Eocene to Miocene palynology of the Amagá Basin (Cauca Valley, Colombia) compared to the Caribbean Region

Abstract

The Cenozoic sedimentary basins of northwestern Colombia contain clues to understanding the evolution of past ecosystems and their possible relationship to regional events, such as the uplift of the northern Andes and the collision of the Panama-Chocó Block with the South American continent. However, these basins have not been thoroughly investigated. This work aims to fill this knowledge gap by performing a comprehensive palynological analysis of the Amagá Basin (Valle del Cauca). Three outcrops were studied to determine their depositional ages, paleoenvironments, and overall floral composition over time. To contextualize our analyses, a comparison was made with two drill holes (ANH-SSJ-Nueva Esperanza-1X and ANH-San Jacinto-1) in the Sinú-San Jacinto Basin in the Colombian Caribbean. The application of the palynological zonation from the Llanos Basin in eastern Colombia was also used to evaluate its suitability in the Amagá Basin. The age of deposition was determined by using several traditional palynostratigraphic markers, including Perisyncolporites pokornyi, Foveotriporites hammeni, Retibrevitricolporites speciosus, Magnastriatites grandiosus, Concavissimisporites fossulatus, Clavainaperturites microclavatus, and Polypodiaceoisporites pseudopsilatus. This assemblage indicates a Middle to Late Eocene, Oligocene, and Miocene age spanning from ~40 to 18 Ma (~22 Myr). However, there are also some differences with respect to the Llanos zonation, such as the absence of Cicatricosisporites dorogensis. Based on this age model, we question whether it is correct to include all these deposits in a single lithostratigraphic unit under the rank of a formation. In the Caribbean, stratigraphic units of similar age exhibit a greater number of key taxa than in the Llanos zonation. In addition, the presence of calcareous microfossils allows for a more detailed determination of the sedimentation age. Furthermore, the discovery of new pollen and spore species could improve the resolution of the biostratigraphy in western Colombia. In the Amagá Basin, only terrestrial palynomorphs were found, which, together with the sedimentary facies, suggest a humid lowland tropical vegetation in fluvial and lacustrine environments.