Palynological record of the upheaval of the Northern Andes: A study of the pliocene and lower quaternary of the Colombian Eastern Cordillera and the early evolution of its high-Andean biota

Abstract

The present paper deals with the stratigraphy and palynology of the Pliocene and Lower Quaternary formations of the central part of the Colombian Eastern Cordillera. Three Plio-Pleistocene formations are distinguished: the Tilata Formation is of Pliocene and Earliest Quaternary, the Subachoque Formation of Early Quaternary, and the Sabana Formation of Middle and Late Quaternary age. Four pollen zones have been distinguished the Tilata Formation, the first three being of Pliocene, the fourth of Earliest Pleistocene age. Zone I shows a purely tropical lowland flora and zone IV a high-mountain flora, whereas zones II and III show floras of intermediate altitude. A gradual elevation by more than 2,000 m has in this way been recorded palynologically for the area of the high plain of Bogotá. Actually, the top level of the Tilatá Formation lies at ca. 2,850 m alt. in the area east of Chocontá. The top level of the Subachoque Formation lies at ca. 2,700 m alt. in the Subachoque and Guasca valleys. The sediments correspond with pollen zone V of Early Quaternary age, exhibiting a high-mountain flora and indicative of glacial conditions. Sediments corresponding with the same pollen zone have been found between 140 m and 195 m below the surface (2,580 m alt.) in a borehole in Bogotá, showing that the wide central part of the high plain may have relatively subsided more than 200 m since the end of the Early Quaternary. The Sabana Formation, whose top has an actual maximum elevation of 2,600 m, corresponds with major pollen zones VI and VII and exhibits a sequence of glacials and interglacials belonging to the Middle and Late Quaternary. The pollen types important for time correlation between the tropical and mountain environments, belong the anemophilous taxa: pollen of Hedyosmum appears in zone II, of Myrica in zone III, of Alnus in zone V–VI and of Quercus in zone VII. During the process of upheaval tropical and subtropical species gradually disappeared from the area owing to the changing ecological conditions, to be gradually replaced by species better adapted to the new circumstances. It is striking that the oldest, cold to temperate flora (zone IV) is still poor in species. Apart from the dominant grasses, Aragoa, Hypericum, Valeriana, Plantago and Polylepis are amongst the oldest high-mountain (páramo) plants. This flora was later gradually enriched both by immigrants from the north (Holarctic) and the south (Antarctic) and, mostly, by endemic elements originated by the evolutionary adaptation of the local neotropical flora. In this way the Colombian Eastern Cordillera provides us, in its Plio-Pleistocene sediments, with a, so far unique, example of the gradual in situ evolution of tropical high-mountain biota. Because of the scarcity of species adapted to the High-Andean environment, it appears that the forest limit in the Early Quaternary was lower and the Andean forest belt narrower than is the case at present. The first glacial periods of the Early Pleistocene were already so severe, that a temporary, additional lowering of the forest limit by, probably, 500–800 m resulted. At the same time fluvio-glacial gravels were deposited in many places, and huge masses of the old tropical weathering products still covering the mountains at that time were removed by solifluction fromthe slopes downwards into the marginal areas of the basin of the high plain. During the Early Quaternary the large Pleistocene lake of the Sabana de Bogotá was formed, and as the result of progressive and probably continuous subsidence, deposits of hundreds of metres of lake sediments accumulated in the basin during the Pleistocene. From the present data it may be concluded that the major ultimate upheaval of the Cordillera took place during the Middle-Late Pliocene, and that it probably ceased before the beginning of the Pleistocene. The average rate of upheaval may be estimated at between 1 and5 mm per year, but it may have been twice as much in the area of the Sierra Nevada del Cocuy.