Pinch Out of Upper Cretaceous-Oligocene Reservoir Units in the Llanos Basin of Colombia: A Result of Flexural Deformation in a Broken Foreland?
German Bayona1, Martin Cortes1, John Jairo Aristizabal2, Carlos Jaramillo3, German Ojeda4,
Andres Reyes-Harker4, Milton Rueda4, and Patricia Villamarin2
1 Corporacion Geologica Ares, Bogota, Colombia
2 ECOPETROL S.A, Bogota, Colombia
3 Smithsonian Tropical Research Institute, Balboa, Panama
4 Instituto Colombiano del Petroleo-ECOPETROL S.A, Bucaramanga, Colombia
Upper Cretaceous-Oligocene strata in the Llanos Foothills and Plains of Colombia wedge out and rest upon older units eastward. Detailed biostratigraphy on reservoir-seal (R-S) units, integrated with structural and geodynamic analyses, indicate growth of the Eastern Cordillera (EC) since Late Cretaceous as the primary control of flexural deformation and reservoir distribution on the Llanos basin.
Width and migration of foreland depozones controlled deposition/preservation/erosion of R-S units. Maastrichtian coarse-grained sandstones (R1) overlaid by coal-bearing mudstones (S1) on the Llanos Foothills record the first pulse of uplift on the EC. During the Paleocene, eastward and northward onlap of medium- to coarse-grained sandstones (R2) on the distal foreland was followed by deposition of fine-grained strata (S2) on tidal flats and coastal plains. The EC supplied sedimentary and metamorphic lithic fragments and contributed to the eastward migration of the flexural wave. Latest Eocene-Oligocene tectonic loading on the EC and flexural uplift on the Llanos Plains interrupted ca. 15 m.y. of tectonic quiescence on the basin during the Eocene. Flexural uplift erosion restricted the record of Eocene coarse-grained sandstones (R3) westward the Llanos Foothills. Subsequent filling of thick coarsening-upward coastal to deltaic successions on the foredeep (S3) covered R3 strata on the Foothills but created new space to accommodate coarse-grained sandstones on the Llanos Plains (R4).
Structural evolution of the EC controlled the final architecture of R-S units on the Llanos basin, and the increasing lacuna to the east. Westward and eastward migration of the flexural wave was a product of tectonic and sedimentary loading, respectively. The former caused erosion on foreland or older strata, whereas the latter controlled deposition.