 to 546, 2013
doi:10.2343/geochemj.2.0275
Geochemistry and chemostratigraphy of the Colón-Mito Juan units
(Campanian–Maastrichtian), Venezuela:
Implications for provenance, depositional conditions,
and stratigraphic subdivision
L. A. MONTILLA,1 M. MARTÍNEZ,2 G. M ÁRQUEZ,3* M. ESCOBAR,4,5 C. SIERRA ,6 J. R. GALLEGO,6
I. ESTEVES7 and J. V. GUTIÉRREZ2
1
PDVSA, División Oriente, Gerencia de Exploración, Puerto La Cruz, Venezuela
Instituto de Ciencias de la Tierra, Universidad Central de Venezuela, Caracas, 3895, 1010-A, Venezuela
3
Departamento de Ingeniería Minera, Mecánica y Energética, Universidad de Huelva, Huelva, 21819 Huelva, Spain
4
CARBOZULIA, Av. 2 No. 55-185, Casa Mene Grande, Maracaibo 4002 A, Venezuela
5
Postgrado de Geología Petrolera, Facultad de Ingeniería, Universidad del Zulia, Maracaibo 4002, Venezuela
6
Departamento de Exploración y Prospección de Minas, Universidad de Oviedo, Mieres, 33600 Asturias, Spain
7
Fundación Instituto Zuliano de Investigaciones Tecnológicas (INZIT), Maracaibo 4001, Venezuela
2
(Received May 4, 2013; Accepted July 25, 2013)
A geochemical and chemostratigraphical study was undertaken on Campanian–Maastrichtian sedimentary rocks (the
Colón-Mito Juan sequence and the upper La Luna Formation) in the southwestern Maracaibo Basin, Venezuela. The
objectives of this work were to determine the paleoenvironmental and physico-chemical characteristics of the Colón-Mito
Juan sequence and its possible subdivision into chemofacies and to study the main chemical differences between the
Colón, Mito Juan, and La Luna Formations within the study region. One hundred and ninety-one rock samples were
collected, and bulk inorganic geochemistry (TiO2, Al2O3, Fe 2O3, MgO, CaO, Na 2O, K2O, P 2O5, C, S, Rb, Cs, Ba, Sr, Th,
U, Y, Hf, Mo, V, Cr, Co, Cu, Ni, Sc, La, Ce, Nd, Sm, Eu, Yb, Lu, As, Sb, Zn, and Be) was analyzed by instrumental neutron
activation analysis or inductively coupled plasma-atomic emission spectroscopy; total sulfur and carbon analyses were
performed by a LECO SC-432 apparatus and coulometry, respectively. Multivariate statistical techniques were applied to
evaluate correlations within this group of variables. Using cluster-constrained analysis, eight subdivisions, or chemical
facies, were defined: two chemofacies differentiating the intervals controlled by biogenic deposition and by the predominant clastic contribution; three chemofacies correlating with the lithologic units (La Luna, Colón, and Mito Juan); and
another three chemofacies related to changes in the paleoredox conditions along the stratigraphic column. All of the units
studied were deposited under a relatively constant climate regime, and the composition of the sediment source showed no
significant changes. The prevailing physico-chemical regime was disoxic-oxic, with a trend of increasing oxygen concentrations towards the top of the column.
Keywords: geochemistry, chemostratigraphy, Colón-Mito Juan sequence, stratigraphic subdivision, Lake Maracaibo
identify source rocks and weathering processes (Cullers,
2000). A chemostratigraphic study, which involves the
characterization of the sedimentary sequence into different units on the basis of major and trace element chemistry (e.g., Pearce et al., 1999) is done when geochemical
data are evaluated in the context of a stratigraphic log.
Chemostratigraphy can be carried out with isotopic data
(e.g., Ehrenberg et al., 2000) or by combining several
chemical indices (Reyment and Hirano, 1999; Reinhardt
and Ricken, 2000). In addition, other features revealed
by chemostratigraphic studies include climatic changes,
paleoredox conditions, stratigraphic correlations,
paleoproductivity, and chemical cyclicity in processes
involving basin sedimentation (Yarincik and Murray,
INTRODUCTION
Integrated geochemical and chemostratigraphical studies of sedimentary rocks allow the determination of
paleoenvironmental conditions and provenance of
sediments (e.g., Armstrong-Altrin et al., 2004). The
geochemistry of clastic sediments is controlled by the
composition of the source rocks, weathering, deposition,
and diagenetic processes (Asiedu et al., 2000; Yan et al.,
2006). Consequently, geochemical tracers can be used to
*Corresponding author (e-mail: [email protected])
Copyright © 2013 by The Geochemical Society of Japan.
537
Maastrichtian in western Venezuela, a time during which
major changes and climatic variations in sedimentation
patterns occurred (Erlich et al., 2000).
The main goals of this study were (1) to establish the
environmental and physico-chemical characteristics of the
Colón-Mito Juan sequence; (2) to chemically differentiate it from the La Luna Formation; (3) to subdivide it
into chemical facies associated with changes in the concentrations of different elements; and (4) to establish the
sedimentary processes that originated these chemofacies.
The literature refers to the Colón and Mito Juan units
as the Colón-Mito Juan sequence, as it is very difficult to
accurately recognize the transitional contact between the
two formations (Savian, 1993). Therefore, it is of interest to establish the stratigraphic level that records the
chemical changes, if present, that help distinguish the two
aforementioned formations.
GEOLOGICAL BACKGROUND
Fig. 1. Sketch map showing the two sampling sites and the
main localities in the study region in the state of Táchira (Venezuela).
2000; Hetzel et al., 2009; among others).
The present study focused on the geochemistry and
chemostratigraphy of Late Campanian to Late
Maastrichtian (76–65 Ma) sedimentary rocks in the western region of the state of Táchira, Venezuela. First, we
studied a sequence consisting of the uppermost part of
the La Luna Formation (Tres Esquinas Member) and the
Colón and Mito Juan units outcropping close to the
Lobaterita River near the locality of San Juan de Colón
(Fig. 1). We then examined a second stratigraphic section of rocks comprising the Táchira Ftanita and Tres
Esquinas members (La Luna Formation) up to the lowest
part of the Colón Formation outcropping in a cut along
the San Pedro de Río-Ureña road (Fig. 1).
The particular case of the Colón Formation is very
interesting because when studying a stratigraphic sequence characterized by a monolithological composition,
according to González de Juana and colleagues (1980),
the variations in chemical profiles are not strongly influenced by lithological changes. Moreover, interest in performing this study in the Colón Formation comes from
the following observations: (1) the formation’s total organic carbon (TOC) values are higher than 1% in some
areas (Malavé, 1994); (2) the formation acts as a caprock
in the petroleum system of the Maracaibo Lake Basin
(Parnaud et al., 1995); and (3) it represents most of the
538 L. A. Montilla et al.
The Lake Maracaibo Basin is located at the southwestern end of the Caribbean Sea in Venezuela, near its border with Colombia. This basin consists of a thick sedimentary cover divided into various sequences conditioned
by tectonic events: a Jurassic rift succession; an Early–
Late Cretaceous passive margin sequence; Late
Cretaceous–Early Paleocene deposits representing a transition to a compressive regime that occurred when collision of the Pacific volcanic arc emplaced the “Lara
Nappes” to the northern edge of the aforementioned basin; Late Paleocene–Middle Eocene foreland basin deposits that formed in front of the volcanic arc; and a Late
Eocene–Pleistocene sequence related to the collision of
the Panama arc with the South American plate (Mann et
al., 2006; Escalona and Mann, 2011).
The sedimentary succession of the southwestern sector within the Lake Maracaibo Basin overlies the
igneous-metamorphic basement and begins with red beds
of the Jurassic La Quinta Formation, which represents
fluvio-lacustrine deposition (González de Juana et al.,
1980). Subsequently, thermal subsidence of the passive
margin of South America extending into the Early Cretaceous led to the deposition of the Río Negro Formation
(coarse-grained, arkosic, and fine-grained sandstones), the
Cogollo Group (limestones and sandstones), the Capacho
Formation (black shales and limestones), and the
Aguardiente unit (shales and sandstones). Subsequently,
the La Luna Formation (organic matter-rich limestones,
shales, and cherty rocks) was deposited during a series of
four marine transgressions of Late Cretaceous age
(Villamil, 1999). These events were followed by the beginning of a regressive succession with the shallow marine deposition of the Campanian–Maastrichtian Colón
Formation (gray shales), which was caused by an oblique
collision between the westward-migrating Caribbean island arc and the passive margin of South America (Lugo
and Mann, 1995). In addition, the Maastrichtian Mito Juan
Formation (sandstones, siltstones, and shales) began to
be deposited in a deltaic environment (Sutton, 1946).
During the Tertiary, paralic to fluvio-estuarine sediment of the Orocué Group (sandstones and siltstones) was
deposited in the Paleocene–Eocene, and the Los Cuervos
Formation (sandstones, siltstones, and shales) was laid
down in a deltaic depositional environment. Overlying
the latter, the Eocene–Early Oligocene Mirador Formation, which consists of sandstones, shales, and siltstones,
was then deposited under fluvio-estuarine conditions.
Finally, Late Oligocene and younger sediments formed
the El Fausto Group (sandstones) and the León unit (shales
and siltstones), as well as the sandy rocks of the Guayabo
Group (González de Juana et al., 1980).
Located in the southwestern Lake Maracaibo Basin,
the Campanian to Early Maastrichtian Colón Formation
(400 to 900 m thick) displays a more sandy lithology toward the base and also toward the top, where this unit
changes concordantly and transitionally to the Mito Juan
Formation with the appearance of interbedded sandstones
and limestones (Sutton, 1946). The Late Maastrichtian
Mito Juan Formation (100 to 300 m thick) is characterized by some gray shales that are lithologically indistinguishable from the clays of the Colón Formation. Thus,
several researchers (e.g., Sievers, 1988) have noted the
difficulty of cartographically separating the Mito Juan
Formation from the Colón unit. With regard to La Luna
Formation horizons, the Coniacian–Santonian Táchira
Ftanita Member (80–100 m thick) consists of regularly
stratified cherts with minor intercalations of siliceous
shale and limestone (Garbán, 2010). Lastly, concordantly
underlying the Colón Formation, the Campanian Tres
Esquinas Member consists of glauconitic limestone that
is rich in silica and phosphates. Despite its small thickness (3–5 m), the member is an important marker bed
throughout the Lake Maracaibo Basin (Stainforth, 1962).
METHODOLOGY
Sampling
One hundred and eighty-three rock samples, taken at
stratigraphic intervals of approximately 2.6 m, were collected from the Colón-Mito Juan sequence near the city
of San Juan de Colón (8°2′ N, 72°16′ W). In addition,
eight rock samples, taken at intervals of about 2.3 m, were
collected from the Táchira Ftanita and Tres Esquinas
members of the La Luna Formation along an outcropping
cut on the road to Ureña (7°57′ N, 72°21′ W), approximately 10 km southwest of the San Pedro de Río village.
The locations of the two sampling sites are shown in Fig.
1.
Analytical procedures
An aliquot (about 100 g) of each sample was crushed
and pulverized using a Shatterbox 5540 with a tungsten
carbide grinding container. Geochemical analyses of six
major/minor elements, expressed as % w/w oxides (TiO2,
Al2O 3, MgO, CaO, K2O, and P2O5), and eight trace elements (Be, Cu, Mo, Ni, Sr, V, Y, and Zn), expressed as
mg/kg, were determined by inductively coupled plasmaatomic emission spectroscopy (ICP-AES) using a PerkinElmer Optima 3000 spectrometer. In addition, two other
major elements, expressed as % w/w oxides (Fe2O 3 and
Na2O), and a further eighteen trace elements (Rb, Cs, Ba,
Th, U, Hf, Cr, Co, Sc, La, Ce, Nd, Sm, Eu, Yb, Lu, As,
and Sb), expressed as mg/kg, were determined by instrumental neutron activation analysis (INAA). Total carbon
(C) and inorganic carbon were measured in a coulometric
carbon analyzer. Total organic carbon (TOC), as weight
percent, was calculated as the difference between C and
inorganic carbon. Sulfur contents were also determined
using a LECO SC-432 apparatus. Two certified reference
materials, Post-Archean Australian Shale (PAAS) and
North American Shale Composite (NASC), were used for
analytical control and data comparison.
Statistical treatment
We performed an exploratory data analysis of our
geochemical dataset prior to statistical treatment. First,
these data are highly multivariate, 36 elements with a
sample size of N = 191. Second, the most widely used
methods of multivariate analysis are all based on the assumption that the variables show a normal or lognormal
distribution (Reiman and Filmoser, 1999). In our case,
descriptive statistics indicate a natural lognormal distribution for element compositions (Dixon and Kronmal,
1965). Data outliers as well as values below the determination limits (VBDLs) were replaced by the corresponding statistical medians and one-half of the determination
limits, respectively. All of the variables showed low numbers of VBDLs (<10%) and outliers (<25%), thus allowing the use of the 36 elemental concentrations for further
statistical treatment. The log-transformed data matrix was
then standardized prior to multivariate statistical analysis through a reported procedure (Reategui et al., 2005)
in order to remove artifacts derived from scale attributes
and to equalize the influence of variables with distinctive variations.
Cluster analysis was applied using the matrix formed
by the log-transformed and standardized data in order to
group the variables. Dissimilarity values were obtained
after calculating squared Euclidean distance measures
using Ward’s minimum variance method (Templ et al.,
2008). A cut-off squared distance of 320 was also selected.
Finally, constrained cluster analysis was carried out to
determine geochemically meaningful zones, or
Geochemistry and chemostratigraphy of the Colón-Mito Juan 539
chemofacies. In the respective dendrograms (see Section
“Results and Discussion”), the samples are arranged in
accordance with their stratigraphic height. The number
of chemical facies depends on the selected cut-off value
(Gill et al., 1993). Multivariate analysis between variables was performed by multi-dimensional scaling (MDS).
Data were processed using the NCSS 2000TM statistical
software package.
RESULTS AND DISCUSSION
Data for the samples from the Colón-Mito Juan se-
Fig. 2. a) Crossplots of several study elements against Al2O3;
b) Berner plot for all the samples from the upper La Luna Fm
and the Colón-Mito Juan sequence.
quence and the Táchira Ftanita and Tres Esquinas Members are listed in Supplementary Tables S1 and S2. The
Colón-Mito Juan sequence is distinguished by three intervals: a lower zone of 222 m (between 18.5 and 240.5
m in the stratigraphic log) dominated by black shales, a
second overlying zone comprised of a 78-m thick interval of gray shales and thin fine-grained sandstones, and,
finally, an uppermost third zone beginning at 318.5 m in
the log that ends at the top of the column and consists of
interbedded gray shales, sandstones, and limestones.
Sedimentary geochemistry
Trends in the geochemical dataset can be partially
observed through crossplots of element pairs, in which
one of the elements is Al (scattergrams of Fralick and
Kronberg, 1997). These diagrams permit evaluation of
the mobility or immobility of each chemical element,
which allows the determination of source area composition, thus reflecting the distinct hydraulic behavior in each
lithology and quantifying sorting in the system (Reategui
et al., 2005). TiO2 and K2O, and to a lesser extent Fe2O3,
are strongly immobile major constituents. Among the trace
elements, Sc, La, Ce, Be, V, Th, Rb, Ni, Na, Cs, Eu, and
Sm, and to a lesser extent Cu, Cr, Mg, Zn, Lu, U, Yb, Sr,
Y, Ba, As, Sb, and Nd, are immobile and similarly affected by sorting. In contrast, P, Ca, Co, Mo, Hf, and S
appear to be highly mobile. These latter elements are either chemically mobilized or added by diagenetic processes (e.g., authigenic mineral formation, organic matter
decomposition). Figure 2a shows the correlation of some
Fig. 3. Several trace element concentrations, normalized to average upper continental crust values, in the samples from the
Colón-Mito Juan sequence and upper La Luna Fm.
540 L. A. Montilla et al.
representative elements (CaO, Cu, Cr, Rb, and Ce) with
Al 2O3. In addition, marine deposition cannot be corroborated by a significant positive correlation between TOC
and S (Fig. 2b; Berner, 1983), possibly because of sulfur
mobility (occurrence of sulfates).
Each lithology was treated as a separate dataset, and
geochemical concentrations of various elements, normalized to average upper continental crustal (UCC) values
(after Wedepohl, 1995), were compared and plotted on a
logarithmic scale, shown in Fig. 3.
As expected, shales and siltstones had higher Al2O3
contents (nearly 16 and 12%, respectively) than
sandstones (5.8%) and cherts (2.4%), reflecting preferential incorporation of clay minerals into the shales and
siltstones. The enrichments in Rb, Cs, V, Cr, Ni, Sc, and
Th in shales could be due to the association with clays
(Bauluz et al., 1994). The trace element Co was significantly more concentrated in the chert samples. The mobility of this element may be governed by redox conditions and by the processes controlling element
remobilization during chert formation. In this regard, Ni/
Co values of nearly 5 have been shown to indicate oxicdisoxic depositional conditions (Ross and Bustin, 2009).
Previously correlated with organic matter preservation
(Zelt, 1985), U had its highest values in samples TLVU
025, TLVU 030, TLVU 035, and TLVU 040 (see Table
S2), which correspond to the Tres Esquinas Member. It
should also be noted that our sandstone showed a higher
Fig. 4. Crossplots of rare earth elements vs. Al2O3.
content of Y and Hf. This observation, together with the
positive correlation between Hf and Ti and the inverse
one between Ti and Y in these rocks, may indicate the
presence of these elements in heavy minerals such as zircon and rutile (Bea, 1996).
Finally, shales were observed to be enriched in light
rare earth elements (LREE), such as La, Ce, and Nd; in
contrast, sandstone showed an enrichment in medium (Sm
and Eu) and heavy rare earth elements (HREE), such as
Tb, Yb, and Lu. This difference may result from the
fractionation of rare earth elements (REE), a process that
usually involves the accumulation of lighter REE in clays,
while heavier ones are concentrated in minerals such as
zircon (Nyakairu and Koeberl, 2001). Our observation is
supported by correlations between ∑REE, LREE, and
HREE with Al 2O 3 (see Fig. 4). Table 1 shows the values
of the ∑REE/Al2O3 ratio. The highest value of this ratio
was recorded in the sandstones, suggesting that a nonclay phase contributes to the content of REE in both
sandstones and limestones. This finding could be attributable to the presence of oxyhydroxides or other heavy
minerals. Chert samples showed the lowest concentrations
of REE because these elements are “diluted” in SiO2
(Garbán, 2010).
Elemental relationships
A Q-mode cluster analysis was performed to establish relationships between elements in the data matrix.
Figure 5 shows the results of the hierarchical clustering
using the dataset from the 191 rock samples and 36 variables. A first group of differentiated elements (Eu, Fe,
Sm, Nd, Ce, La, Th, Sc, Lu, Yb, Na, and Hf) is mostly
rare earths, these being associated with oxyhydroxydes
such as goethite or other oxides. A second association is
comprised of Zn, Ni, V, Cr, Mg, Ti, Rb, Cs, K, Al, and
Be, which are governed by clay minerals (illite, kaolinite,
and others) and trace elements adsorbed onto clays. A third
group (C, P, Mo, U, Cu, Ba, and Sb) displays a remarkable relationship with redox conditions, being associated
Table 1. Main REE values, paleoweathering indices, and average elemental ratios for each lithology
and the three references (PAAS, NASC, and UCC). Standard deviations are shown in parentheses.
Shales
Sandstones
Siltstones
Cherts
Limestones
PAAS
NASC
UCC
∑REE (mg/kg)
LREE (mg/kg)
231.36 (21.58)
169.08 (54.70)
214.84 (30.88)
43.23 (29.49)
131.35 (28.81)
160.70
136.34
128.56
224.48 (21.49)
160.16 (49.57)
207.13 (30.96)
41.45 (28.46)
125.18 (27.97)
155.60
130.79
124.50
HREE (mg/kg)
4.72 (0.76)
5.64 (1.79)
5.86 (0.95)
1.15 (0.78)
4.21 (1.40)
3.23
3.52
2.54
∑REE/Al2 O3
CIA
CIW
Th/Sc
14.47
29.20
18.06
18.00
24.48
8.50
8.07
8.50
78.67
42.45
77.73
—
—
75.30
65.91
56.93
90.07
46.51
87.48
—
—
88.32
77.99
65.23
1.26 (0.12)
1.43 (0.28)
1.46 (0.18)
1.16 (0.66)
1.46 (0.25)
0.91
0.83
0.97
PAAS, Post-Archean Australian Shale; NASC, North American Shale Composite; UCC, Upper Continental Crust.
Geochemistry and chemostratigraphy of the Colón-Mito Juan 541
Fig. 6. Al2O3–K2O–CaO+Na2O plot of sandstones and shales
of the Colón-Mito Juan sequence and upper La Luna Fm.
Fig. 5. Groups of variables provided by Q-mode hierarchical
cluster analysis of the data matrix from the Colón-Mito Juan
sequence and upper La Luna Fm.
with organic matter (Mo), primary productivity (P and
Ba), or fixed as a result of highly reducing conditions.
Lastly, Ca, Y, Sr, Co, S, and As were observed to be associated with carbonates and sulfates, and these elements
appear to have been mobilized during diagenetic or
postgenetic processes.
Paleo-weathering conditions and provenance
Two dimensionless weathering indexes, the chemical
index of alteration (CIA) and the chemical index of weathering (CIW) (Nesbitt and Young, 1982; Harnois, 1988),
have been widely used to quantify relative weathering in
source regions of sediments. High CIA and CIW values
for the shales and siltstones (see Table 1) of the ColónMito Juan sequence may suggest moderate to intense
weathering as part of the first cycle of sedimentation in
the source area of the precursor materials for the sedimentary rocks under study (Young and Nesbitt, 1999).
Therefore, a humid and warm paleoclimatic environment,
without discarding small local variations, can be inferred
(Erlich et al., 2000).
The Al2O 3–K 2O–CaO+Na2O plot (Fig. 6) shows that
the analyzed shales and siltstones are formed mostly by
illite, suggesting moderate chemical weathering of the
source area of the sediment (average CIA of 79%). The
presence of this mineral in the shales is supported by the
positive correlation between K and Rb because these cations bind to clays such as illite (Young and Nesbitt, 1999).
Considering that the average Th/Sc values exceed 1
and that the Th/Sc standard deviations are low for all
lithologies (see Table 1), the samples generally cluster
along a relatively straight trend located in the continental
542 L. A. Montilla et al.
Fig. 7. a) and b) Th/Co vs. La/Sc plot and Hiscott diagram
(Cr/V vs. Y/Ni), respectively, applied to the Colón-Mito Juan
sequence and the upper La Luna Fm.
Fig. 8. Stratigraphic subdivisions through constrained clustering based on a) redox processes controlling the concentrations of Eu, Fe, Sm, Nd, Ce, La, Th, Sc, Lu, Yb, Na, and Hf; b)
reactions that control clay-associated elements such as Zn, Ni,
V, Cr, Mg, Ti, Rb, Cs, K, Al, and Be; and c) processes related to
C, P, Mo, U, Cu, Ba, and Sb. Lithologies: L, shale; S, sandstone; Lm, limestone; F, chert.
domain. Th/Sc values higher than those of the PAAS,
NASC, and UCC references (see Table 1) indicate a source
of felsic composition (Young and Nesbitt, 1999).
Figure 7a shows a Th/Co vs. La/Sc diagrammatic rep-
Fig. 9. a), b), and c) Chemostratigraphic profiles for the three first groups of elements, respectively, obtained from Q-mode
hierarchical cluster analysis for the generalized stratigraphic column.
resentation (López et al., 2005) for the Colón-Mito Juan
sequence. This plot allows discrimination of source rocks
based on a felsic (rich in Th and La, depleted in Sc and
Co) or basic affinity (low Th/Co and La/Sc ratios). Moreover, most samples in the Hiscott diagram (Y/Ni vs. Cr/
V; Fig. 7b) plot around the felsic field and show similar
low Cr/V ratios; however, the Y/Ni ratios vary widely.
This observation could be explained by an additional sediment source, namely recycled sedimentary rocks. As a
result of recycling processes, Ni may be preferentially
depleted from sediments, thus increasing the Y/Ni ratio
and promoting scattering in the values; Y, Cr, and V are
immobile and affected similarly by sorting (Dinelli et al.,
1999). In our case, felsic metamorphic sources yielded
sediments to the Colón-Mito Juan sequence and the
Táchira Ftanita and Tres Esquinas Members. However, it
has been demonstrated that numerous metal ratios show
significant differences in metamorphic and granitic rocks
(Piovano et al., 1999). Despite this drawback, we used
these diagrams as indicators of provenance. However, they
must be interpreted with caution.
Furthermore, several authors (Amstrong-Altrin et al.,
2004; among others) have reported that mafic and felsic
rocks have low and high values, respectively, in the ratio
of LREE/HREE. In our case, for all samples and litholo-
gies, LREE values were clearly higher than those of HREE
(see Table 1), thus corroborating the felsic origin.
Chemostratigraphy
Figure 8 shows the division of the generalized
stratigraphic column through constrained clustering based
on a) redox reactions controlling the concentration of elements (Eu, Fe, Sm, Nd, Ce, La, Th, Sc, Lu, Yb, Na, and
Hf) adsorbed or predominantly associated with
oxyhydroxides or other oxides; b) processes related to
those elements (Zn, Ni, V, Cr, Mg, Ti, Rb, Cs, K, Al, and
Be) mostly bound to the clay minerals; and c) redox reactions that control elements (C, P, Mo, U, Cu, Ba, and
Sb) associated with organic matter or high-potential reduction processes. The selected cut-off values were 200,
100, and 50, respectively.
First, two chemofacies were determined, and these are
identified as O-I (TLVU 005 to TLVU 020) and O-II
(TLVU 025 to TCMJ 002). The geochemical profiles for
REE and Fe (Fig. 9a) and also Na values (see Table S1)
indicate that these elements showed a tendency to increase
in the interval between 2 m from the bottom (TLVU 025)
and the top of the stratigraphic log (499.5 m). Therefore,
the O-I/O-II boundary is coincident with the contact between the Tres Esquinas and Táchira Ftanita members (see
Geochemistry and chemostratigraphy of the Colón-Mito Juan 543
Fig. 8).
Other changes in the trends of several geochemical
profiles (Fe, La, Ce, Nd, Sm, Th, Sc, Eu, Na, Hf, Yb, and
Lu) were detected within the O-II chemofacies at the same
stratigraphic level, a level coinciding with one of the previously defined lithological boundaries (approximately
240.5 m in the log); this can be interpreted as a change in
the sedimentation pattern. Furthermore, Fe, La, Ce, Nd,
Sm, Th, Sc, and Eu (elements associated with oxides and
oxyhydroxides) were enriched in the clay fractions, in
contrast to Na, Hf, Yb, and Lu (elements bound to heavy
minerals), which had high values in the sandstone horizons, as was the case of the highest concentrations of Yb
and Lu observed in the Tres Esquinas Member resulting
from hydraulic conditions.
Multivariate analysis enabled the division of the sequence into three chemofacies, identified as A-I (TLVU
005 to TLVU 040), A-II (TCMJ 900 to TCMJ 400), and
A-III (TCMJ 395 to TCMJ 005) from the bottom to the
top of the log (Fig. 8). These divisions are associated with
lithological variations: A-I corresponds to the La Luna
Fm., A-II is characterized by lithologic homogeneity
(black shales of the Colón unit s. str.), and A-III corresponds to a progradational sequence characteristic of the
Mito Juan Fm. (alternation of sandstones, siltstones, and
shales), permitting chemical differentiation of the Colón
and Mito Juan units. The geochemical profiles of Zn, Ni,
V, Cr, Mg, Ti, Rb, Cs, K, Al, and Be show an enrichment
at the La Luna-Colón contact at a stratigraphic height of
18.5 m (Fig. 9b). However, higher concentrations of these
elements were also detected in the shaly interval located
between the cherty and sandstone levels in the boundary
between the Táchira Ftanita and Tres Esquinas members
(2 m in stratigraphic height), thus confirming an association with clays. In coherence with differences in the sedimentation pattern in the Colón-Mito Juan boundary (variation in energy conditions) related to a relative reduction
in clay content, another change in trace element composition can be observed at 240.5 m in the log.
Additionally, constrained cluster analysis led to the
identification of the last three chemofacies (see Fig. 8):
R-I (TLVU 005 to TCMJ 635), R-II (TCMJ 630 to TCMJ
540), and R-III (TCMJ 535 to TCMJ 002), from bottom
to top, based on the geochemical profiles of the elements
C, P, Mo, U, Cu, Sb, and Ba (Fig. 9c). Generally, these
profiles suggest less reducing conditions for the ColónMito Juan sequence compared to the upper La Luna Formation. This notion is also supported by a slight decrease
in TOC in the log (see Table S1). The Tres Esquinas Member showed the maximum enrichment in U, Cu, Sb, Ba,
and P, indicating that deposition occurred during a period of maximum transgression and high primary productivity. This was accompanied by an abrupt decrease in
the anoxicity of the water as a result of at-depth (succes-
544 L. A. Montilla et al.
sive upwelling events) and near-surface mixing processes
caused by Late Companion–Early Maastrichtian tectonic
episodes that occurred on the northern edge of the Lake
Maracaibo Basin and impeded water circulation (Lugo
and Mann, 1995). The interval defined as R-II (from approximately 110 to 158 m in stratigraphic height) is characterized by a decrease in the concentrations of Cu, Ba,
and Sb and an increase in the Mo and TOC values. This
interval may indicate a period of rapid redox changes related to variations in water oxygen content. The R-III interval begins at 158 m in the log and ends at the top of
the section, indicating a zonal redox change defined by
elements such as Cr, Ni, Zn, and V.
On the whole, our approach allowed us to differentiate two zones characterized by the input of either
siliciclastic materials or biogenic siliceous sediments, the
latter being identified as the chert-rich Táchira Ftanita
Member (Garbán, 2010). In addition, a glauconite-rich
phosphorite unit was identified as the Tres Esquinas Member (Parra et al., 2003). Furthermore, the
chemostratigraphic profiles of the clay-associated elements indicated the contact between the Colón and Mito
Juan formations, the latter formation being a deltaic
sedimentation unit of interbedded gray shales, finegrained sandstones, and, occasionally, carbonates. Finally,
a series of redox changes were detected within the
monolithological black shaly interval; these changes can
be explained by either variations in oxygen levels in the
water or by subsidence, which would have caused a lower
water level. It is also interesting to note that the redox
changes did not affect redox-sensitive elements equally.
CONCLUSIONS
The outcroppings studied here were found to be representative of the Colón and Mito Juan units on the basis
of the lithologies identified and their correlation with information in the literature. The contact between the lower
and middle lithologic intervals within the Colón-Mito
Juan sequence is proposed to mark the Colón-Mito Juan
boundary.
The geochemical profiles reflected the lithological
compositions (sandstones, siltstones, shales, limestones,
and cherts) found in the study stratigraphic horizons. We
propose that these profiles indicate intense weathering
processes for the source area of the felsic-origin sediments
and successive changes in redox conditions of the
depositional environment, with increasingly oxidizing
conditions towards the top.
Chemostratigraphically, both the paleoweathering conditions and the source area of the sediments remained
uniform along the formations studied. We identified eight
chemical facies: two (O-I and O-II) indicate sections controlled by biogenic and clastic deposition, respectively;
three (A-I, A-II, and A-III) show coherence with changes
in lithologic facies and coincide with the La Luna, Colón,
and Mito Juan formations, respectively; and three (R-I,
R-II, and R-III) represent changes in the redox conditions
throughout the column.
Acknowledgments—This research was funded by the Consejo
de Desarrollo Científico y Humanístico (Universidad Central
de Venezuela) through the projects CDCH-03.32.4412/99,
CDCH-03.32.4412/00, and CDCH-03.30.4702/99.
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S UPPLEMENTARY MATERIALS
URL (http://www.terrapub.co.jp/journals/GJ/archives/
data/47/MS275.pdf)
Tables S1 and S2