LARGE CORDILLERAN FANS IN CENTRAL COLOMBIA: THE RECORD OF GLACIAL-MELT AND CATASTROPHIC DRAINAGE OF THE BOGOTA LAKE?

The western flank of the eastern Cordillera of Colombia preserves at least three moderately to deeply dissected, low slope, low rugosity fans (Fusa, Anapoima and La Mesa). These large, nearly flat areas strongly contrast with the rugged topography of the Cordillera Oriental and have been attributed to fluvial deposits draining the western flank of the eastern Cordillera. These fans are located immediately west of the Sumapaz highlands of the central part of the eastern Cordillera, a range with maximum elevations of up to 4,700 m above sea level, which during glacial periods had glaciers reaching as low as 3,350 m above sea level. This range drains directly to the west (Fusa fan) and north to the Bogota plateau at 2,600m above sea level, which in turn drains to the west (Anapoima and La Mesa fans). A large lake developed in the Bogota plateau from late Pliocene to Pleistocene times, draining the surrounding glacially covered peaks. Preliminary sedimentologic observations suggest high viscosity flow conditions for the Fusa fan, with very large, probably glacial blocks, floating in a massive, muddy matrix. The combined area of these three fans is now 130 km², with the largest preserved area in the less dissected Fusa fan (105 km²). The conditions for the development of large, glacially induced floods were all in place during the glacial periods, with large mountain glaciers, a large glacial lake and large altitudinal gradients to the west (from 2,600 m to less than 500 m above sea level). Several alternatives can be considered, including a seismogenic triggering mechanism if all three fans are shown to be simultaneous events.