Late Eocene marine incursion in north-western South America

During the late Eocene in the Colombian Subandean basins, one of the most important oil-bearing rocks of the country was deposited: the Mirador Formation. Palaeogeographical models have interpreted a typically fluvial environment for the lower Mirador Formation and marginal to marine environments for its upper layers. The potential of marine influence in the upper Mirador beds and the overlying lower Carbonera formation as a correlation tool, and the palaeogeographic distribution of the event have not yet been defined. In order to determine the palaeogeography of this probable incursion, 80 wells and four sections were analysed using palynological techniques. The presence of a marine influence in the sediments was determined by using a Salinity Index (SI) that describes the negative relationship between continental and marine palynomorphs. The marine influence has been recognized in two areas: the first, in the Putumayo basin, and the second, in the Eastern Cordillera and Central Llanos Foothills. In the Putumayo basin the Salinity Index pattern reveals a southern provenance of the marine incursion, flooding the Colombian territory in a South–North trend through the Ecuadorian coast. The marine influence of the Eastern Cordillera and the Central-Eastern Llanos Foothills is more difficult to explain. We propose a possible corridor through the proto-Lower Magdalena Valley that connected the Caribbean Sea and the Central Llanos Foothills. Palaeogeographic models for the late Eocene of north-western South America should consider this marine incursion and its geographical distribution.