Geochemistry and Chemostratigraphy of the Colón-Mito Juan Units

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. 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, Venezuela 2Instituto 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, Spain 4CARBOZULIA, Av. 2 No. 55-185, Casa Mene Grande, Maracaibo 4002 A, Venezuela 5Postgrado de Geología Petrolera, Facultad de Ingeniería, Universidad del Zulia, Maracaibo 4002, Venezuela 6Departamento 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 (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, 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 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 predomi- nant 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

INTRODUCTION identify source rocks and weathering processes (Cullers, 2000). A chemostratigraphic study, which involves the Integrated geochemical and chemostratigraphical stud- characterization of the sedimentary sequence into differ- ies of sedimentary rocks allow the determination of ent units on the basis of major and trace element chemis- paleoenvironmental conditions and provenance of try (e.g., Pearce et al., 1999) is done when geochemical sediments (e.g., Armstrong-Altrin et al., 2004). The data are evaluated in the context of a stratigraphic log. geochemistry of clastic sediments is controlled by the Chemostratigraphy can be carried out with isotopic data composition of the source rocks, weathering, deposition, (e.g., Ehrenberg et al., 2000) or by combining several and diagenetic processes (Asiedu et al., 2000; Yan et al., chemical indices (Reyment and Hirano, 1999; Reinhardt 2006). Consequently, geochemical tracers can be used to and Ricken, 2000). In addition, other features revealed by chemostratigraphic studies include climatic changes, paleoredox conditions, stratigraphic correlations, *Corresponding author (e-mail: [email protected]) paleoproductivity, and chemical cyclicity in processes Copyright © 2013 by The Geochemical Society of Japan. involving basin sedimentation (Yarincik and Murray,

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 differenti- ate 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 The Lake Maracaibo Basin is located at the southwestern end of the Caribbean Sea in Venezuela, near its bor- Fig. 1. Sketch map showing the two sampling sites and the der with Colombia. This basin consists of a thick sedi- main localities in the study region in the state of Táchira (Ven- mentary cover divided into various sequences conditioned ezuela). 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 colli- 2000; Hetzel et al., 2009; among others). sion of the Pacific volcanic arc emplaced the “Lara The present study focused on the geochemistry and Nappes” to the northern edge of the aforementioned ba- chemostratigraphy of Late Campanian to Late sin; Late Paleocene–Middle Eocene foreland basin de- Maastrichtian (76–65 Ma) sedimentary rocks in the west- posits that formed in front of the volcanic arc; and a Late ern region of the state of Táchira, Venezuela. First, we Eocene–Pleistocene sequence related to the collision of studied a sequence consisting of the uppermost part of the Panama arc with the South American plate (Mann et the La Luna Formation (Tres Esquinas Member) and the al., 2006; Escalona and Mann, 2011). Colón and Mito Juan units outcropping close to the The sedimentary succession of the southwestern sec- Lobaterita River near the locality of San Juan de Colón tor within the Lake Maracaibo Basin overlies the (Fig. 1). We then examined a second stratigraphic sec- igneous-metamorphic basement and begins with red beds tion of rocks comprising the Táchira Ftanita and Tres of the Jurassic La Quinta Formation, which represents Esquinas members (La Luna Formation) up to the lowest fluvio-lacustrine deposition (González de Juana et al., part of the Colón Formation outcropping in a cut along 1980). Subsequently, thermal subsidence of the passive the San Pedro de Río-Ureña road (Fig. 1). margin of South America extending into the Early Creta- The particular case of the Colón Formation is very ceous led to the deposition of the Río Negro Formation interesting because when studying a stratigraphic se- (coarse-grained, arkosic, and fine-grained sandstones), the quence characterized by a monolithological composition, Cogollo Group (limestones and sandstones), the Capacho according to González de Juana and colleagues (1980), Formation (black shales and limestones), and the the variations in chemical profiles are not strongly influ- Aguardiente unit (shales and sandstones). Subsequently, enced by lithological changes. Moreover, interest in per- the La Luna Formation (organic matter-rich limestones, forming this study in the Colón Formation comes from shales, and cherty rocks) was deposited during a series of the following observations: (1) the formation’s total or- four marine transgressions of Late Cretaceous age ganic carbon (TOC) values are higher than 1% in some (Villamil, 1999). These events were followed by the be- areas (Malavé, 1994); (2) the formation acts as a caprock ginning of a regressive succession with the shallow main the petroleum system of the Maracaibo Lake Basin rine deposition of the Campanian–Maastrichtian Colón (Parnaud et al., 1995); and (3) it represents most of the Formation (gray shales), which was caused by an oblique

538 L. A. Montilla et al. collision between the westward-migrating Caribbean is- Analytical procedures land arc and the passive margin of South America (Lugo An aliquot (about 100 g) of each sample was crushed and Mann, 1995). In addition, the Maastrichtian Mito Juan and pulverized using a Shatterbox 5540 with a tungsten Formation (sandstones, siltstones, and shales) began to carbide grinding container. Geochemical analyses of six be deposited in a deltaic environment (Sutton, 1946). major/minor elements, expressed as % w/w oxides (TiO2, During the Tertiary, paralic to fluvio-estuarine sedi- Al2O3, MgO, CaO, K2O, and P2O5), and eight trace element of the Orocué Group (sandstones and siltstones) was ments (Be, Cu, Mo, Ni, Sr, V, Y, and Zn), expressed as deposited in the Paleocene–Eocene, and the Los Cuervos mg/kg, were determined by inductively coupled plasma- Formation (sandstones, siltstones, and shales) was laid atomic emission spectroscopy (ICP-AES) using a Perkin- down in a deltaic depositional environment. Overlying Elmer Optima 3000 spectrometer. In addition, two other the latter, the Eocene–Early Oligocene Mirador Forma- major elements, expressed as % w/w oxides (Fe2O3 and tion, which consists of sandstones, shales, and siltstones, Na2O), and a further eighteen trace elements (Rb, Cs, Ba, was then deposited under fluvio-estuarine conditions. Th, U, Hf, Cr, Co, Sc, La, Ce, Nd, Sm, Eu, Yb, Lu, As, Finally, Late Oligocene and younger sediments formed and Sb), expressed as mg/kg, were determined by instru- the El Fausto Group (sandstones) and the León unit (shales mental neutron activation analysis (INAA). Total carbon and siltstones), as well as the sandy rocks of the Guayabo (C) and inorganic carbon were measured in a coulometric Group (González de Juana et al., 1980). carbon analyzer. Total organic carbon (TOC), as weight Located in the southwestern Lake Maracaibo Basin, percent, was calculated as the difference between C and the Campanian to Early Maastrichtian Colón Formation inorganic carbon. Sulfur contents were also determined (400 to 900 m thick) displays a more sandy lithology to- using a LECO SC-432 apparatus. Two certified reference ward the base and also toward the top, where this unit materials, Post-Archean Australian Shale (PAAS) and changes concordantly and transitionally to the Mito Juan North American Shale Composite (NASC), were used for Formation with the appearance of interbedded sandstones analytical control and data comparison. and limestones (Sutton, 1946). The Late Maastrichtian Mito Juan Formation (100 to 300 m thick) is character- Statistical treatment ized by some gray shales that are lithologically indistin- We performed an exploratory data analysis of our guishable from the clays of the Colón Formation. Thus, geochemical dataset prior to statistical treatment. First, several researchers (e.g., Sievers, 1988) have noted the these data are highly multivariate, 36 elements with a difficulty of cartographically separating the Mito Juan sample size of N = 191. Second, the most widely used Formation from the Colón unit. With regard to La Luna methods of multivariate analysis are all based on the as- Formation horizons, the Coniacian–Santonian Táchira sumption that the variables show a normal or lognormal Ftanita Member (80–100 m thick) consists of regularly distribution (Reiman and Filmoser, 1999). In our case, stratified cherts with minor intercalations of siliceous descriptive statistics indicate a natural lognormal distri- shale and limestone (Garbán, 2010). Lastly, concordantly bution for element compositions (Dixon and Kronmal, underlying the Colón Formation, the Campanian Tres 1965). Data outliers as well as values below the determi- Esquinas Member consists of glauconitic limestone that nation limits (VBDLs) were replaced by the correspond- is rich in silica and phosphates. Despite its small thick- ing statistical medians and one-half of the determination ness (3–5 m), the member is an important marker bed limits, respectively. All of the variables showed low num- throughout the Lake Maracaibo Basin (Stainforth, 1962). bers of VBDLs (<10%) and outliers (<25%), thus allow- ing the use of the 36 elemental concentrations for further statistical treatment. The log-transformed data matrix was METHODOLOGY then standardized prior to multivariate statistical analy- Sampling sis through a reported procedure (Reategui et al., 2005) One hundred and eighty-three rock samples, taken at in order to remove artifacts derived from scale attributes stratigraphic intervals of approximately 2.6 m, were col- and to equalize the influence of variables with distinc- lected from the Colón-Mito Juan sequence near the city tive variations. of San Juan de Colón (8°2′ N, 72°16′ W). In addition, Cluster analysis was applied using the matrix formed eight rock samples, taken at intervals of about 2.3 m, were by the log-transformed and standardized data in order to collected from the Táchira Ftanita and Tres Esquinas group the variables. Dissimilarity values were obtained members of the La Luna Formation along an outcropping after calculating squared Euclidean distance measures cut on the road to Ureña (7°57′ N, 72°21′ W), approxi- using Ward’s minimum variance method (Templ et al., mately 10 km southwest of the San Pedro de Río village. 2008). A cut-off squared distance of 320 was also selected. The locations of the two sampling sites are shown in Fig. Finally, constrained cluster analysis was carried out to 1. determine geochemically meaningful zones, or

Geochemistry and chemostratigraphy of the Colón-Mito Juan 539 chemofacies. In the respective dendrograms (see Section quence and the Táchira Ftanita and Tres Esquinas Mem- “Results and Discussion”), the samples are arranged in bers are listed in Supplementary Tables S1 and S2. The accordance with their stratigraphic height. The number Colón-Mito Juan sequence is distinguished by three in- of chemical facies depends on the selected cut-off value tervals: a lower zone of 222 m (between 18.5 and 240.5 (Gill et al., 1993). Multivariate analysis between vari- m in the stratigraphic log) dominated by black shales, a ables was performed by multi-dimensional scaling (MDS). second overlying zone comprised of a 78-m thick inter- Data were processed using the NCSS 2000TM statistical val of gray shales and thin fine-grained sandstones, and, software package. 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. RESULTS AND DISCUSSION Data for the samples from the Colón-Mito Juan se- 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 proc- Fig. 2. a) Crossplots of several study elements against Al2O3; b) Berner plot for all the samples from the upper La Luna Fm esses (e.g., authigenic mineral formation, organic matter and the Colón-Mito Juan sequence. 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 content of Y and Hf. This observation, together with the Al2O3. In addition, marine deposition cannot be corrobo- positive correlation between Hf and Ti and the inverse rated by a significant positive correlation between TOC one between Ti and Y in these rocks, may indicate the and S (Fig. 2b; Berner, 1983), possibly because of sulfur presence of these elements in heavy minerals such as zir- mobility (occurrence of sulfates). con and rutile (Bea, 1996). Each lithology was treated as a separate dataset, and Finally, shales were observed to be enriched in light geochemical concentrations of various elements, normal- rare earth elements (LREE), such as La, Ce, and Nd; in ized to average upper continental crustal (UCC) values contrast, sandstone showed an enrichment in medium (Sm (after Wedepohl, 1995), were compared and plotted on a and Eu) and heavy rare earth elements (HREE), such as logarithmic scale, shown in Fig. 3. Tb, Yb, and Lu. This difference may result from the As expected, shales and siltstones had higher Al2O3 fractionation of rare earth elements (REE), a process that contents (nearly 16 and 12%, respectively) than usually involves the accumulation of lighter REE in clays, sandstones (5.8%) and cherts (2.4%), reflecting prefer- while heavier ones are concentrated in minerals such as ential incorporation of clay minerals into the shales and zircon (Nyakairu and Koeberl, 2001). Our observation is siltstones. The enrichments in Rb, Cs, V, Cr, Ni, Sc, and supported by correlations between ∑REE, LREE, and Th in shales could be due to the association with clays HREE with Al2O3 (see Fig. 4). Table 1 shows the values ∑ (Bauluz et al., 1994). The trace element Co was signifi- of the REE/Al2O3 ratio. The highest value of this ratio cantly more concentrated in the chert samples. The mo- was recorded in the sandstones, suggesting that a non- bility of this element may be governed by redox condi- clay phase contributes to the content of REE in both tions and by the processes controlling element sandstones and limestones. This finding could be attrib- remobilization during chert formation. In this regard, Ni/ utable to the presence of oxyhydroxides or other heavy Co values of nearly 5 have been shown to indicate oxic- minerals. Chert samples showed the lowest concentrations disoxic depositional conditions (Ross and Bustin, 2009). of REE because these elements are “diluted” in SiO2 Previously correlated with organic matter preservation (Garbán, 2010). (Zelt, 1985), U had its highest values in samples TLVU 025, TLVU 030, TLVU 035, and TLVU 040 (see Table Elemental relationships S2), which correspond to the Tres Esquinas Member. It A Q-mode cluster analysis was performed to estab- should also be noted that our sandstone showed a higher lish 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 remark-

Fig. 4. Crossplots of rare earth elements vs. Al2O3. able 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.

∑ ∑ REE (mg/kg) LREE (mg/kg) HREE (mg/kg) REE/Al2O3 CIA CIW Th/Sc

Shales 231.36 (21.58) 224.48 (21.49) 4.72 (0.76) 14.47 78.67 90.07 1.26 (0.12) Sandstones 169.08 (54.70) 160.16 (49.57) 5.64 (1.79) 29.20 42.45 46.51 1.43 (0.28) Siltstones 214.84 (30.88) 207.13 (30.96) 5.86 (0.95) 18.06 77.73 87.48 1.46 (0.18) Cherts 43.23 (29.49) 41.45 (28.46) 1.15 (0.78) 18.00 ——1.16 (0.66) Limestones 131.35 (28.81) 125.18 (27.97) 4.21 (1.40) 24.48 ——1.46 (0.25) PAAS 160.70 155.60 3.23 8.50 75.30 88.32 0.91 NASC 136.34 130.79 3.52 8.07 65.91 77.99 0.83 UCC 128.56 124.50 2.54 8.50 56.93 65.23 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. Fig. 7. a) and b) Th/Co vs. La/Sc plot and Hiscott diagram Lastly, Ca, Y, Sr, Co, S, and As were observed to be asso- (Cr/V vs. Y/Ni), respectively, applied to the Colón-Mito Juan ciated with carbonates and sulfates, and these elements sequence and the upper La Luna Fm. 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 Colón- Mito 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 Fig. 8. Stratigraphic subdivisions through constrained clus- (Erlich et al., 2000). tering based on a) redox processes controlling the concentra- The Al2O3–K2O–CaO+Na2O plot (Fig. 6) shows that tions of Eu, Fe, Sm, Nd, Ce, La, Th, Sc, Lu, Yb, Na, and Hf; b) the analyzed shales and siltstones are formed mostly by reactions that control clay-associated elements such as Zn, Ni, illite, suggesting moderate chemical weathering of the V, Cr, Mg, Ti, Rb, Cs, K, Al, and Be; and c) processes related to source area of the sediment (average CIA of 79%). The C, P, Mo, U, Cu, Ba, and Sb. Lithologies: L, shale; S, sand- presence of this mineral in the shales is supported by the stone; Lm, limestone; F, chert. positive 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 1 domain. Th/Sc values higher than those of the PAAS, and that the Th/Sc standard deviations are low for all NASC, and UCC references (see Table 1) indicate a source lithologies (see Table 1), the samples generally cluster of felsic composition (Young and Nesbitt, 1999). along a relatively straight trend located in the continental Figure 7a shows a Th/Co vs. La/Sc diagrammatic rep-

542 L. A. Montilla et al. 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 gies, LREE values were clearly higher than those of HREE sequence. This plot allows discrimination of source rocks (see Table 1), thus corroborating the felsic origin. based on a felsic (rich in Th and La, depleted in Sc and Co) or basic affinity (low Th/Co and La/Sc ratios). More- Chemostratigraphy over, most samples in the Hiscott diagram (Y/Ni vs. Cr/ Figure 8 shows the division of the generalized V; Fig. 7b) plot around the felsic field and show similar stratigraphic column through constrained clustering based low Cr/V ratios; however, the Y/Ni ratios vary widely. on a) redox reactions controlling the concentration of el- This observation could be explained by an additional sedi- ements (Eu, Fe, Sm, Nd, Ce, La, Th, Sc, Lu, Yb, Na, and ment source, namely recycled sedimentary rocks. As a Hf) adsorbed or predominantly associated with result of recycling processes, Ni may be preferentially oxyhydroxides or other oxides; b) processes related to depleted from sediments, thus increasing the Y/Ni ratio those elements (Zn, Ni, V, Cr, Mg, Ti, Rb, Cs, K, Al, and and promoting scattering in the values; Y, Cr, and V are Be) mostly bound to the clay minerals; and c) redox re- immobile and affected similarly by sorting (Dinelli et al., actions that control elements (C, P, Mo, U, Cu, Ba, and 1999). In our case, felsic metamorphic sources yielded Sb) associated with organic matter or high-potential re- sediments to the Colón-Mito Juan sequence and the duction processes. The selected cut-off values were 200, Táchira Ftanita and Tres Esquinas Members. However, it 100, and 50, respectively. has been demonstrated that numerous metal ratios show First, two chemofacies were determined, and these are significant differences in metamorphic and granitic rocks identified as O-I (TLVU 005 to TLVU 020) and O-II (Piovano et al., 1999). Despite this drawback, we used (TLVU 025 to TCMJ 002). The geochemical profiles for these diagrams as indicators of provenance. However, they REE and Fe (Fig. 9a) and also Na values (see Table S1) must be interpreted with caution. indicate that these elements showed a tendency to increase Furthermore, several authors (Amstrong-Altrin et al., in the interval between 2 m from the bottom (TLVU 025) 2004; among others) have reported that mafic and felsic and the top of the stratigraphic log (499.5 m). Therefore, rocks have low and high values, respectively, in the ratio the O-I/O-II boundary is coincident with the contact be- of LREE/HREE. In our case, for all samples and litholo- tween the Tres Esquinas and Táchira Ftanita members (see

Geochemistry and chemostratigraphy of the Colón-Mito Juan 543 Fig. 8). sive upwelling events) and near-surface mixing processes Other changes in the trends of several geochemical caused by Late Companion–Early Maastrichtian tectonic profiles (Fe, La, Ce, Nd, Sm, Th, Sc, Eu, Na, Hf, Yb, and episodes that occurred on the northern edge of the Lake Lu) were detected within the O-II chemofacies at the same Maracaibo Basin and impeded water circulation (Lugo stratigraphic level, a level coinciding with one of the pre- and Mann, 1995). The interval defined as R-II (from ap- viously defined lithological boundaries (approximately proximately 110 to 158 m in stratigraphic height) is char- 240.5 m in the log); this can be interpreted as a change in acterized by a decrease in the concentrations of Cu, Ba, the sedimentation pattern. Furthermore, Fe, La, Ce, Nd, and Sb and an increase in the Mo and TOC values. This Sm, Th, Sc, and Eu (elements associated with oxides and interval may indicate a period of rapid redox changes re- oxyhydroxides) were enriched in the clay fractions, in lated to variations in water oxygen content. The R-III in- contrast to Na, Hf, Yb, and Lu (elements bound to heavy terval begins at 158 m in the log and ends at the top of minerals), which had high values in the sandstone hori- the section, indicating a zonal redox change defined by zons, as was the case of the highest concentrations of Yb elements such as Cr, Ni, Zn, and V. and Lu observed in the Tres Esquinas Member resulting On the whole, our approach allowed us to differenti- from hydraulic conditions. ate two zones characterized by the input of either Multivariate analysis enabled the division of the se- siliciclastic materials or biogenic siliceous sediments, the quence into three chemofacies, identified as A-I (TLVU latter being identified as the chert-rich Táchira Ftanita 005 to TLVU 040), A-II (TCMJ 900 to TCMJ 400), and Member (Garbán, 2010). In addition, a glauconite-rich A-III (TCMJ 395 to TCMJ 005) from the bottom to the phosphorite unit was identified as the Tres Esquinas Mem- top of the log (Fig. 8). These divisions are associated with ber (Parra et al., 2003). Furthermore, the lithological variations: A-I corresponds to the La Luna chemostratigraphic profiles of the clay-associated ele- Fm., A-II is characterized by lithologic homogeneity ments indicated the contact between the Colón and Mito (black shales of the Colón unit s. str.), and A-III corre- Juan formations, the latter formation being a deltaic sponds to a progradational sequence characteristic of the sedimentation unit of interbedded gray shales, fine- Mito Juan Fm. (alternation of sandstones, siltstones, and grained sandstones, and, occasionally, carbonates. Finally, shales), permitting chemical differentiation of the Colón a series of redox changes were detected within the and Mito Juan units. The geochemical profiles of Zn, Ni, monolithological black shaly interval; these changes can V, Cr, Mg, Ti, Rb, Cs, K, Al, and Be show an enrichment be explained by either variations in oxygen levels in the at the La Luna-Colón contact at a stratigraphic height of water or by subsidence, which would have caused a lower 18.5 m (Fig. 9b). However, higher concentrations of these water level. It is also interesting to note that the redox elements were also detected in the shaly interval located changes did not affect redox-sensitive elements equally. between the cherty and sandstone levels in the boundary between the Táchira Ftanita and Tres Esquinas members CONCLUSIONS (2 m in stratigraphic height), thus confirming an association with clays. In coherence with differences in the sedi- The outcroppings studied here were found to be rep- mentation pattern in the Colón-Mito Juan boundary (vari- resentative of the Colón and Mito Juan units on the basis ation in energy conditions) related to a relative reduction of the lithologies identified and their correlation with in- in clay content, another change in trace element compo- formation in the literature. The contact between the lower sition can be observed at 240.5 m in the log. and middle lithologic intervals within the Colón-Mito Additionally, constrained cluster analysis led to the Juan sequence is proposed to mark the Colón-Mito Juan identification of the last three chemofacies (see Fig. 8): boundary. R-I (TLVU 005 to TCMJ 635), R-II (TCMJ 630 to TCMJ The geochemical profiles reflected the lithological 540), and R-III (TCMJ 535 to TCMJ 002), from bottom compositions (sandstones, siltstones, shales, limestones, to top, based on the geochemical profiles of the elements and cherts) found in the study stratigraphic horizons. We C, P, Mo, U, Cu, Sb, and Ba (Fig. 9c). Generally, these propose that these profiles indicate intense weathering profiles suggest less reducing conditions for the Colón- processes for the source area of the felsic-origin sediments Mito Juan sequence compared to the upper La Luna For- and successive changes in redox conditions of the mation. This notion is also supported by a slight decrease depositional environment, with increasingly oxidizing in TOC in the log (see Table S1). The Tres Esquinas Mem- conditions towards the top. ber showed the maximum enrichment in U, Cu, Sb, Ba, Chemostratigraphically, both the paleoweathering con- and P, indicating that deposition occurred during a pe- ditions and the source area of the sediments remained riod of maximum transgression and high primary produc- uniform along the formations studied. We identified eight tivity. This was accompanied by an abrupt decrease in chemical facies: two (O-I and O-II) indicate sections con- the anoxicity of the water as a result of at-depth (succes- trolled by biogenic and clastic deposition, respectively;

544 L. A. Montilla et al. three (A-I, A-II, and A-III) show coherence with changes Táchira Ftanita Member (Late Cretaceous), La Luna For- in lithologic facies and coincide with the La Luna, Colón, mation, Western Venezuela: paleoceanographic and and Mito Juan formations, respectively; and three (R-I, paleoenvironmental implications. Dr. Sci. Thesis, Central R-II, and R-III) represent changes in the redox conditions University of Venezuela, 402 pp. (in Spanish). throughout the column. Gill, D., Shomrony, A. and Fligelman, H. (1993) Numerical zonation of log suites and logfacies recognition by multivariate clustering. AAPG Bull. 77, 1781–1791. Acknowledgments—This research was funded by the Consejo González de Juana, C., Iturralde de Arozarena, J. M. and Picard, de Desarrollo Científico y Humanístico (Universidad Central X. (1980) Geology of Venezuela and the Petroliferous Ven- de Venezuela) through the projects CDCH-03.32.4412/99, ezuelan Basins. 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