Abstract: Campanian-Oligocene Depocenter Evolution and Tectonostratigraphic Development of Northwestern South America

Villamil, T.; P. Restrepo and K. Svela - Conoco Inc.

The position of the central axis of deposition over Colombian and Venezuelan continental crust has varied markedly though time. The axis of the depocenter migrated from west to east from Late Cretaceous to Oligocene, but at times, secondary drainage divides were established by local uplift events. In late Oligocene times with initial inversion of the Eastern Cordillera, the central axis of deposition divided into two permanent main axes, the proto Magdalena and the proto Orinoco systems ( 1). The west to east migration of the central axis of deposition had a tectonic origin and it happened in combination with tectonically driven changes in accommodation space. The evolution of depocenters can be divided into the following phases: (A, ACAD) The Albian-Campanian basin was open to the west and the central axis of deposition was not confined to a position between mountains. This basin can be considered as a retro-arc post-extensional sag that behaved like a passive margin. (B, CCAD) The axis of Campanian and early Maastrichtian depocenter was located along or west of the present Central Cordillera but migrated to the east pushed by the uplift of the mountain chain. (C, MCAD) The central axis of late Maastrichtian deposition runs approximately over the present day western foothills of the Eastern Cordillera, possibly crosses over the Santander massif and continues into the Maracaibo Lake. Accommodation space decreased from Campanian to Maastrichtian times, as reflected in a regional facies change from distal marine shales to shallow marine sandstones and carbonate buildups. In K-T boundary times parts of the eastern margin of the Eastern Cordillera were uplifted by initial inversion of deeply-rooted Jurassic and early Cretaceous normal faults leading to a secondary axis of deposition located east of the uplifted region (PSAD). This K-T boundary uplift caused the erosion of the upper portions of the Cretaceous in the eastern foothills of the Eastern Cordillera ( 1). (D, EPCAD) In Paleocene times the central axis of deposition was located along the spine of the Eastern Cordillera and extended into the Maracaibo Lake. Accommodation space decreased from Maastrichtian to Paleocene times, this is expressed as a regional facies change from shallow marine sandstones and isolated carbonates to coastal depositional systems. In many regions of Colombia, the base of coastal deposits is an unconformity or a paraconformity. (E, LPCAD) In latest Paleocene times, the central axis of deposition shifted to eastern regions of the Eastern Cordillera and accommodation space decreased leading to deposition of a sand-prone unit. (F, EECAD) The Early Eocene central axis of deposition was located along the present-day Llanos foothills; accommodation space decreased and the regional and well-recognized Middle Eocene unconformity began to develop. In mid Eocene times the axis of deposition probably continued to shift eastwards but accommodation space decreased dramatically, this unconformity marks the climax of the pre-Andean Orogeny in northern South America. Deposition during middle Eocene was dominant in the Maracaibo Lake area where large amounts of sediment derived from vast exposed areas accumulated in a large deltaic system. (G, LECAD) The Late Eocene central axis of deposition was confined to the present position of the Llanos foothills. Late Eocene deposition reflects a regional increase in accommodation space and deposition of the principal reservoirs of rich petroleum basins (i.e., the Mirador, La Paz, upper Misoa formations). In Oligocene times inversion of the Eastern Cordillera divided the main depocenter into two central axes (H and H? in Fig. 1). Accommodation space diminished in uplifted regions and continued to increase in the depocenters allowing sporadic marine ingressions to the present position of the Llanos foothills. As uplift of the Eastern Cordillera continued, the eastern depocenter axis (proto Orinoco) migrated east and the western depocenter axis (proto Magdalena) migrated west. This process continued through the rest of the Cenozoic.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil