cooper etal,1995

23
ABSTRACT The Middle Magdalena Valley, Eastern Cordillera, and Llanos basin constituted a major regional sedimentary basin from the Triassic to the middle Miocene. Basin development began during the Triassic to the earliest Cretaceous with a syn- rift megasequence related to the separation of North and South America in the proto-Caribbean. The synrift megasequence began with deposition in a continental environment that became paralic and shallow marine in the Early Cretaceous. Basin development continued into the Cretaceous in a back-arc setting east of the Andean subduction zone. The back-arc megasequence was dominated by shallow-marine sedimentation and produced an excellent regional source rock during the Turonian–Coniacian. Marine deposition was abruptly terminated during the early Maastrich- tian due to the final accretion of the Western Cordillera. Accretion of the Western Cordillera created the early pre-Andean foreland basin megasequence of late Maastrichtian to early Eocene age. This depo- sitional episode consists of coal-rich alluvial plain, coastal plain, and estuarine deposits throughout the Middle Magdalena Valley, Eastern Cordillera, and eastern Llanos basin. The megasequence was terminated by middle Eocene deformation in the Magdalena Valley, which ended sediment deposi- tion throughout Colombia. Loading effects of this deformation reestablished the basin, in which the late pre-Andean foreland basin megasequence was deposited, until the early Miocene. This megasequence also consists of alluvial plain, coastal plain, and estuarine deposits, including the primary reservoir in the Llanos Foothills—the upper Eocene Mirador Formation. The megasequence also includes a series of four major grossly coarsening-upward cycles in the Llanos basin; these cycles corre- spond to changes in sea level, sediment supply, and foreland basin loading. The mudstone in the lowermost of these cycles is the regional seal in the Llanos basin and Foothills. The middle Miocene onset of Andean deforma- tion in the Eastern Cordillera isolated the Middle Magdalena Valley from the Llanos basin. The defor- mation was dominated by inversion of the basin- controlling faults; the resultant loading of the litho- sphere created the accommodation space for the Andean foreland basin megasequence. A major transgression into the Llanos basin coeval with this deformation caused deposition of marine mud- stones in the lower part of the megasequence. However, the majority of the Andean foreland basin megasequence consists of the Guayabo Formation, a classic molasse sequence, deposited in a high- energy, coarse-grained, bed-load–dominated fluvial system that was supplied by the developing moun- tains of the Eastern Cordillera. INTRODUCTION The physiography of Colombia is dominated by the Andes mountains in the west and the Amazon/Orinoco basin in the east. The Colombian Andes form three separate ranges, the Western, Central, and Eastern Cordilleras, which merge south- ward into a single range. The Cauca valley separates 1421 AAPG Bulletin, V. 79, No. 10 (October 1995), P. 1421–1443. ©Copyright 1995. The American Association of Petroleum Geologists. All rights reserved. 1 Manuscript received June 10, 1994; revised manuscript received March 14, 1995; final acceptance June 6, 1995. 2 PanCanadian Petroleum, 150 9th Ave. SW, Calgary, Alberta, Canada T2P 2S5. 3 BP Exploration (Colombia) Ltd., Carrera 9A no. 99-02, Piso 9, A.A. 59824, Bogotá, Colombia. We wish to thank BP Exploration (Colombia) Ltd. for permission to publish this paper and Olga Lucia Daza for drafting the figures. We are also grateful to Jaime Buitrago and Kevin Biddle for constructive and helpful reviews and to John Weissenberger for reading numerous drafts of the revised manuscript. An extended bibliography is available on diskette (Macintosh or Windows). Send your check, made out to AAPG, for $3.00 to Publications Manager, P.O. Box 979, Tulsa, Oklahoma 74101-0979. Basin Development and Tectonic History of the Llanos Basin, Eastern Cordillera, and Middle Magdalena Valley, Colombia 1 M. A. Cooper, 2 F. T. Addison, 3 R. Alvarez, 3 M. Coral, 3 R. H. Graham, 3 A. B. Hayward, 3 S. Howe, 3 J. Martinez, 3 J. Naar, 3 R. Peñas, 3 A. J. Pulham, 3 and A. Taborda 3

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Page 1: Cooper Etal,1995

ABSTRACT

T h e M i d d l e M a g d a l e n a Va l l e y, E a s t e r nCordillera, and Llanos basin constituted a majorregional sedimentary basin from the Triassic to themiddle Miocene. Basin development began duringthe Triassic to the earliest Cretaceous with a syn-rift megasequence related to the separation ofNorth and South America in the proto-Caribbean.The synrift megasequence began with depositionin a continental environment that became paralicand shallow marine in the Early Cretaceous. Basindevelopment continued into the Cretaceous in aback-arc setting east of the Andean subductionzone. The back-arc megasequence was dominatedby shallow-marine sedimentation and producedan excellent regional source rock during theTuronian–Coniacian. Marine deposition wasabruptly terminated during the early Maastrich-tian due to the final accretion of the WesternCordillera.

Accretion of the Western Cordillera created theearly pre-Andean foreland basin megasequence oflate Maastrichtian to early Eocene age. This depo-sitional episode consists of coal-rich alluvial plain,coastal plain, and estuarine deposits throughoutthe Middle Magdalena Valley, Eastern Cordillera,and eastern Llanos basin. The megasequence was

terminated by middle Eocene deformation in theMagdalena Valley, which ended sediment deposi-tion throughout Colombia. Loading effects ofthis deformation reestablished the basin, inwhich the late pre -Andean foreland basinmegasequence was deposited, until the earlyMiocene. This megasequence also consists ofa l luvia l pla in, coasta l pla in, and estuarinedeposits, including the primary reservoir in theLlanos Foothills—the upper Eocene MiradorFormation. The megasequence also includes aseries of four major grossly coarsening-upwardcycles in the Llanos basin; these cycles corre-spond to changes in sea level, sediment supply,and foreland basin loading. The mudstone in thelowermost of these cycles is the regional seal inthe Llanos basin and Foothills.

The middle Miocene onset of Andean deforma-tion in the Eastern Cordillera isolated the MiddleMagdalena Valley from the Llanos basin. The defor-mation was dominated by inversion of the basin-controlling faults; the resultant loading of the litho-sphere created the accommodation space for theAndean foreland basin megasequence. A majortransgression into the Llanos basin coeval with thisdeformation caused deposition of marine mud-stones in the lower part of the megasequence.However, the majority of the Andean foreland basinmegasequence consists of the Guayabo Formation,a classic molasse sequence, deposited in a high-energy, coarse-grained, bed-load–dominated fluvialsystem that was supplied by the developing moun-tains of the Eastern Cordillera.

INTRODUCTION

The physiography of Colombia is dominated bythe Andes mountains in the west and theAmazon/Orinoco basin in the east. The ColombianAndes form three separate ranges, the Western,Central, and Eastern Cordilleras, which merge south-ward into a single range. The Cauca valley separates

1421AAPG Bulletin, V. 79, No. 10 (October 1995), P. 1421–1443.

©Copyright 1995. The American Association of Petroleum Geologists. Allrights reserved.

1Manuscript received June 10, 1994; revised manuscript received March 14,1995; final acceptance June 6, 1995.

2PanCanadian Petroleum, 150 9th Ave. SW, Calgary, Alberta, CanadaT2P 2S5.

3BP Exploration (Colombia) Ltd., Carrera 9A no. 99-02, Piso 9, A.A.59824, Bogotá, Colombia.

We wish to thank BP Exploration (Colombia) Ltd. for permission topublish this paper and Olga Lucia Daza for drafting the figures. We are alsograteful to Jaime Buitrago and Kevin Biddle for constructive and helpfulreviews and to John Weissenberger for reading numerous drafts of therevised manuscript.

An extended bibliography is available on diskette (Macintosh orWindows). Send your check, made out to AAPG, for $3.00 to PublicationsManager, P.O. Box 979, Tulsa, Oklahoma 74101-0979.

Basin Development and Tectonic History of the LlanosBasin, Eastern Cordillera, and Middle Magdalena Valley,Colombia1

M. A. Cooper,2 F. T. Addison,3 R. Alvarez,3 M. Coral,3 R. H. Graham,3 A. B. Hayward,3

S. Howe,3 J. Martinez,3 J. Naar,3 R. Peñas,3 A. J. Pulham,3 and A. Taborda3

Page 2: Cooper Etal,1995

the Western and Central Cordilleras, and theMagdalena Valley separates the Central and EasternCordilleras. East of the Eastern Cordillera is theLlanos, a savanna that is part of the catchment areafor the Orinoco River.

Early workers made detailed age determinationsusing ammonites from Cretaceous beds around theSierra Nevada del Cocuy and Villa de Leiva (vonBuch, 1839; Lea, 1840; Orbigny, 1842). The firstdetailed maps of the Eastern Cordillera were pro-duced by Hettner (1892), and maps of the LlanosFoothills were produced by Hubach (1957).Advances in Colombian geological knowledge havebeen closely linked to mineral exploration andexploitation. Emeralds are in the Lower Cretaceousand coal is in the Upper Cretaceous and LowerTertiary of the Eastern Cordillera and Guajira(Figure 1). Oil and gas exploration has been con-centrated in the Llanos, Putumayo, and Magdalenabasins.

Bürgl (1961) produced a reconstruction ofCretaceous and Tertiary paleogeography and relat-ed sedimentation to tectonics, establishing achronological correlation of the Cretaceous andTertiary nomenclature. He compiled Cretaceousthicknesses (Ramirez, 1953; Morales and theColombian Petroleum Industry, 1956; Renz, 1956;Bürgl, 1960) and observed considerable variabilityin thickness in the Sabana de Bogotá. He interpret-ed these differences in thickness as being due tofour stages of marine transgression during theUpper Jurassic and Cretaceous. These stages pro-gressively onlapped the Guyana shield. Since thework of Bürgl (1961), the stratigraphic interpreta-tion of the Eastern Cordillera has developed slow-ly. Etayo (1964, 1979), Etayo et al. (1969), andFabre (1987) refined the understanding ofCretaceous shoreline changes, and numerousauthors have identified continental sediments inthe Jurassic and Lower Cretaceous prior to themarine transgressions (Cediel, 1968; Mojica andDorado, 1987). The majority of recent literaturethat integrates stratigraphy and tectonics has beenfocused on the Magdalena Valley (Butler andSchamel, 1988, 1989; Schamel, 1991; Montgomery,1992). Significant papers on the tectonics of theEastern Cordillera are Campbell (1974), Colletta etal. (1990), and Dengo and Covey (1993).

In this paper, we present a sequence stratigraphyfor the Middle Magdalena Valley, Eastern Cordillera,and Llanos basin. The sequence stratigraphy isrelated to the deformation that has af fectedColombia since the Triassic and provides a frame-work for interpreting basin evolution. The paper isalso intended to provide the regional context forthe companion paper by Cazier et al. (1995),which discusses the petroleum geology of theCusiana field.

REGIONAL TECTONIC FRAMEWORK ANDBASIN DEVELOPMENT

Major tectonic events that influenced develop-ment of the Colombian basins are all closely tied tothe evolution of the active margin of western SouthAmerica. In this paper, we concentrate on basindevelopment from the Cretaceous onward in theMiddle Magdalena Valley, Eastern Cordillera, andthe Llanos basin (Figure 1). Models of tectonic evo-lution prior to the Late Cretaceous are uncon-strained by ocean f loor magnetic anomaly data(Pardo-Casas and Molnar, 1987) and thus are specu-lative.

The basement of Colombia is divisible into threezones separated by major sutures (Suarez, 1990):(1) the Precambrian Guyana shield in the east; (2)the Central Province of Precambrian–earlyPaleozoic metamorphic rocks, which underliesthe Central and Eastern Cordilleras; and (3) accret-ed oceanic crustal fragments and subduction-relat-ed sediments and volcanics, which form theWestern Cordillera (Barrero, 1979; Alvarez, 1983;Duque-Caro, 1990). Megard (1987) interpretedaccretion of the western terrane along theRomeral suture (Figure 1) as a series of discretecollisions commencing in the Early Cretaceousand ending in the Eocene. The suture betweenthe Guyana shield and Central Province is theBorde Llanero, which approximately coincideswith the Llanos Foothills thrust front (Suarez,1990). Following the work of Butler and Schamel(1988, 1989), Montgomery (1992) suggested thatthe distribution of Cretaceous and Tertiary basinsin central Colombia was possibly controlled byreactivation of old faults.

During the Triassic, Jurassic, and earliestCretaceous, Colombia was peripherally affected byrifting related to eventual separation of North andSouth America in the proto-Caribbean (Jaillard etal., 1990). Maze (1984) proposed an alternativemechanism for the extension in a back-arc setting,which, given the oblique nature of the subductionzone, may have had a transtensional component.Both mechanisms probably contributed to theextension. Exact timing of rifting onset is difficultto determine because the synrift continental clas-tics are difficult to date. The depocenters wereestablished throughout the Eastern Cordillera andUpper Magdalena with marginal basins in theLlanos and Putumayo basins (Figures 1, 2). In theEastern Cordillera, two rift basins developed(Figure 2), the Cocuy basin in the east and theTablazo-Magdalena basin in the west (Etayo et al.,1969). Between the two depocenters is the intra-basinal Santander high (Figure 2) , whichincludes the Santander and Floresta massifs andpersists south of Tunja as a zone of thinned Lower

1422 Llanos Basin Development

Page 3: Cooper Etal,1995

Cretaceous stratigraphy (Etayo et al., 1969). This sys-tem of basins was active into the Early Cretaceous,when considerable accommodation space was creat-ed in the Eastern Cordillera (Hebrard, 1985; Fabre,1987) allowing thick Lower Cretaceous deposition.Shallow-water sedimentation through much of theCretaceous suggests that deposition approximatelykept pace with subsidence. Early Cretaceous exten-sion and subsidence may have been due to back-arcstretching behind the subduction zone off the west-ern coast of South America. Subduction is believedto have intensified in the Late Jurassic and Berriasianbased on the presence of calc-alkaline plutons of thisage in the eastern part of the Central Cordillera(McCourt et al., 1984).

A Valanginian–Barremian hiatus in igneous activi-ty is interpreted to have resulted from accretion ofthe Amaime terrane, composed of Upper Jurassicto Lower Cretaceous oceanic crust along theRomeral suture (Aspden and McCourt, 1986).Megard (1987) suggested that subduction shiftedwestward following the accretion, as did plutonicactivity, which peaked in the Campanian andSantonian (Aspden and McCourt, 1986). The subsi-dence rate in the back-arc basin decreased in thepost-Cenomanian. Magmatic arcs produced duringsubduction do not appear to have created emer-gent landmasses west of the back-arc basin, exceptin the Upper Magdalena Valley where immature,

continental clastics were deposited until marineconditions were established in the Aptian. Theimplication is that an emergent barrier related tosubduction developed in southern Colombia, butdid not persist northward into the Tablazo-Magdalena basin. The Central Cordillera remainedsubmerged until the Maastrichtian, although Bürgl(1961) believed there was a submarine barrierbetween the Western Andes (his eugeosyncline)and the nonvolcanic Eastern Andes (his mio-geosyncline). First indications of western sedimentprovenance (granitic and volcanic pebbles derivedfrom the Central Cordillera) are in the earlyMaastrichtian Cimarrona Formation on the easternmargin of the southern Middle Magdalena Valley.

Four major occurrences of deformation havebeen recognized in the Tertiary of centralColombia: Late Cretaceous–early Paleocene, middleEocene, late Oligocene–early Miocene, and lateMiocene–Pliocene (Bürgl, 1961; Ben Avraham andNur, 1987).

Late Cretaceous–early Paleocene deformationresulted from the final accretion of the WesternCordillera (McCourt et al., 1984) (Figure 3). This

Cooper et al. 1423

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Page 4: Cooper Etal,1995

deformation marks a significant change in deposi-tional environments throughout the EasternCordillera, Magdalena basin, and Llanos basin(Figures 4, 5) from marine to continental in the incip-ient foreland basin (Van der Hammen, 1961). Prior tothis deformation, deposition since the EarlyCretaceous was entirely marine except for the shore-line facies on the Guyana shield margin and some flu-vial sedimentation in the Upper Magdalena. LateCretaceous–early Paleocene deformation was restrict-ed to the Western and Central Cordilleras except forsome deformation and uplift in the Sierra Nevadadel Cocuy (Figure 2) (Fabre, 1987). The amount of

compressional deformation generated during theaccretion may have been limited by the obliqueconvergence of the Nazca and South Americanplates until 49 Ma (Pardo-Casas and Molnar, 1987).

Middle Eocene deformation created folds andthrusts in the Middle Magdalena Valley. These foldsare truncated and unconformably overlain byupper Eocene clastics (Morales and the ColombianPetroleum Industry, 1956). This deformation maybe related to an increase in convergence ratebetween 49 and 42 Ma (Daly, 1989).

Changes in plate tectonic motions documented inthe late Oligocene to early Miocene (Pilger, 1984;

1424 Llanos Basin Development

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Figure 3—Sequential model of regionaltectonic development for the EasternCordillera, Middle Magdalena Valley,and the Llanos basin.

Page 5: Cooper Etal,1995

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Page 6: Cooper Etal,1995

1426 Llanos Basin Development

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Page 7: Cooper Etal,1995

Ben-Avraham and Nur, 1987) did not cause anydeformation in the Eastern Cordillera or the Llanos.Deformation of this age has been described in theCauca Valley (Alfonso et al., 1989) and in theMagdalena Valley where the reactivation of the mid-dle Eocene structures created an upper Oligoceneunconformity (Schamel, 1991). Collision of theChoco terrain with the northwestern margin ofSouth America also occurred during the middleMiocene (Duque-Caro, 1990), which may have con-tributed to loading and initiated deformation in theEastern Cordillera.

Major deformation of the Eastern Cordillera andLlanos Foothills began at approximately 10.5 Maand resulted from Panama’s collision with SouthAmerica. During this deformation phase, theEastern Cordillera was uplifted and eroded. Oldextensional faults were inverted and new compres-sional structures developed. On the western flankof the Eastern Cordillera and in the MagdalenaValley, middle Eocene folds were reactivated(Butler and Schamel, 1989).

Erosional deposits from the Eastern Cordilleraare preserved in the Guayabo Formation in theLlanos basin. Deformation and uplift are still active,periodically causing earthquakes in the LlanosFoothills. Studies of the Pliocene Tilata Formationsuggest that 1000–2000 m of the uplift occurred atapproximately 3.5 Ma (Van der Hammen, 1957;Hooghiemstra, 1984). These data were used byDengo and Covey (1993) to time basement-involved deformation in the Eastern Cordillera;however, the Tilata rests with a pronounced angu-lar unconformity on a variety of older strata, indi-cating that some deformation preceded depositionof the Tilata.

REGIONAL STRATIGRAPHIC FRAMEWORKAND BASIN EVOLUTION

Our basin stratigraphic model is based on pub-lished well-log, core, seismic, and outcrop dataacquired by BP during exploration of the LlanosFoothills, combined with regional studies of theLlanos basin and Eastern Cordillera. The regionaldatabase includes logs from over 170 explorationand development wells; 25,000 km of seismicdata; 40 biostratigraphic analyses from individualwells; regional outcrop and mapping studies inthe Eastern Cordillera and Llanos basin; andnumerous published papers. Schamel (1991) pub-lished a stratigraphic correlation for the Upperand Middle Magdalena Valley with a number oftransgressive/regressive cycles after Macellari(1988). Dengo and Covey (1993) produced alithostratigraphic correlation from the MiddleMagdalena to the Llanos along the line of their

regional cross section. Our model considers a larg-er area and subdivides the Cretaceous and Tertiaryinto more sequences.

Bürgl (1961) proposed that gentle, verticalCretaceous movements caused cycles of sedimenta-tion, beginning with shallow-water deposits fol-lowed by bathyal and littoral sediments. He did notrecognize angular unconformities within theCretaceous in the Andean zones, but did describestratigraphic gaps, condensed sequences, andisopach changes that he interpreted as the conse-quence of syndepositional folding. This deforma-tion was considered to have initiated the present-day mountain ranges and five synclinoria basins inwhich Tertiary and Quaternary sediments weredeposited.

In this paper, we have synthesized the confusinglithostratigraphic nomenclature in Colombian geo-logical literature to produce chronostratigraphicsummaries of the Llanos basin, Eastern Cordillera,and the Magdalena Valley (Figures 4, 5). These sum-maries are based on a sequence stratigraphy devel-oped for the Cusiana field (Figures 1, 2) and adja-cent areas of the Llanos basin (Figure 6). We thenapplied the sequence stratigraphy throughout theLlanos basin, Eastern Cordillera, and MiddleMagdalena Valley by combining selective fieldwork, biostratigraphic analyses, and interpretationof published and publicly available data. Thechronostratigraphy was correlated over such awide area to develop a regional model of basin evo-lution. This methodology is justified because sub-basins existed only after the 10.5-Ma deformation inthe Eastern Cordillera. Cretaceous sequences areprefaced with a “K” and Tertiary sequences with a“T” (Figure 6). The data diskette for this paper,available from AAPG, contains the raw data used toconstruct the chronostratigraphic correlations andthe gross depositional environment maps.

Synrift Megasequence (Triassic to SequenceK20) and Older Sequences

Upper Cretaceous sediments normally rest direct-ly on a Paleozoic sedimentary and metamorphicbasement in the Llanos. Triassic–Lower Cretaceousrocks are absent in the Llanos except for small,localized (synrift?) packages in isolated wells(Numpaque, 1986). In the Eastern Cordillera, thicksequences of Lower Cretaceous and Jurassic rocksare exposed (Figure 2), although thicknesses arevariable due to extension on faults controlling depo-sition (Cediel, 1968; Mojica and Dorado, 1987).Two depocenters can be recognized in the EasternCordillera (Figure 2), the Cocuy basin in the eastand the Tablazo-Magdalena basin in the west, sepa-rated by the Santander high (Etayo et al., 1969)

Cooper et al. 1427

Page 8: Cooper Etal,1995

(Figure 7). The K10 sequence is dominated by con-tinental red beds in the Tablazo-Magdalena basinand by shallow-marine sediments in the Cocuybasin. The sequence is not present in the UpperMagdalena Valley. The base of the K20 sequencemarks a change from continental sediments of K10to shallow-marine sedimentation in the Tablazo-Magdalena basin (Figure 4). In the Upper Magdalena

Valley, the continental sandstones of the YaviFormation were deposited during deposition of K20following a hiatus in the Middle and Late Jurassicand K10. The Ibague fault, an ancestral Triassic–Jurassic strike-slip fault, controlled facies distribu-tion during the Berriasian and Valanginian (Geotec,1992). The Ibague fault may have continued to con-trol facies during deposition of K20. Clastics erodedfrom the Guyana shield were efficiently ponded inthe sediment sink of the Cocuy basin and graduallyshale out to the southwest, suggesting a sedimententry point near the northern end of the rift(Figures 7, 8). On the Santander high, whichappears to lose elevation to the south, shallow-marine carbonates were deposited (Figures 7, 8).Shallow water depths, combined with starvation ofcoarse clastics, caused restricted marginal marinemudstones to accumulate on the western margin ofthe Santander high (Paja Formation; Morales and theColombian Petroleum Industry, 1956) (Figures 7, 8).In the Tablazo-Magdalena basin, the sequence is rep-resented by organic-rich marine mudstones (Villetaand La Luna formations). The map of gross deposi-tional environments (GDE) for the K20 sequence isvery similar to that of the K30 sequence, which hasbeen designated using sea level curve data (Haq etal., 1987) and regional facies patterns (Figures 4, 5).The boundary between K20 and K30 is consideredto be the boundary between the synrift and back-arc megasequences, coinciding with the westwardjump in the subduction zone after accretion of theAmaime terrane (Aspden and McCourt, 1986;Megard, 1987). The megasequence boundary couldbe placed at the base of K20, but this was rejectedbecause there are substantial thickness changesacross the basin-controlling faults within the K20sequence.

1428 Llanos Basin Development

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GEOLOGICAL FORMATION

HIATUS

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C7

C8

Figure 6—Cusiana field area stratigraphy showing typi-cal gamma-ray log response. The gamma-ray log hasbeen displayed twice by reversing the scaling for theright curve and, hence, low gamma-ray intervals arewhere the two curves are widely separated. The reservoir(R), cap rock (C), and source (S) intervals are indicated.The log signature is color-filled based on the systemstract interpretation. The correlation of the sequencenomenclature proposed here with conventional, indus-try stratigraphic terminology for the Llanos basin isillustrated in the column at the right of the figure.

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CLASTIC INPUT FROM GUYANA SHIELD

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Figure 7—Block diagram illustrating the separation ofthe Cocuy and Tablazo-Magdalena basins during theLower Cretaceous (sequences K20–K40) and the influ-ence of the Santander high on facies distribution.

Page 9: Cooper Etal,1995

Back-Arc Megasequence (Sequences K30–K90)

In the southern part of the Tablazo-Magdalenabasin the K30 and K40 sequences are dominatedby organic-rich marine mudstones and occasionalthin limestones and sandstones (Villeta, Tablazo,San Gil, Simiti, and Salto formations). Most workersassume that these sediments were deposited in adeep-marine basin. However, an alternative modelis that the basin became restricted and anoxic dueto continued starvation from coarse clastic input.The other potential source of coarse clastics for theTablazo-Magdalena basin during deposition of K30and K40 was the Upper Magdalena Valley, whichhas thick sequences of Valanginian–Barremian flu-vial and coastal plain sands (Yavi Formation). Thedepositional environment of the sands becamemarine in the Aptian (Caballos Formation, Figure 5)

and intermittently supplied submarine fans to theTablazo-Magdalena basin (Gallo, 1979). This pat-tern of shallow-marine sedimentation continuedthroughout the Cretaceous, with accommodationspace being produced continually by extension inthe back-arc basin.

In the Cocuy basin, deposition of K30 was char-acterized by minor pulses of shallow-marine sandsderived from the Guyana shield, as in the FomequeFormation (Figure 4). During K40 deposition, agradual rise in sea level, combined with continuedsubsidence, caused a regional transgression. Thistransgression established a shallow-marine silici-clastic shelf over a wide area, including theSantander high (Une Formation; Hubach, 1931;Herngreen et al., 1990). These sandstones progres-sively onlapped farther eastward onto the Guyanashield during K50 deposition. The K50 sands havealso been referred to as the Une Formation, but aresignificantly younger (Figure 4). In the EasternCordillera and westward, the sequence becomesincreasingly muddy and eventually passes intomarine mudstones with subordinate thin carbon-ates (Simiti and San Gil formations) (Figure 4). Thistransition reflects the increasingly distal nature ofthe primary source for the clastic sediments,which, throughout K30–K50 deposition, was theGuyana shield (Figure 4). The K40 sequence marksthe end of the intrabasinal Santander high as a sig-nificant barrier to sediment movement.

In the Turonian–early Coniacian (91–88 Ma),sequence K60 was initiated by global sea level rise(Haq et al., 1987) that, combined with anoxicupwelling (Villamil and Kauffman, 1993), deposit-ed marine mudstones, cherts, and phosphates(Figures 4, 9). K60 contains prolific source rocks;for example, the Villeta Shale in the UpperMagdalena Valley (Beltrán and Gallo, 1968) (Figure5) and the La Luna Formation of the MiddleMagdalena Valley and western Venezuela (Garner,1925; Talukdar et al., 1986) (Figure 4). The K60sequence has been penetrated only by the Medina1 well in the Llanos Foothills, but field work donein the Eastern Cordillera and well data to the east inthe Llanos basin indicate that it is almost certainlypresent throughout the Foothills area (GachetáFormation; Miller, 1979). The K60 mudstones areoil prone and are the main petroleum source rockfor the Llanos Foothills and basin (Fabre, 1987;Palmer and Russell, 1988; Droszd and Piggott, inpress) and the Magdalena Valley (Zumberge, 1984).The K60 onlapped the Guyana shield and over-stepped the basal Cretaceous sands to establish amore easterly littoral facies belt (Figure 9).

Anoxic conditions during K60 were terminatedby a fall in relative sea level in the Coniacian–earlySantonian (88–85 Ma). The fall in relative sea levelshifted deposition into a northeast-trending basin

Cooper et al. 1429

R. Casanare

R. Me t a

R.Ca

uca

R. Magdalena

Bogotá

R . Putumayo

K20

400

1900

1000

095

400

1000

1800

095

1000

1900

1800

1000

0 500 km

Figure 8—K20 (138–122 Ma) gross depositional environ-ment (GDE) map drawn within the highstand systemstract at approximately 125 Ma. This map and all subse-quent GDE maps have been drawn using present-daygeographical positions; the key for the color scheme isthe same as in Figures 4 and 5. The red arrows indicatethe interpreted direction of sediment supply into thebasin for this and all subsequent gross depositionalenvironment maps.

Page 10: Cooper Etal,1995

system in the Eastern Cordillera, which extendednorthward to the Maracaibo basin. The Llanosbasin was on the eastern margin of this basinal sys-tem. The K70 and K80 sequences were depositedon the shallow-marine shelf created by the fall insea level (Figures 4, 5, 10). K70 and K80 equateapproximately with the Guadalupe Group (Hettner,1892; Hubach, 1931; Pérez and Salazar, 1978).Sequences K70 and K80 represent two majorcycles of westward shoreline progradation, aggra-dation, and retrogradation dominated by high-ener-gy, quartz-rich, shoreface sandstones derived fromthe Guyana shield. Sequence K70 began with alower forced regression systems tract (sensuPosamentier et al., 1992) of shallow-marine sands(lower Guadalupe Sandstone and Dura formations)and ended with a transgressive systems tract(Guadalupe Shale and lower Plaeners). Sand progra-dation into the basin began again as sea level beganto drop. The upper and lower Plaeners of theGuadalupe Group in the Eastern Cordillera aresiliceous, locally phosphatic, silts, mudstones, andporcellanites interpreted to be the result of

upwelling at the shelf edge (Follmi et al., 1992). Inthe western highlands of the Eastern Cordillera theK70 is represented by distal mudstones of the LaLuna Formation (Figure 4).

The K80 is divided into a sand-dominated,forced regression systems tract (Santonian–earlyCampanian Upper Guadalupe Sandstone Form-ation) overlain by shale-dominated highstandand transgressive systems tracts. The shales havebeen mistakenly identi f ied as the Maastrich-tian–Pa leocene Guaduas Formation (Figure 3)(Sarmiento, 1992) in some of the earlier wells inthe Llanos Foothills; e.g., the Medina 1. Recentlyacquired data by BP has conclusively datedthese youngest Cretaceous rocks in the LlanosFoothills as Campanian in age (Pulham, 1994).The sands at the base of K80 extend westbeyond Tunja, but in the western highlands ofthe Eastern Cordillera are represented by marineshales (La Luna Formation). In the EasternCordillera, the K80 sand is the middle sand unitof the Guadalupe Group (Labor Formation). K80sandstones form the oldest proven commercial

1430 Llanos Basin Development

R. Casanare

R. Me t a

R.Ca

uca

R. Magdalena

Bogotá

R . Putumayo

K60

400

1900

1000

095 400

1000

1800

095

1000

1900

1800

1000

0 500 km

Figure 9—K60 (89–84 Ma) gross depositional environ-ment map drawn within the transgressive systems tractat approximately 88.5 Ma. Key for the color scheme isthe same as in Figures 4 and 5.

R. Casanare

R. Me t a

R.Ca

uca

R. Magdalena

Bogotá

R . Putumayo

K80

400

1900

1000

095

400

1000

1800

095

1000

19001800

1000

0 500 km

Figure 10—K80 (79.5–73.5 Ma) gross depositional envi-ronment map drawn within the highstand systems tractat approximately 76 Ma. Key for the color scheme is thesame as in Figures 4 and 5.

Page 11: Cooper Etal,1995

reservoir unit in the Llanos Foothills. The highlymature quartzarenites were deposited as a relative-ly uniform sheet over a shallow shelf that extendedfrom the Llanos into the Eastern Cordillera. A risein relative sea level in the early Campanian effec-tively starved the K80 shelf of sand and capped thesequence with a condensed marine mudstone inthe Eastern Cordillera (upper Plaeners Formation;Figures 4, 5, 10).

The K90 sequence is early Maastrichtian and iscomposed of a sand-dominated transgressive sys-tems tract (Tierna Formation) overlain by a poor-ly developed, shale-dominated highstand systemstract. K90 is present throughout the EasternCordillera, but as in the sequences below, thesands shale out and are represented by the UmirFormation in the Middle Magdalena Val ley(Figure 4). The sequence is not present in wellsin the Llanos Foothi l ls , where few rel iableMaastrichtian deposits have been reported (G.Eaton, 1992, personal communication). The K90correlates with the hiatus above K80 in theLlanos Foothills.

Early Pre-Andean Foreland BasinMegasequence (Sequences T10–T20)

The final episode of accretion in the WesternCordillera began near the end of the earlyMaastrichtian and resulted in a fundamental changeto the nonmarine deposition of the pre-Andeanforeland basin megasequence (Figures 4, 5). TheT10 sequence is dominated in the EasternCordillera by the coastal and alluvial-plain mud-stones and coals of the Guaduas Formation(Hettner, 1892). These rocks were dated bySarmiento (1992) as late Maastrichtian and earlyPaleocene. This sequence is not present in theLlanos basin and Foothills, but is correlative with ahiatus of approximately 14 m.y., spanning theCretaceous–Tertiary boundary. Dengo and Covey(1993) considered this sequence to persist to theeast of the Guaicaramo fault, but this does notagree with the available biostratigraphic data(Pulham, 1994). The T10 sequence shows a system-atic northward and eastward thinning; suddenthickness changes occurring across major faults in

Cooper et al. 1431

R. Casanare

R. Me t a

R.Ca

uca

R. Magdalena

Bogotá

R . Putumayo

T20

400

1900

1000

095

400

1000

1800

095

1000

1900

1800

1000

0 500 km

Figure 11—T20 (61–55 Ma) gross depositional environ-ment map drawn within the highstand systems tract atapproximately 58 Ma. Key for the color scheme is thesame as in Figures 4 and 5.

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R. Me t a

R.Ca

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R. Magdalena

Bogotá

R . Putumayo

T30

400

1900

1000

095 400

1000

1800

095

1000

1900

1800

1000

0 500 km

Figure 12—T30 (40.5–34 Ma) gross depositional environ-ment map drawn within the transgressive systems tractat approximately 35 Ma. Key for the color scheme is thesame as in Figures 4 and 5.

Page 12: Cooper Etal,1995

the Eastern Cordillera are considered to result fromlater erosional truncation (Sarmiento, 1992).However, the thinning may represent progressiveonlap onto the eastern hinterland. The T10 is pres-ent in the eastern highlands of the EasternCordillera immediately west of the Guaicaramofault system, but is absent in the Foothills. Thisplacement suggests some degree of fault control onT10 deposition, possibly due to differential subsi-dence across the fault. In the Magdalena Valley theT10 is represented by a series of shales and occa-sional sands (Lisama Formation, Figure 4).

Deposition began again in the Llanos Foothillsin the late Paleocene at approximately 60 Ma inresponse to a transgression that extended forelandbasin deposition across the Llanos basin. The T20sequence extends farther east than the underlyingT10, possibly due to a combination of transgres-sion and early loading of the protoforeland basindue to deformation in the Central and Western

Cordilleras (Figures 4, 5, 11). The Barco Formationreservoir (Notestein et al., 1944) forms the basaltransgressive systems tract of T20. It is predomi-nantly a highly mature, sandstone-rich, estuarinedeposit derived from the Guyana shield. Marineinf luence is strong throughout the BarcoFormation in the area of the Cusiana field with arelatively abrupt upward transition into more het-erolithic coastal and alluvial-plain deposits. BasalT20 sandstones are developed throughout theLlanos Foothills, Eastern Cordillera (SoachaFormation in part), and Magdalena Valley (Figures4, 11). Conformity exists between T10 and T20 inthe Eastern Cordillera and the Middle MagdalenaValley. T20 sandstone deposition ended as the latePaleocene transgression weakened and a relativesea level highstand was established (∼59 Ma).Subsequent regression shifted the regional shore-line gradually westward. Coarse clastics appear tohave bypassed the Llanos Foothills and Eastern

1432 Llanos Basin Development

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T70

T60

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T20

K80

K60

K50

K70

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La María 1

Entrerríos 1Surimena 1

Simón 1 Guarrojo 1

ST-GU 15

Sequence Boundary

Maximum Flooding Surface

0

1000

2000

Depth in Feet

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ern

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iller

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Cusiana Field

4˚ North

Llan

osF

oo

thill

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Figure 13—Correlation of stratigraphic units K50–T70 in the Llanos basin based on seven representative wells. Thelog signatures are gamma ray in API units; the gamma-ray curve has been plotted twice by reversing the scale forthe right curve. Wide separation of the two curves indicates a low gamma-ray response of the formation, which iscorrelative with sands in the cuttings. The locations of this correlation section and of the correlation section of Fig-ure 16 are shown on the inset map.

Page 13: Cooper Etal,1995

Cordillera in the middle of T20 deposition, wherea regressive, mud-dominated coastal plain systemwas established (lower Los Cuervos Formation)(Notestein et al., 1944). These mudstones mayhave some source potential in the Llanos Foothills(Droszd and Piggott, in press).

A major drop in relative sea level at about 54 Ma(the top of T20) resulted in a shift in deposition tothe west and north (Catlin et al., 1994). In theLlanos the depositional hiatus lasted almost 16 m.y.and resulted in a disconformity with no apparentangular component (cf. Dengo and Covey, 1993)(Figure 4). The fluvial systems in the upper T20 layin the Eastern Cordillera (Picacho and Bogota for-mations). These bypass systems are in the samecycle of deposition in which the Misoa “C” delta inVenezuela was deposited (Catlin et al., 1994).Thickness variations within T20 (Naar and Coral,1993) imply continued extension on the Cusiana-Tamara fault system in the Llanos Foothills. Earliestmiddle Eocene sediments are not present through-out Colombia (Figures 4, 5) due to deformation inthe Magdalena related to change in the directionor rate of subduction (Daly, 1989). The deforma-tion produced thrust and fold structures in theMagdalena Valley. The hiatus corresponds to theunconformity at the base of the GualandayFormation in the Magdalena Valley and separatestwo pre-Andean megasequences (Figures 4, 5)(Corrigan, 1967). Although some uncertaintyexists regarding the exact length of the hiatus, theregional plate tectonic data have been used to timethe deformation (Daly, 1989).

Late Pre-Andean Foreland BasinMegasequence (Sequences T30–T70)

Deposition in the Llanos was renewed in the lat-est middle Eocene (∼40.5 Ma) in response to atransgression that spread southward and eastwardfrom the foreland basin (Figures 4, 12). The T30transgression was significantly more extensive thanthe earlier T20 f looding and caused the T30 toonlap much farther to the east onto the Guyanashield (Figure 13). Initial T30 deposition consistedof marine-influenced, sandstone-rich, fluvial andestuarine valley-fill deposits contained in muddiercoastal plain sediments (Pulham, 1994). Coarse andoften pebbly, fluvial and alluvial fan sandstones arethe dominant component of T30 deposited over awide area of the Llanos basin and Foothills (MiradorFormation; Notestein et al., 1944). The T30sequence contains the most important reservoirunits in the Cusiana, Cupiagua, and Volcanera fields(Cazier et al., 1995). The middle T30 in the Cusianafield contains a distinctive, muddy alluvial-plainunit that is at least subregionally extensive in theLlanos Foothills and western Llanos basin(McCollough, 1991; Pulham, 1994). Continuedtransgression eventually submerged the alluvialplain and established a shallow-marine shelf acrossthe Cusiana area. The upper T30 comprises heavilybioturbated estuarine parasequences punctuatedby sandstone-rich, estuarine valley-fill deposits. Inthe Cusiana field, major flooding (∼34 Ma) effec-tively ended sand deposition (Figure 13). All of thecoarser grained T30 sandstones in the Llanos

Cooper et al. 1433

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Zone A Zone B Zone CAlways Marine

Central Cordillera

W E

Erosion

Magdalena Valley

Prog.Retrog.

Eastern Cordillera

MAX SLMIN SL

Prog.Retrog.

Llanos Guyana Shield

Erosion

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Figure 14—Zonation of the Llanos foreland basin modified from the generic model of Posamentier and Allen (1993)with the addition of zone C where parasequence patterns resemble those in zone B, but with opposite polarity. ZoneA is located where the maximum rate of eustatic fall is less then the rate of subsidence and zone B is located wheresubsidence rates are less than the maximum rate of eustatic fall. In zone C the relationship between subsidence andeustatic change will be the same as in zone B, but the sediment supply will be from the opposite direction.

Page 14: Cooper Etal,1995

Foothills are extremely mature quartzarenites simi-lar in composition to those of the underlying T20.Locally, fine-grained litharenites occur in highstandcoastal and alluvial-plain deposits that overlie trans-gressive, estuarine, and valley-fill sediments. Fabre(1987) concluded that the Santander massif wasalready being eroded at this time and may be thesource for the lithic component in the sediments.Significant thickness variations in T30 (Moreno andVelazquez, 1993; Naar and Coral, 1993) (Figure 13),may be a result of fault control or may be due to thewestward thickening into the foreland basin. In theMagdalena Valley, T30 sediments are termed (inpart) the Gualanday, Esmeraldas, and La Paz forma-tions that range from late Eocene to middleOligocene in age (Figures 4, 5) and occur above adramatic angular unconformity (Corrigan, 1967).The Gualanday and La Paz formations contain feld-spathic and lithic material and local conglomerates incontrast to the mature quartzarenites of the MiradorFormation to the east. This compositional change isinterpreted to be the result of sediment supply from

the volcanics and intrusives of the Central Cordillerainto the Magdalena Valley (Figure 12).

After T30, four major cycles of marine-inf lu-enced, lower coastal-plain deposition accumulat-ed in the Llanos basin and Foothills (T40–T70),traditionally termed the Carbonera Formation(Notestein et al., 1944). However, due to erosion,only sparse outcrops of these sequences occur inthe Eastern Cordillera (Concentracion Formation).In the Middle Magdalena, these sequences are rep-resented (in part) by the Esmeraldas, Mugrosa,Colorado, and La Cira formations. SequencesT40–T70 (∼34–16.5 Ma) correlate to industry usagein the Llanos basin as shown in Figure 6. Thesesequences are separated at maximum flooding sur-faces, which are more correlative through the basinthan sequence boundaries. Thus, the sequences arenot true sequences in the sense of Mitchum et al.(1977), but are genetic stratigraphic units in thesense of Galloway (1989). The sequences recordeasterly migration of foreland basin subsidence,which culminated with the onset of EasternCordillera deformation. T40–T70 are correlatablethroughout the Llanos basin (Figure 13), displayinga gradual increase in sand percentage and becom-ing increasingly continental with proximity to theGuyana shield. The sequences all thicken graduallywestward due to increasing accommodation spacein the foreland basin axis. Well and seismic dataalso indicate continued episodic normal displace-ment on the Cusiana fault system during T40–T70deposition (Figure 13). The extension results fromlithospheric loading that reactivated preexistingfaults.

Each sequence consists of a mud-dominatedhighstand systems tract, followed by a thin, forcedregression systems tract, and ends with a sand-prone transgressive systems tract that culminates ina maximum flooding surface. Droszd and Piggott(in press) suggested that the mudstones in the high-stand systems tract of T40 may be the source rockfor one of the component oils in the Volcanera andCupiagua fields, which they typed to a Tertiarysource. During T30–T70 the major source of sedi-ment for the Llanos basin was the Guyana shieldand, as a result, parasequences prograde westwardinto the basin. The gross pattern that results is ofonlap onto the Guyana shield. An exception is T50,which does not onlap as far east as either T40 orT60 (Figure 13). Protracted peneplanation since atleast the Jurassic created low relief in the Llanosbasin; this relief was susceptible to shifts in grossdepositional environments caused by changes insediment supply, accommodation space resultingfrom loading, and global eustatic sea level changes.

The coarse clastics of the Gualanday, Mugrosa,and Doima formations in the Magdalena Valley indi-cate that the depocenter in the Magdalena and the

1434 Llanos Basin Development

R.Ca

uca

R. Magdalena

Bogotá

T80

R. Casanar e

R. Meta

R. P utumayo

400

1900

1000

095 400

1000

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095

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1900

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1000

0 500 km

Figure 15—T80 (16–10.5 Ma) gross depositional environ-ment map drawn within the highstand systems tract atapproximately 15 Ma. Key for the color scheme is thesame as in Figures 4 and 5.

Page 15: Cooper Etal,1995

western part of the Eastern Cordillera was an effi-cient sink for the sediments being derived from thewest. Posamentier and Allen (1993) recently present-ed a model that divided a foreland basin into twodepositional zones. This model has been modified toaccount for sediment supply from the Guyana shieldinto the foreland basin (Figure 14).

Andean Foreland Basin Megasequence(Sequences T80–T90)

During the middle Miocene, the global rise insea level (Haq et al., 1987) coincided with the firstsignificant deformation and uplift in the EasternCordillera. This deformation isolated the MiddleMagdalena Valley from the Llanos basin. The resul-tant loading tectonically enhanced the highstandsystems tract, causing deposition of the T80 mud-stones (Léon Formation of Notestein et al., 1944).Evidence for at least partial emergence of theEastern Cordillera is more sand in the T80 in thewestern Foothills than in the east (Figures 15, 16).The T80 marine mudstones extend farther east-ward than any of the older sequences. The easternonlapping edge of the T80 onto the Guyana shieldis marked by a change in facies to shoreface sandsand marginal-marine facies dominated by coarseclastics (Figure 15). Additional evidence for defor-mation, uplift, and erosion of the Eastern Cordilleraduring T80 deposition is a correlative unconformi-ty between the Honda and Real formations in theMiddle Magdalena Valley (Figure 4).

The Léon Formation illustrates the problems ofthe lithostratigraphic correlation schemes. TheLéon in the Llanos falls within T80, whereas theLéon Formation in Venezuela is Oligocene (Boesi etal., 1988) and is correlative with the T50 mudstonesof the Llanos basin. Bürgl (1955) suggested that a

widespread transgression occurred during the lateOligocene based on marine shales of the Léon andLa Cira formations, an erroneous conclusion drivenby miscorrelation of the Llanos and VenezuelanLéons. In the absence of definitive biostratigraphicdata, the top of the T80 is defined by a color changeof the mudstones from gray to red, which reflectsthe last vestige of marine influence in the system.

In the Llanos basin, approximately 3000–3500 mof T90 coarse continental clastics were depositedfrom about 10 to 2 Ma (Guayabo Formation;Hubach, 1957). This phase of deposition recordsuplift of the Eastern Cordillera (Van der Hammen etal., 1973) immediately west of the Foothills and theend of migration of the foreland basin axis (Figure17). The Guyana shield is no longer the provenanceof the sediment because Cretaceous clasts erodedfrom the Eastern Cordillera occur within T90(Moreno and Velazquez, 1993). In the Magdalenabasin, T90 is represented by the Honda and Realgroups. Deposition of the T90 molasse causedrapid late-stage burial of the Late Cretaceous–earlyTertiary stratigraphic section in the MagdalenaValley and in the Llanos. The deposition of T90placed the K60 and Tertiary source rocks in the oilwindow at approximately 5 Ma when generationbegan in the Llanos Foothills (Cazier et al., 1995).

STRUCTURAL EVOLUTION AND STYLE OF THEANDEAN DEFORMATION

In the Eastern Cordillera and Llanos basin, majortectonic elements are the Las Salinas-Bituima faultsystem and the Guaicaramo fault system thatbound the Eastern Cordillera, the faults boundingthe Santander massif, and the Cusiana-Tamara faultsystem that separates the Llanos Foothills from theLlanos basin (Figure 18).

Cooper et al. 1435

GRCusiana 4

3500

4000

4500

5000

5500

6000

6500

7000

7500

GRCupiagua 1

Increasing sand content

3500

3000

4000

4500

5000

5500

6000

6500Top T70

Top T80

GRVolcanera 1

NorthSouth

1000

1500

2000

2500

3000

3500

4000

4500

5000

5000

Figure 16—Correlation of T80 in the Cusiana 4, Cupiagua 1, andVolcanera 1 wells in the LlanosFoothills using the top of T70 as thedatum. The log signatures aregamma ray in API units; thegamma-ray curve has been plotted twice by reversing the scale for the right curve. Wide separation of the two curves indicates a low gamma-rayresponse of the formation, which is correlative with sands in the cuttings. Location of the sectionline is shown on the inset map ofFigure 13.

Page 16: Cooper Etal,1995

Structural geometry of the orogene is illustratedby a regional cross section from the MiddleMagdalena Valley, through the towns of Villa deLeiva and Tunja, and the Cusiana field (Figures 2,18), which includes a Middle Miocene restorationprior to the main Andean deformation. The crosssection is constrained by published geologicalmaps, field traverses, and well and seismic data. Thesection was bed length and area balanced using theGEOSEC software package. Total shortening is approx-imately 68 km, significantly less than the estimate ofshortening published by Dengo and Covey (1993).The section is similar to that of Colletta et al.(1990). The most important geometric observationis dramatic thickening of Jurassic (and older sedi-ments?) and the Lower Cretaceous in the hangingwalls of some of the faults. The Guaicaramo,Arcabuco, and Las Salinas-Bituima fault systemsshow thickening in their hanging walls. Thickeningof the Lower Cretaceous is the result of continuedoblique extension in the back-arc on these faultsand creation of accommodation space in the Cocuyand Tablazo-Magdalena basins. The location of thefaults can be inferred by examining geologic maps

of the region and comparing the published strati-graphic thicknesses from the hanging walls andfootwalls. Inversion of these faults (Cooper andWilliams, 1989) during the main deformation phase(beginning at ∼10.5 Ma) controlled style and distri-bution of compressional structures, uplift, and ero-sion of the Eastern Cordillera. Figure 18 also illus-trates the truncation of the Paleocene and olderstrata by the middle Eocene unconformity due tomiddle Eocene deformation in the MiddleMagdalena Valley. This relationship is not shown onthe cross section published by Colletta et al.(1990).

The earliest thickening across the Cusiana-Tamara fault system occurred in the LateCretaceous K80 sequence. The thickening isshown by differences in thickness betweenCusiana field wells and the wells in the immediateforeland (Figures 13, 19). The Guaicaramo fault sys-tem that bounded the Cocuy basin controlled thedramatic thickening of Lower Cretaceous sedi-ments from the foreland into the Eastern Cordillera(Ulloa and Rodríguez, 1981; Hebrard, 1985). TheCusiana-Tamara fault system may have had an earli-er extensional history as an extensional footwallcollapse of the Guaicaramo fault system during theEarly Cretaceous rifting and back-arc subsidence.This interpretation differs from that of Colletta etal. (1990), who did not recognize an early exten-sional history of the Cusiana-Tamara fault system.Movement continued episodically from the LateCretaceous until deposition of the T80 sequence.This phase of normal displacement on the Cusiana-Tamara fault system accommodates lithosphericflexure in response to loading by accretion of theWestern Cordillera and deformation of the Centraland Eastern Cordilleras. Similar faults have beendescribed in other foreland basins (Kittler andNeumayer, 1983).

Other inversion structures can be recognized inthe Eastern Cordillera (Figure 18). For example, thefootwall of the Pesca fault carries folds with a wave-length of 1–2 km whose limbs are locally cut by bothfore- and backthrusts. In the hanging wall, a homo-clinal dip panel extends 10 km to the west, suggest-ing deep detachment of the Pesca fault. BetweenTunja and Villa de Leiva, tight, faulted folds onceagain developed, ending on the southeastern limbof the Arcabuco anticline. West of the Arcabucoanticline the structure is dominated by folds thathave a wavelength of 10 km, implying a deepdetachment. The area from the Arcabuco anticlineto the Las Salinas fault is substantially above regionalelevation, even in the syncline cores, and is inter-preted as the inverted hanging wall of the Arcabucofault. The Arcabuco fault controls the western mar-gin of the Santander high. The Pesca fault is inter-preted as a footwall shortcut of the inverted

1436 Llanos Basin Development

R. Casanare

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Figure 17—T90 (10.5 Ma–present) gross depositionalenvironment map drawn at approximately 10 Ma. Keyfor the color scheme is the same as in Figures 4 and 5.

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Cooper et al. 1437

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Arcabuco fault, which implies that the Santanderhigh is allochthonous (cf. Colletta et al., 1990). Thesmaller wavelength folds and their associated faultsdeveloped to accommodate slip where deeperdetaching faults splay into shale-prone LowerCretaceous sequences. The Cusiana-Tamara fault sys-tem may have originated as a system of extensionalfootwall-collapse faults based on thickness changesof the Cretaceous and Tertiary sequences across thefault. The alternative is to interpret the faults as foot-wall shortcuts to the inversion of the Guaicaramofault. Colletta et al. (1990) also recognized the signif-icant role of these inverted extensional faults in thedeformation history of the Eastern Cordillera; how-ever, these workers did not describe the footwallshortcuts and their relationship to intense, locallyobserved deformation. Dengo and Covey (1993)interpreted much of the structural elevation as beingdue to thin-skinned deformation detaching withinthe K60 and K30; as a result, their estimate of short-ening is significantly higher than that of the sectionpresented here. On the western margin of the

Eastern Cordillera, the Las Salinas-Buituima fault sys-tem that forms the boundary with the MiddleMagdalena Valley is also characterized by footwallshortcuts involving basement (Schamel, 1991).

In the Llanos Foothills west-northwest–east-southeast compression caused inversion along theCusiana-Tamara fault system (Figure 19). The thin-skinned Yopal fault, which detaches within T40,overrides the Cusiana fault to the north and buriesthe branch line with the latter fault (Figure 20).West of the frontal inversion structures is a systemof major regional synclines including the Nunchiaand Zamaricote synclines (Figure 20). The westernlimbs of the synclines are elevated by a series ofstructures that involve the Late Cretaceous andearly Tertiary sedimentary sequences. These struc-tures can be modeled as a series of basement-involved or thin-skinned duplex horses detachingin the K60 shale. The duplex model is based onrepetition of T30 in the El Morro well (Naar andCoral, 1993) and the short wavelength and geome-try of the upper horse, which outcrops to the

1438 Llanos Basin Development

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north (Moreno and Velazquez, 1993). The duplex istruncated to the west by the out-of-sequenceGuaicaramo fault. The Guaicaramo fault is definedin this paper as the inverted extensional fault thatoriginally controlled the Cocuy basin. The fault iscomplicated in outcrop by a number of faults with-in the hanging wall that detach toward the base ofthe Cretaceous. One of these detached faults isfolded by the Monterallo anticline to the west ofthe El Morro 1 well (Figure 20).

The Foothills contain both deep-detaching base-ment-involved faults, which have inverted anddeveloped footwall shortcut faults, and thin-skinned thrusts detaching within the Cretaceousand Tertiary. This pattern of deformation is alsocharacteristic of the Eastern Cordillera and theMiddle Magdalena Valley (Figures 2, 18).

CONCLUSIONS

Mesozoic and Tertiary evolution of the MiddleMagdalena Valley, Eastern Cordillera, and Llanosbasin is closely tied to the tectonics of the marginof western South America. Conventional strati-graphic analyses of the basin have been based onlithostratigraphic correlations with limited bio-stratigraphic control. The chronostratigraphy pre-sented here is based on an extensive database ofcompiled biostratigraphic and lithological datafrom wells and outcrops. The sedimentary succes-sion is divided into five megasequences, each relat-ed to a discrete period of deformation.

The Triassic–Barremian synrift megasequencewas dominated by continental clastic sedimentsuntil an Early Cretaceous transgression establishedmarine conditions. The synrift megasequenceincludes two Cretaceous sequences, K10 and K20,which thicken dramatically into the hanging wallsof major basin-controlling faults. Shallow-marineclastics, derived from the Guyana shield, wereponded in the Cocuy basin by the intrabasinalSantander high. The western Tablazo-Magdalenabasin was dominated by fine-grained marine mud-stones with minor submarine fan deposition.

The Cretaceous back-arc megasequence wasdeposited in a basin behind the Andean subductionzone. In the middle Cretaceous a marine transgres-sion established a shallow-marine siliciclastic shelf

Cooper et al. 1439

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Figure 20—Summary surface geology map of the LlanosFoothills showing key wells and line of cross section forFigure 19. The map is based on the 1:100,000 map sheets211 Tauramena and 193 Yopal, proprietary data collect-ed for BP by Geotec Ltda., and mapping by Moreno andVelazquez (1993), Naar and Coral (1993), and Arangoand Venegas (1994).

Page 20: Cooper Etal,1995

over a wide area (K40), including the intrabasinalSantander high. Continued relative sea level rise,combined with anoxic upwelling, resulted in thedeposition of a succession of marine mudstones,cherts, and phosphates (K60), which are prolificsource rocks in the Llanos basin and MiddleMagdalena Valley. The overlying Santonian–earlyMaastrichtian sequences (K70–K90) comprise aseries of high-energy quartz-r ich sandstonesderived from the Guyana shield. The sequencesare widespread, comprising a series of majorcycles of shoreline progradation, aggradation, andretrogradation. K80 sandstones are the oldest sig-nificant reservoir units in the Llanos basin andFoothills.

Accretion of the Western Cordillera at the end ofthe Cretaceous resulted in a fundamental changefrom marine to nonmarine deposition (theK90/T10 boundary) and development of the pre-Andean foreland basin. The pre-Andean forelandbasin is divided into two megasequences by middleEocene deformation in the Magdalena Valley, aresult of changes in direction and rate of subduc-tion (Daly, 1989).

In the Llanos basin, both pre-Andean forelandbasin megasequences contain mature quartz-richfluvial sands derived from the Guyana shield (e.g.,T30, Mirador Formation). In contrast, in the MiddleMagdalena Valley feldspathic and lithic fluvial sandsderived from the Central Cordillera dominate. Inthe Llanos basin and Eastern Cordillera the youngersequences form a series of major grossly coarsen-ing-upward cycles separated by maximum floodingsurfaces (T40–T70). In this distal foreland basin set-ting, the sediment supply was from the east, notfrom the orogenic hinterland to the west.

In the middle Miocene, a global rise in sea levelcoincided with the first significant deformation anduplift in the Eastern Cordillera and initiated theAndean foreland basin megasequence. The marinemudstone (T80) deposited onlapped far to the eastacross the Guyana shield. Increasing sand contentwithin T80 to the west documents the initial partialemergence of the Eastern Cordillera and the resul-tant isolation of the Llanos and Middle MagdalenaValley. The final depositional episode is the deposi-tion of a thick, coarse, continental clastic sequence(T90) derived from the Eastern Cordillera duringdeformation and uplift, which caused the sourcerocks to generate hydrocarbons.

Andean deformation within the EasternCordillera is dominated by the inversion of theearlier Cretaceous extensional faults, resulting inmajor folds, with wavelengths of 10 km, thatdeveloped above deep, detaching planar faults.Only where the deep detaching faults splay intothe overlying cover did short-wavelength foldsdevelop.

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Cooper et al. 1441

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Mark Cooper

Mark Cooper has a B.Sc. degreein geology from Imperial Collegeand a Ph.D. from Bristol University.He taught at University CollegeCork prior to joining BP in 1985.He has worked on many basinsworldwide, has published over 35papers, and is an advisory editorfor the Journal of the GeologicalSociety . He is now with Pan-Canadian Petroleum in Calgary.

Fergus Addison

Fergus Addison received a B.Sc.degree in geology from the Uni-versity of Aston in Birmingham anda Ph.D. in geophysics from theUniversity of Newcastle upon Tyne,England. He joined BP in 1982 andis currently manager of LlanosFoothills exploration for BPExploration Colombia.

Ricardo Alvarez

Ricardo Alvarez received a B.Sc. degree in geologyfrom the Colombian National University in Bogotá in1986. He joined BP Exploration in 1987 and is currentlybusiness planning team leader for BP ExplorationColombia.

Mario Coral

Mario Coral received a B.Sc. degreein geology from the ColombianNational University in Bogotá in1993. He joined BP in the sameyear and is currently working as ageophysicist on the developmentof the Cupiagua oil field inColombia.

R. H. Graham

Rod Graham graduated from the University of Walesin 1966, obtained a Ph.D. from Imperial College in 1969,and taught at University College Swansea for thirteenyears before joining BP in 1984 as a structural geologist.He has published many papers on structural geology.

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Van der Hammen, T., J. H. Werner, and H. van Dommelen, 1973,Palynological record of the upheaval of the northern Andes: astudy of Pliocene and lower Quaternary of the ColombianEastern Cordillera and the early evolution of its high-Andeanbiota: Paleobotany and Palynology, v. 16, p. 1–122.

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von Buch, L., 1839, Petrifications recueillis en Amerique par M. A.de Humboldt et par Charles Degenhardt: Reprinted, 1885, inEwald, Roth and Dames, Leopold von Buch GesammelteSchriften, 4, Band, 2, Halfte, p. 519–542.

Zumberge, J. E., 1984, Source rocks of the La Luna Formation(Upper Cretaceous) in the Middle Magdalena Valley, Colombia,in J. G. Palacas, ed., Petroleum geochemistry and source rockpotential of carbonate rocks: AAPG Studies in Geology 18, p. 127–133.

1442 Llanos Basin Development

ABOUT THE AUTHORS

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Cooper et al. 1443

Anthony B. Hayward

Tony Hayward received a B.Sc.degree and a Ph.D. in geology fromthe University of Edinburgh. Hejoined BP Exploration in 1982 andis currently exploration managerfor BP Exploration Colombia.

S. Howe

Spencer Howe received a B.Sc.degree in applied physics andchemistry and an M.Sc. degree ingeophysics from the University ofDurham, England. He joined BPExploration in 1984 and is current-ly working on Llanos Foothillsexploration in Colombia.

Jaime Martinez

Jaime Martinez received a B.Sc.degree in geology from theColombian National University inBogotá in 1991. He joined BPExploration in 1992 and is current-ly working on Llanos Foothillsexploration.

Joaquín Naar

Joaquín Naar received a B.Sc.d e g r e e i n g e o l o g y f r o m t h eColombian National University inBogotá in 1993. He joined BP inthe same year and is currentlyworking as a development geolo-gist on the Cusiana oil field inColombia.

Ricardo Peñas

Ricardo Peñas received a B.Sc.degree in geology from theColombian National University inBogotá in 1990. He joined BP in1992 and is currently working asan exploration geologist in thebusiness planning team for BPExploration Colombia.

Andy Pulham

Andy Pulham received a B.Sc.degree in physical geography andgeology at Liverpool University anda Ph.D. in geology at SwanseaUniversity. He has worked for BPExploration in Aberdeen, London,Houston, and Bogotá. He is cur-rently working as a sedimentolo-gist with responsibility for reser-voir description in the Cusianafield.

Adriana Taborda

Adriana Taborda received a B.Sc.degree in geology from theColombian National University inBogotá in 1984. She has worked inthe Colombian oil industry as a geo-physicist since then, and joined BPin 1989. She is currently working asa development geophysicist on theCusiana oil field in Colombia.