By JAMES PINDELL,
This month's column, the second in a four-part series dealing with regional plate kinematics ("Arm Waving, or Underutilized Exploration Tool?"), is titled "Kinematics: Key to Unraveling Basin Histories."
Kinematics a Key To Unlocking Plays
Pindell, Kennan and Barrett are with Tectonic Analysis in Sussex, England, and Houston. Pindell is also adjunct associate professor at Rice University, Houston.
Click on a figure below to see an enlarged view.
Last month's Geophysical Corner outlined some of the principles and methods of kinematic analysis as a means of better deciphering the structural history of basins. In this second of our four-article series, we will apply some of the principles of kinematic analysis to the first of our example areas: the northern Andes of Colombia and western Venezuela.
We also will illustrate some of the uses and benefits of this analysis to petroleum geology and exploration in continental settings.
When applied to continental areas, kinematic analysis can provide map-view palinspastic reconstructions of deformed regions prior to the deformation(s), analogous to balancing cross sections in the vertical plane.
Two very useful applications of continental block kinematics for exploration are:
Here we show a set of simple steps for restoring the northern Andean ranges and basins for Early Oligocene and earlier time, prior to the majority of "Andean" deformation. Note that variations in the reconstruction will derive from applying different numbers of steps (accuracy can be increased by accounting for more fault motions between more blocks), and also from adjusting various input parameters, such as magnitudes of strike-slip offset on certain fault zones.
A reference frame is needed to begin: In this example, Andean motions are assessed relative to the Guyana Shield.
First, we address the relative motion of the Maracaibo Block by assessing displacement in the Mérida Andes, which separate the Maracaibo Block and the Shield.
Figure 1 shows the dextral offset across the Mérida Andes of the "Eocene thrustbelt," which came to rest in the Early Oligocene, measured by many as about 150 kilometers. In addition, shortening in the Mérida Andes has been estimated as about 40 kilometers. Thus, in the Early Oligocene, the Maracaibo Block lay significantly farther southwest relative to the Shield than it does today.
In figure 2, we construct a tie line between the Shield and Maracaibo Block by performing vector addition of the strike-slip (150 kilometers) and thrust (40 kilometers) components. Because we wish to restore the accrued offset (155 kilometers), we draw the tie lines opposite to the real-life sense of fault displacements, i.e., moving back in time.
Having defined the Oligocene paleoposition of Maracaibo relative to the Shield, our next concern is the Perijá Range, deformation of which accounts for movements between the Maracaibo Block and the Santa Marta Massif Block.
Estimates of post-Early Oligocene Perijá shortening are roughly 25 kilometers along an azimuth of east-southeast/west-northwest, as shown by the Perijá vector in figure 2. Thus, displacing Santa Marta Massif to the west-northwest of Maracaibo by 25 kilometers gives the Early Oligocene position of Santa Marta relative to both Maracaibo Block and Guyana Shield.
Next, the Santa Marta strike-slip fault displaces the Santa Marta Massif Block from the northern part of Colombia's Central Cordillera. Left-lateral offset of about 110 kilometers (figures 1, 2) is believed to have occurred on this fault zone since the Late Oligocene.
This strain is transferred into the Eastern Cordillera along the south-southeast continuation of the fault, where it is called the Bucaramanga Fault. Interestingly, the Bucaramanga Fault is flanked by the high, compressive topography of Santander Massif; this is because the Bucaramanga Fault defines the boundary between the Central Cordillera and the Maracaibo Block, not the Santa Marta Block.
For simplicity in figure 2, the trend shown for the Bucaramanga Fault (in orange) defines only the total strain between those blocks, i.e. the sum of the strike-slip and orthogonal components of relative motion.
Finally, we restore Colombia's Guajira Block, also relative to the Santa Marta Block, by removing about 125 kilometers of dextral shear on the Oca Fault in order to realign the western flanks of continental basement in the two blocks prior to fault displacement.
With just these simple considerations, and assuming that only minor vertical axis rotation of these blocks has occurred during their relative motions, we can now fill out other tie lines in the vector "nest" of figure 2 to define offsets between other pairs of blocks in the system.
For example, the total strain in the Eastern Cordillera since the Oligocene is seen to be roughly 200 kilometers toward the east-southeast (red tie line). This can then be broken down into components of orthogonal and strike-parallel strain of 180 kilometers (blue line) and 100 kilometers (green line), respectively, which translates geologically into shortening (180 kilometers) and dextral shear (100 kilometers), moving forward in time.
We note that this value of shortening (180 kilometers) falls in the middle of the range of published values of estimated shortening in Eastern Cordillera. Thus, vector nests such as figure 2 can be used to help choose between alternative balanced cross section models assessing shortening, because different assumptions of depths to detachments or degrees of basement involvement produce very different modeled shortening values.
In addition, it also allows detection and estimation of the strike-slip component, which usually cannot be seen in cross sections. Our inferred dextral shear in the Eastern Cordillera is supported by seismicity, GPS data and field observations.
The known limit of pre-Mesozoic continental crust has been identified in figure 3 to show the pre-Andean geometry of the northern Andes "autochthon," to which a number of oceanic terranes have been accreted in the Cenozoic. Additional information can now be added to better focus the picture.
We can, for example, draw the occurrence of Eocene formations, sedimentary facies and paleoenvironments on our reconstruction in order to build palinspastically accurate models of regional Eocene depositional systems. This practice also allows better sequence stratigraphic interpretation and correlation at the regional scale, which is helpful to determining migration pathways through the strata.
Also, the depositional models can be compared more meaningfully to modern analogues and analyzed for implications concerning reservoir potential, such as sand body orientation, sinuosity, flow direction, sand grain provenance and sediment maturity.
Finally, the reconstruction also allows a better interpretation of Cretaceous source rock character, quality and original areal extent.
Using the same block/plate restoration technique, we can depict Eocene-aged structures and the Eocene position of the Caribbean Plate relative to South America, to better understand the driving forces of Eocene sedimentation patterns and deformation.
Figure 4 thus shows the Caribbean Plate driving an Eocene foredeep basin in the northern Maracaibo area -- much like today's Persian Gulf -- which caused an important early hydrocarbon maturation event in western Venezuela and Colombia's Cesar Basin.
Figure 4 also shows depositional systems with important reservoir facies belts at the Middle to Late Eocene boundary, as well as the existence, continuity and origin of an Eocene "Maracaibo Tar Belt" in western Venezuela (also recognized in Middle to Late Eocene field sections).
The concept of this "textbook" foredeep basin for the Eocene of Maracaibo Basin had remained darkly veiled for decades by today's grossly different geography.
Next month, we will use plate kinematics to reconstruct Africa and South America, and to progressively close the Atlantic Ocean during Mesozoic times, in order to set the stage for tracing the evolution of the Gulf of Mexico and the Florida/Bahamas region in our fourth article of the series.