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Cenozoic Tectono-Stratigraphic Relationships between the Cesar Sub-Basin and the Southeastern Lower Magdalena Valley Basin of Northern Colombia*
By
Alejandro Mora1 and Alberto Garcia1
Search and Discovery Article #30046 (2006)
Posted November 27, 2006
*Adapted from extended abstract prepared for presentation at AAPG Annual Convention, Houston, Texas, April 9-12, 2006
1Drummond Ltd. Bogotá, Colombia ([email protected])
Introduction
The
boundary between the Cesar Sub-basin (CS) and the southeastern Lower Magdalena
Valley Basin (LMV) of northern Colombia comprises a poorly understood area that
has been controlled mainly by the activity of the Santa Marta-Bucaramanga Fault
System (SMBF). Several regional studies have been made in both the CS (ICP/Ecopetrol,1990;
Intera-Bioss, 1995; Ecopetrol/Geopetrocol, 1998) and in the LMV (Reyes,
Montenegro and Gómez, 2000), but none of them has studied the boundary between
both basins. Other papers have studied the evolution of the different tectonic
blocks of northern Colombia (Hernández, Rámirez and Reyes, 2003), without
providing much detail concerning the boundaries between the blocks. The scarce
seismic coverage of the area between the CS and the eastern LMV, the poor
quality of the seismic data, and the lack of integration of the data from both
basins are the main causes of its poor geological knowledge. One of the very few
previous attempts to integrate data from both basins was carried out by Baquero,
Chamorro, and Aya (1990), but none of this data has been published. Due to the
hydrocarbon and coalbed gas exploration activities that have been carried out by
Drummond Ltd. (DLTD) in recent years, new data of the boundary between the CS
and eastern LMV has been obtained. According to the seismic interpretation and
mapping, the kinematic behavior of the SMBF changed dramatically in the
Oligocene, and this is the main cause of the contrasting stratigraphic and
structural configurations of both basins. A proposal of new models for the
tectonic evolution of northern Colombia is beyond the scope of this paper,
mainly because there are extensive areas that have very few and poor data. The
main objective of this article is to present a new structural model of the area
which resulted from the reprocessing, interpretation, and mapping of the
available 2D seismic data and from an attempt to integrate the data from the
eastern LMV and the CS.
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Geological Setting
The Andes Mountains in Colombia are divided
into three branches or “Cordilleras” with different tectonic histories
and configurations (Figure 1). However,
toward the north the topographic expression of the Western and Central
Cordilleras ends against the LMV, whereas the Eastern Cordillera splits
almost at the same latitude into two branches that form the Santander
Massif and the Sierra de Perijá (SP) in the west and the Mérida Andes in
the east. These two branches form part of the southern half of the
Maracaibo Tectonic Block, a triangular-shaped block that also comprises
the Sierra Nevada de Santa Marta (SM), the Cesar-Ranchería Basin (CRB),
and the Maracaibo Basin (MB), and it is limited by the SMBF in the west,
the Oca-El Pilar The stratigraphy of both basins is different mainly due to the action of the SMBF. DLTD has studied in detail the stratigraphy of the main depocenter of the CS in the La Loma Block called La Loma Syncline (Figure 2), in which Cretaceous to Lower Paleogene and some Neogene units have been preserved. The main oil targets in the CS are Cretaceous carbonates and thin sandstones, whereas the coal-bearing unit is the Cuervos Formation of Late Paleocene age (Figure 3). The post-Paleocene deposits have not been studied much; they consist of fluvial, coarse-grained, poorly consolidated deposits. By contrast, in the eastern LMV, the Cretaceous to Paleocene units are absent whereas the post-Paleocene deposits are much thicker and correspond to deposition in more distal fluvial, deltaic, and even deep-marine settings in the main basin depocenters. In this area, the sedimentary infill consists of the deltaic Oligocene-lower Miocene Cienaga de Oro Formation, which is the main oil target, the marine Miocene Porquero Formation, and younger shallow-marine to continental units. This post-Paleocene succession was unconformably deposited on top of the pre-Cretaceous crystalline basement. The Structure of the Boundary between the Eastern Lower Magdalena Valley Basin and the Cesar Sub-Basin The database that was used for this study consists of more than 2300 km of 2D seismic, data from 7 wells in the CS and 7 wells in the eastern LMV (Figure 2), satellite images, Digital Elevation Models (DEMs), and regional geological maps and reports. Additionally, DLTD’s coal exploration database, including more than 1300 shallow wells (less than 2000 ft), was also used. An important aspect was the reprocessing of 567 km of 2D seismic that was done by DLTD; this allowed a much better imaging of the southwest part of the CS, toward the SMBF. High-resolution seismic data was also acquired by DLTD. Figure 4 is a strike seismic line that shows the main reflectors and seismic packages that were identified and mapped in the CS. Two main seismic packages that are separated by angular unconformities were identified. The older seismic package, which includes Cretaceous to Eocene(?) units, is preserved in the CS under an angular unconformity of probable Late Eocene- Early Oligocene age (Figure 4). The younger seismic package includes post-Eocene(?) deposits that are equivalent to the Tertiary sedimentary infill of the LMV, as will be shown later. This means that an intense deformation episode, probably in Late Eocene-Early Oligocene times, folded the CS, uplifted the basement, and caused the removal of the Cretaceous to Eocene (?) units in the southwest part of the CS.
The time map shown in
Figure 5 (base Cretaceous map) illustrates the structure of the
pre-Cretaceous basement in the study area. The general structure of the
CS consists of NW-verging, SW-NE-trending thrust faults with strike-slip
influence, and a main depocenter, La Loma Syncline. Most of the SW-NE
faults curve and change their orientation as they approach the SMBF,
resembling a horsetail splay structure that could be suggestive of the
northern termination of the Bucaramanga Between both basin areas, a boundary or transition zone affected by the activity of the SMBF that displays structures with a completely different orientation (SSE-NNW) can be subdivided into two different structural domains, one in the south and one in the north. The southern domain comprises a basement high that has been an active topographic barrier since Oligocene times (Figures 4 and 5), and the northern domain consists of a basement depression through which both basin areas have been connected also since Oligocene times. This connection is evident in the map (Figure 6) that represents the base of the post-Eocene deposits and in the seismic lines shown in Figures 7 and 8. The map shows how the post-Eocene(?) sedimentation that took place in the core of the La Loma Syncline continued toward the LMV in the west, passing north of the basement high. Additionally, it shows that, while the LMV has been an area with relatively continuous sedimentation since the Oligocene, most of the CS has been a positive element that has undergone erosion and has been a sediment source area. In fact, the seismic lines in Figure 8 display the pre-Oligocene paleo-topography over which the late Oligocene to Quaternary tectono-sequences of the eastern LMV were deposited. The seismic patterns suggest that sedimentation began in a lowstand fluvial to shallow-marine prograding wedge, probably during latest Oligocene times. Considering the slope of the paleo-topographic surface, valley incision probably occurred up-slope toward the NE. Lower Miocene and younger sequences were deposited on top of the initial lowstand wedge, but their detailed description and identification are beyond the scope of this study.
Toward the eastern LMV there is a prominent
structure that reflects activity during the Neogene. It is a
NW-SE-trending graben of transtensional origin, consisting, according to
several authors (e.g. Reyes, Montenegro and Gómez, 2000), of two main
faults, the Algarrobo-Ariguaní
Discussion and Conclusions
Considering the previous description of the
structure of the transition between the CS and the eastern LMV, there
are several issues that have to be discussed regarding the SMBF as the
boundary between the two basins. In terms of the structure and
configuration, the so-called SMBF, between the CS and the eastern LMV,
displays noteworthy changes at different stratigraphic levels,
indicating that its kinematics have changed during each of its activity
episodes. This study documents only the most recent (post-Paleocene)
episodes, with the most intense of them taking place in the Late
Eocene-Early Oligocene. In that time, the CS was a downthrown block that
was folded and eroded toward the area occupied today by the LMV. The
intense deformation caused the removal of the Cretaceous to Eocene (?)
units. Unfortunately, the low seismic resolution and quality do not
allow clear identification and tracing within the pre-Cretaceous
basement of the
All the aforementioned indicates that the SMBF
is a mature-stage wrench zone that was responsible for the intense
structural deformation that affected pre-Oligocene units; what indicates
that it was an active boundary between the CS and the eastern LMV during
the late Eocene- early Oligocene. By contrast, since Oligocene times the
role of the SMBF as an active
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