Structure of the Offshore Sinu Accretionary Wedge. Northern Colombia

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1 Structure of the Offshore Sinu Accretionary Wedge. Northern Colombia J. FLINCH, J. AMARAL*, A. DOULCET, B. MOULY, C. OSORIO* AND J. M. PINCE. Total CSTJF Pau, France, * TEPMA Colombia, Bogota. ABSTRACT The Sinu Accretionary Wedge is located along the northern Colombia Caribbean margin an is partially exposed onshore (Lower Magdalena valley and San Jacinto mountains) and offshore extending from Uraba to the offshore Santa Marta area and joints the northern accretionary wedge of Venezuela in the north. The Offshore and younger part of the prism is much wider and develope than the inner «starved» onshore part. The Proto-Magdalena, Plato and other sedimentary systems contributed to a high sediment supply triggered by the surrection of the Central and Eastern Andean Cordillera since Upper Miocene time, when the offshore part of the prism developed. The offshore Sinu accretionary wedge illustrates the effect of strong lateral (i. e. along strike) variations of syn-tectonic sediments on the structural style of the prism. The thickness of syn-sedimentary strata is huge along the Proto-Magdalena delta (Offshore Cartagena area) which results in a blanketing effect of the deformation (i. e. false image of non-deformed strata). Along strike variations of sediment supply induce lateral changes of the critical taper angle. To re-equilibrate and reach a more stable profile (lower than the critical angle) the wedge collapses through normal faulting and toe-thrusting (i. e. thrusts that accommodate rear extension). Gravitationally induced toe-thrusts are superimposed to pre-existing thrust imbricates related to compression. This complex structure becomes more complicated by the presence of a ductile level of overpressure Oligocene shale that rise as shale ridges. Listric normal faults, toe-thrusts and shale ridges control fore-arc basins like the San Bernardo basin (SW from Cartagena). The present day structure of the offshore Sinu accretionary wedge is the result of NW-vergent compressional imbricates related to the B-subduction process overprinted by extensional and compressional gravitational structures. REGIONAL SETTING Northern Colombia is occupied by a very extensive Accretionary Complex that extends along the Caribbean Sea (i. e. Offshore Sinu Accretionary Wedge) which represents the most external part of the Northern Colombia Accretionary Wedge. The Prism is associated to the B type subduction of the Caribbean Plate underneath the South American plate. Presentday seismicity is associated with this plate contact. Following the Obduction of the Caribbean Oceanic Plateau (Campanian-Maastrichthian time) Accretion took place along the Sinu-Lower Magdalena area since that time until present day. An Inner Accretionary Wedge made by Upper Cretaceous to Oligocene strata floored by oceanic crust is exposed along the San Jacinto and Sinu areas. This reduced starved part of the Prism contrast with the well-developed Offshore prism «Offshore Sinu Accretionary Wedge» or «Outer Prism» consisting of mostly Miocene to Pleistocene imbricates. The prism extends from Uraba Basin in the south and joints the northern accretionary wedge of Venezuela in the north. The Proto-Magdalena and other sedimentary systems contribute to a high sediment supply triggered by the surrection of the Central and Eastern Cordillera of the Colombian Andes from Upper Miocene time on. The shifting of the Magdalena drainage system and associated delta and deep sea fan plays a major role on the interactions between tectonics and sedimentation. The accretionary wedge of northern Colombia can be subdivided according to its age of accretion and structural characteristics into Inner and Outer complex (Fig. 1). The most internal and older part of the Northern Colombia Accretionary Wedge is exposed along the San Jacinto Mountains and Sinu area, west from the Plato-San Jorge Basin. This Inner Accretionary Wedge consist of west-vergent thrust related imbricates involving the Cretaceous Oceanic Crust a thin Cretaceous sedimentary cover represented by the Cansona Formation and pelagic and flysch type Paleogene to Lower Miocene sediments. Growth strata depocenters shift from east to west as accretion progresses towards the west (Flinch 2003). Seismic, well and surface geological data can be used to characterize the internal structure of the prism in the southern San Jacinto region. This contrast with the northern Barranquilla area where the image is chaotic. The offshore and more widespread part of accretionary wedge extends along the offshore Caribbean Sea from Uraba to Santa Marta and is occupied by three well-defined structural provinces 76

2 Fig. 1. Major tectonic provinces of northern Colombia. Onshore area modified from Flinch

3 Fig. 2. Structural sketch of the Offshore Cartagena area. STRUCTURE OF THE OFFSHORE SINU ACCRETIONARY COMPLEX Three well-defined structural provinces, from west to east occupy the offshore Sinu accretionary wedge: 1. Undeformed Abyssal Plain floored by the oceanic crust and its volcanic cover. This unit is characterized by chaotic reflectors local highs that could represent seamounts and a thin cover of undeformed or parallel reflectors locally affected by normal faulting. 2. «Frontal Imbricates» consisting of thin Upper Cretaceous? to Oligocene thin section overlain by a thick Miocene and specially Pliocene-Pleistocene section. Most of the deformation is Pliocene in age but the prism is still active today. Seismic expression west to northwest vergent thrust related imbricates well-bedded seismic facies. Growth strata can be clearly seen in the area. 3. «Chaotic Unit», this is the part of the prism affected by normal faulting and toe-thrusting, shale ridges are common in this domain which is characterized by transparent seismic facies. This part of the prism is made up by older strata, mostly Oligocene overpressure shale and Miocene sediments. This unit is overlain by thick sedimentary fore-arc basin like the San Bernardo Basin, which is controlled by normal faults and shale ridges. This area corresponds with bottom sea evidences of gas escape and mud-volcanoes of large dimensions. Toe thrusting is also a common mechanism to re-equilibrate the wedge s unstable profile (i. e. critical taper of Davis et al. 1983). STRUCTURE OF THE OFFSHORE CARTAGENA AREA The structure of the offshore prism strongly changes along the offshore Cartagena area, coinciding with the location of the Proto- Magdalena Delta and deep-sea Fan (i. e. Pliocene to Pleistocene depositional system) (Fig. 2). The frontal imbricates of the accretionary prism change from NNE-SSW to an almost E-W trend. The thickness of syn-tectonic growth strata strongly changes from being very thin south and north from the Proto-Magdalena to extremely thick along the Proto-Magdalena delta. For more datails on the stratigraphy of this area see Pince et al. this volume. The apparent structural change north of the Sinu block along the so-called Canoas Fault of the classical geologist seems to be in fact the effect of the onlap of the Pliocene to Pleistocene Magdalena Delta and deep-sea fan. This «blanketing effect» due to high sediment supply results in an apparent discontinuity of the wedge imbricates. Very thin growth strata in the south contrast with an extremely thick syn-sedimentary section along the offshore Magdalena delta. 78

Despite of a backthrust of the Pliocene Magdalena deltaic strata above the underlying wedge imbricates, normal faulting locally occurs in the contact between these two units

Widespread normal faulting occurs along the Proto-Magdalena delta plain representing a way to accommodate thick sediment supply (Fig. 4). Fig. 3.

4 Despite of a backthrust of the Pliocene Magdalena deltaic strata above the underlying wedge imbricates, normal faulting locally occurs in the contact between these two units (i. e. frontal imbricates and Magdalena onlap area) (Fig. 3). Widespread normal faulting occurs along the Proto-Magdalena delta plain representing a way to accommodate thick sediment supply (Fig. 4). Fig. 3. Line-drawing of a dip line through the Proto-Magdalena delta area offshore from Cartagena illustrating Triangle geometry between the wedge imbricates and the overlying deltaic units. Fig. 4. Line drawing of a strike line illustrating the Pliocene to Pleistocene Magdalena delta and its related normal faulting system. 79

RESTORATION OF THE DEFORMATION A tentative balanced-cross section was done to evaluate the kinematic evolution of the area specially the sequence of thrust emplacement.

5 RESTORATION OF THE DEFORMATION A tentative balanced-cross section was done to evaluate the kinematic evolution of the area specially the sequence of thrust emplacement. Figure 5 illustrates several restored stages of deformation along the offshore part of the prism just south of Cartagena (see figure 2 for location). This exercise suggest that we do not have a clearly defined piggy back or break back sequence of emplacement and that initial thrust emplacement (i. e. Frontal Imbricates) was followed by gravitational collapse which resulted in the generation of toe-thrust and normal faults that cross-cut previously existing compressional imbricates. STRUCTURAL EVOLUTION Following Upper Cretaceous obduction of the Cretaceous oceanic crust (and its Cretaceous sedimentary cover) of the Proto-Caribbean plate above the South-American plate, subduction took place along the Northern Colombian margin (Flinch et al. 2000) (Fig. 6). Since that time until present west-vergent accretion took place along the margin. Accretion migrated from east to west being mostly Paleocene to Oligocene in the Inner Accretionary Wedge, exposed onshore along the San Jacinto and Sinu areas, and becoming Neogene towards the offshore Outer Accretionary Wedge. In the so-called Chaotic unit accretion took place during Miocene time and in the frontal Imbricates mostly during Pliocene to Pleistocene time. Due to the dynamics and internal thickening of the prism, the accretionary wedge becomes unstable reaching the critical taper (Davis et al. 1983) and collapses. This collapse mostly Pleistocene to present in age overprinted previous compressional structures and it is linked to an Oligocene shale detachment (shale tectonics is also common in this domain). So part of the Outer Accretionary Wedge is the result of gravitational collapse that overprints preexisting compressional structures. Accretion migrates from east to west being mostly Paleocene to Oligocene in the San Jacinto area and becoming Neogene towards the offshore. In the present day chaotic unit accretion took place during Miocene time and in the frontal Imbricates mostly during Pliocene to Pleistocene time. Due to the dynamic and internal thickening of the prism becomes unstable (critical taper) and collapses. This collapse, mostly Pleistocene to present, in age overprints previous compressional structures an it is associated to an Oligocene shale detachment (shale tectonics is also common in this domain). Fig. 5. Tentative restoration of the deformation along the offshore Outer Accretionary wedge south from Cartagena. See Fig. 1 for location. 80

6 81

Fig. 6. A NW-SE trending cross-section and a palaeogeographic sketch indicate the structural evolution of northern Colombia from Upper Cretaceous to Present. See Fig. 1 for location.

7 Fig. 6. A NW-SE trending cross-section and a palaeogeographic sketch indicate the structural evolution of northern Colombia from Upper Cretaceous to Present. See Fig. 1 for location. Maps modified from Mann CONCLUSIONS 1. The Offshore Sinu Accretionary Wedge is characterized by A Chaotic Unit involving Paleogene to Miocene Strata (in the east) and the "Frontal Imbricates" made of W to NW vergent thrust imbricates involving Miocene to Pleistocene Rocks (in the west). 2. The so-called Canoas Ft. is in fact a Lateral Ramp area onlap by the Proto-Magdalena Delta. 3. Folding and thrusting is less evident along the Proto-Magdalena area due to high sediment rate during deformation. 4. Deformation is mostly Pliocene/Pleistocene in the "Frontal Imbricates" further East Pleistocene Extension overprints previous deformation. 5. Normal faulting and toe-thrusting represent ways to re-equilibrate the prim unstable profile to the critical taper. DISCUSSION Structural interpretation of the offshore seismic data suggests that part of the wedge has gravitationally collapsed. Normal faulting and toethrusting has been superimposed on the pure compressional structure of the accretionary wedge. This complex structural evolution strongly complicates the way to evaluate rates of shortening associated to convergence related to B-type subduction and results in problems to restore deformation. The data presented here illustrates huge along strike thickness variations of syn-cinematic strata which results in lateral variations of the Critical Taper (Davis et al. 1983). In this sense this area offers an exceptional way to study the effect of lateral variations of slope and loading within a constant convergent margin. Our work does not support the presence of the socalled Canoas Fault proposed as a contact between the Proto-Magdalena fan and the prism imbricates, instead we propose a lateral ramp model with onlap of the fan younger strata. This contact is locally resolved as a triangle zone. The data presented here has also important implications on the palaeogeographic and plate tectonic evolution of the Caribbean plate. Late Cretaceous obduction in northern Colombia 82

8 makes more consistent and homogeneous the eastern boundary of the Caribbean plate, which becomes the so-called «Great Arc of the Caribbean». The transition between obduction to subduction marks a major even on the evolution of the Caribbean plate and is well illustrated in this area. To extrapolate this model to other parts of the Caribbean may be a subject of further discussion. Colombian Margin. Tectonophysics, v. 202, p REFERENCES Davis, D., Suppe, J. and Dahlen, F. A., Mechanics of fold-and-thrust belts and Accretionary wedges. Journal of Geophysical Research, 88, p Flinch, J. F., Grand, M. V. and Casero, P., Accretion and Obduction along the Sinu-Lower Magdalena area (Northern Colombia). Memorias VII Congreso Bolivariano, Exploracion Petrolera en las Cuencas Subandinas. pp Flinch, J. F Structural Evolution of the Sinu- Lower Magdalena area (Northern Colombia). in Bartolini, C., Buffler, R., and Blickwede, J., eds., The Gulf of Mexico and Caribbean Region: Hydrocarbon Habitats, Basin Formation and Plate Tectonics, American Association of Petroleum Geologists Memoir (in press). Chapter pages and 20 figures. Ingeominas, 1997, Atlas geológico digital de Colombia, Scale 1/ , Ministerio de Minas y Energia, República de Colombia, Sheets 1, 3, 4 and 6. Mann, P Caribbean Sedimentary Basins : Classification and Tectonic setting from Jurassic to Present, in P. Mann, ed., Caribbean Basins. Sedimentary Basins of the World 4. Elsevier, p Pirmez, C., Flood, R. D. and Ercilla, G. 1998, Contrasting Submarine Fans off the Amzon and Magdalena Rivers. Extended Abstracts Volume, AAPG International Conference and Exhibition Rio de Janeiro, Brazil, p Ruiz, C., Davis, N., Bentham, P., Price, A. and Carvajal, D. 2000, Structure and Tectonic Evolution of the South Caribbean Basin, Southern Offshore Colombia : a Progressive Accretionary System, Memoria VII Simposio Bolivariano, Exploracion Petrolera en las Cuencas Subandinas, Sociedad Venezolana de Geologos, Caracas, September 2000, p Vernette, G., Mauffret, A., Bobier, C., Briceno, L. and Gayet, J.,1992, Mud diapirism, fan sedimentation and strike-slip faulting, Caribbean 83

Colombia s Offshore*

PS A Seismic-Structural Interpretation, on the Identification of Possible Causes in the Formation of Gas Chimneys in Colombia s Offshore* Tatiana Mayorga 1, Andrés E. Calle 2, Freddy M. Niño 2, Jorge Rubiano