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The Maastrichtian-Cenozoic southern Middle Magdalena Valley Basin of Colombia contains a unique record of unconformities, strata, and structure, from which we extract the histories of exhumation of the Central Cordillera, to the west, and evolution of the Eastern Cordillera fold-and-thrust belt, to the east. This study integrates field-based analyses of stratigraphy, laboratory analyses of provenance, fission-track thermochronology, vitrinite-reflectance data, volcanic-ash geochronology, and studies of synorogenic geometries and structure displayed in seismic data. A major unconformity, the Middle Magdalena Valley unconformity, formed by eastward migration of Central Cordillera uplift during Late Cretaceous to early Eocene time, which transformed a Maastrichtian marine basin into a Paleocene depositional piedmont area This transformation is recorded by a coarsening-upward sequence of marine shales to alluvial-fan conglomerates, which was partly eroded during further early Eocene propagation of Central Cordillera deformation. Cessation of this phase of uplift led to formation of a pediment surface, the Middle Magdalena Valley unconformity, which was buried by westward-onlapping middle Eocene to Neogene alluvial deposits. Middle Eocene to Neogene facies, paleoflow, and unconformities were controlled by Eastern Cordillera deformation. In the Eastern Cordillera foot-hills, growth strata and thermal history reveal two phases of folding of middle Eocene-Oligocene and late Miocene ages, prior to intense Pliocene-Pleistocene uplift. Two unconformities of early late Miocene and Pliocene-Pleistocene ages occur to the west of the Eastern Cordillera and record flexural tilting related to episodes of Eastern Cordillera loading.

This study addresses multiple controls on foreland-basin accommodation and contributes to enhanced understanding of the evolution of the northern Andes. The Middle Magdalena Valley Basin (MMVB), Eastern Cordillera, and Llanos Basin are part of a Late Cretaceous–Cenozoic foreland-basin system, east of the Colombian Central Cordillera. Mechanical modeling indicates that the primary control on complex distributions of sedimentary thicknesses, facies, and unconformities was lithospheric flexure in response to crustal loads from the Central and Eastern Cordilleras. Shorter-wavelength folding and paleoaltitude determined the local character of strata. Our mechanical modeling consists of the application of orogenic and sedimentary loads extracted from geologic data on a continuous elastic lithosphere. The results validate two major basin configurations. The first configuration was a Maastrichtian–early Eocene foreland basin coupled with Central Cordillera uplift. Growth strata record continuous sedimentation in the Eastern Cordillera, whereas regional unconformities in the Llanos Basin (distal foreland basin) reflect isostatic adjustments of the basin's amplitude and wavelength to Central Cordillera episodic uplift and tectonic quiescence. The second major basin configuration was characterized by Central Cordillera erosion since middle Eocene times recorded by a regional pediment surface. In the absence of Central Cordillera effective loading, loads from onlapping sediments and Eastern Cordillera piggyback sub-basins provoked post–middle Eocene accommodation in the MMVB and Llanos Basin. Intensified Eastern Cordillera uplift during the Neogene produced basinal tilting recorded by unconformities in the MMVB. This study highlights the importance of assessing the causes of tectonic accommodation as a foundation for interpretation of the evolution of large foreland and intermontane basins.

Development of the Colombian Middle Magdalena Valley Basin (MMVB) was determined by late Cretaceous-early Eocene uplift of the Central Cordillera to the west, and subsequent transferal of deformation to the Eastern Cordillera to the east. These phases are separated in the tectono-stratigraphic record by a major unconformity, the Middle Magdalena Valley unconformity (MMVU). Paleocene coastal to alluvial facies underneath the MMVU were deposited in a foreland basin coupled to Central Cordillera kilometer-scale uplift. The middle Eocene to Neogene continental strata that onlap the MMVU document transformation of the MMVB into an interior basin due to Eastern Cordillera deformation recorded by growth strata in seismic lines and changing provenance and paleoflow patterns. Exhumation histories of the MMVB bounding ranges are further constrained by apatit-fission-track and vitrinite reflectance thermochronology and volcanic ash chronology. This study indicates that the MMVB is fundamentally related to evolution of the entire Andean margin of South America.