Petrographic aspects of a sequence of Quaternary volcanic ashes from the Laguna de Fuquene area, Colombia, and their stratigraphic significance

A core section from the Fuquene area (Cordillera Oriental, Colombia) with a total length of 60 m shows a predominantly peaty-clay zone in the upper 12 m containing 15 laminae and thin layers of fine-grained, pyroclastic material. These 15 layers consist of crystal-vitric ash derived from dacitic-andesitic magmas. Mineralogic data and refractive index (r.i.) values of the associated glass shards make it possible to distinguish a lower group of mica-rich and an upper group of pyroxene-bearing ash layers. The successive layers illustrate a progressive increase of hydration of the shards with time and with decreasing particle size. The frequency distribution curves of the r.i. values of 8–4- and 75–50-μm shards are in general rather similar. Provisional pollen data and some radiocarbon dates allow comparison of the petrographic properties of these ashes with those of some ash occurrences in a nearby palynologically investigated core section. This comparison indicates that the mica-rich ashes were produced between about 44,000 and 20,000 yr B.P. and that the production of the pyroxene-bearing ashes took place after about 20,000 yr B.P. The petrographic features of a number of ashes, collected from sections and cores at different sites of the Sabana de Bogotá, appear to be comparable with those established in the sequence of Fuquene ashes. This suggests that these petrographic characteristics may be of use in correlation studies in the Sabana de Bogotá and adjacent areas.