Basin Development and Tectonic History of the Llanos Basin. Cooper 36

Basin Development and Tectonic History of the Llanos Basin, Colombia M. A. Cooper F. T. Addison R. Alvarez A. B. Hayward S. Howe A. J. Pulham A. Taborda BP Exploration (Colombia) Ltd. Bogotá, Colombia

Abstract

T

he Llanos basin lies east of the Eastern Cordillera in northeastern Colombia. Basin development commenced with a Triassic–Jurassic synrift megasequence related to the separation of North and South America in the Caribbean. Basin development continued in the Cretaceous as a back-arc megasequence behind the Andean subduction zone. Marine deposition was abruptly terminated during the early Maastrichtian due to final accretion of the Western Cordillera. The accretion of the Western Cordillera created the pre-Andean foreland basin megasequence (Paleocene–early Miocene), which covered the Magdalena Valley, Eastern Cordillera, and Llanos basin. This megasequence is dominated by fluviodeltaic strata. The overlying Andean foreland basin megasequence commenced with deformation in the Central Cordillera and Magdalena Valley. The Andean foreland basin megasequence also includes the Guayabo Formation, which is a classic molasse sequence shed from the developing mountains of the Eastern Cordillera as deformation moved eastward into the Llanos foothills. The deformation in the Llanos foothills is a combination of inversion of preexisting extensional faults and thin-skinned thrusting.

Resumen

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a Cuenca de la Llanos se encuentra localizada al oriente de la Cordillera Oriental en el nororiente de Colombia. El desarrollo de la cuenca comenzó con una megasecuencia de “synrift” Triasica-Jurasica relacionada con la separación de Norte y Suramerica en el Caribe. El desarrollo de la cuenca continuó durante el Cretácico con una megasecuencia de “back-arc” en frente de la zona de subducción de los Andes. La sedimentación marina terminó abruptamente durante el Maestrichtiano temprano debido a la acreción final de la Cordillera Occidental. La acreción de la Cordillera Occidental dió origen a una megasecuencia pre-andina de antepaís (Paleoceno a Mioceno Inferior) la cual cubrió el Valle del Magdalena, la Cordillera Oriental y la Cuenca de los Llanos. Esta megasecuencia está dominada por sedimentos fluvio-deltaicos. La megasecuencia andina de antepaís suprayacente comenzó con la deformación en la Cordillera Central y el Valle del Magdalena. La megasecuencia andina de antepaís también incluye la Formación Guayabo la cual es una clásica molasa proveniente de la erosión de la naciente Cordillera Oriental a medida que la deformación se movió hacia el este en el piedemonte de los Llanos. La deformación en el piedemonte de los Llanos es una combinación de inversión de las fallas extensionales pre-existentes y cabalgamientos de escamación delgada.

Cooper, M. A., F. T. Addison, R. Alvarez, A. B. Hayward, S. Howe, A. J. Pulham, and A. Taborda, 1995, Basin development and tectonic history of the Llanos basin, Colombia, in A. J. Tankard, R. Suárez S., and H. J. Welsink, Petroleum basins of South America: AAPG Memoir 62, p. 659–665.

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INTRODUCTION The physiography of Colombia is dominated by the Andes mountains in the western half of the country and by the Amazon-Orinoco basin in the east. The Colombian Andes are split into three ranges—the Western, Central, and Eastern Cordilleras—which to the south merge into a single range in Ecuador. To the east of the Eastern Cordillera is Los Llanos, an elevated savannah that is part of the catchment area for the Rio Orinoco (Figure 1). Major work that has been done on the stratigraphy, tectonics, and regional tectonic setting of Colombia include Hettner (1892), Hubach (1957), Bürgl (1961), Etayo-Serna (1979), Fabre (1983), McCourt et al. (1984), Pilger (1984), Aspden and McCourt (1986), Ben Avraham and Nur (1987), Megard (1987), PardoCasas and Molnar (1987), Burke (1988), Butler and Schamel (1988), and Montgomery (1992). The major tectonic events that have influenced the development of the Llanos basin are all closely tied to the development of the active margin of western South America. The regional structural evolution is divisible into eight major events: 1. Triassic–Early Cretaceous—Rift basins developed as a result of the separation of North and South America as the Caribbean opened (~235–130 Ma); this is the synrift megasequence. 2. Barremian–Maastrichtian—A prolonged period of episodic extension occurred on a series of extensional faults (e.g., the Guaicáramo fault system) along with passive regional subsidence in a backarc basin setting (~125–74 Ma); this is the back-arc megasequence. 3. Maastrichtian–early Paleocene—The final event in the accretion of the Western Cordillera caused uplift and erosion of the Central Cordillera (~74–65 Ma); this is the onset of the pre-Andean foreland basin megasequence. 4. middle Eocene:—An early compressional deformation event affected the Magdalena Valley and the western margin of the Eastern Cordillera (~49–42 Ma) due to an increase in convergence rates of the Nazca and South American plates (Pardo-Casas and Molnar, 1987; Daly, 1989). 5. late Eocene–late Oligocene—A prolonged period of subsidence and localized normal faulting occurred in response to flexure of the lithosphere in the foreland basin created by the deformation load of the Western and Central Cordilleras (~39–29 Ma). 6. late Oligocene–early Miocene—Deformation in the Cauca and Magdalena valleys caused continued subsidence in the Llanos basin (~29–16.5 Ma). 7. middle Miocene—A phase of rapid subsidence occurred as deformation, uplift, and erosion commenced in the Eastern Cordillera and established the foreland basin depocenter in the Llanos foothills (~16.5–10.5 Ma); this is the Andean foreland basin megasequence. 8. late Miocene–Recent—The latest phase of compression and inversion associated with the formation of

Figure 1—Map of major tectonic provinces and sutures in Colombia.

the frontal fold and thrust belt of the Eastern Cordillera (10.5 Ma–present day). The aim of this paper is to present a brief review of the basin development, chronostratigraphy, and structural style of the Llanos basin and Llanos foothills to provide a regional context for the recently discovered Cusiana giant field.

REGIONAL STRATIGRAPHIC FRAMEWORK AND BASIN EVOLUTION The basin stratigraphic model has been developed on the basis of published data and the log, core, seismic, and outcrop data acquired by BP during regional studies of the Llanos basin and exploration of the Llanos foothills. The chronostratigraphic summary of the Llanos basin (Figure 2) is based on a sequence stratigraphic scheme originally developed for the Cusiana field and adjacent areas of the foreland. This stratigraphic scheme was subsequently extended throughout the Llanos basin by careful correlation of the well logs and using available biostratigraphic data. Comparison of the sequence stratigraphy with the conventional industry lithostratigraphic scheme for the Llanos basin is illustrated in Figure 3. The sedimentary rocks in the Llanos foothills and Llanos basin were deposited in a basin that evolved from a back-arc basin in the Late Cretaceous to a foreland basin in the early Tertiary and whose depocenter moved

Basin Development and Tectonic History of the Llanos Basin, Colombia

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Figure 2—Chronostratigraphic summary diagram for the Llanos basin and foothills based on well and outcrop data. The sea level curve of Haq et al. (1987) has been adjusted to the BP time scale.

progressively eastward throughout the Tertiary. The resulting stratigraphy is a highly punctuated succession of Upper Cretaceous–lower Tertiary strata, representing periodic marginal deposition in major contemporaneous depocenters to the west and north. The stratigraphic record becomes more complete westward in the Eastern Cordillera and the Magdalena Valley, although Tertiary rocks are only sparsely preserved in the Eastern Cordillera because of late Miocene and Pliocene uplift and erosion. The following basin evolution model has been developed on the basis of available published information and by integrating recent BP well, seismic, and outcrop data. Rocks older than the Late Cretaceous include a succession of Paleozoic metamorphic and sedimentary rocks that have only been penetrated in a few wells in the Llanos. Triassic–Lower Cretaceous rocks are absent in the area except for possible small, localized synrift sequences. Upper Cretaceous strata thus generally rest directly on Paleozoic basement. Upper Cretaceous deposition of the back-arc megasequence was initiated in Cenomanian time (98–91 Ma) during a regional transgression that drowned the exposed Paleozoic rocks of the Llanos region and

resulted in deposition of a series of basal, shallow marine and shoreline Cretaceous sandstones, which progressively onlapped farther eastward onto the Guyana shield (Figure 2). These sandstones are depositional sequence K50 (Une Formation equivalent) (Hubach, 1931). In the Turonian–early Coniacian (91–88 Ma), global sea level rise (Haq et al., 1987), combined with anoxic upwelling conditions, resulted in deposition of a succession of marine mudstones, cherts, and phosphates (Figure 2). These sediments formed an excellent marine source rock (sequence K60, Gacheta Formation) (Miller, 1979) in the Llanos area. This sequence is the equivalent of prolific source rocks such as the Villeta Shale Formation in the Upper Magdalena Valley (Beltrán and Gallo, 1968) and the La Luna Formation of the Middle Magdalena Valley and western Venezuela. Sequence K60 deposition was terminated by a fall in relative sea level in the Coniacian–early Santonian (88–85 Ma). The Llanos foothills area was on the eastern margin of the basinal system. Sequences K70 and K80 (which equate approximately with the Guadalupe; Hettner, 1892) were deposited at this time and represent two major cycles of eastward shoreline progradation, aggradation, and retrogradation. They are dominated by high-

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Cooper et al. TYPE LOG SIGNATURE PROPOSED SEQUENCES

AGE

GAMMA RAY 0m

RESISTIVITY

CONVENTIONAL FORMATION NAMES FARALLONES

PLIOCENE

1500 m

LOWER

GUAYABO

1000 m

LATE

T90

UPPER

500 m

LEON

MIDDLE

MIOCENE

2000 m

T80

2500 m

EARLY

T70

C1

T60 T50

3500 m

CARBONERA

3000 m

C2 C3 C4 C5 C6

OLIGOCENE

C7

T40

L. CRETACEOUS

4000 m LATE EOCENE

T30

PALEOCENE

CAMPANIAN

T20 K80

SANTONIAN

K70

CONIACIANTURONIAN

K60

CENOMANIANALBIAN

K50

C8 MIRADOR LOS CUERVOS

BARCO 4500 m

5000 m

GUADUAS U. GUADALUPE GUADALUPE SHALE L. GUADALUPE

GACHETA SHALES UNE

Figure 3—Comparison of stratigraphic schemes for the Llanos foothills and Llanos basin. The symbols in the sequences column are the same as in Figure 5. The symbols in the stylized resistivity log column indicate the depositional environments and are the same as in Figure 2.

energy, quartz-rich shoreface sandstones supplied from the Guyana shield to the west and exhibit a widespread distribution across the Llanos basin and foothills. The Campanian K80 sandstones form the oldest proven commercial reservoir unit in the foothills. At the top of

sequence K80 is a shale unit that has been mistakenly identified as the Maastrichtian–Paleocene Guaduas Formation (Figure 3) (Sarmiento, 1992) in some of the earlier wells in the Llanos foothills (e.g., Medina-1). Recently acquired data by BP has conclusively dated these youngest Cretaceous rocks in the foothills as Campanian. The final accretion event in the Western Cordillera commenced at the end of the Cretaceous. A relative drop in sea level, probably linked to the onset of compression to the west, resulted in a fundamental change in the nonmarine deposition of the pre-Andean foreland basin megasequence. Sequence T10 is not present in the Llanos basin and foothills, being represented by a hiatus of about 20 m.y. that spans the Cretaceous–Tertiary boundary. Renewed deposition commenced about 60 Ma in the late Paleocene in response to a far-reaching transgression. The Barco Formation (Notestein et al., 1944) forms the basal transgressive part of sequence T20, which was laid down on a major unconformity surface. It mainly comprises sandstone-rich estuarine deposits. Marine influence is strong throughout the Barco Formation in the Cusiana field area in the Llanos foothills, but at the top of the formation there is a relatively abrupt upward transition into more heterolithic coastal plain and alluvial plain deposits. T20 sandstone deposition ended as the late Paleocene transgression weakened and a relative highstand in sea level was established (~59 Ma). During the subsequent regression, the regional shoreline gradually shifted westward. The sediments laid down during this sea level highstand were muddy lower coastal plain deposits (Los Cuervos Formation) (Notestein et al., 1944). In the Llanos, a hiatus of up to 20 m.y. separates sequences T20 and T30 and appears to correlate with a similar unconformity in the Middle Magdalena Valley. Deposition in the area was renewed in the middle–late Eocene (~39 Ma). Deposition of sequence T30 (Mirador Formation) (Notestein et al., 1944) began in response to a far-reaching transgression that came out of the foreland basins to the west and north. Initial T30 deposition included marine-influenced, sand-rich, valley fill deposits that passed upward into muddier coastal plain sediments. Continued transgression eventually submerged this middle Mirador alluvial plain and established a shallow marine shelf across the Cusiana area. Offshore muds and sandy bioturbated shoreface progradational cycles punctuated by sand-rich fluvial and estuarine valley fill deposits comprised latest Eocene deposition, forming the upper part of the Mirador Formation. All of the coarser grained sandstones in the Mirador Formation in the Llanos foothills are extremely mature quartz arenites. After sequence T30 deposition, four major cycles of marine-influenced lower coastal plain deposition occurred in the Llanos basin and foothills (sequences T40–T70). These sequences are ~34–16.5 Ma and are traditionally termed the Carbonera Formation (Notestein et al., 1944). These cycles are bounded by widespread maximum flooding surfaces. Each cycle consists of a

Basin Development and Tectonic History of the Llanos Basin, Colombia N 6º

ºW 72

Tamara-1

N 6º

M

Pauto-2

NUNCHI A SYNC LINE

F A ULT

SYSTE

º 72 'W 30

RI O P A UTO

RI

O T OCARIA

ºW 72

N

El Morro-1

Cupiagua-1

YOPAL FAULT

YOPAL

N 5º

Cusiana-2A LINE OF SECTION

ºW 73

Cusiana-4

RIO CUSIANA

CUSIANA F A LT U

CHITAM RIO E NA

CUSIA NA AN TICLINE

Leticia-1

STRUCTURAL EVOLUTION AND STYLE

º 72 'W 30

N 5º

0

Medina-1

LIN E IO ANTIC GU A V

The boundary between the Eastern Cordillera and the Llanos foothills is the Guaicaramo fault system (Figure 4). The foothills are about 15–20 km wide and are separated from the foreland to the north and south of the area by the inverted Cusiana-Yopal fault system. In the central part, the inversion faults lie beneath the thinskinned Yopal fault system (Figure 4), which detaches in the lower part of sequence T40. The earliest documented extension on the Cusiana-Yopal fault system occurred in the Late Cretaceous during deposition of sequence K80 (upper Guadalupe Formation). This is based on the differences in thickness of K80 measured in the Cusiana field wells and wells in the immediate foreland (e.g., Leticia-1). It is likely, however, that the fault trend had an earlier extensional history during Early Cretaceous rifting and back-arc subsidence, given the dramatic thickening of the Lower Cretaceous strata from the foreland toward the Eastern Cordillera (Hebrard, 1985; Ulloa and Rodriguez, 1981). Extension continued episodically from Late Cretaceous to middle Miocene time (sequence T80, Leon Formation) and was punctuated by hiatuses and tectonic quiescence. The extensional movements on the Cusiana fault system can be seen on seismic data and are also demonstrable from the thickness of the sequences in the foreland wells as compared to wells within the

YOPAL FAUL T SYS TEM

PAZ DE ARIPORO

GUAICARAMO

mud-dominated highstand systems tract followed by a thin, forced retrogradational systems tract, and ending with a sand-prone transgressive systems tract that culminates with the maximum flooding surface. The sequences are thus not true sequences in the sense of Mitchum et al. (1977), but are genetic stratigraphic units in the sense of Galloway (1989). Type 1 sequence boundaries (Mitchum et al., 1977) have not yet been recognized. The units can be correlated throughout the Llanos basin and show a gradual increase in the sand percentage as the Guyana shield provenance area is approached. In the middle Miocene, the global rise in sea level coincided with the first significant deformation and uplift in the Eastern Cordillera and hence with a significant loading event that tectonically enhanced the relative rise in sea level and the highstand systems tract that resulted (T80 mudstone, Leon Formation) (Notestein et al., 1944). Evidence for partial emergence of the Eastern Cordillera is that sequence T80 becomes more sand prone in the western part of the foothills, suggesting that there was a supply of coarse clastics derived from the west. The final depositional episode in the Llanos was the deposition of about 3000 m of coarse continental clastics in sequence T90 (Guayabo Formation) (Hubach, 1957) from ~10 to 2 Ma ago. This last phase of deposition marks uplift of the Eastern Cordillera immediately west of the foothills and migration of the foreland basin axis to the current location of the Eastern Cordillera foothills (Figure 2). Deposition of this molasse unit caused rapid late stage burial of the Upper Cretaceous–lower Tertiary stratigraphic section in the foothills and Llanos basin.

663

5

10

15

20 km

GUAYABO LEON CARBONERA MIRADOR + CRETACEOUS ANTICLINE SYNCLINE THRUST FAULT

Figure 4—Surface geologic map of the Llanos foothills showing major structures and the location of the cross section and wells referred to in the text.

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NW

SE Guaicaramo Fault System

Cusiana-4

Cusiana-2A

Leticia-1

Cusiana Yopal Fault Fault

0

5

10 Km

Figure 5—Cross section through the Cusiana field in the Llanos foothills. See the “proposed sequences” column in Figure 3 for the key to the stratigraphic units.

hanging wall of the fault. This phase of extension on the Cusiana fault system accommodates the flexure of the lithosphere in response to the load imposed by the regional compressional deformation. As deformation in the Eastern Cordillera migrated eastward, the foothills became involved in the frontal fold and thrust belt (R. Herrera, 1971, personal communication; Colletta et al., 1990). Relatively simple compression along a WNW-ESE trending azimuth caused inversion along the Cusiana-Yopal fault system (Figure 5). The thin-skinned Yopal fault, which detaches within sequence T40, overrides the Cusiana fault to the north and buries the branch line with the latter fault. To the west of these frontal inversion structures is a system of major regional synclines. The western limbs of the synclines are elevated by a series of structures that involve the Upper Cretaceous and lower Tertiary sedimentary sequences. These structures can be modeled as a series of basement involved or thin-skinned (Gacheta Shale, K60 detachment) duplex horses (Figures 4, 5), largely based on the evidence of fault repetition of the Mirador in the El Morro-1 well. The recent paper by Dengo and Covey (1993) contains a cross section through the Unete-1 well that implies a deeper detachment for the Yopal fault than is evident from our studies of the foothills (Figure 5), which are based on an extensive seismic and well database. The other difference in the work by Dengo and Covey (1993) is the absence of inversion on the basement-involved Cusiana-Yopal fault system.

CONCLUSIONS The tectonic history of the region records an initial phase of localized Triassic–Jurassic rifting followed by an Early Cretaceous back-arc basin that became less active by the middle Cretaceous. In the Late Cretaceous, the collision of the Western Cordillera initiated the foreland basin megasequence that has dominated to the present day. The tectonic evolution strongly influenced stratigraphic development of the basin. The back-arc megasequence is characterized by marine strata that are more basinal to the west. The collision event in the Late Cretaceous reestablished continental and coastal plain environments throughout the basin. The pre-Andean foreland basin megasequence shows strongly cyclical sediment patterns with the alternation of continental, coastal plain, and marginal marine environments. This is interpreted to be the result of the highly peneplained Guyana shield hinterland, which had a low paleoslope gradient and was thus susceptible to rapid flooding and regression. This in turn allowed the rapid migration of facies belts within the systems tracts. The main Andean deformation phase caused inversion of preexisting extensional faults and thin-skinned thrust structures. Acknowledgments We wish to thank BP Exploration (Colombia) Ltd. for permission to publish this work, Michel Coudèyre of Total for the type log signatures used, and Andres Tovar for drafting the figures.

Basin Development and Tectonic History of the Llanos Basin, Colombia

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Authors’ Mailing Addresses M. A. Cooper PanCanadian Petroleum Ltd. 150 9th Ave S.W. P.O. Box 2850 Calgary, Alberta T2P 2S5 Canada F. T. Addison R. Alvarez A. B. Hayward S. Howe A. J. Pulham A. Taborda BP Exploration (Colombia) Ltd. Carrera 9A, 99-02, Bogotá Colombia