MIOCENE SURFACE UPLIFT AND PALEOENVIRONMENTAL EVOLUTION OF THE NUNCHíA SYNCLINE, NORTHERN ANDES, COLOMBIA

Clastic Miocene units in the Llanos basin of Colombia are related to syntectonic sedimentation of the Eastern Cordillera. Upper Carbonera (C4 to C1), León and lower Guayabo Formations, representing almost the entire Miocene epoch, have been analyzed in the Nunchía syncline. This is the frontal structure in the eastern foothills and is bounded by the Yopal thrust to the east and the Guaicaramo fault to the west.

2300 meters of stratigraphic sections in both limbs of the syncline were measured and interpreted. Facies analysis of Upper Carbonera, León and lower Guayabo Formations record the transition from a marginal marine to a nonmarine environment in the area. Five facies associations were interpreted: estuarine sandy channels, tidal plain siltstones and mudstones, prograding sand bars, braided fluvial sandstones and flood plain mudstones and siltstones. The history of deposition for this segment starts in the Carbonera Formation with tidal mud flats and sand bars giving way to deltaic channels, showing an initial progradation of the nonmarine environment. Leon Formation shows the same progradational pattern from estuarine plains to sandy bars. The next unit, the lower Guayabo, transitions to a braided fluvial system.

Analysis of 91 paleocurrent measurement stations took from trough crossbedded strata and ripple marks in León and lower Guayabo Formations show two segments with different fluvial transport direction. The lower segment, including the upper 150 meters of León and the first 150 meters of Guayabo, has been deposited by a transverse drainage system flowing to the east. In the upper segment, 750 meters of Guayabo, there is a switch to a subsequent drainage pattern flowing largely to the south. This could be controlled by an early movement of the Yopal thrust developing a topographic barrier east of the Nunchia syncline. If true, the Yopal fault initiated in the late Miocene, inferred by the deposition of the Guayabo Formation.