Feb03 Tectonic Evolution San Jacinto

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Abstract

The San Jacinto Fold Belt is interpreted to be an inverted riftor graben on the northwestern continental margin of SouthAmerica. Tectonic evolution of the San Jacinto Fold Belt fromCretaceous to present is complex and related to the develop-ment of the Caribbean Plate and its interaction with the Cocos,Nazca and South American plates. Tectonism originated in theUpper Cretaceous as a small rift or graben as a result of theopening of the Atlantic. Early sedimentation was characterized

by pelagic and hemipelagic sediments, including UpperCretaceous basalts followed by Palaeocene-Middle Mioceneturbiditic deposits derived from the continent. During theMiddle Eocene, lateral compressional stresses caused the firstphase of inversion. Previous normal faults were reactivated ashigh angle reverse faults. The subduction zone shifted to the

west of the continental margin and the Upper Eocene forma-tions were deposited. At the end of the Eocene a regionaltectonic change occurred and sedimentation started in thenewly-formed San Jorge and Plato basins to the east. FromOligocene to Miocene the San Jacinto Fold Belt emerged in someplaces while other locations recorded sedimentation. UpperMiocene-Pliocene rifting in the east generated sedimentation inadjacent basins and in some places in the San Jacinto area.Finally, from Pliocene to Pleistocene, uplift, folding and faultingoccurred in the San Jacinto belt and the adjacent western Sin

belt because of Andean tectonic activity.

Geology

The San Jacinto Fold Belt (SJFB) is located in the LowerMagdalena Valley of northwest Colombia, South America,

between the Sin and San Jorge-Plato basins (Figure 1). The SJFBtrends N20E for a length of 360 km and is approximately 20 kmwide. On the surface it is expressed by three anticlinoria which

are separated by low areas covered by recent sediments. Ourwork is mainly focussed in the central, San Jacinto anticlinorium

area.

Some regional and local studies have been completed in theLower Magdalena Valley (mainly in the San Jorge and Plato

basins), where there is oil and gas production from Tertiaryreservoirs (Haffer, 1964; Duque-Caro, 1984; Gulf, 1983; Duque-Caro et al. 1987; Reyes and Camargo, 1995). However, a struc-tural model and an understanding of the tectonic evolution ofthe San Jacinto Fold Belt do not exist. In addition, the relation-ship between the San Jacinto Fold Belt and its adjacent basins isnot clear.

Although some wells have been drilled in the area, knowl-edge of the basin is not well constrained because most of the

wells are very shallow and do not reach the Cretaceous rocks orthe basement.

The geological position of the SJFB makes it vital to under-stand the regional plate kinematics of the Caribbean area(Figure 2). The SJFB is located at the junction of four tectonicplates: the Caribbean, Cocos, Nazca and South American plates(Pennington, 1981; Kellogg et al., 1985; Mann, 1999).

To the north, the Caribbean plate is moving eastward relativeto the Americas, accommodated by right-lateral strike-slip faults

along the South American plate boundary and left-lateral strike-slip faults along the North American plate boundary. Oceaniclithosphere of the North and South American plates is beingconsumed along the Caribbean and the Lesser Antillas subduc-tion zone. The Cocos and Nazca plates are subducting beneaththe western and southwestern edges of the South AmericanPlate (Mann, 1999).

According to published models, this margin of the SouthAmerican plate has passed through three main tectonic regimesa strike-slip margin, an orthogonal A-type subduction zone and

Tectonic Evolution of theSan Jacinto Fold Belt, NW ColombiaMiryam Caro and Deborah Spratt; Fold-Fault Research Project, University of Calgary

Figure 1. Tectonic map of northern Colombia showing the major geologicalprovinces (after Case et al., 1990). The displayed seismic lines are annotated A, Band C.

Figure 2. Major crustal provinces of the Caribbean (after Mann, 1999).

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February, 2003 CSEG RECORDER 37

an oblique B-type subduction zone (Pennington, 1981; Gulf, 1983;Kellogg et al., 1985). This evolution was accompanied byNeogene strike-slip movement on presently active strike-slipfaults. In the first phase, from Upper Cretaceous to MiddleEocene, the Cansona and San Cayetano formations (Figure 3)were accreted. In the second stage, folding and high angle imbri-cate thrust faults were formed because of A-type subductionduring the Middle Eocene. In the third stage, development of aforearc basin took place when B-type subduction occurred

between the Caribbean plate and the accreted South Americanplate (Middle Eocene to present) (Duque-Caro, 1984; Gulf, 1983).

On a regional scale, the sedimentation and tectonic evolutionof the SJFB was controlled by four main features, the Romeralfault, the Sin lineament, the Bucaramanga-Santa Marta fault andthe Oca fault (Figure 1).

The Romeral fault forms the eastern border of the SJFB. Itcontinues southward as the boundary of the South Americanplate. This prominent feature separates western and north-western Colombia from the main South American Plate and sepa-rates oceanic crust to the west from continental crust to the east.

In the study area the Romeral fault limited sedimentation to thewest until Lower Oligocene, when sedimentation started in theSan Jorge basin. From Upper Cretaceous to Early Eocene, theRomeral fault acted as a megashear with left lateral movement(Gulf, 1983, Escalante, 1990). During Early-Middle Miocene themovement changed to high angle, imbricate reverse faultingwhich deformed the Upper Cretaceous rocks.

The Sin lineament trends N20E, parallel to the Romeral faultseparating the SJFB from the Sin belt to the west. The expressionof this fault is not clear at the surface as it lies below Quaternaryalluvial deposits. However, this lineament marks a change instructural style between the San Jacinto and Sin belts. Also, theSinu lineament defines the western limit of mud volcanism,which is absent in the San Jacinto Fold Belt.

The Bucaramanga-Santa Marta fault is a left-lateral strike-slipfault active since ?Early Tertiary with a maximum rate ofdisplacement during the Miocene (Gulf, 1983). In the EarlyMiocene, the Santa Marta massif was uplifted as a horst along theBucaramanga-Santa Marta and Oca faults. The Bucaramanga-

Santa Marta fault forms the boundary between the Plato andCesar basins and is still active.

The Oca fault is a right-lateral strike-slip fault with approxi-mately 65 km of displacement. It is contemporaneous with theBucaramanga-Santa Marta fault. The maximum rate of displace-ment occurred during the Miocene and major displacementalong the Oca fault commenced in Late Oligocene (Driscoll andDiebold, 1999).

Major Unconformities

Sediment distribution and non-deposition or erosion areimportant in the depositional and tectonic history of the San

Jacinto Fold Belt and in its adjacent basins. The unconformitiesreflect tectonic events and, although they cannot be easily identi-fied in the seismic profiles in the SJFB due to poor imaging andstructural complexities, they are documented widely in strati-graphic studies through the Lower Magdalena Valley (Duque-Caro, 1984; Gulf, 1983).

The main unconformities in the study area are Middle EoceneMiddle-Upper Eocene, Lower Oligocene, Middle Miocene, andUpper Pliocene (Figure 3). The Middle Eocene unconformityrepresents the first compressional event and it is well docu-mented in the northwestern part of South America (Escalante1990; Driscoll and Diebold, 1999). The Middle-Upper Eocene

unconformity reflects the second phase of deformation in thearea.

Reworked microfauna of the Eocene-age San JacintoFormation in the Oligocene Carmen Formation contain evidenceof the Lower Oligocene unconformity. The Middle Mioceneunconformity is the second regional unconformity in north-western South America.

The Upper Pliocene unconformity records a drastic change inthe sedimentary environment from marine to continental

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Figure 3. Stratigraphic column representative of the San Jacinto Fold Belt.

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deposits. This unconformity correlates with the Andean orogenyin the Eastern Cordillera of Colombia. It is recorded at 3.2 Ma andmarks the beginning of large amounts of sedimentation of theSincelejo Formation to the west.

Generally, the SJFB unconformities demonstrate that environ-ments began to change before each erosional interval.

Sedimentation resumed afterwards in the form of basal conglom-erates derived from nearby continental slopes. Tectonism appearsto have been responsible for most of the unconformities.However, other mechanisms, such as relative sea-level fluctua-tions, changes in the bottom water circulation or water-mass

boundaries, could also be responsible in part for interruptingdeeper-water deposition. Moreover, all these mechanisms can belinked together with agents such as regional tectonism, readjust-ment of crustal plate positions and, possibly, climatic cycles.

Seismic Data Interpretation

The interpretation started with the surface geology wheregeological maps were available and we concentrated on a struc-tural interpretation. Detailed stratigraphic picks could not befollowed laterally, therefore only the main unconformities wereinterpreted. In areas where particularly poor resolution and noiseare apparent in the seismic profiles, the interpretation was basedon the regional context.

Seismic line A (Figure 4) is a northwest-southeast representa-tive structural seismic section (87.4 km) across the southern San

Jacinto Fold Belt. Three wells are close to this line but only one isin the SJFB. From west to east this line shows the Sin basin, San

Jacinto Fold Belt, the Romeral fault system and San Jorge basin.Surface sediments are mainly Quaternary sediments. In the Sin

basin, seismic reflections in the synclines are very clear whereasthe associated anticlines are difficult to see (Figure 4). Thisphenomenon has been attributed to diapirism (Duque-Caro,1984). In the centre of the line, the SJFB is present, showing thepossibly inverted San Jacinto graben covered by Upper Mioceneand Pliocene sediments.

Line B (Figure 5) runs (from west to east) across the middlepart of the San Jacinto Fold Belt. The SJFB is located in the middlepart of the line, where it is difficult to differentiate any structuresor any reflectors. The western part of the line corresponds to theSin belt, which has some interpretable reflections. The faultingobserved in this line is covered by Quaternary sediments. It islikely that some structures are present in the SJFB but poor reso-lution makes it impossible to identify any. The east dippingreflections in the eastern part of the line belong to the rocksdeposited in the Plato basin, where deposition started in LowerOligocene.

Line C (Figure 6) runs almost parallel to line B and continuesfurther to the east. The seismic data clearly show the basin-

bounding fault, dipping eastward, which separates the Platbasin from the San Jacinto Fold Belt. The faults located below themain unconformity, from SP 900 to 1500, could be the expressionof the Romeral fault system. It is difficult to identify the basementdue to poor resolution at depth. The basement should be conti-nental to the east of this fault system and oceanic to the west. Thetwo wells that project on to this line only reached the lower partof the Lower Oligocene Cinaga de Oro Formation.

Tectonic Evolution

The new model (Figure 7) for the tectonic evolution of the SJFBis based on the regional interpretation of seismic data, surfacegeology, integration of the sparse well data and some localstudies. Further work is needed to support, or negate, this model

Figure 4. Line A. Note the typical faulting related to the San Jacinto rift.

Figure 5. Line B. Note the highly folded and faulted Cansona and Cayetano forma-tions (greens).



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The San Jacinto Fold Belt was a continental margin of theColombian Caribbean in the Upper Cretaceous which was likelydeformed and accreted to the South American continent as aresult of the tectonic interaction of the Caribbean, Cocos, Nazca,North American and South American plates. Evolution of theSJFB commenced in the Upper Cretaceous when pelagic andhemipelagic sedimentation took place on the deep ocean floor.The regional tectonics involved a volcanic arc setting in the senseof Macdonald (1993). Sedimentation was restricted to a small rift

or graben, limited to the east by the Romeral fault system orpalaeosuture, and to the west by the Sin lineament. To the eastof the Romeral fault, the Cansona and San Cayetano formationswere not deposited. Continental crust is present in the eastwhereas to the west oceanic crust exists (Duque-Caro, 1984;Escalante, 1990).

It is difficult to determine how the Romeral fault actedthrough time because of superposition of structural events andlater sedimentation in the San Jorge-Plato basins. According toGulf (1983), the Romeral fault or Cansona megasuture was amegashear from Late Cretaceous to Early Eocene with left lateral

movement. Another interpetation is that the Romeral fault is aneasterly dipping Lower Cretaceous thrust fault.

The origin of the proposed rift or graben is the opening of theAtlantic Ocean in the Upper Cretaceous. At this time the Nazcaplate began to subduct under the western margin of the SouthAmerican plate, generating a volcanic arc with the Romeral faultas a suture zone (Kellogg et al., 1985; Pindell et al., 1988;Escalante, 1990; Toto and Kellogg, 1992; Reyes and Camargo,1995).

The San Cayetano turbidites or flysh syn-rift deposits weredeposited from Paleocene to Lower Eocene, followed by deposi-

tion of the Maco Formation conglomerates. During this time thecontinent emerged, including the Santa Marta Massif and theCentral Cordillera (Duque-Caro, 1984).

During the Middle Eocene, lateral compressional stressescaused inversion of the rift. Previous normal faults of the riftwere reactivated as high angle reverse faults. Possibly, this upliftpermitted deposition of the Chengue Formation. At this time theSin lineament clearly started to separate the abyssal shelf fromthe uplifted San Jacinto rift, or graben, and the subduction zonemigrated to the west of the continental margin.

During the Upper Eocene the San Jacinto Formation wasdeposited unconformably over the Chengue Formation, probablydue to small pulses of uplift in the SJFB. At the end of the Eocenea regional tectonic change occurred. The Serrana de Perij, theSantander and the Santa Marta massifs record evidence forpulses of uplifting (Kellogg, 1985). In the Oligocene distensionstarted east of the San Jacinto rift in the continental platform andsedimentation of the San Jorge and Plato basins commenced.

During the Oligocene to Miocene, either uplift of the SanJacinto rift continued or the San Jacinto rift remained emergentThe Middle Miocene unconformity also supports the San Jacintorift remaining emergent during this time interval. In this area the

Figure 6. Line C. The interpreted faults belong to the Romeral fault system. Notethe difficulty in interpreting horizons below the first unconformity.

Figure 7. Sketch of the tectonic evolution of northwestern Colombia.



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Cinaga de Oro Formation (Lower Oligocene Lower Miocene)and the Porquero Formation (Lower Miocene) were notdeposited. However, on the flanks of the San Jacinto rift or grabenthe upper part of the Cinaga de Oro and Carmen formationswere deposited. These two formations are the first post-riftdeposits in the area. The Lower Oligocene unconformity appearsto mark the end of the San Jacinto rift.

From Upper Miocene to Pliocene, rifting to the east, produced by lateral compressive stresses due to separation of the SantaMarta massif, generated sedimentationof the Plato and San Jorge basins. TheCerrito Formation was deposited insome parts of the San Jacinto Fold Belt.

From Pliocene to Pleistocene anevent of uplift, folding and faultingoccurred in the San Jacinto and Sin

belts. This event has been correlatedwith the Andean Orogeny. During thistime an increment of continental sedi-

mentation occurred in the Plato andSan Jorge basins. In the Sin beltdiapiric events are associated with thisuplift. The abyssal shelf migrated towest of the Sin basin, where pelagicand turbidite sequences were beingdeposited.

Discussion

This study interprets the San JacintoFold Belt as an inverted paleo-rift orgraben developed on the continentalmargin of the South American plateand initiated in a volcanic arc setting.Other interpretations are possible. Thelack of information in both the subsur-face and at the surface makes it diffi-cult to either support or negate thismodel. The few wells in the area arevery shallow and do not assist in thevisualization of the three dimensionsof this basin.

Alternative models include i) conti-nental basin uplifted and folded in thetwo main compressional events, ii)

trench deposits inverted by thrustingand uplift, iii) strike-slip pull-apart basin, and iv) an accretionary wedgemodel. The last model is the one used

by most authors and companies,however there is no clear evidence inthe seismic sections for an accretionarywedge.

The fore-arc or volcanic arc settingincludes an accretionary complex,trench- slope basins and fore-arc basins

(Macdonald, 1993). Sediments typical of these basins include insitu pelagic and hemipelagic sedimentation in a deep ocean floorenvironment, such as black shales, red clays, cherts, siliceous andcalcareous oozes. There should be a clastic supply from the conti-nental side of the complex, a clastic supply from one end of thetrench (not represented in the SJFB to date) and sediments carriedalong the trench by turbidity currents. Stratigraphically, many of

the features typical of a fore-arc or volcanic arc setting are presentin the SJFB.

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The features imaged in the seismic lines A and B lead us topropose that the rift model started with a volcanic arc on a conti-nental margin. There are some very steeply dipping reverse andnormal faults resembling a rift. Also, the thrust faults mapped inthe San Jacinto Fold Belt have a very steep dip: 70 at the surface.Texas Petroleum (1991) reports two normal faults where thrustfaults are mapped. It is possible that these are reactivated thrust

faults with more recent normal motion. More detailed field workis needed to unravel problems such as these.

The three main blocks of the San Jacinto Fold Belt have expe-rienced different tectonic events. The San Jacinto anticlinorium ismore strongly deformed than the others. It also has the highestmountains in the area (800 m). Many further questions exist. Wasthe inversion Middle Eocene in age? Was inversion accompanied

by transpression? Was motion on the Romeral fault reverse orstrike-slip during the inversion? What was the effect of migrationof the Caribbean Plate to the east during the Eocene? Accordingto Pindell et al. (1988) the Caribbean plate migrated about 1100km eastward, beginning in the Eocene. How was this motionaccommodated? Another point of discussion involves tectonic

versus eustatic processes. How did tectonism affect the sedimen-tation and vice versa?

Hydrocarbon Potential

In the San Jacinto Fold Belt some wells have been drilled butno oil and gas accumulations of economic importance have beendiscovered. Ecopetrol drilled a well which produced from anEocene limestone (5.5 MMCFD) but two other nearby wells weredry. One well produced 100 BODP from the San CayetanoFormation in two years.

The Cansona Formation, with 5.6 TOC, type-II kerogen, is thebest source rock in the SJFB. The turbidites of the San Cayetano,Maco and San Jacinto formations are reservoir rocks. The mainproblem for hydrocarbon potential in the SJFB is the seal and the

timing of structures versus migration. Oil was generated in theSJFB, but it migrated before the younger structures were formed

It is possible that some structures remain with oil, especiallywhere unconformities cover the older stratigraphy. Howeverthese traps could be small. More seismic data, wells and fieldwork are required to discover these possible traps.

Conclusions

Evolution of the San Jacinto Fold Belt was controlled by exten-sional and compressional events related to the tectonic evolutionof the Caribbean area and the subduction of the Cocos and Nazcaplates beneath the western and southwestern edges of the SouthAmerican plate.

The San Jacinto Fold Belt began its development in the UpperCretaceous with the deposition of pelagic and hemipelagic sedi-ments in a small rift or graben generated by the opening of theAtlantic. It is limited by the Romeral fault to the east and by theSin lineament to the west. Lateral stresses during the MiddleEocene inverted the rift. Post-rift sedimentation and a final eventof uplift occurred from Pliocene to Pleistocene. During this timethe deposition was mainly in adjacent basins.

The main unconformities are Middle Eocene, Middle-UpperEocene, Lower Oligocene, Middle Miocene and Upper Pliocene.More intraformational unconformities are recorded in the area.

The San Jacinto Fold Belt stratigraphic sequence shares manycharacteristics with volcanic arc sequences. These include in situpelagic and hemipelagic sediments clastics supplied from thecontinent and sediments carried along the trench by turbiditycurrents.

Previously an accretionary wedge model was used frequentlyhowever the available seismic sections show no clear evidence



Miryam Caro received her B.Sc. in Geology in 1986 from the Universidad Nacional de Colombia. She workedfor Ecopetrel-ICP Colombia from 1986-2001 as an exploration geologist specializing in structural modelling andseismic data interpretation. She is currently pursuing an M.Sc. with the Fold-Fault Research Project at theUniversity of Calgary.

Deborah Spratt is Professor of Geology at the University of Calgary and a director of the Fold-Fault ResearchProject. Her research interests include combining detailed mapping with interpretation of seismic, well log, radar,gravity and aeromagnetic data to construct balanced cross-sections and palinspastic maps through complex struc-tural belts; documenting 3-D changes in geometries and structural styles, measurement and analysis of fracturedensities, lengths, distributions and fillings in folded and faulted reservoir units; compilation of ramp angles andflat lengths for stratigraphic intervals cut by faults in outcrop and seismic sections; determination of controllingfactors such as lithofacies changes, inherited structures and variations in rock strength and kinematic modellingand strain analysis of fault-related structures. She has taught many structural, field, seismic interpretation, engi-

neering and physical geoscience courses at the University of Calgary and has lead numerous field seminars in the Rockies for studentsfrom several universities, the petroleum industry and in conjunction with CSPG, CSEG, GAC, GSA and CGU conferences.


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for an accretionary wedge. More work is needed to integrate allreliable geological and geophysical data, such as stratigraphy, sedi-mentology, magnetic and gravity surveys, before the best modelwill be achieved. An improved comprehension of the San JacintoFold Belt tectonic processes would improve the development of itshydrocarbon potential.

In the long term, more seismic and well data should be acquiredin the SJFB to improve the comprehension of its stratigraphy andstructural evolution and to improve exploration results.

Acknowledgements

We gratefully acknowledge the financial and technical supportfor this project by Ecopetrol-ICP and sponsors of the Fold-FaultResearch Project. R

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Duque-Caro, H., 1984, Structural style, diapirism and accretionary episodes of Sin-SanJacinto Terrane, South Western Caribbean border: Geological Society of America, Memoir162, 303-316.

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Pindell, J., Cande, S. C., Pitman III, W. C., Rowley, D., Dewey, J., Labrecque, J., andHaxby W., 1988, A plate-kinematic framework for models of Caribbean evolutionTectonophysics, 155, 121-138. Elsevier Science Publishers B. V., Amsterdam.

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Toto, El A. & Kellogg, J. N., 1992, Structure of the Sinu-San Jacinto Fold Belt an activeaccretionary prism in northern Colombia: Journal of South American Earth Sciences, 5(2)211-222.

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