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Structural Interpretation of the Monagas Foreland Thrust Belt, Eastern

Venezuela*

By

L.S. Cobos1

 

Search and Discovery Article #30031 (2005)

Posted February 10, 2005

 

*Adapted from extended abstract prepared for presentation at AAPG Annual Convention, Dallas, Texas, April 17-21, 2004.

 

1Universidad Simón Bolívar, Caracas, Venezuela; California State University, Bakersfield, CA ([email protected]).

 

Summary 

The Monagas Foreland Thrust Belt, located in the Eastern Venezuelan Basin (EVB), is the result of a Neogene compression related to the oblique collision between Caribbean and South-American plates (Figure 1). This paper presents a possible structural model for the Monagas foreland thrust belt of Eastern Venezuela, resulting from the interpretation of an integrated geological-geophysical data set from both the surface and subsurface. The subsurface data consisted of 1000 Km of 2D and 700 Km2 of 3D seismic data (Figure 2), correlated to well-log stratigraphy and biostratigraphy from about 30 wells; as well as regional and residual gravimetric maps for the northern part of the Eastern Venezuela Basin. The surface data consisted of two regional surface structural cross section constructed from an integrated surface geologic map (Figure 3) (PDVSA Exploration) and 14 general stratigraphic columns distributed along the different outcropping provinces of the Serrania del Interior Ranges.

 

 

uSummary

uFigure captions

uStructure

uTectono-stratigraphy

uStratigraphic implications

uAcknowledgments

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uSummary

uFigure captions

uStructure

uTectono-stratigraphy

uStratigraphic implications

uAcknowledgments

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uSummary

uFigure captions

uStructure

uTectono-stratigraphy

uStratigraphic implications

uAcknowledgments

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uSummary

uFigure captions

uStructure

uTectono-stratigraphy

uStratigraphic implications

uAcknowledgments

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uSummary

uFigure captions

uStructure

uTectono-stratigraphy

uStratigraphic implications

uAcknowledgments

uReferences

 

 

 

 

 

Figure Captions

Figure 1. Geodynamic setting of Eastern Venezuela Basin (EVB). Present day plate structure of the Caribbean region. Subduction type B to the east between Atlantic and Caribbean-South American plates. At the north, subduction type B of the Caribbean Plate. In the south, Subduction type A of the South American plate. (Modified from Jacome 2001, after DeMets et al. 1994, Kellogg, et al. 1995, Mascle and Letouze, 1990, and Audemard and Lugo, 1996.) Topographic and bathymetric map are from The National Geophysical Data Base, 1988.

Figure 2. Seismic data base--1000 km 2-D and 700 km2 3D.

Figure 3. Surface geology base map.

Figure 4. Map of tectonic provinces. The final results provided by this study, enabled the separation of the Pirital block in two different sub-blocks: Pirital-Cerro Corazon block and Manresa block (see section C-C’). (Map modified after Cobos, 2002, and Chaplet, 2001.)

Figure 5. Strike section showing the different tectonic sheets laterally. Note that the Pirital-Tala thrust systems (system in blue) are composed of various thrust families separated by lateral ramps. This explains the differences between dip sections that lie in different tectonic sheets. This new interpretation of the Pirital block enabled us to develop the proposition of a new lateral ramp within the Pirital block, delimiting two laterally different tectonic provinces: the Pirital-Cerro Corazon block and the Manresa block.

Figure 6. NW-SE dip seismic cross section showing the principal seismic termination and interpretation of four main unconformities.

Figure 7. Surface structural cross section across the Interior Ranges, showing the structural styles of the outcropping ranges. The thrusts painted in red correspond to the youngest-short wavelength thrust system.

Figure 8. Same cross-section as Figure 5. Three main thrust systems (red, blue, and green) with different basal detachment, emplaced at different periods of time forming structures with different wavelength. The oldest system in red is interpreted to be bisected (in two), after the first emplacement of the Pirital thrust system (painted in blue), burying some of the structures with depth to the south (Monagas giant oil field) and exposing the others in the outcropping ranges. The oldest system (painted in green) would reactivate the main Pirital thrust, uplifting and rotating the Pirital high and forming the Morichito Basin.

Figure 9. Spatial correlation of the stratigraphic sequences, the structural model, and the biostratigraphic data, based on a strike section (Figure 5). The integration of the paleo-environmental curves was obtained through the compendium of the biostratigraphic data available for the respective wells. Note that the sequence M2 shows dramatic changes in paleo-environments from South to North and also from West to East.

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Structural Interpretation 

Three main tectonic provinces define the Monagas foreland thrust belt: The Interior Ranges -“Serrania del Interior,” the Pirital block, and the Monagas foothills.  

The integration of subsurface, seismic-structural interpretation, and surface structural profiles enabled the description and characterization of the structural styles for each of these three main tectonic provinces. Three main thrust systems were interpreted to have been emplaced at different periods. The youngest thrusts (highlighted in red color) generated smaller short-wavelength anticlines. The oldest thrust systems (assigned in blue and green colors) generated wider structures reactivating and deforming previous thrusts.  

Three-dimensional correlation of regional seismic profiles tied to surface features shows that not only three different thrust systems can be identified in cross sections, but also two different families of thrusts can be correlated in a map view. These two different sets of thrusts are bounded by their respective lateral ramps. The systems are named here as the First Pirital thrust system to the west, and the Second Pirital thrust system to the east (Figure 4). The southeastern end of the very well known “Urica fault zone” has been interpreted as the western lateral ramps of the First Pirital system. The subsurface interpretation of a new system of lateral ramps (Figure 5) to the east of Urica and to the south of the also known “San Francisco fault” was the principal criteria to divide the Pirital system into two separate families of thrusts. As a result, it is proposed here that the Pirital block should be divided into two different blocks: the Manresa block and the Pirital-Cerro Corazon block (Figure 4).

 

Tectono-Stratigraphic Interpretation 

Seismic stratigraphy and biostratigraphic data allowed the documentation of three main unconformities (Figure 6), each one of which dates the emplacement of the three thrust systems interpreted in this study.  

The oldest unconformity, dated early Miocene, documents the emplacement of the first tectonic pulse recognized. High frequency-short wavelength asymmetric anticlines characterize the structural style for this period of deformation. Currently, most of these structures are exposed to the north, in the outcropping ranges (Figure 7). To the south, in the subsurface, they are buried below a thick column of foreland sediments and form the giant oil fields typical of the Northern Monagas foredeep.  

Middle Miocene sediments onlapping erosional truncations (Figure 6) provide evidence and timing for a second tectonic event. The thrust system (Pirital thrust system), associated with this event, intercepted, uplifted, folded, and reactivated the previous thrusts, exposing some of the oldest faults in outcrops to the north and burying the rest of them in subsurface to the south. The basal detachment of the Pirital thrust system has been interpreted to lie within pre-Cretaceous rocks. Thus, more than 5 km of Cretaceous and pre-Cretaceous strata have been folded, uplifted, and transported to the south for more than 40 km of average displacement.  

Upper Miocene strata onlapping middle Miocene sediments give evidence for the emplacement of a third thrust system, with its basal detachment interpreted to lie at the top of the basement (Figure 8, system in green). Major thrusts reactivated and deformed the Pirital structure, causing the rotation and uplifting of the Pirital high and the creation of the Morichito basin. It is also proposed in this study that some of the major faults within the Monagas basin, such as the Urica fault and the San Francisco fault, constitute the lateral ramps of this last major event.

 

Stratigraphic Implications 

Structural-stratigraphic integration (Figure 9) allowed the spatial correlation of the tectono-stratigraphic sequences. The regional strike section (Figure 5) was the key to this integration. The lack of lateral correlation within some stratigraphic units was better understood when realizing that the Pirital block is composed of more than one thrust systems.  

The tectono-stratigraphic evolution of the Pirital block, according to the model presented in this study, enables a better understanding of the Morochito basin history. Traditionally, this basin had been interpreted as a piggy back basin, formed after the emplacement of the Pirital thrust. However, the interpretation of two different unconformities within this basin, the evidence of a deeper and younger thrust system, reactivating and deforming the previous Pirital thrusts, together with the interpretation of seismic-stratigraphic relations tied to biostratigraphic data, indicate that the Morichito basin has more than one episode of formation. As opposed to traditional interpretations, here two different tectono-stratigraphic sequences are defined within the Morichito basin, named sequences M2 and M3, middle Miocene and late Miocene, respectively (Figure 9).

 

Acknowledgments 

I want to thank Jose Humberto Sanchez and his exploration team from PDVSA, who proposed the project, tutored and followed the process to its end. This work was possible thanks to the kind orientation and advice of my mentors from PDVSA Exploration, Raul Ysaccis and Felipe Audemard. Special thanks to my academic mentor Franklin Yoris from Simón Bolívar University, for his support and guidance.

 

References 

Audemard, F.E., and Lugo, J., 1996, Petroleum geology of Venezuela: AAPG Annual Meeting, Caracas, Venezuela, 1996, Short Course.

Duerto, L., and McClay, K., 2002, 3D geometry of shale diapirs in the Eastern Venezuela Basin: Search and Discovery Article # 10026 (2002). Adapted for online presentation from poster session by the authors at the AAPG Convention, Houston, Texas, March, 2002.

Jacome, M.I., 2001, The formation of the Monagas foreland basin: Eastern Venezuela: Ph.D. Thesis, University of Liverpool, Liverpool, England, U.K., 204 p.

National Geophysical Data Center, 1988, ETOPO-5, bathymetry/topography data. Data Announcement 88-MGG-02: National Oceanic Atmospheric Administration, US Department of Commerce, Washington, DC. 

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