Abstract: Exploration in Overthrust Belts: An Example from the Upper Magdalena Valley, Colombia, S.A.

By

Kim Robert Butler
Consulting Geologist

Hydrocarbon production from the Canadian and other A-type overthrust belts has been the impetus for far reaching exploration in similar orogenic belts. The Andean overthrust belts have been one such region.

The Upper Magdalena Valley is a Neogene age foredeep basin situated between the Central and Eastern Cordillera of Colombia. Up to 2600 meters(8000 ft.) of non-marine molasse sediments overlie a series of east-directed thrust faults, These faults disrupt a pre-Miocene eastward thinning clastic sedimentary basin. This older basin contains two distinct sedimentary cycles. The first cycle on top of basement is a Late Cretaceous marine transgressive-regressive sequence. The second cycle is a Paleogene non-marine molasse sequence.

The thrustbelt that developed across part of the pre- Miocene sedimentary wedge is typified by a sequence of east-verging, i.e. cratonward migrating, basement-rooted thrusts. Part of the crustal shortening is transferred into the sedimentary cover to create an adjacent, but aerially limited decollement thrustbelt. Younger decollement thrusts fringe structurally lower basement faults that lie to the east. Other than the extensive basement involved nature of these structures, this overthrust belt illustrates typical thrustbelt features, i.e. ramp anticlines, small duplex zones, and folded thrust sequences.

An outline of the structural style and development of the Upper Magdalena Valley illustrates common thrustbelt structures within a unique section of Andean geology. For purposes of demonstrating the integrated approach that is used in exploration in these structurally complex areas, examples from surface geology, seismic, gravity, and geochemical surveys will be shown across mature oil fields and exploration plays.

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