PALYNOLOGY OF THE CENOZOIC AMAGÁ BASIN (NORTHWESTERN SOUTH AMERICA): BIOSTRATIGRAPHY AND PALEOENVIRONMENTS

NW Colombian sedimentary basins can be useful to understand the geologic evolution of the Northern Andes. A stratigraphic and palynological study was developed in the Cenozoic fill of the Amagá intermontane basin (NW Colombia), in order to determine its age and depositional environments. It is composed by a thick (~1400 m) succession of sandstones, conglomerates, mudrocks and some coal beds. Fifty palynological samples were collected in three stratigraphic sections, from base to top they are: 1) San Nicolas (54 m), 2) Santa Fé de Antioquia (250 m) and 3) La Nuarque creek (150 m).

The San Nicolás section did not provide enough microfossils to conduct the study. The co-occurrence of Foveotriporites hammeni, Psilastephanocolporites fissilis, Retistephanoporites crassiannulatus, Spyrosyncolpites spiralis, Striatopollis catatumbus, Polypodiaceoisporites? fossulatus, Perisyncolporites pokornyi, Retibrevitricolporites speciosus and Mauritidiites franciscoi in Santa Fé de Antioquia section suggest a Middle Eocene age. In contrast, the presence of Clavainaperturites microclavatus, Concavissimisporites fossulatus, Retitriletes Sommeri, Foveotriletes ornatus, Mauritidiites franciscoi, Polypodiaceoisporites pseudopsilatus, Polypodiisporites usmensis, Striatopollis catatumbus, Retitricolporites simplex, Crassiectoapertites columbianus and Perisyncolporites pokornyi in La Nuarque section suggest an age not older than the early Miocene. The presence of subvolcanic rocks that intrude the studied sediments indicate a minimum age of ~11 Ma. Therefore the time interval covered by the sedimentary fill in the area is ~30 Ma. Considering that this basin was formed during the Andean uplift, it is possible that some unidentified unconformities can be present in the basin. The presence of M. franciscoi (Mauritia, Palmae), P. pokornyi (Malpighiaceae), S. catatumbus (Fabaceae), S. spiralis (Passifloraceae?), Sapotaceae, Bombacaceae, Onagraceae, ferns and freshwater algae (e.g. Chomotriletes minor), indicates a lowland humid tropical forest, probably associated with fluvial plains, peat bogs and lacustrine environments. Three new species were identified and described.