Geochemical Journal, Vol. 47, pp. 537 to 546, 2013 doi:10.2343/geochemj.2.0275
identify source rocks and weathering processes (Cullers,2000). A chemostratigraphic study, which involves thecharacterization of the sedimentary sequence into differ-ent units on the basis of major and trace element chemis-try (e.g., Pearce et al., 1999) is done when geochemicaldata 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 severalchemical indices (Reyment and Hirano, 1999; Reinhardtand Ricken, 2000). In addition, other features revealedby chemostratigraphic studies include climatic changes,paleoredox conditions, stratigraphic correlations,paleoproductivity, and chemical cyclicity in processesinvolving basin sedimentation (Yarincik and Murray,
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. MÁRQUEZ,3* M. ESCOBAR,4,5 C. SIERRA,6 J. R. GALLEGO,6
I. ESTEVES7 and J. V. GUTIÉRREZ2
1PDVSA, División Oriente, Gerencia de Exploración, Puerto La Cruz, Venezuela2Instituto de Ciencias de la Tierra, Universidad Central de Venezuela, Caracas, 3895, 1010-A, Venezuela
3Departamento de Ingeniería Minera, Mecánica y Energética, Universidad de Huelva, Huelva, 21819 Huelva, Spain4CARBOZULIA, Av. 2 No. 55-185, Casa Mene Grande, Maracaibo 4002 A, Venezuela
5Postgrado de Geología Petrolera, Facultad de Ingeniería, Universidad del Zulia, Maracaibo 4002, Venezuela6Departamento de Exploración y Prospección de Minas, Universidad de Oviedo, Mieres, 33600 Asturias, Spain
7Fundación Instituto Zuliano de Investigaciones Tecnológicas (INZIT), Maracaibo 4001, Venezuela
(Received May 4, 2013; Accepted July 25, 2013)
A geochemical and chemostratigraphical study was undertaken on Campanian–Maastrichtian sedimentary rocks (theColón-Mito Juan sequence and the upper La Luna Formation) in the southwestern Maracaibo Basin, Venezuela. Theobjectives of this work were to determine the paleoenvironmental and physico-chemical characteristics of the Colón-MitoJuan sequence and its possible subdivision into chemofacies and to study the main chemical differences between theColón, Mito Juan, and La Luna Formations within the study region. One hundred and ninety-one rock samples werecollected, and bulk inorganic geochemistry (TiO2, Al2O3, Fe2O3, MgO, CaO, Na2O, K2O, P2O5, 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 neutronactivation analysis or inductively coupled plasma-atomic emission spectroscopy; total sulfur and carbon analyses wereperformed by a LECO SC-432 apparatus and coulometry, respectively. Multivariate statistical techniques were applied toevaluate correlations within this group of variables. Using cluster-constrained analysis, eight subdivisions, or chemicalfacies, were defined: two chemofacies differentiating the intervals controlled by biogenic deposition and by the predomi-nant clastic contribution; three chemofacies correlating with the lithologic units (La Luna, Colón, and Mito Juan); andanother three chemofacies related to changes in the paleoredox conditions along the stratigraphic column. All of the unitsstudied were deposited under a relatively constant climate regime, and the composition of the sediment source showed nosignificant changes. The prevailing physico-chemical regime was disoxic-oxic, with a trend of increasing oxygen concen-trations towards the top of the column.
Keywords: geochemistry, chemostratigraphy, Colón-Mito Juan sequence, stratigraphic subdivision, Lake Maracaibo
INTRODUCTION
Integrated geochemical and chemostratigraphical stud-ies of sedimentary rocks allow the determination ofpaleoenvironmental conditions and provenance ofsediments (e.g., Armstrong-Altrin et al., 2004). Thegeochemistry of clastic sediments is controlled by thecomposition of the source rocks, weathering, deposition,and diagenetic processes (Asiedu et al., 2000; Yan et al.,2006). Consequently, geochemical tracers can be used to
538 L. A. Montilla et al.
2000; Hetzel et al., 2009; among others).The present study focused on the geochemistry and
chemostratigraphy of Late Campanian to LateMaastrichtian (76–65 Ma) sedimentary rocks in the west-ern region of the state of Táchira, Venezuela. First, westudied a sequence consisting of the uppermost part ofthe La Luna Formation (Tres Esquinas Member) and theColón and Mito Juan units outcropping close to theLobaterita River near the locality of San Juan de Colón(Fig. 1). We then examined a second stratigraphic sec-tion of rocks comprising the Táchira Ftanita and TresEsquinas members (La Luna Formation) up to the lowestpart of the Colón Formation outcropping in a cut alongthe San Pedro de Río-Ureña road (Fig. 1).
The particular case of the Colón Formation is veryinteresting because when studying a stratigraphic se-quence characterized by a monolithological composition,according to González de Juana and colleagues (1980),the variations in chemical profiles are not strongly influ-enced by lithological changes. Moreover, interest in per-forming this study in the Colón Formation comes fromthe following observations: (1) the formation’s total or-ganic carbon (TOC) values are higher than 1% in someareas (Malavé, 1994); (2) the formation acts as a caprockin the petroleum system of the Maracaibo Lake Basin(Parnaud et al., 1995); and (3) it represents most of the
Maastrichtian in western Venezuela, a time during whichmajor changes and climatic variations in sedimentationpatterns occurred (Erlich et al., 2000).
The main goals of this study were (1) to establish theenvironmental and physico-chemical characteristics of theColón-Mito Juan sequence; (2) to chemically differenti-ate it from the La Luna Formation; (3) to subdivide itinto chemical facies associated with changes in the con-centrations of different elements; and (4) to establish thesedimentary processes that originated these chemofacies.
The literature refers to the Colón and Mito Juan unitsas the Colón-Mito Juan sequence, as it is very difficult toaccurately recognize the transitional contact between thetwo formations (Savian, 1993). Therefore, it is of inter-est to establish the stratigraphic level that records thechemical changes, if present, that help distinguish the twoaforementioned formations.
GEOLOGICAL BACKGROUND
The Lake Maracaibo Basin is located at the southwest-ern end of the Caribbean Sea in Venezuela, near its bor-der with Colombia. This basin consists of a thick sedi-mentary cover divided into various sequences conditionedby tectonic events: a Jurassic rift succession; an Early–Late Cretaceous passive margin sequence; LateCretaceous–Early Paleocene deposits representing a tran-sition to a compressive regime that occurred when colli-sion of the Pacific volcanic arc emplaced the “LaraNappes” to the northern edge of the aforementioned ba-sin; Late Paleocene–Middle Eocene foreland basin de-posits that formed in front of the volcanic arc; and a LateEocene–Pleistocene sequence related to the collision ofthe Panama arc with the South American plate (Mann etal., 2006; Escalona and Mann, 2011).
The sedimentary succession of the southwestern sec-tor within the Lake Maracaibo Basin overlies theigneous-metamorphic basement and begins with red bedsof the Jurassic La Quinta Formation, which representsfluvio-lacustrine deposition (González de Juana et al.,1980). Subsequently, thermal subsidence of the passivemargin of South America extending into the Early Creta-ceous led to the deposition of the Río Negro Formation(coarse-grained, arkosic, and fine-grained sandstones), theCogollo Group (limestones and sandstones), the CapachoFormation (black shales and limestones), and theAguardiente unit (shales and sandstones). Subsequently,the La Luna Formation (organic matter-rich limestones,shales, and cherty rocks) was deposited during a series offour marine transgressions of Late Cretaceous age(Villamil, 1999). These events were followed by the be-ginning of a regressive succession with the shallow ma-rine deposition of the Campanian–Maastrichtian ColónFormation (gray shales), which was caused by an oblique
Fig. 1. Sketch map showing the two sampling sites and themain localities in the study region in the state of Táchira (Ven-ezuela).
Geochemistry and chemostratigraphy of the Colón-Mito Juan 539
collision between the westward-migrating Caribbean is-land arc and the passive margin of South America (Lugoand Mann, 1995). In addition, the Maastrichtian Mito JuanFormation (sandstones, siltstones, and shales) began tobe deposited in a deltaic environment (Sutton, 1946).
During the Tertiary, paralic to fluvio-estuarine sedi-ment of the Orocué Group (sandstones and siltstones) wasdeposited in the Paleocene–Eocene, and the Los CuervosFormation (sandstones, siltstones, and shales) was laiddown in a deltaic depositional environment. Overlyingthe latter, the Eocene–Early Oligocene Mirador Forma-tion, which consists of sandstones, shales, and siltstones,was then deposited under fluvio-estuarine conditions.Finally, Late Oligocene and younger sediments formedthe El Fausto Group (sandstones) and the León unit (shalesand siltstones), as well as the sandy rocks of the GuayaboGroup (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 to-ward the base and also toward the top, where this unitchanges concordantly and transitionally to the Mito JuanFormation with the appearance of interbedded sandstonesand limestones (Sutton, 1946). The Late MaastrichtianMito Juan Formation (100 to 300 m thick) is character-ized by some gray shales that are lithologically indistin-guishable from the clays of the Colón Formation. Thus,several researchers (e.g., Sievers, 1988) have noted thedifficulty of cartographically separating the Mito JuanFormation from the Colón unit. With regard to La LunaFormation horizons, the Coniacian–Santonian TáchiraFtanita Member (80–100 m thick) consists of regularlystratified cherts with minor intercalations of siliceousshale and limestone (Garbán, 2010). Lastly, concordantlyunderlying the Colón Formation, the Campanian TresEsquinas Member consists of glauconitic limestone thatis rich in silica and phosphates. Despite its small thick-ness (3–5 m), the member is an important marker bedthroughout the Lake Maracaibo Basin (Stainforth, 1962).
METHODOLOGY
SamplingOne hundred and eighty-three rock samples, taken at
stratigraphic intervals of approximately 2.6 m, were col-lected from the Colón-Mito Juan sequence near the cityof San Juan de Colón (8°2′ N, 72°16′ W). In addition,eight rock samples, taken at intervals of about 2.3 m, werecollected from the Táchira Ftanita and Tres Esquinasmembers of the La Luna Formation along an outcroppingcut on the road to Ureña (7°57′ N, 72°21′ W), approxi-mately 10 km southwest of the San Pedro de Río village.The locations of the two sampling sites are shown in Fig.1.
Analytical proceduresAn aliquot (about 100 g) of each sample was crushed
and pulverized using a Shatterbox 5540 with a tungstencarbide grinding container. Geochemical analyses of sixmajor/minor elements, expressed as % w/w oxides (TiO2,Al2O3, MgO, CaO, K2O, and P2O5), and eight trace ele-ments (Be, Cu, Mo, Ni, Sr, V, Y, and Zn), expressed asmg/kg, were determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) using a Perkin-Elmer Optima 3000 spectrometer. In addition, two othermajor elements, expressed as % w/w oxides (Fe2O3 andNa2O), 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 instru-mental neutron activation analysis (INAA). Total carbon(C) and inorganic carbon were measured in a coulometriccarbon analyzer. Total organic carbon (TOC), as weightpercent, was calculated as the difference between C andinorganic carbon. Sulfur contents were also determinedusing a LECO SC-432 apparatus. Two certified referencematerials, Post-Archean Australian Shale (PAAS) andNorth American Shale Composite (NASC), were used foranalytical control and data comparison.
Statistical treatmentWe performed an exploratory data analysis of our
geochemical dataset prior to statistical treatment. First,these data are highly multivariate, 36 elements with asample size of N = 191. Second, the most widely usedmethods of multivariate analysis are all based on the as-sumption that the variables show a normal or lognormaldistribution (Reiman and Filmoser, 1999). In our case,descriptive statistics indicate a natural lognormal distri-bution for element compositions (Dixon and Kronmal,1965). Data outliers as well as values below the determi-nation limits (VBDLs) were replaced by the correspond-ing statistical medians and one-half of the determinationlimits, respectively. All of the variables showed low num-bers of VBDLs (<10%) and outliers (<25%), thus allow-ing the use of the 36 elemental concentrations for furtherstatistical treatment. The log-transformed data matrix wasthen standardized prior to multivariate statistical analy-sis through a reported procedure (Reategui et al., 2005)in order to remove artifacts derived from scale attributesand to equalize the influence of variables with distinc-tive variations.
Cluster analysis was applied using the matrix formedby the log-transformed and standardized data in order togroup the variables. Dissimilarity values were obtainedafter calculating squared Euclidean distance measuresusing 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 todetermine geochemically meaningful zones, or
540 L. A. Montilla et al.
Fig. 3. Several trace element concentrations, normalized to average upper continental crust values, in the samples from theColón-Mito Juan sequence and upper La Luna Fm.
Fig. 2. a) Crossplots of several study elements against Al2O3;b) Berner plot for all the samples from the upper La Luna Fmand the Colón-Mito Juan sequence.
chemofacies. In the respective dendrograms (see Section“Results and Discussion”), the samples are arranged inaccordance with their stratigraphic height. The numberof chemical facies depends on the selected cut-off value(Gill et al., 1993). Multivariate analysis between vari-ables was performed by multi-dimensional scaling (MDS).Data were processed using the NCSS 2000TM statisticalsoftware package.
RESULTS AND DISCUSSION
Data for the samples from the Colón-Mito Juan se-
quence and the Táchira Ftanita and Tres Esquinas Mem-bers are listed in Supplementary Tables S1 and S2. TheColón-Mito Juan sequence is distinguished by three in-tervals: a lower zone of 222 m (between 18.5 and 240.5m in the stratigraphic log) dominated by black shales, asecond overlying zone comprised of a 78-m thick inter-val of gray shales and thin fine-grained sandstones, and,finally, an uppermost third zone beginning at 318.5 m inthe log that ends at the top of the column and consists ofinterbedded gray shales, sandstones, and limestones.
Sedimentary geochemistryTrends in the geochemical dataset can be partially
observed through crossplots of element pairs, in whichone of the elements is Al (scattergrams of Fralick andKronberg, 1997). These diagrams permit evaluation ofthe mobility or immobility of each chemical element,which allows the determination of source area composi-tion, thus reflecting the distinct hydraulic behavior in eachlithology and quantifying sorting in the system (Reateguiet al., 2005). TiO2 and K2O, and to a lesser extent Fe2O3,are strongly immobile major constituents. Among the traceelements, Sc, La, Ce, Be, V, Th, Rb, Ni, Na, Cs, Eu, andSm, and to a lesser extent Cu, Cr, Mg, Zn, Lu, U, Yb, Sr,Y, Ba, As, Sb, and Nd, are immobile and similarly af-fected by sorting. In contrast, P, Ca, Co, Mo, Hf, and Sappear to be highly mobile. These latter elements are ei-ther chemically mobilized or added by diagenetic proc-esses (e.g., authigenic mineral formation, organic matterdecomposition). Figure 2a shows the correlation of some
Geochemistry and chemostratigraphy of the Colón-Mito Juan 541
representative elements (CaO, Cu, Cr, Rb, and Ce) withAl2O3. In addition, marine deposition cannot be corrobo-rated by a significant positive correlation between TOCand S (Fig. 2b; Berner, 1983), possibly because of sulfurmobility (occurrence of sulfates).
Each lithology was treated as a separate dataset, andgeochemical concentrations of various elements, normal-ized to average upper continental crustal (UCC) values(after Wedepohl, 1995), were compared and plotted on alogarithmic scale, shown in Fig. 3.
As expected, shales and siltstones had higher Al2O3contents (nearly 16 and 12%, respectively) thansandstones (5.8%) and cherts (2.4%), reflecting prefer-ential incorporation of clay minerals into the shales andsiltstones. The enrichments in Rb, Cs, V, Cr, Ni, Sc, andTh in shales could be due to the association with clays(Bauluz et al., 1994). The trace element Co was signifi-cantly more concentrated in the chert samples. The mo-bility of this element may be governed by redox condi-tions and by the processes controlling elementremobilization during chert formation. In this regard, Ni/Co values of nearly 5 have been shown to indicate oxic-disoxic depositional conditions (Ross and Bustin, 2009).Previously correlated with organic matter preservation(Zelt, 1985), U had its highest values in samples TLVU025, TLVU 030, TLVU 035, and TLVU 040 (see TableS2), which correspond to the Tres Esquinas Member. Itshould also be noted that our sandstone showed a higher
content of Y and Hf. This observation, together with thepositive correlation between Hf and Ti and the inverseone between Ti and Y in these rocks, may indicate thepresence of these elements in heavy minerals such as zir-con and rutile (Bea, 1996).
Finally, shales were observed to be enriched in lightrare earth elements (LREE), such as La, Ce, and Nd; incontrast, sandstone showed an enrichment in medium (Smand Eu) and heavy rare earth elements (HREE), such asTb, Yb, and Lu. This difference may result from thefractionation of rare earth elements (REE), a process thatusually involves the accumulation of lighter REE in clays,while heavier ones are concentrated in minerals such aszircon (Nyakairu and Koeberl, 2001). Our observation issupported by correlations between ∑REE, LREE, andHREE with Al2O3 (see Fig. 4). Table 1 shows the valuesof the ∑REE/Al2O3 ratio. The highest value of this ratiowas recorded in the sandstones, suggesting that a non-clay phase contributes to the content of REE in bothsandstones and limestones. This finding could be attrib-utable to the presence of oxyhydroxides or other heavyminerals. Chert samples showed the lowest concentrationsof REE because these elements are “diluted” in SiO2(Garbán, 2010).
Elemental relationshipsA Q-mode cluster analysis was performed to estab-
lish relationships between elements in the data matrix.Figure 5 shows the results of the hierarchical clusteringusing the dataset from the 191 rock samples and 36 vari-ables. A first group of differentiated elements (Eu, Fe,Sm, Nd, Ce, La, Th, Sc, Lu, Yb, Na, and Hf) is mostlyrare earths, these being associated with oxyhydroxydessuch as goethite or other oxides. A second association iscomprised of Zn, Ni, V, Cr, Mg, Ti, Rb, Cs, K, Al, andBe, which are governed by clay minerals (illite, kaolinite,and others) and trace elements adsorbed onto clays. A thirdgroup (C, P, Mo, U, Cu, Ba, and Sb) displays a remark-able relationship with redox conditions, being associatedFig. 4. Crossplots of rare earth elements vs. Al2O3.
∑REE (mg/kg) LREE (mg/kg) HREE (mg/kg) ∑REE/Al2O3 CIA CIW Th/Sc
Table 1. Main REE values, paleoweathering indices, and average elemental ratios for each lithologyand the three references (PAAS, NASC, and UCC). Standard deviations are shown in parentheses.
PAAS, Post-Archean Australian Shale; NASC, North American Shale Composite; UCC, Upper Continental Crust.
542 L. A. Montilla et al.
with organic matter (Mo), primary productivity (P andBa), or fixed as a result of highly reducing conditions.Lastly, Ca, Y, Sr, Co, S, and As were observed to be asso-ciated with carbonates and sulfates, and these elementsappear to have been mobilized during diagenetic orpostgenetic processes.
Paleo-weathering conditions and provenanceTwo dimensionless weathering indexes, the chemical
index of alteration (CIA) and the chemical index of weath-ering (CIW) (Nesbitt and Young, 1982; Harnois, 1988),have been widely used to quantify relative weathering insource regions of sediments. High CIA and CIW valuesfor the shales and siltstones (see Table 1) of the Colón-Mito Juan sequence may suggest moderate to intenseweathering as part of the first cycle of sedimentation inthe source area of the precursor materials for the sedi-mentary 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 Al2O3–K2O–CaO+Na2O plot (Fig. 6) shows thatthe analyzed shales and siltstones are formed mostly byillite, suggesting moderate chemical weathering of thesource area of the sediment (average CIA of 79%). Thepresence of this mineral in the shales is supported by thepositive correlation between K and Rb because these cati-ons bind to clays such as illite (Young and Nesbitt, 1999).
Considering that the average Th/Sc values exceed 1and that the Th/Sc standard deviations are low for alllithologies (see Table 1), the samples generally clusteralong a relatively straight trend located in the continental
domain. Th/Sc values higher than those of the PAAS,NASC, and UCC references (see Table 1) indicate a sourceof felsic composition (Young and Nesbitt, 1999).
Figure 7a shows a Th/Co vs. La/Sc diagrammatic rep-
Fig. 5. Groups of variables provided by Q-mode hierarchicalcluster analysis of the data matrix from the Colón-Mito Juansequence and upper La Luna Fm.
Fig. 6. Al2O3–K2O–CaO+Na2O plot of sandstones and shalesof the Colón-Mito Juan sequence and upper La Luna Fm.
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 Juansequence and the upper La Luna Fm.
Fig. 8. Stratigraphic subdivisions through constrained clus-tering based on a) redox processes controlling the concentra-tions 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 toC, P, Mo, U, Cu, Ba, and Sb. Lithologies: L, shale; S, sand-stone; Lm, limestone; F, chert.
Geochemistry and chemostratigraphy of the Colón-Mito Juan 543
resentation (López et al., 2005) for the Colón-Mito Juansequence. This plot allows discrimination of source rocksbased on a felsic (rich in Th and La, depleted in Sc andCo) or basic affinity (low Th/Co and La/Sc ratios). More-over, most samples in the Hiscott diagram (Y/Ni vs. Cr/V; Fig. 7b) plot around the felsic field and show similarlow Cr/V ratios; however, the Y/Ni ratios vary widely.This observation could be explained by an additional sedi-ment source, namely recycled sedimentary rocks. As aresult of recycling processes, Ni may be preferentiallydepleted from sediments, thus increasing the Y/Ni ratioand promoting scattering in the values; Y, Cr, and V areimmobile and affected similarly by sorting (Dinelli et al.,1999). In our case, felsic metamorphic sources yieldedsediments to the Colón-Mito Juan sequence and theTáchira Ftanita and Tres Esquinas Members. However, ithas been demonstrated that numerous metal ratios showsignificant differences in metamorphic and granitic rocks(Piovano et al., 1999). Despite this drawback, we usedthese diagrams as indicators of provenance. However, theymust be interpreted with caution.
Furthermore, several authors (Amstrong-Altrin et al.,2004; among others) have reported that mafic and felsicrocks have low and high values, respectively, in the ratioof 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.
ChemostratigraphyFigure 8 shows the division of the generalized
stratigraphic column through constrained clustering basedon a) redox reactions controlling the concentration of el-ements (Eu, Fe, Sm, Nd, Ce, La, Th, Sc, Lu, Yb, Na, andHf) adsorbed or predominantly associated withoxyhydroxides or other oxides; b) processes related tothose elements (Zn, Ni, V, Cr, Mg, Ti, Rb, Cs, K, Al, andBe) mostly bound to the clay minerals; and c) redox re-actions that control elements (C, P, Mo, U, Cu, Ba, andSb) associated with organic matter or high-potential re-duction processes. The selected cut-off values were 200,100, and 50, respectively.
First, two chemofacies were determined, and these areidentified as O-I (TLVU 005 to TLVU 020) and O-II(TLVU 025 to TCMJ 002). The geochemical profiles forREE and Fe (Fig. 9a) and also Na values (see Table S1)indicate that these elements showed a tendency to increasein 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 be-tween the Tres Esquinas and Táchira Ftanita members (see
Fig. 9. a), b), and c) Chemostratigraphic profiles for the three first groups of elements, respectively, obtained from Q-modehierarchical cluster analysis for the generalized stratigraphic column.
544 L. A. Montilla et al.
Fig. 8).Other changes in the trends of several geochemical
profiles (Fe, La, Ce, Nd, Sm, Th, Sc, Eu, Na, Hf, Yb, andLu) were detected within the O-II chemofacies at the samestratigraphic level, a level coinciding with one of the pre-viously defined lithological boundaries (approximately240.5 m in the log); this can be interpreted as a change inthe sedimentation pattern. Furthermore, Fe, La, Ce, Nd,Sm, Th, Sc, and Eu (elements associated with oxides andoxyhydroxides) were enriched in the clay fractions, incontrast to Na, Hf, Yb, and Lu (elements bound to heavyminerals), which had high values in the sandstone hori-zons, as was the case of the highest concentrations of Yband Lu observed in the Tres Esquinas Member resultingfrom hydraulic conditions.
Multivariate analysis enabled the division of the se-quence into three chemofacies, identified as A-I (TLVU005 to TLVU 040), A-II (TCMJ 900 to TCMJ 400), andA-III (TCMJ 395 to TCMJ 005) from the bottom to thetop of the log (Fig. 8). These divisions are associated withlithological variations: A-I corresponds to the La LunaFm., A-II is characterized by lithologic homogeneity(black shales of the Colón unit s. str.), and A-III corre-sponds to a progradational sequence characteristic of theMito Juan Fm. (alternation of sandstones, siltstones, andshales), permitting chemical differentiation of the Colónand Mito Juan units. The geochemical profiles of Zn, Ni,V, Cr, Mg, Ti, Rb, Cs, K, Al, and Be show an enrichmentat the La Luna-Colón contact at a stratigraphic height of18.5 m (Fig. 9b). However, higher concentrations of theseelements were also detected in the shaly interval locatedbetween the cherty and sandstone levels in the boundarybetween the Táchira Ftanita and Tres Esquinas members(2 m in stratigraphic height), thus confirming an associa-tion with clays. In coherence with differences in the sedi-mentation pattern in the Colón-Mito Juan boundary (vari-ation in energy conditions) related to a relative reductionin clay content, another change in trace element compo-sition can be observed at 240.5 m in the log.
Additionally, constrained cluster analysis led to theidentification of the last three chemofacies (see Fig. 8):R-I (TLVU 005 to TCMJ 635), R-II (TCMJ 630 to TCMJ540), and R-III (TCMJ 535 to TCMJ 002), from bottomto top, based on the geochemical profiles of the elementsC, P, Mo, U, Cu, Sb, and Ba (Fig. 9c). Generally, theseprofiles suggest less reducing conditions for the Colón-Mito Juan sequence compared to the upper La Luna For-mation. This notion is also supported by a slight decreasein TOC in the log (see Table S1). The Tres Esquinas Mem-ber showed the maximum enrichment in U, Cu, Sb, Ba,and P, indicating that deposition occurred during a pe-riod of maximum transgression and high primary produc-tivity. This was accompanied by an abrupt decrease inthe anoxicity of the water as a result of at-depth (succes-
sive upwelling events) and near-surface mixing processescaused by Late Companion–Early Maastrichtian tectonicepisodes that occurred on the northern edge of the LakeMaracaibo Basin and impeded water circulation (Lugoand Mann, 1995). The interval defined as R-II (from ap-proximately 110 to 158 m in stratigraphic height) is char-acterized by a decrease in the concentrations of Cu, Ba,and Sb and an increase in the Mo and TOC values. Thisinterval may indicate a period of rapid redox changes re-lated to variations in water oxygen content. The R-III in-terval begins at 158 m in the log and ends at the top ofthe section, indicating a zonal redox change defined byelements such as Cr, Ni, Zn, and V.
On the whole, our approach allowed us to differenti-ate two zones characterized by the input of eithersiliciclastic materials or biogenic siliceous sediments, thelatter being identified as the chert-rich Táchira FtanitaMember (Garbán, 2010). In addition, a glauconite-richphosphorite unit was identified as the Tres Esquinas Mem-ber (Parra et al . , 2003). Furthermore, thechemostratigraphic profiles of the clay-associated ele-ments indicated the contact between the Colón and MitoJuan formations, the latter formation being a deltaicsedimentation unit of interbedded gray shales, fine-grained sandstones, and, occasionally, carbonates. Finally,a series of redox changes were detected within themonolithological black shaly interval; these changes canbe explained by either variations in oxygen levels in thewater or by subsidence, which would have caused a lowerwater level. It is also interesting to note that the redoxchanges did not affect redox-sensitive elements equally.
CONCLUSIONS
The outcroppings studied here were found to be rep-resentative of the Colón and Mito Juan units on the basisof the lithologies identified and their correlation with in-formation in the literature. The contact between the lowerand middle lithologic intervals within the Colón-MitoJuan sequence is proposed to mark the Colón-Mito Juanboundary.
The geochemical profiles reflected the lithologicalcompositions (sandstones, siltstones, shales, limestones,and cherts) found in the study stratigraphic horizons. Wepropose that these profiles indicate intense weatheringprocesses for the source area of the felsic-origin sedimentsand successive changes in redox conditions of thedepositional environment, with increasingly oxidizingconditions towards the top.
Chemostratigraphically, both the paleoweathering con-ditions and the source area of the sediments remaineduniform along the formations studied. We identified eightchemical facies: two (O-I and O-II) indicate sections con-trolled by biogenic and clastic deposition, respectively;
Geochemistry and chemostratigraphy of the Colón-Mito Juan 545
three (A-I, A-II, and A-III) show coherence with changesin 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 conditionsthroughout the column.
Acknowledgments—This research was funded by the Consejode Desarrollo Científico y Humanístico (Universidad Centralde Venezuela) through the projects CDCH-03.32.4412/99,CDCH-03.32.4412/00, and CDCH-03.30.4702/99.
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