The fluvial and volcaniclastic deposits of the Miocene Gigante Formation of the southern Neiva Basin record the transition from fluvial deposition to a type of sedimentation strongly influenced by volcanic activity of the Central Cordillera volcanic arc. The lower part of the formation, reflecting the influence of uplift of the Central Cordillera to the south and west of the basin, consists of mixed-load sediments deposited by a braided river system. In middle Miocene times, this braided, north-flowing paleo-Magdalena River was forced eastward by coalescing volcanic aprons that probably originated along the front of a reactivated thrust fault. Sedimentation processes on these aprons were dominated by debris flows and hyperconcentrated flows as well as by unconfined sheetfloods on the more proximal parts of the aprons and by unconfined and confined sheetfloods and overbank deposition on the dstal parts of the aprons grading into the alluvial plain. Average sedimentation rates on the volcanic aprons measured between 0.36 and 0.42 mm/year. When volcanism abated, the paleo-Magdalena River returned to its former course. During this period, fluvial deposition was influenced by renewed uplift of the Garz��n Massif to the east of the basin. �� 1992.