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Geochemistry and Chemostratigraphy of the Colón-Mito Juan Units

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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 concen- trations 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 con- centrations 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 inter- est 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 southwest- ern 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 tran- sition 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 ma- in 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 ele- ment 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)
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