The Central Cordillera and Eastern Cordillera of the northern Andes form the western and eastern flanks of the north-trending Middle Magdalena Valley basin. Previous estimates for the timing of initial exhumation of the two cordilleras range from ~100 to ~10 Ma. Accurately constraining the spatial and temporal distribution of deformation in Colombia has implications for the shortening history of the Andean convergent margin and the prediction of rapid lateral facies changes in sedimentary basins in close proximity to sediment sources. This study applies sandstone petrographic point counts, field sedimentological analyses of basin fill, and paleocurrent measurements of trough cross-stratification, clast imbrication, and flute casts to provide new insights into the tectonic history of the flanks of the Middle Magdalena Valley basin. Between the lower and upper Paleocene strata of the Lisama Formation, paleocurrent orientations show a shift from northward to eastward transport. This change in sediment dispersal coincides with a shift from a cratonic (Amazonian) to orogenic (Andean) sediment source, as recorded by published U-Pb detrital zircon geochronological results (Nie et al. 2010), suggesting initial uplift of the Central Cordillera by mid-Paleocene time. Later in the basin’s history, establishment of an alluvial-plain system with meandering-channel deposits is recorded in lower–middle Eocene strata of the lower La Paz Formation. Consistent eastward paleocurrents characterize mid-Paleocene through uppermost Eocene strata, indicating a continuous influence of western sediment source areas. However, within the upper middle Eocene succession (~40 Ma), at the boundary between the lower and upper La Paz Formation, sandstone compositions show a dramatic decrease in lithic content. This compositional change is accompanied by a facies shift to amalgamated fluvial channels, reflecting changes in both the composition and proximity of the western sediment source. We attribute these changes to the growing influence of the exhumed La Cira/Infantas paleohighs off the western flank of the present-day Nuevo Mundo syncline. In the uppermost Eocene strata of the Esmeraldas Formation, paleocurrents show a switch to dominantly westward transport that persisted through the Neogene. In addition, deposits show a contemporaneous decrease in the amount of coarse-grained channel deposits. These changes are interpreted to reflect the onset of exhumation in the Eastern Cordillera. The lack of a significant change in sandstone compositions at this boundary suggests a compositional similarity between strata uplifted by the Lisama structure and the Eastern Cordillera. These data support and further refine previous thermochronologic and provenance studies which suggest that uplift-induced exhumation of the Central Cordillera and Eastern Cordillera commenced by mid-Paleocene and late Eocene–early Miocene time, respectively. / text

Structural analysis of surface and subsurface data from the Middle Magdalena Valley basin and Eastern Cordillera fold-thrust belt to construct a kinematic model for its Cenozoic structural and stratigraphic evolution. The La Salina west-vergent thrust system marks the boundary between the Paleogene foreland basin of the Middle Magdalena basin and the Eastern Cordillera fold-thrust belt. New low-temperature thermochonological and sandstone petrographic analyses provide constraints on ages of thrust deformation and sediment dispersal. Apatite fission track (AFT) and U-Th/He thermochronological results show the timing of three structural events along the La Salina fault system: (1) late Eocene-early Oligocene (~43–35 Ma) initial hanging wall exhumation; (2) continued middle Miocene (~15 Ma) exhumation; and (3) continued but more rapid late Miocene (~12–3 Ma) hanging wall exhumation. Vitrinite reflectance results provide estimates of maximum burial depths for the hanging wall of the La Salina fault ranging from 4 to 6 km., this depth of burial estimates constrain the basin geometry during its late Eocene to late Miocene evolution. The eastern hanging wall of the La Salina fault displays a broad anticline-syncline pair affecting Cretaceous to Eocene strata with no significant faulting, whereas the western footwall contains a complex series of tight, thrust-related folds in Eocene-Quaternary strata. For foreland basin province, a proposed triangle zone accommodates a small amount of east-west shortening (< 1000 m) along the frontal thrust system by east-vergent backthrusting within a broader passive-roof duplex. East-west shortening in the Cenozoic stratigraphic section was also accommodated by detachment folding, which produced localized areas of steep dips. In the proposed kinematic restoration, the most recent phase of deformation represents out-of-sequence reactivation of the La Salina fault that is consistent with irregular crosscutting relationships of some footwall structures. Earliest exhumation by ~45–30 Ma in the Eastern Cordillera fold-thrust belt province matches (1) an increased proportion of sedimentary lithic fragments; and (2) a high degree of compositional maturity (Q88F4Lf8). Exhumation since ~15 Ma in the foreland province coincides with (1) the highest accumulation rates observed for the upper Miocene Real Group; and (2) a decrease in compositional maturity (Q55F8Lf36). / text

This article presents key data and discussion about the initial peopling of northwestern South America, highlighting its strategic significance in the continental context, cultural variability and landscape change over time. In Colombia there is very little relevant information regarding occupations prior to 12,000 BP; in contrast, the information is abundant relative to a number ofearly contexts during the Pleistocene-Holocene transition. The variability among unifacial and bifacial lithic assemblages reported in different physiographic regions indicates alternative models on early settlement to regional level. Considering the importanceof the inter-Andean valley of the Magdalena River, this article emphasizes the archaeological contexts and paleoenvironmental information, highlighting environmental impacts associated not only to global climate change, but in particular of the effects ofvolcanism. The article stresses the findings and chronologies of bifacial lithic assemblages of the Magdalena Valley in relation to the early archaeology of northwestern South America. / En este artículo se presentan los principales datos y reflexiones asociados al poblamiento inicial del noroccidente de Sudamérica debido a su significado estratégico en el marco continental, se destacará la variabilidad cultural y se enfatizarán los cambios depaisajes en el transcurso del tiempo. En Colombia existe muy poca información relevante relacionada con las ocupaciones anterioresa 12.000 AP; en contraste, la información es abundante en cuanto al número de contextos tempranos durante la transición Pleistoceno-Holoceno. La variabilidad entre los conjuntos líticos unifaciales y bifaciales reportados en distintas regiones fisiográficas plantea propuestas de modelos alternativos sobre el poblamiento temprano a escala regional. Considerando la importancia del valle interandino del río Magdalena, se recalcan los contextos arqueológicos e información paleoambiental. Asimismo, se destacan los impactos ambientales que demuestran por qué no perduraron las evidencias del Pleniglacial, lo que podría deberse a efectos asociados no solo al cambio climático global, sino, en particular, a causa del vulcanismo. Este trabajo resalta los hallazgos y cronologías de los conjuntos líticos bifaciales del valle del Magdalena en relación con la arqueología temprana del noroccidente de Sudamérica.