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​​Sedimentitas marinas del Neógeno en la bahía de Tumaco, Nariño

 Volume 3 Chapter 2

Chapter 2

Formation and Evolution of the Lower Magdalena Valley Basin and San Jacinto Fold Belt of Northwestern Colombia: Insights from Upper Cretaceous to Recent Tectono–Stratigraphy 

Josué Alejandro MORA–BOHÓRQUEZ  , Onno ONCKEN, Eline LE BRETON, Mauricio IBAÑEZ–MEJIA , Gabriel VELOZA, Andrés MORA, Vickye VÉLEZ, and Mario DE FREITAS

Citation is suggested as: 

Mora–Bohórquez, J.A., Oncken, O., Le Breton, E., Ibañez–Mejia, M., Veloza, G., Mora, A., Vélez, V. & De Freitas, M. 2020. Formation and evolution of the Lower Magdalena Valley Basin and San Jacinto fold belt of northwestern Colombia: Insights from Upper Cretaceous to recent tectono–stratigraphy. In: Gómez, J. & Mateus–Zabala, D. (editors), The Geology of Colombia, Volume 3 Paleogene – Neogene. Servicio Geológico Colombiano, Publicaciones Geológicas Especiales 37, p. 21–66. Bogotá.​

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Using a regional geological and geophysical dataset, we reconstructed the stratigraphic evolution of the Lower Magdalena Valley Basin and San Jacinto fold belt of northwestern Colombia. Detailed interpretations of reflection seismic data and new geochronology analyses reveal that the basement of the Lower Magdalena Basin is the northward continuation of the basement terranes of the northern Central Cordillera and consists of Permian – Triassic metasedimentary rocks intruded by Upper Cretaceous granitoids. Structural analyses suggest that the NE–SW strike of faults in basement rocks underlying the northeastern Lower Magdalena is inherited from a Jurassic rifting event, while the ESE–WNW—striking faults in the western part originated from a Late Cretaceous to Eocene strike–slip and extensional episode. The Upper Cretaceous to lower Eocene sedimentary rocks preserved in the present–day San Jacinto fold belt were deposited in a submarine, forearc basin formed during the coeval oblique convergence between the Caribbean and South American Plates. A lower to middle Eocene angular unconformity at the top of the upper Paleocene to lower Eocene San Cayetano sequence, the termination of the activity of the Romeral Fault System, and the cessation of arc magmatism are all interpreted to indicate the onset of low–angle orthogonal subduction of the Caribbean Plateau beneath South America between 56 and 43 Ma. Flat subduction of the plateau has continued to the present and would be the main cause of amagmatic post–Eocene deposition and formation of the Lower Magdalena Valley Forearc Basin. Extensional reactivation of inherited, pre–Oligocene basement faults was crucial for the tectonic segmentation of the basin and the formation of its two depocenters (Plato and San Jorge). Late Oligocene to early Miocene fault–controlled subsidence allowed initial infill of the Lower Magdalena, while uplift of Andean terranes made possible the connection of the Lower and Middle Magdalena Valleys, and the formation of the largest Colombian drainage system (Magdalena River system). This drainage system started delivering enormous amounts of sediments in middle Miocene times, as fault–controlled subsidence was gradually replaced by sedimentary loading. Such dramatic increase in sedimentation and the huge volume of sediment being delivered to the trench caused the formation of forearc highs in San Jacinto and of an accretionary prism farther to the west. Our results highlight the fundamental role of plate kinematics, inherited basement structure and sediment flux on the evolution of forearc basins such as the Lower Magdalena and San Jacinto.

Keywords: forearc basin, basement, flatslab subduction, tectonostratigraphy, Lower Magdalena, San Jacinto fold belt, Caribbean, subsidence,   sedimentation.