Structural Interpretation of the Monagas Foreland Thrust Belt, Eastern
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]).
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.
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).
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.
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).
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.
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