UNGRD

With reference to the eruption of the Nevado del Ruíz Volcano on november 13, 1985, and the destruction caused by subsequent mudflows (debris. flows s.e)along the length of the banks of the Lagunilla and Gualí Rivers, aregional study has been undertaken in order to evaluate the geological risks for the lower parts of the extreme NE (NE end)of the departamento del Tolima, Colombia (in the western half of the southern end of the Meddle Magadalena Valley). The present study of the startigraphic sections indicates that the Lagunilla and Recio river valleys, as well as the flatlands of Mariquita and Guayabal are constituted by accumulations of succesive alluvial fans, which were formed, for the most part, as a result of mudflows along the riverberds which descend directly from the Nevado del Ruíz volcano (Lagunilla, Recio and Gualí). such mudflows represent catastrophic events generated by sudden slides and/or partial melting of the glaciers of the eastern slope of the Central Cordillera, in the area corresponding to the National Park of the Nevados which includes the Ruíz, Cisne, Santa Isabel, Quindío and Tolima volcanoes. The present study of the general sedimentological properties of the sediments accumulates on november 13, 1985 in Armero and Mariquita has allowed a clear identification of older mudflows in the region. These flows share the following characteristics : the chaotic mixture of polygenic materials of varied sizes; the frequent combination of angular clasts (almost always predominant)with subrounded to rounded clasts; and, finaly the poor expression of their primary internal structures). In the stratigraphic columns there appear not only the mudflows, but frequently also important banks of gray sandy-conglomeratic sediments, which thin pyroclastic beds, constituted by ashes and lapilli, which testify the previous explosions of the volcanoes of this sector of the Central Cordillera. At the present time, these pyroclastic beds have not been dated (AU). / ARMERO / GUAYABAL / MARIQUITA / VOLCAN NEVADO DEL RUIZ / TOLIMA