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ADDISON, F., M. COOPER, A. HAYWARD, S. HOWE, and J. O'LEARY, BP Exploration Co. (Colombia) Ltd., Santafe de Bogota, Colombia

ABSTRACT: Evolution of the Llanos Basin and the Deformation of the Eastern Cordillera, Colombia

The Llanos Basin is located on the flank of the Eastern Cordillera in northeast Colombia. Basin development commenced with the deposition of a synrift Triassic and Jurassic megasequence related to the separation of North and South America in the Caribbean. Basin development continued with the Cretaceous Back Arc Megasequence deposited in a back arc basin behind the Andean subduction zone. Three major sequences can be recognized corresponding to extensional pulses in the Tithonian, Albian, and the Santonian which control thickness and facies distributions.

The primary reservoir in the basin is the Late Eocene Mirador Formation which was deposited in a fluvial system which prograded from the Guyana Shield to the west-northwest. This was deposited as part of the Pre-Andean Foreland Basin Megasequence (Bartonian to Serravallian) which developed as a result of the onset of uplift and deformation in the Central Cordillera. This megasequence covered the Magdalena Valley the Eastern Cordillera and the Llanos Basin. In the foothills of the Eastern Cordillera the Mirador Formation begins to show evidence of marine influence and was probably deposited in a series of shoreface sands and offshore bar complexes in the Cordillera.

The Pre-Andean Foreland Basin Megasequence also includes the Eocene-Oligocene Carbonera Formation which was deposited in a low every fluvial system that was mud dominated. Within the Carbonera Formation a series of major, grossly coarsening upward cycles can be recognized which are separated by maximum flooding surfaces that approximate to time lines. These cycles correspond to the early phases of development of the Central Cordillera with each pulse being seen as an influx of coarser clastics to the basin. The Miocene Leon Formation is a marine shale which probably represents the major thrust sheet loading in the Central Cordillera creating a deeper, but relatively sediment starved marine basin.

The Andean Foreland basin megasequences includes the overlying Guayabo Formation which is a classic "molasse" sequence deposited in a high energy fluvial system with the coarse clastic detritus being shed from the developing mountains of the Eastern Cordillera to the west-northwest.

The deformation style in the Eastern Cordillera is a mixture of thin-skinned thrust structures and the inversion of the thick-skinned basement involved extension faults. The thin-skinned thrust features include ramp anticlines, duplexes and faults that develop "out-of-sequence." The main detachment levels in the sequence are the Cretaceous Gacheta Shale and the Carbonera shales at the base of the lowest coarsening upward sequence. The inverted extensional faults can be recognized by the thickening of the Lower Tertiary and Cretaceous in the hangingwall and the larger wavelength of the inversion structures. The relative timing of the two structural styles is problematic but preliminary analysis indicates that the inversion structures probably developed first. The inversion structures i clude the Cusiana field, a giant oil and gas-condensate discovery currently being appraised by BP and their partners Triton, Total, and Ecopetrol.

AAPG Search and Discovery Article #90988©1993 AAPG/SVG International Congress and Exhibition, Caracas, Venezuela, March 14-17, 1993.