The changing course of the Orinoco River during the Neogene: A review

Recent studies have revealed that the rich fresh-water vertebrate fossil fauna of the Urumaco Formation in northwestern Falcon, of Late Miocene age, belongs biogeographically to the Orinoco River system. This finding, together with other evidence, can be used to chart the changing course of the Orinoco River during the Neogene and thus date the uplift of various mountain ranges that affected the change in this course. Presence of the Misoa delta in the Middle Eocene of the Maracaibo Basin has been suggested as evidence for a large river, running in a south-north direction and draining the Central Cordillera of Colombia and the Guayana Highlands. The late Middle Eocene uplifting of Western Venezuela changed the paleogeographic setting, and a new delta-building shifted to the south, represented by the extensive Carbonera Formation of Late Eocene to Oligocene age in the Llanos Basin of Colombia and Southwestern Venezuela. In the earliest Miocene, the Falcon Basin, situated to the east of the Maracaibo Basin, was primarily the site of marine shale sedimentation. Stratigraphic and microfaunal evidence suggests the possibility of the presence of the proto-Orinoco River in the western margin of the basin. In any case, from the mid Early Miocene until the end of the Middle Miocene, a fluvio-deltaic sequence was deposited in the northwestern Falcon, with a total thickness of 4.7 km. The Urumaco Formation overlies this sequence and its vertebrate fauna indicates that the river which built the prior delta was definitely the proto-Orinoco. The deformation and uplift of the Eastern Cordillera of Colombia and of the southwestern end of the Merida Andes caused the distal course of the river to be deflected to a west-east direction. A late Middle Miocene age for this change of course is precisely dated on marine microfaunal evidence. The Late Miocene sedimentary record of the Orinoco River delta appears east of Maturin, in the Eastern Venezuela Basin, and Pliocene and Pleistocene deltaic sediments are especially evident in Trinidad and the Columbus Basin.