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Journal of South American Earth Sciences xxx (2012) 1e12

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Journal of South American Earth Sciences

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Three-dimensional structural evolution and kinematics of the PiedemonteLlanero, Central Llanos foothills, Eastern Cordillera, Colombia

Obi Egbue*, James KelloggDepartment of Earth and Ocean Sciences, University of South Carolina, Columbia, SC 29208, USA

a r t i c l e i n f o

Article history:Received 6 April 2011Accepted 4 April 2012

Keywords:Triangle zoneActive-roof duplex zonesEastern CordilleraLlanos foothillsThin-skinned thrustingThick-skinned basement-upliftImbricate fault-propagation folds

* Corresponding author. Nomac Services LLC, a COklahoma City, OK, USA. Tel.: þ1 8035860328.

E-mail addresses: [email protected], oegbue@g

0895-9811/$ e see front matter � 2012 Elsevier Ltd.doi:10.1016/j.jsames.2012.04.012

Please cite this article in press as: Egbue, OCentral Llanos foothills, Eastern Cordillera, C

a b s t r a c t

The Piedemonte Llanero is a wedge duplex zone in the Llanos foothills on the eastern flank of the EasternCordillera of Colombia. It is located northeast of the Cusiana and Cupiagua hydrocarbon fields. The area ischaracterized by a series of moderate to high-angle duplexes with east-southeast verging thin-skinnedand thick-skinned tectonics. We present a structural model constrained by 2-D and 3-D seismic reflectiondata, surface geology, and well data. The structural analysis is based on backward modeling (kinematicrestoration) and forward modeling using transfer-flexural slip and fault slip fold algorithms. The struc-tures are significantly tighter in the northern segment compared to the southern segment of the over-thrust trend. We estimate approximately 17 km of shortening in the northern duplex zone, and about26 km total shortening for the southern duplex zone. We propose that thin-skinned in-sequenceimbricate thrust stack deformation produced most of the shortening. The main Andean deformation (80%of the total shortening) commenced in the Piedemonte area about 6 Ma with rapid shortening and upliftin the area resulting in the development of an active-roof duplex structure as the cover was bulldozedforward by a horse block (Monterralo anticline) ramping up to a detachment at the base of C2, thenramping to the surface as the Yopal thrust. Later the horse blocks in the wedge rose, underthrusting thecover in a passive-roof duplex triangle zone. This was followed by an out-of-sequence Laramide-stylethick-skinned basement-uplift of the range which produced much of the structural relief of the EasternCordillera. Cenozoic deformation in the Eastern Cordillera has been primarily range-normal, but hasinvolved an increasing component of mountain-parallel right-lateral shear in the last 2 Ma.

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1. Introduction

The Piedemonte Llanero in the central Llanos foothills is a broadzone covering the area of Fig. 1 and containing a triangle zone. TheNEeSW trending central Llanos foothills form the frontal thrustzone along the eastern flank of the Eastern Cordillera of Colombia.The central Llanos foothills are bordered by the Yopal thrust faultsystem and the Llanos basin to the east, and by the Guaicaramofault system and the Eastern Cordillera to the west. The foothills aredivided into three main structural domains that exhibit increasingstructural complexity northeastward along the mountain front(Villamil et al., 2004; Martinez, 2006) (Fig. 2): a frontal invertednormal fault domain (Cusiana), a transition bedding plane thrustdomain (Cupiagua), and a wedge duplex domain (Piedemonte).Within the Piedemonte Llanero, the main surficial structures are

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., Kellogg, J., Three-dimensioolombia, Journal of South Am

the Nunchia syncline that folds outcropping Oligocene to Recentage rocks, and the Monterralo anticline, that folds outcroppingCretaceous to Oligocene age rocks (Fig. 1). The main hydrocarbonaccumulations in the Piedemonte Llanero are in the sandy units ofthe Late Cretaceous Guadalupe Group, Eocene Mirador Formationand Paleocene Barco Formationwithin the Floreña, Pauto, Dele, andVolcanera structures (Figs. 3e7). These wedge zone structures holdconsiderable hydrocarbon reserves due to the duplication ofreservoirs in the duplexes.

Exploration in the Piedemonte Llanero began in the early 1990swith the acquisition of 2-D seismic reflection lines and the drillingof several exploratory wells. Well data reveals tight asymmetricfolds with high dip angles (usually exceeding 50�). The quality ofthe 2-D seismic images is generally poor due to the complexity ofthe subsurface structures, rugged terrain, and the poor acousticproperties of the Mesozoic rocks, resulting in low signal-to-noiseratio in the acquired seismic data. Recent 3-D seismic reflectiondata have improved on the seismic image quality.

This work offers new insight into the 3-dimensional structureand kinematics of the Piedemonte Llanero. We present the first

nal structural evolution and kinematics of the Piedemonte Llanero,erican Earth Sciences (2012), doi:10.1016/j.jsames.2012.04.012

Delta:1_given nameDelta:1_given nameDelta:1_given nameDelta:1_given nameDelta:1_surnameDelta:1_given nameDelta:1_given nameDelta:1_given namemailto:[email protected]:[email protected]/science/journal/08959811http://www.elsevier.com/locate/jsameshttp://dx.doi.org/10.1016/j.jsames.2012.04.012http://dx.doi.org/10.1016/j.jsames.2012.04.012http://dx.doi.org/10.1016/j.jsames.2012.04.012

Fig. 1. Geologic map of the Llanos foothills study area showing the major structural features, the locations of cross-sections and wells used in the study. Geological data wasprovided by BP Exploration Company Colombia.

O. Egbue, J. Kellogg / Journal of South American Earth Sciences xxx (2012) 1e122

interpretation with multiple geologic profiles of the wedge duplexzone constrained by previously unpublished 2-D and 3-D seismicreflection data, well data, and detailed surface geology. The struc-tural analysis is based on kinematic restoration and forwardmodeling using Transfer-flexural slip and Fault slip fold algorithmsin Paradigm GeoSec�. We propose a thin-skinned in-sequenceimbricate thrust stack deformation for the Piedemonte Llanero

Fig. 2. Schematic cross-section illustrating the different structural styles in the Llanosfoothills (modified after Villamil et al. (2004) and Martinez (2006)). From right to left:a frontal inverted normal fault zone (Cusiana), a transition bedding plane thrust zone(Cupiagua), and a duplex zone (Piedemonte).

Please cite this article in press as: Egbue, O., Kellogg, J., Three-dimensioCentral Llanos foothills, Eastern Cordillera, Colombia, Journal of South Am

followed by thick-skinned out-of-sequence thrusting and uplift. Wepresent the first published shortening estimates for the Piede-monte Llanero wedge duplex zone. The main Andean deformation(80% of the total shortening) commenced in the Piedemonte areaabout 6 Mawith rapid shortening and uplift in the area resulting inthe development of an active-roof duplex structure as the coverramped to the surface as the Yopal thrust. Our multiple profiles ofthe Piedemonte Llanero show that structures are significantlytighter with less shortening in the northern segment compared tothe southern segment of the wedge duplex. Unlike previousinterpretations, we recognize the importance of fault-propagationfolding in the formation of the wedge duplex structures. We alsonote, for the first time in the foothills, evidence for trishear fault-propagation folding, in which decreased displacement along theblind El Moro fault is accommodated by heterogeneous shear ina triangular zone radiating outward from the fault tip. We alsopropose an increasing component of mountain-parallel right-lateral shear in the last 2 Ma.

2. Regional geologic setting

The Eastern Cordillera represents the leading edge of deforma-tion in the northern Andes (Corredor, 2003). It is interpreted asa wide extensional basin that evolved into a foreland basin and was


Fig. 3. Stratigraphic summary diagram for the Llanos foothills showing typical gamma ray log response, the key lithostratigraphic units/formations, proven reservoir rocks, sourcerocks, and regional seals. Stratigraphic information is from BP Exploration Company Colombia.

O. Egbue, J. Kellogg / Journal of South American Earth Sciences xxx (2012) 1e12 3

inverted during the Cenozoic Andean orogeny (Colletta et al., 1990;Dengo and Covey, 1993; Cooper et al., 1995; Roeder andChamberlain, 1995; Mora et al., 2006). The inversion of pre-existing extensional structures has been reported for numerous


other orogens (for example, Coward et al., 1989; Grier et al., 1991;Lowell, 1995).

During the Mesozoic era, the area of the modern EasternCordillera was an extensional basin. Northwestern South America


Fig. 4. End-member kinematic models for cover displacement and horse blockdisplacement. (a) Passive-roof duplex (Banks and Warburton, 1986), where the cover isunderthrusted by the horse blocks (b) Active-roof duplex, where the cover is bulldozedforward by the horse blocks. Models are from Couzens-Schultz et al. (2003).


was affected by rifting related to the break up of Pangea and theeventual separation of North and South America (Pindell andDewey, 1982; Ross and Scotese, 1988; Cediel et al., 2003). West-ward propagating intracontinental rifting between Yucatan andnorthern South America led to the proto-Caribbean seaway.However, since synrift continental clastic sediments are difficult todate, the exact timing of the onset of rifting is not well constrained,and basin geometries are still uncertain. Rifting in Colombiaestablished depocenters throughout the Eastern Cordillera area.Pre-rift units include LowereUpper Paleozoic rocks. The QuetameGroup, made up of possible Precambrian or CambrianeOrdovicianphyllitic rocks with a minimum thickness of 915 m (Campbell,1962) represents the lower unit of the pre-rift metamorphicrocks. An alternative mechanism for the rifting is back-arc exten-sion due to subduction rollback, with which the South Americancraton did not keep pace (Maze, 1984; Pindell and Erikson, 1993;Pindell and Tabbutt, 1995).

By the Early Cretaceous, considerable accommodation wascreated in the Eastern Cordillera. Late Jurassic to Early Cretaceousrifting generated basaltic intrusions, thinning of the lithosphere,and shallowing of the mantle (Ojeda, 1996). The rifting phase wasfollowed by thermal subsidence throughout the Cretaceous period(Pindell and Tabbutt, 1995; Ojeda, 1996).

From the Early Cretaceous to Late Cretaceous period, depositionin the Eastern Cordillera area was almost entirely marine. The UneFormation (Fig. 3) was deposited from Early Cretaceous until theCenomanian and is composed of sandstones deposited in shallowmarine conditions (Cooper et al., 1995; Linares et al., 2009). It isconsiderably thicker in the foothills and thins eastward in theLlanos basin. In the Late Cretaceous, global sea level rise (Haq et al.,


1987) combined with anoxic upwelling, created a favorable envi-ronment for deposition of the marine mudstones, cherts, andphosphates that make up the Gacheta Formation, a prolific sourcerock that is contemporaneous with La Luna Formation (Villamil andKauffman, 1993; Pindell and Tabbutt, 1995). A drop in sea levelduring the CampanianeMaastrichtian accompanied the depositionof the Guadalupe Group on a shallow marine shelf (Linares et al.,2009). The Guadalupe Group is predominantly a progradationalsandstone unit that progressively thins toward the east. LateCretaceous subsidence in the Eastern Cordillera area was mostlydriven by lithospheric cooling, with additional input from waterloading associated with eustatic sea level rise, and horizontalcompressional stresses resulting from the collision of oceanicterranes in western Colombia (Sarmiento-Rojas et al., 2006).

Deformation resulting from the accretion of the oceanic terranesof the Western Cordillera from Late Cretaceous to early Paleocenemarked a significant change from a shallow marine to continentaldepositional environment in an incipient foreland basin (Van derHammen, 1961; Sarmiento-Rojas et al., 2006). This foreland basinlikely developed as a result of the topographic load of the CentralCordillera, a volcanic arc that formed as part of the Farallon platesubduction complex along the west coast of Colombia (Cooperet al., 1995). Starting in the middle Paleocene, the Barco Forma-tion, a sandstone rich fluvial to estuarine deposit that is sourcedfrom the Guyana shield was deposited (Cooper et al., 1995). In thelate Paleocene, the Los Cuervos Formation, which is mostly shalewith intercalations of sandstones, siltstones, and coal beds (Linareset al., 2009), was deposited in a regressive mud dominated coastalplain setting (Cooper et al., 1995).

In the Eocene, the Mirador Formation was deposited in twostages interrupted by a regional unconformity (Martinez, 2006),with tidal channel sandstones overlapping fluvial channel sand-stones (Linares et al., 2009). The Carbonera Group (C8eC1), a cyclicsequence of continental to transitional sandstones and marineshales, was deposited in the late Eocene to late Miocene. A globalrise in sea level following the deposition of the Carbonera Group inthe middle Miocene (Haq et al., 1987) led to the deposition of theLeon Formation shales.

Based on detrital-zircon ages from the Carbonera Group in theeastern foothills of the Eastern Cordillera, Horton et al. (2010)concluded that uplift-induced recycling of basin fill was wellunder way by the latest Oligocene to early Miocene (26e23 Ma).However paleobotanical data (Wijninga, 1996; Gregory-Wodzicki,2000) and apatite fission-track ages and low e T thermochronol-ogy (Mora, 2007; Parra et al., 2009b; Mora et al., 2010b) indicatethat most of the shortening and uplift in the central and easternflank of the Eastern Cordillera occurred in the last 6 Ma. Paleobo-tanical data imply rapid uplift at a rate on the order of 0.6e3 mm/a (Gregory-Wodzicki, 2000).

By late Oligocene to early Miocene, the Farallon plate hadbroken up into the Nazca and Cocos plates and a new spreadingcenter formed, resulting in increased convergence rates at theMiddle and South American trenches (Wortel and Cloetingh, 1981;Wortel, 1984; Lonsdale, 2005). About 12 Ma the Panama arc begancolliding with the northern Andes, leading to the uplift of theEastern Cordillera, which became a sediment source (Gregory-Wodzicki, 2000). The timing, direction, and magnitude of short-ening indicate that the Panama arc collisionwas the probable causeof the Andean orogeny in the Eastern Cordillera. The thin litho-sphere and relatively ductile sediments of the Eastern Cordilleraand the foothills were caught in a vice between the rigid SouthAmerican backstop and the rigid Panama arc indenter. The collisioninitiated the inversion of Mesozoic extensional structures, uplift ofthe Eastern Cordillera and development of east-verging compres-sional structures in the Llanos foothills (Van der Hammen, 1961;


Fig. 5. 3-D seismic reflection volume through the Piedemonte showing some of the main structures in the duplex zone.


Cooper et al., 1995; Roeder and Chamberlain, 1995; Taboada et al.,2000; Sarmiento, 2001; Parra et al., 2009a; Mora et al., 2010b).Lateral changes in Mesozoic stratigraphic thicknesses suggest thatthe thrust faults that now define the eastern andwestern borders ofthe Eastern Cordillera were originally normal faults (Sarmiento-Rojas et al., 2006; Mora et al., 2006, 2009).

Deformation and uplift in the Eastern Cordillera is still ongoingtoday, resulting in earthquakes mainly in the foothills area. Recenterosion of the Eastern Cordillera is evident from the coarse grainedcontinental clastics of the Guayabo formation in the Llanos foothillsand Llanos basin (Cooper et al., 1995). GPS studies show thatPanama is actively colliding with South America at an average rateof 25 mm/a (Trenkamp et al., 2002). However, present-day short-ening in the Eastern Cordillera appears to have slowed to at most12 � 4 mm/a and on the east flank of the Cordillera to only4 � 2 mm/a (Trenkamp et al., 2002).

Approximately 2 Ma, the aseismic Carnegie ridge, which wasformed by the Galapagos hotspot at the CocoseNazca spreadingcenter, arrived at the ColombiaeEcuador trench (Egbue andKellogg, 2010). Increased coupling in the trench due to thesubduction of the buoyant aseismic Carnegie ridge is driving thepresent-day northeastward tectonic escape of the northern Andes.GPS results show that the northern Andes are escaping toward thenortheast at a rate of about 6 mm/a (Trenkamp et al., 2002).Geologic estimates indicate that slip rates of 4e10 mm/a continuedback to about 2 Ma (Egbue and Kellogg, 2010; Egbue, 2011). In theEastern Cordillera area, the rate of range-normal shortening stillexceeds the rate of escape, but range-parallel right-lateral shear hasbecome increasingly important in the last 2 Ma (Egbue and Kellogg,2010).

3. Structure of the Eastern Cordillera

The Eastern Cordillera of Colombia is an asymmetrical bivergentfan of thrusts toward the Middle Magdalena Valley basins and the


Llanos basin, containing a thick Jurassic and lower Cretaceousdepocenter (Campbell and Burgl, 1965; Julivert, 1970; Campbell,1974; Cooper et al., 1995). Thickness estimates for the Cretaceoussection in the Eastern Cordillera are not well controlled and varywidely. Restrepo-Pace (1989) estimated the thickness of the totalCretaceous section as approximately 5 km, based on structural andbiostratigraphic analysis of the lower Cretaceous rocks along theeastern flank of the Cordillera. On the western flank of the Cordil-lera, Cardozo (1988) measured the Cretaceous section as about8 km. Campbell and Burgl (1965) estimated the thickness of thetotal Cretaceous stratigraphic column as over 11 km near Bogotá. Itis important to note that thicknesses measured from outcrops inthe Eastern Cordillera are likely exaggerated due to structuralrepetition as demonstrated by Restrepo-Pace (1989).

Numerous structural models have been proposed for the EasternCordillera. Based on a retrodeformed cross-section through theEastern Cordillera, Colletta et al. (1990) proposed that the Cordillerawas formed by the inversion of two deep Upper JurassiceLowerCretaceous basins during MioePliocene times. In this model, thestructure is controlled by low angle thrusts and reactivated normalfaults that constitute frontal ramps. They estimated a total short-ening of at least 105 km with a decollement at a depth of about20 km. Dengo and Covey (1993) proposed that basement-detached(“thin-skinned”) shortening was followed by uplift on high-anglebasement-involved reverse faults (“thick-skinned”) deformationand estimated approximately 150 km of shortening (40%) froma regional retrodeformable cross-section. Cooper et al. (1995)proposed that deformation in the Llanos foothills was predomi-nantly inversion of pre-existing extensional faults by basement-involved “thick-skinned” listric reverse faults. They estimatedonly 68 km of shortening.

Martinez (2006) proposed two main phases of Cenozoic defor-mation in the Llanos foothills, incipient shortening during thedeposition of the Lower Carbonera (C8eC6) and the main Andeanorogeny, starting around 7e5 Ma. He proposed an overthrust


Fig. 6. Depth slices through the Piedemonte showing the major structural features at(a) 2270 m, El Morro anticline, (b) 4300 m, Floreña anticline, and (c) 5280 m, Pautoanticline.


stacked duplex model for the wedge zone with short back limbs onthe stacked thrusts. However, Martinez did not publish cross-sections for the northern or southern parts of the wedge zone thatdemonstrate variations in shortening along strike, quantify short-ening, or provide a structural explanation for the latest uplift of theCordillera. Mora et al. (2010a) published one regional cross-sectionof the Llanos foothills showing sequential deformation based onapatite and zircon fission-track analysis and regional structuralrelationships. Mora et al. (2010a) show most of the deformationoccurring in the last 8 Ma, but the profile lacks detail and does notshow control from seismic, well data, and surface geology.

4. Triangle zones, passive and active-roof duplexes

Triangle zones are a common structural feature found at theleading edges of fold and thrust belt systems, located between themost recent exposed thrust and the frontal syncline in the foreland


(Boyer and Elliott, 1982; Jones, 1982; Banks and Warburton, 1986;Vann et al., 1986; Álvarez-Marrón et al., 1993; Valderrama et al.,1996). The Piedemonte Llanero has been interpreted as an active-roof duplex and “triangle zone” by Martinez (2006). Some authors,however, restrict the use of the term “triangle zone” to passive-roofduplexes only (e.g., Jones, 1996).

4.1. Passive-roof duplex

In a passive-roof duplex (Fig. 4a), the cover is underthrust by thehorse blocks, most of the shortening is accommodated by delami-nating the cover and no significant displacement is transmittedforelandward of the duplex leading edge (Jones, 1982; McMechan,1985; Banks and Warburton, 1986). The cover is “passively upliftedby thrust wedging beneath (Jones, 1996) and may be folded orthrust to accommodate shortening above the roof thrust. TheFoothills in Alberta, Canada is an example of a passive-roof duplextriangle zone.

4.2. Active-roof duplex

In an active-roof duplex (Fig. 4b; Couzens-Schultz et al., 2003)the cover is bulldozed forward by the horse blocks. An importantamount of shortening is transferred to the cover and accommo-dated by structures forelandward from the duplex leading edge (nosignificant delamination occurs). Active-roof duplex zones arefound in the southern Pyrennes, Spain and the Virginia Appala-chians (Couzens-Schultz et al., 2003).

Duplex zones produce multiple trapping configurations, but aredifficult to image seismically because of steep bedding and faultdip angles. Exploration in these areas has therefore, relied heavilyon structural models, and drilling usually reveals structuralsurprises. The Piedemonte Llanero duplex zone is bounded by theGuaicaramo thrust fault to the west, the bedding plane detach-ment of the Nunchia syncline above and to the east, and theYopal fault at the base (Fig. 7; Martinez, 2006). Based on seismicreflection profiles and well data from the Piedemonte, we proposethat the structure developed as an active-roof duplex during thePlioceneePleistocene, followed by passive-roof delamination anduplift of the cover over the thrust wedge.

5. Data & Methodology

For this research, we used a recent 3-D seismic reflection datasetprovided by BP Exploration Company Colombia. The migrated 3-Ddata has a lateral coverage of 17 � 37 km with 939 NW-SE inlinesand 850 NE-SW cross-lines. The survey configuration consisted of17 shots per km2 in a subsurface cell of 20 � 40 m, with a nominal48-fold multiplicity and 60,000 traces per km2. For the source,5e10 kg of dynamite was used in a 17 m deep single hole witha total of 17 shot points per km2. The data was processed by CGGVeritas Argentina for BP Exploration Company Colombia.

BP Exploration Company Colombia also provided the authorswith several 2-D seismic reflection lines that were acquired in the1990s. For this study, we selected seismic line PT90-379 (Line 7b inFig. 1) that traverses the triangle zone. The location of this seismicline allows us to constrain our interpretation using well data fromtwo deviated wells located adjacent to the profile (Fig. 1) andextend the interpretation outside the area covered by the 3-Dseismic survey. Some of the limitations of the 2-D seismic reflec-tion data related to the difficulty in imaging complex subsurfacestructures and low signal-to-noise ratio are resolved with the new3-D data (Fig. 5). Depth slices through the 3-D volume (Fig. 6)illustrate three structures in map view.


Fig. 7. Geologic cross-sections through the Piedemonte Llanero with the seismic reflection profiles, well data, and surface geology used to constrain the interpretations. Cross-sections were constructed using Paradigm GeoSec 2D. (a) 3-D Inline 201, (b) 2-D Line PT90-379, and (c) 3-D Inline 608. Locations of the cross-sections are shown in Fig. 1.


The following interpretations are constrained by a detailedsurface geology map of the Llanos foothills and well data (beddingdips and formation tops. Bedding dips are from Stratigraphic High-resolution Dipmeter Tool (SHDT), Ultrasonic Borehole Imager (UBI),Fullbore Formation MicroImager (FMI), surface geology, andseismic profiles. Where two wells penetrate clearly imaged seismicreflectors in the Nunchia Syncline (Fig. 7b and c), the dipmeter datacorrelates well with the dips of the depth-corrected seismicreflectors. This correlation supports the reliability of the dipmeterdata to reflect true bedding dips with errors of a few degrees. Time-to-depth conversion of the migrated 3-D and 2-D seismic reflectiondata was performed using Hampson-Russell Strata�. The velocitymodel used for the depth conversion was built using P-wavevelocities from eight deviated wells in the study area. Preliminaryinterpretations of the seismic depth sections were then developedusing Schlumberger Petrel�. Balanced cross-section construction,structural analyses, and modeling were performed using ParadigmGeoSec 2D and 3-D Canvas. Transfer-flexural slip and Fault slip foldalgorithms were used for the kinematic restoration and forwardmodeling in GeoSec 2D.

5.1. Data preparation

Selected inlines were extracted from the 3-D seismic volumeusing Schlumberger Petrel. They were then exported as 2-D SEGYfiles. The exported inlines 201 (Fig. 1, line 7a) and 608 (line 7c) and2-D line (PT90-379; line 7b) and adjacent deviated wells withformation tops and dipmeter data were then imported into Geo-Sec 2D. Following this, well data (trajectory, formation tops, andbedding dips) were projected onto adjacent seismic cross-sections. The program automatically converts the true dipmeasurements associated with thewell data into apparent dips onthe section plane, and the 3-D deviated boreholes are projectedinto a 2-D deviated borehole trace. We created a stratigraphiccolumn for the Piedemonte Llanero in GeoSec 2D. The thicknessesassigned to each stratigraphic unit were based on data from BPExploration Company Colombia and Bayona et al. (2008). Groundelevation and surface geologic information (strike and dip


measurements, geologic contacts, and fault traces) for the Piede-monte were entered into GeoSec 2D and projected onto the cross-sections.

5.2. Cross-section construction

Our interpretations of the seismic reflection cross-sections areconstrained by well data and surface geology. Constant beddingthicknesses and layer parallel slip, the initial structural approxi-mations, weremodified as needed. Uniform dip domain boundariesand consistent dip domain geometries were chosen whereverpossible. The simplest structural solutions with the minimumamount of shortening consistent with the data were selected. Thegreatest sources of error in the interpretations were the poorseismic resolution and limited well control for the cross-sections.As mentioned previously, the lack of resolution is due to thecomplex structural geometry, the high dip angles, and low signal-to-noise ratio. After constructing the cross-sections, we builta 2.5-D model of our interpretation of seismic line PT90-379 (line7b) using Paradigm 3D Canvas.

5.3. Kinematic restoration

Following our interpretation and cross-section construction,we restored line PT90-39 using the Transfer-Flexural slip modulein GeoSec 2D to test the feasibility of our structural model. TheTransfer-Flexural Slip algorithm folds rocks along sliding surfacesthat are parallel, or close to parallel, to bedding. The section waskinematically restored to the top of the Eocene Mirador Forma-tion. We chose line 2D PT90-379 (line 7b) for restorationbecause, unlike inlines 201 (7a) and 608 (7c), this profile clearlyimages the relatively undeformed footwall block (Llanos basin).The footwall block provides us with a pre-deformation referenceand target surfaces for retrodeformation. Finally the restoredsection was forward modeled using Transfer-flexural slip andFault slip fold algorithms until the present-day deformation wasduplicated.



6. Structural interpretation

In order to better understand the structural geometry and alongstrike variations in the Piedemonte Llanero triangle zone, weinterpreted three NWeSE dip seismic cross-sections from thenorthern and southern segments of the overthrust trend (Fig. 7).The cross-sections traverse the Guaicaramo fault and the Nunchiasyncline and are constrained by detailed surface geology, well data,and 3-D and 2-D seismic reflection profiles. Line PT90-379 (Fig. 7b)is 21 km long and based on a 2-D reflection profile that extends intothe Llanos basin. In-lines 201 (Fig. 7a) and 608 (Fig. 7c) are bothabout 17 km long, and are from the 3-D volume. Given the seismicambiguities, the availability of well control was key in determiningwhich seismic lines were selected for interpretation and modeling.

6.1. Northern cross-sections

6.1.1. Inline 201This cross-section (Fig. 7a) is located in the northern segment of

the Piedemonte Llanero (Fig. 1). It traverses a structural high in thenorthwestern corner that thrusts Cretaceous rocks over the Mon-terralo anticline along the NE-SW striking Chameza fault. It alsotraverses the Guaicaramo fault and extends into the Nunchiasyncline. The main structural features in the cross-section are theeast-verging Monterralo, El Morro, Floreña, and Pauto anticlinesand the Nunchia syncline. Deformation in this cross-sectioninvolves sediments from the Lower Cretaceous (Fomeque Forma-tion and older) to the upper Miocene to Recent Guayabo Formation.The main fault detachments are in the shaly units of the basalCarbonera Group, the Gacheta Formation, and the Lower Creta-ceous. There is seismic evidence for a number of major unconfor-mities within the lower Carbonera Group, including anunconformity at the base of C5 in the Nunchia syncline. Theseunconformities are also observed in the wells. There isa pronounced lateral variation in the thicknesses of the lowermembers of the Carbonera Group, especially C7. The Cretaceoussection is considerably thicker in the imbricated tectonic wedgethan in the footwall block (Llanos basin). We interpret the Mon-terralo and El Morro anticlines as fault-propagation folds (Suppeand Medwedeff, 1984), while the Floreña and Pauto anticlines areinterpreted as fault-bend folds (Suppe, 1983). Monterralo and ElMorro are asymmetrically faulted anticlines with steep overturnedforelimbs (up to 85�) and elongated backlimbs with estimatedramp dips of 40�e50�. The El Morro forelimb is overturned in thewell. The overturned forelimbs suggest that the folds formed asthe result of fault-propagation followed by forelimb breakthroughfaulting. Monterralo and El Morro have been shortenedapproximately 5.5 km and 2.5 km, with about 4 km of verticalrelief on each structure. The deeper structures, the Floreña andPauto anticlines, are characterized by elongated moderatelydipping forelimbs and shorter backlimbs. We estimateapproximately 5 km and 4 km of shortening for Floreña andPauto structures respectively with vertical reliefs of about 3 and2 km. We estimated ramp dips of approximately 45� degrees forFloreña and 20� for Pauto. The minimum total shortening neededto produce the four imbricated anticlines in the cross-section isapproximately 17 km. In addition there was at least 6e8 km ofshortening on the Chameza fault plus a minimum of 4 kmadditional slip on the Yopal thrust (see Fig. 7b) from a beddingplane detachment at the base of C2 for aminimum total of 27 km. InFig. 7a the Chameza fault is depicted as an out-of-sequence faultwhich cuts the core of the Monterralo anticline. However, theChameza fault may be just as easily interpreted as an in-sequencefault that gently ramps up from a detachment at the base of C7 toa detachment at the base of C2 and was subsequently uplifted by


the Monterralo anticline. The Yopal thrust bedding plane slip at thebase of C2 postdated deposition of the lower Guayabo Formation.The base C2 slip formed a roof décollement for the duplex (Fig. 4)that may have been the upper flat for the Monterralo ramp anti-cline breakthrough fault. Approximately 3 km of the 11.5 km duplexzone shortening in the El Morro, Floreña and Pauto anticlines istaken up in the Yopal thrust. The remaining 8.5 km is predicted tohave gone into a) displacement prior to the base C6 unconformityon the El Moro and Floreña breakthrough faults and b) backslip orpassive-roof delamination on the base C6 backthrust at the roof ofthe triangle zone. The southeast dipping Lower Cretaceous unit inthe lower northwest corner of the cross-section represents thepredicted forelimb of an out-of-sequence basement-involved rampanticline rooted deep in the lower crust below the Eastern Cordil-lera. The predicted deep southeast dipping forelimb is also shownin the regional interpretations of Dengo and Covey (1993), Cortéset al. (2006), Bayona et al. (2008), and Mora et al. (2010a). Dengoand Covey (1993) and Bayona et al. (2008) explain it as the fore-limb of a mid crustal fault-bend fold similar to our model. TheCortés et al. (2006) and Mora et al. (2010a) listric reverse faultmodels provide a structural explanation for the uplift, but not forthe southeast dipping limb. This late stage “thick-skinned”Laramide-style uplift (Bayona et al., 2008; see De Toni and Kellogg,1993 for “thick-skinned” deformation references and examples)provides 5e6 km of the structural relief for most of the EasternCordillera.

6.1.2. Line PT90-379This cross-section (Fig. 7b) is located in the northern segment of

the Piedemonte Llanero south of inline 201. This is a 2-D seismicprofile that extends beyond the 3-D survey volume, traversing theYopal fault and extending into the Llanos basin. This cross-sectionhas better well constraint than inline 201. In this cross-section,just like in inline 201, the Monterralo, El Morro, and Floreña anti-clines are interpreted as fault-propagation folds, while the Pautostructure is interpreted as a fault-bend fold. The structure inter-preted from two wells that penetrate the overturned forelimb ofthe ElMorro anticline can bemodeled by trishear fault-propagationfolding (Erslev, 1991; Shaw et al., 2005), in which decreaseddisplacement along the fault is accommodated by heterogeneousshear in a triangular zone radiating outward from the fault tip. Weestimate at least 2.6 km of shortening for the Monterralo structure,3 km of shortening for the El Morro structure and about 4 km ofvertical relief for each. We predict ramp dips of approximately40�e50� for Monterralo and El Morro based primarily on surfacedips exposed up plunge to the south southwest. The deeperFloreña structure is a tight fault-propagation fold witha breakthrough fault at the top of its overturned forelimb. Wepredict a ramp dip of about 50�, shortening of about 2.3 km, andvertical relief of over 2 km. Well data shows that the Pautoanticline is a gentle fold, probably produced by a fault-bend. Thefold was faulted into two levels: Upper Pauto and Lower Pautowith a total vertical relief of about 1.3 km and 30�e40� ramp dip.The Upper Pauto has been shortened approximately 1.5 km andthe Lower Pauto about 3.0 km. The beds in the Llanos basin arerelatively undeformed and dip toward the northwest due toflexural loading of the lithosphere by the weight of the growingmountain belt (Salvador, 1991; Ojeda and Whitman, 2002). TheNunchia syncline is asymmetrical, formed by the Yopal ramp thrustto the southeast and the wedge of the triangle zone to the north-west. At least 4 km of the 6 km minimum slip on the Yopal fault,ramps up from a bedding plane detachment at the base of C2 in theCarbonera Group, which may have ramped up from the Monterralobreakthrough fault. The remaining 2 km of displacement on theYopal fault ramps up from a detachment in the Lower Cretaceous


Fig. 8. Perspective view of the northern segment of the Piedemonte Llanero illus-trating the horse block geometry of the triangle zone. This 2.5-D model was generatedusing Paradigm GeoSec 2D and 3D Canvas.


rocks, inverting a Lower Cretaceous normal fault (Mora et al.,2010a). Of the 10 km wedge zone shortening in the El Morro,Floreña and Pauto anticlines, the remaining 8 km is predicted tohave gone into (a) displacement prior to the base C6 unconformityand (b) backslip on the base C6 backthrust at the roof of the trianglezone. The Yopal fault trace marks the eastern edge of the Piede-monte Llanero, separating the Llanos foothills from the relativelyundeformed Llanos basin. With 6 km of slip on the Chameza fault,we estimate minimum total shortening in the cross-section as24.5 km. The southeast dipping Lower Cretaceous unit in the lowernorthwest corner of the cross-section is the predicted forelimb ofa basement-involved ramp anticline uplifting the Eastern Cordillera5e6 km (Dengo and Covey, 1993; Bayona et al., 2008).

6.2. Southern cross-section

6.2.1. Inline 608This dip cross-section (Fig. 7c) is located in the southern

segment of the Piedemonte Llanero and is parallel to inline 201. It isconstrained by the 3-D seismic volume, surface geology, and wellcontrol of the Dele, Pauto, and Volcanera structures below theNunchia syncline. This cross-section traverses a syncline on thehanging wall of the Chameza fault, the Monterralo anticline, theGuaicaramo fault, and the Nunchia syncline. The wells reveal thatthe basal members of the Carbonera Group (C7 and C8) thickensubstantially northwestward. There is also evidence of an uncon-formity at the base of C5. Along this profile theMonterralo anticlineis interpreted as a fault-bend fold with a forelimb breakthroughfault, and an estimated 30�e35� ramp dip. Structural shortening isestimated as approximately 6.5 km with about 5 km of verticalrelief. Based on well data, the El Morro anticline is modeled asa fault-propagation fold, but it is not as tightly folded as in thetwo northern cross-sections. It has a ramp dip of about 30� and hasbeen shortened approximately 6 kmwith 4e5 km of vertical relief.The Floreña structure dies out southward and does not extend intothis cross-section. This section reveals two new structures: the Deleand Volcanera anticlines, with the Pauto structure sandwichedbetween them. These three deeper structures are modeled asgentle fault-bend folds characterized by elongated backlimbs andmoderately dipping forelimbs. Dele has been shortened approxi-mately 1.4 km on a 45� ramp producing about 1 km of vertical relief.Pauto has been shortened about 7 km on a 28� ramp producing2 km of vertical uplift. Volcanera, the deepest structure, has a lowangle 14� ramp dip, 1.3 km vertical relief, and about 5.5 km ofshortening. Extrapolating from the middle profile, 4 km of beddingplane slip from the base of C2 is assumed for the Yopal thrust. TheYopal thrust is also displaced an additional 2 km from the under-lying triangle zone. Of the 20 kmwedge shortening in the El Morro,Dele, Pauto, and Volcanera anticlines, the remaining 18 km is pre-dicted to have gone into a) displacement prior to the base C6unconformity and b) backslip or passive-roof delamination on thebase C6 backthrust at the roof of the triangle zone. With an esti-mated shortening of at least 5 km on the Chameza fault, totalshortening for the section is over 35 km. This shortening estimate issignificantly higher than the estimates for the northern cross-sections, but is highly dependent on relatively unconstrainedmodels of footwall structures. As in the two northern cross-sections, the southeast dipping Lower Cretaceous unit in thelower northwest corner of the cross-section is the predicted fore-limb of a lower crustal basement-involved ramp anticline (Dengoand Covey, 1993; Bayona et al., 2008).

We interpret the Piedemonte duplex zone as a series of imbri-cated fault-propagation folds and fault-bend folds with steeplydipping, sometimes overturned forelimbs. Well data shows thatstratigraphic units in the El Morro structure are inverted in the


forelimbs with variable bedding thicknesses suggesting trisheardeformation (Erslev, 1991; Shaw et al., 2005). Andean age defor-mation in the Piedemonte Llanero duplex zone began with a post-early Guayabo Fm (Pliocene-Pleistocene) active-roof duplex, as thecover was bulldozed forward by a horse block (Monterralo anti-cline) ramping up to a detachment at the base of C2 (not shown inFigs. 8 and9), then ramping to the surface as the Yopal thrust. Laterthe horse blocks in the triangle zone rose, underthrusting the coverin a passive-roof duplex. The cover back thrust took place ona detachment at the base C6 unconformity. The complexity of thestructures increases from the southern segment of the trianglezone (Fig. 7c) to the northern segment (Fig. 7a and b) with foldinggenerally tighter in the northern cross-sections. The amount ofshortening in the triangle zone appears to decrease from the southto the north, withmore of the deformation in the northern segmentgoing into folding and uplift. The basal members of the CarboneraGroup (C7 and C8) thicken northwestward (Fig. 7c). This thickeningmight represent pre-C6 (Oligocene) foreland basin deposition andflexural loading to the west. This deposition was followed byfolding observed under the base C6 angular unconformity in allthree sections. The steep eastern flank of the Nunchia syncline isthe hanging wall of the Yopal fault ramp, while the gentler westernflank is the passive-roof of the duplex stack in the triangle zone.

Thin-skinned in-sequence imbricate thrust stack deformationproduced most of the shortening in the Eastern Cordillera. Subse-quent Laramide-style “thick-skinned” basement-uplift producedmuch of the structural relief in the range. The SE dipping units inthe lower left corner of all three cross-sections compose the fore-limb of a ramp anticline rooted deep in the crust. The basementreverse faulting uplifted the Eastern Cordillera and to a lesserextent the Monterralo anticline and triangle zone, as seen in thekinematic forward model of the Piedemonte Llanero (Fig. 9). Fig. 8shows a simplified 2.5-D model for the northern segment of thePiedemonte Llanero based on cross-section PT90-379 (Fig. 7b). Thisperspective view for the top of theMirador formation illustrates thehorse block geometry within the duplex zone.

7. Cenozoic structural evolution

Fig. 9 shows the structural evolution of the Piedemontetriangle zone in the central Llanos foothills based on kinematicforward modeling of 2-D seismic line PT90-379. The PiedemonteLlanero and central Llanos foothills evolved through several


Fig. 9. Tectonic evolution of the Piedemonte Llanero (from Late Eocene to Present)based on retrodeformation of seismic line PT90-379 (Fig. 7b). Kinematic restorationand forward modeling were performed using Transfer-flexural slip and Fault slip foldalgorithms in Paradigm GeoSec 2D. Note that this figure is simplified and does notshow the Late MioceneePleistocene base C2 active-roof décollement for the duplexzone and subsequent base C6 passive-roof décollement (Figs. 4 and 7).



stages of major Cenozoic deformation, following the depositionof the basal members of the Carbonera Group (C8eC6) approxi-mately 35 Ma.

7.1. Late Oligocene (25 Ma)

As noted earlier, based on detrital-zircon ages from the Car-bonera Group in the eastern foothills of the Eastern Cordillera,Horton et al. (2010) concluded that uplift-induced recycling ofbasin fill was well under way by the latest Oligocene to earlyMiocene (26e23 Ma). Corredor (2003) estimated over 50 km ofOligocene ENE-WSW shortening in the northern Eastern Cordil-lera. The Oligocene age deformation is defined in the presenteastern foothills area by the base C6 unconformity (Fig. 7),marking an important stage of Cenozoic crustal shortening in thepre-Eastern Cordillera foreland basin (Corredor, 2003). The baseC6 unconformity is a striking feature apparent in all threeprofiles in Fig. 7 as a seismic angular unconformity, usually asa zone of abrupt changes in well dips, and by pronounced vari-ations in thicknesses of C7. The late Oligocene folding andthrusting and subsequent erosion produced the observed C7thickness variations in the study area. This period marks theearliest stages of deformation in the Monterralo, El Morro, andFloreña structures. This Oligocene deformation event is alsorecognized in the northern Eastern Cordillera by Corredor (2003)based on geologic mapping, seismic reflection profiles, andbiostratigraphic data. Our evolutionary model is similar to that ofMartinez (2006), but we attempt to quantify shortening andpropose a structural explanation for the latest uplift of theCordillera. Late Oligocene shortening in the Piedemonte Llanerois estimated as approximately 4 km, and represents about15e20% of the total shortening in the area for the Cenozoic.Between 25 and 10 Ma there was very little tectonic activity inthe area (e.g., Corredor, 2003). The upper members of the Car-bonera Group and the Leon Formation were deposited duringthis period.

7.2. Late MioceneePleistocene Andean deformation (10 Ma topresent)

The main Andean deformation commenced in the Piedemontearea about 6 Ma (Late Guayabo Formation age) with rapidshortening and uplift in the area resulting in the development ofthe duplex structures and folding of the roof thrust to createa frontal syncline in the foothills with the breakthrough of theYopal thrust to the surface. The deformation age is demonstratedby the conformable folding of the lower Guayabo Fm with noevidence of syn-sedimentary deformation. As noted earlier, thisdeformation stage began with a bedding plane thrust at the baseof C2 that ramped to the surface as the Yopal fault. This beddingplane thrust may have been the upper flat for the break throughfault ramping up under the Monterralo fold (Fig. 7). Southeast ofthe Yopal fault, the Llanos basin remained largely undeformedwith the Mesozoic normal faults inhibiting progradation alongbedding plane detachments. During this stage of deformation,sediments of the Guayabo formation eroded from the rapidlyuplifting Eastern Cordillera to the west were deposited in thedeveloping Nunchia syncline and Llanos basin. Deposition of theGuayabo formation, shortening and uplift continued in the areathroughout the Quaternary. The Pauto structure developed, andthe Monterralo, El Morro, and Floreña folds became tighter. Thissection through the Piedemonte has undergone a minimum of17 km (40%) shortening (not including 6 km slip on the Chamezafault) since the Late Eocene.



7.3. Basement block thick-skinned deformation

One of the latest structural events affecting the Piedemonte areaduring the Andean stage of deformation was the thick-skinnedbasement block uplift of the Eastern Cordillera. There is no directevidence for this in the seismic cross-sections. This may be due tothe lack of deep seismic image resolution underneath the imbricatethrust stack in the Piedemonte. The basement block crustal scalefault-bend fold is proposed as the most likely solution to explainthe relatively uniform structural uplift of the entire mountain rangeto the northwest of the Foothills, similar to the solutions of Dengoand Covey (1993) and Bayona et al. (2008).We estimate that thisuplift accounts for approximately 5e6 km of the total of 10 km ofstructural uplift relative to the flanking basins. This structure isassumed to be a crustal scale ramp anticline analogous to theWind River Range (Berg, 1962; Smithson et al., 1978) and theVenezuelan Merida Andes (Kellogg and Bonini, 1985; De Toni andKellogg, 1993). The basement block listric fault models of Cortéset al. (2006) and Mora et al. (2010a) explain the uplift, but notthe southeast dipping forelimbs shown in their cross-sections.Low-angle basement thrust models can require as much as190 km of shortening (e.g., Roeder and Chamberlain, 1995). Weassume a conservative solution with late stage mountain scalefault-bend folding that explains the observed uplift and forelimb.For a fault-bend fold with 5 km vertical relief, a forelimb dip of 23�,and 12� upper ramp dip (Fig. 7b), the structurewould require 10 kmof additional late-stage shortening on a 29� lower ramp (Suppe,1983). There does not appear to be good evidence for an outcrop-ping crustal ramp thrust as in the eroded Wind River Range, so wepostulate a blind wedge fault geometry similar to theMerida Andes(De Toni and Kellogg, 1993). The SE dipping forelimb of the crustalramp is shown in the lower left side of the three cross-sections.

8. Conclusion

The cross-sections and 3-D model presented here providesa newmodel for the kinematic evolution of structures in the duplexzone of the Piedemonte Llanero, based on new 3-D seismic andwelldata. The duplex zone is characterized by a series of as many as fiveeast-verging stacked thrust sheets. To the northeast, the structuresbecome tighter, developing steeper forelimbs, and in someinstances they become overturned. We have modeled the Piede-monte duplex zone structures as fault-propagation folds and fault-bend folds characterized by back limbs truncated by ESE-dippinglimbs which limit the predicted shortening. The deformation ofthe foothills occurred in several stages that controlled its evolutionand present-day geometry. The main Andean deformation (80% ofthe total shortening) commenced about 10 Ma with rapid short-ening and uplift in the area resulting in the development of anactive-roof duplex structure. Later the horse blocks in the trianglezone rose, underthrusting the cover in a passive-roof duplex. Thelatest stagewas an out-of-sequence Laramide-style “thick-skinned”basement-uplift of the range which provided most of the uplift inthe area. The Piedemonte area has been shortened by over 40%since the late Eocene. In the last 2 Ma range-parallel right-lateralshear has become an increasingly important component of struc-tural deformation in the Piedemonte. Further drilling will help todefine the complex structural evolution of the wedge duplex zone,and further GPS measurements will map the present-day strain inthe Piedemonte.

Acknowledgments

This research was supported by a grant from BP ExplorationCompany Colombia (now Equion Energia Limited). The authors are


grateful to Hector Aguirre, Juan Carlos Alzate, and Roberto Linaresof Equion Energia Ltd. for helpful discussions and Equion EnergiaLtd. for making the data available for this study. Andrés Mora andan anonymous reviewer provided thorough reviews that improvedthe manuscript. Camelia Knapp and David Barbeau of the Depart-ment of Earth and Ocean Sciences at the University of South Car-olina provided helpful comments and recommendations. MikeWaddell and John Shafer of the Earth Sciences and ResourcesInstitute at the University of South Carolina kindly granted usaccess to geophysical software and applications. We acknowledgeParadigm Geophysical, Schlumberger, and CGGVeritas for theirsoftware donations through the University Grants Program.

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Three-dimensional structural evolution and kinematics of the Piedemonte Llanero, Central Llanos foothills, Eastern Cordille ...1. Introduction2. Regional geologic setting3. Structure of the Eastern Cordillera4. Triangle zones, passive and active-roof duplexes4.1. Passive-roof duplex4.2. Active-roof duplex

5. Data & Methodology5.1. Data preparation5.2. Cross-section construction5.3. Kinematic restoration

6. Structural interpretation6.1. Northern cross-sections6.1.1. Inline 2016.1.2. Line PT90-379

6.2. Southern cross-section6.2.1. Inline 608

7. Cenozoic structural evolution7.1. Late Oligocene (25 Ma)7.2. Late Miocene–Pleistocene Andean deformation (10 Ma to present)7.3. Basement block thick-skinned deformation

8. ConclusionAcknowledgmentsReferences

journal of south american earth sciences - andean geophysical and... · late jurassic to early...

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