Basin Development and Tectonic History of the Llanos Basin, Eastern Cordillera, and Middle Magdalena Valley, Colombia

Abstract The Middle Magdalena Valley, Eastern Cordillera, and Llanos basin constituted a major regional sedimentary basin from the Triassic to the middle Miocene. Basin development began during the Triassic to the earliest Cretaceous with a synrift megasequence related to the separation of North and South America in the proto-Caribbean. The synrift megasequence began with deposition in a continental environment that became paralic and shallow marine in the Early Cretaceous. Basin development continued into the Cretaceous in a back-arc setting east of the Andean subduction zone. The back-arc megasequence was dominated by shallow-marine sedimentation and produced an excellent regional source rock during the Turonian-Coniacian. Marine deposition was abruptly terminated during the early Maastrichtian due to the final accretion of the Western Cordillera. Accretion of the Western Cordillera created the early pre-Andean foreland basin megasequence of late Maastrichtian to early Eocene age. This depositional episode consists of coal-rich alluvial plain, coastal plain, and estuarine deposits throughout the Middle Magdalena Valley, Eastern Cordillera, and eastern Llanos basin. The megasequence was terminated by middle Eocene deformation in the Magdalena Valley, which ended sediment deposition throughout Colombia. Loading effects of this deformation reestablished the basin, in which the late pre-Andean foreland basin megasequence was deposited, until the early Miocene. This megasequence also consists of alluvial plain, coastal plain, and estuarine deposits, including the primary reservoir in the Llanos Foothills—the upper Eocene Mirador Formation. The megasequence also includes a series of four major grossly coarsening-upward cycles in the Llanos basin; these cycles correspond to changes in sea level, sediment supply, and foreland basin loading. The mudstone in the lowermost of these cycles is the regional seal in the Llanos basin and Foothills. The middle Miocene onset of Andean deformation in the Eastern Cordillera isolated the Middle Magdalena Valley from the Llanos basin. The deformation was dominated by inversion of the basin-controlling faults; the resultant loading of the lithosphere created the accommodation space for the Andean foreland basin megasequence. A major transgression into the Llanos basin coeval with this deformation caused deposition of marine mudstones in the lower part of the megasequence. However, the majority of the Andean foreland basin megasequence consists of the Guayabo Formation, a classic molasse sequence, deposited in a high-energy, coarse-grained, bed-load-dominated fluvial system that was supplied by the developing mountains of the Eastern Cordillera.