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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.
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Structural InterpretationThree 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
Tectono-Stratigraphic InterpretationSeismic 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 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).
AcknowledgmentsI 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.
ReferencesAudemard, 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. |