PALYNOFLORA FROM THE VOLCANICLASTIC SEQUENCE OF ARANZAZU: A SINGULAR RECORD OF NEOGENE NEOTROPICAL VEGETATION IN THE COLOMBIAN CENTRAL CORDILLERA

Paleopalynologic studies in the Colombian Andes (Northern South America) have been mainly focused on its eastern region (e.g. Llanos Orientales, Eastern Cordillera and Magdalena Valley). In these areas biostratigraphic and paleoecological research has been conducted in order to infer climatic and tectonic changes during the Cenozoic. In contrast, similar studies in Western Colombia (Central Cordillera and the Cauca Valley) are rare and unpublished.

This paper presents the results of a palynologic study conducted in the Aranzazu Volcaniclastic Sequence, located in the Central Cordillera in Colombia (1960 masl). This unit has ~240 m thick, and is composed of tuffaceous sandstones, pyroclastic, and mud flows interbedded with lignites, which were accumulated in fluvial environments with volcanic influence. This is the only reported unit in the Western flank of the Colombian Central Cordillera that presents organic-rich deposits associated with volcaniclastic materials, a situation that allows the integration of radiometric dating and palynologic studies.

Fifty samples were collected in organic-rich levels, mainly lignites. The palynological assemblage is dominated by spores (e.g. Laevigatosporites spp., Psilatriletes spp. and Polipodiisporites spp.), and palynomorphs typical from the Andean forest: Hedyosmum, Ilex, Podocarpus, Sapium, Asteraceae, Ericaceae, Poaceae and Cyatheacea. Tropical and sub-Andean forest taxa such as Bombacaceae, Palmae and Malpighiaceae are also represented in a low proportion. Two radiometric U/Pb ages in zircons from volcanic deposits at the bottom and the top of the study interval indicate Piacenciense (~2.9-3.0 Ma) age. The pollen diagram shows changes in the relative percentages of Andean and sub-Andean forest taxa, which is probably related to climatic variations. These data provide information, hitherto unknown about flora of the Pliocene in NW South America. The palynologic association supports the existence of Pliocene mountain basins filled with fluvial and volcanic sediments, which were deformed and dissected in recent times.