The Upper Magdalena Valley is bounded on the west and east by thrust faults that carry 'basement' lithologies: Precambrian metamorphics (Garzon Gneiss) or Mesozoic igneous and volcano-sedimentary rocks (Saldaña Fm. et al.). Many prospects drilled beneath these basement overthrusts have failed because trap geometries were not as prognosed. To improve our prognoses of these subthrust structures (and avoid becoming "Saldaña victims") we need not only better seismic imaging but also a better understanding of how the basin-bounding thrust systems evolved. The Acevedo Block lies on the eastern side of the southern Upper Magdalena basin. Subthrust prospects have been identified beneath the Garzon-Suaza fault system, which forms the western edge of the Eastern Cordillera there. Seismic, geological mapping, and thermochronometric studies show that the lowest basement-carrying thrust is relatively steep and has a small displacement, that the thrust system developed 'out-of-sequence', and that most of the structural development - on both sides of the valley - occurred during the Neogene. In addition, depth conversion of the seismic and the geology to the south show that the subthrust block is a planar, gently northwest-dipping panel >15 km across. The important result for local exploration is that the trap geometry is not an anticline beneath a shallow, fartravelled basement sheet - as would be implied by restoring a 'piggy-back' thrust sequence - but is a dipping panel that terminates updip against basement. It is possible that similar traps exist elsewhere along the margins of the Upper Magdalena Valley.


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