Middle Magdalena Basin

Basin development

During the Jurassic period, Pangea began to pull apart causing separation of North America from South America. This rifting produced a subduction zone where the Nazca plate was subducting to the east under the South American plate. Part of this subducting plate was the Baudo-Island Arc separated from the South American continent by the marginal Colombian Sea. The formation of the extensional back-arc basin associated with this subduction is the origin of the Middle Magdalena Basin in the late Jurassic. Throughout the Cretaceous, the basin experienced thermal subsidence and five transgressive-regressive cycles as part of a marine megacycle.

In the Paleocene, the rate of subduction increased causing the marginal Colombian Sea to close and the Baudo-Island Arc to collide with the South American continent. This caused accretion of the Western Cordillera and uplift of the Central Cordillera transforming the back-arc basin into the pre-Andean foreland basin. Around the time of the Oligocene, the Nazca plate increased its subduction to the east while the South American plate experienced a westward pull. This caused the Andean orogeny in the Miocene and uplift of the Eastern Cordillera in the Pliocene. Now the Middle Magdalena basin is an intermontane basin situated between the uplifted Central and Eastern Cordilleras.

Geologic structures

Faulting in the Middle Magdalena Basin is primarily reverse and thrust faulting. Reverse faulting is high angle in the west and low angle in the eastern and central areas of the basin with normal faults also developing along the eastern margin. These thrust faults formed from thrusting from the eastern margin of the Central Cordillera in the Eocene and the western margin of the Eastern Cordillera in the Miocene. The major thrust faults in the Middle Magdalena Basin include the Infantas thrust, La Salina thrust, and Cantagallo thrust fault. The basin is structurally bounded by the Palestina fault, a dextral strike slip system, in the west and the Bucaramanga fault, a sinistral strike slip system, in the east.

The major surface structures of the Middle Magdalena Basin are asymmetric synclines and basement cored anticlines which formed as a result of thrusting from the Eastern Cordillera and Central Cordillera. The thrusting initiated faulting in the Pre-Mesozoic basement. The faults then pushed through the Jurassic layers to the Cretaceous ductile stratigraphy. The faults then form horizontally at the ductile-brittle transition for 10–20 km before cutting through the upper brittle stratigraphy. The resulting structure is a syncline against the hanging wall of the fault next to an inclined anticline. Key folds in the basin for hydrocarbon exploration include the Nuevo Mundo and Guaduas syncline. As suggested by the formation of the folds, both of these synclines are bounded by thrust faulting and anticlines.

Stratigraphy

The stratigraphy of the Middle Magdalena Basin can be divided into three sequences separated by angular unconformities. The basement of these sequences is Pre-Mesozoic metaclastics and sediments which are now exposed on the surface of the Central Cordillera as a result of its deformation and uplift. This geologic basement is at most 15 km deep with faulted sections shifted up to approximately 10 km in depth. The unconformity that separates the basement from the first sequences indicates the approximate time rifting began.

First sequence

The first sequence was deposited in the Jurassic during the rifting which caused the initial formation of the basin. This Jurassic formation is called the Giron group which consists of siltstones and rhyolitic tuffs. During this period, the basin also experienced granitic plutonism along its western margins. The Jurassic-Cretaceous angular unconformity separating the first and second sequence is representative of the post-rift boundary.

Second sequence

The middle sequence represents the formations deposited throughout the Cretaceous and early Paleocene. The oldest formations in this sequence are the Tambor and Los Santos formations of conglomerates and sandstones indicate a continental to fluvial depositional environment. In the early Cretaceous, the sea level began to rise and formed a shallow marine environment with siltstone and shale deposits in the Cumbre formation. The sea level continued to rise throughout the middle Cretaceous when the Tablazo and Salto limestones and Simiti shale were deposited. The La Luna formation represents the peak of the sea level with deep marine deposits of limestone, chert, and shale. The sea level then began to fall returning the environment to shallow marine with deposition of the Umir formation of shales and sandstones. Finally, the Paleocene saw the deposition of the Lisama formation of deltaic mudstones and sandstones. This entire middle sequence indicates a marine megacycle consisting of five transgressive-regressive cycles. The angular unconformity between the second and third sequences is a result of erosion from the accretion of the Western Cordillera.

Third sequence

The final sequence represents deposition from the early Tertiary to present day. Within this sequence there are three subsequences that are a result of deformation and uplift of the Central and Eastern Cordilleras. The first subsequence consists of the Chorro group with the La Paz and Esmeraldas formations and the Chuspas group with the Mugrosa and Colorado formations all deposited during the Eocene-Oligocene. These groups consist of fluvial sandstones, mudstones, siltstones, and shales and are a result of erosion fn the Central Cordillera. The second subsequence is the Miocene Real formation consisting of fluvial sandstones and conglomerates. Similar to the Real formation, the final subsequence is the Pliocene Mesa formation, which is composed of sandstones and conglomerates deposited from the Eastern Cordillera uplift. On the surface of the basin are Pleistocene alluvial fan deposits.