INVERSION STRUCTURES IN THE BUENAVISTA ANTICLINE. LLANOS FOOTHILLS. COLOMBIA

Based on geological mapping and structural modeling of the Villavicencio area we identified structural elements that can be used to divide the area in 3 structural domains bounded by major faults (Mirador y Guayuriba). Each domain includes structures with similar geometries in terms of attitude, extent and wave length. The Buenavista anticline is an overturned structures with basement involved oriented in a N30E direction. The central domain has open folds in a N30-50E direction whereas the western domain has close to tight folds oriented in a N10E direction. The eastern domain coincides with the overturned limb of the anticline, in which cinematic indicators suggest an additional shear to the one expected in overturned folds. In a similar way, the stratigraphic sequences in each domain also have differences in terms of thicknesses and facies of the conglomeratic units. To analyze these structural and stratigraphic changes serial cross sections were constructed. Theses showed that using the top of the Une sandstone as a reference, there is a difference in at least 1000 m. in the level of the basement between the central and the eastern domain, and a difference in 500 m. between the central and the western domain. These differences in the thicknesses of the sequences and the level of basement suggest that the faults between domains acted as normal faults during early cretaceous times. In addition, the sequences are interpreted to have been deposited in Fan Deltas with marked differences between the blocks, showing the process of formation and infill of basin between normal faults. Currently, the Buenavista Anticlines constitutes a basement high with a difference of 5000 m. between the levels its outcrops and the one drilled by the Anaconda-1 well. This difference generated by folding and thrusting, shows that the Mirador Fault constitutes an inversion fault. In contrast, the Guayuriba fault preserves its displacement as a normal fault, and the tight folds in the western domain suggest a process of buttressing against the fault. These show that the Mirador Fault is the principal inversion structures and that the Guayuriba Fault acted as a rigid block that was passively transported in the hanging wall of the Mirador Fault.