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Abstract: Fold and Thrust Belt Along the Western Flank of the Eastern Cordillera of Colombia: Style, Kinematics and Timing Constraints Derived From Seismic Data and Detailed Surface Mapping

Restrepo-Pace, P. A. - Conoco Colombia Inc.; Fabio Colmenares, Camilo Higuera, and Marcela Mayorga - Geosearch Ltd.; and Jairo Leal - Texaco Colombia

Direct evidence for polyphase deformation along the western flank of the Eastern Cordillera of Colombia along the Middle Magdalena Valley has been obtained from detailed surface mapping and seismic data. The sequence of events and related evidence is summarized as follows:

Latest Campanian to Maastrichtian uplift of the Central Cordillera signaled by the presence of extensive alluvial fans, i.e., Cimarrona and Puerto Romero Formations. These consist largely of coarse clastic conglomerates and sandstones containing milky quartz pebbles most likely to be derived from basement rocks to the west. The Late Maastrichtian unconformity that strips-out the Cimarrona deposits locally, may indicate incipient structuration along the western flank of the Eastern Cordillera. A well developed east dipping Late Paleocene-E. Eocene unconformity recognized seismically throughout the Middle Magdalena Valley, is represented along the Eastern Cordillera mountain front by a deeply scouring unconformity beveling a westerly vergent thin-skinned belt of tightly spaced folds and thrusts. Synkinematic deposits containing large clastic fragments mainly of Cretaceous cherts are preserved as piggybacks of the main thrust sheets (Pocara and the base of La Paz Formations). The relief generated by this foldbelt constrained the axis of late Eocene sediments to run along the present crestal zone of the Eastern Cordillera (Villamil and Restrepo-Pace, 1997) and caused east tilting of the Middle Magdalena Valley basement by flexural loading. At the thrust front, a series of intra-cutaneous wedges were developed at some point in Late Miocene time as evidenced by onlapping relationships within the Miocene Real deposits at the top of its fluvial section. Late Miocene reactivation of some of the west vergent structures, in a break-back sequence, exhumed and refolded the Late Paleocene-E. Eocene foldbelt. In the field, tilted synkinematic deposits and the refolded Late Paleocene- E. Eocene unconformity may be observed. In the hinterland, major inversion of the Mesozoic extensional faults occurred, with concomitant deposition of molassic sediments in the Middle Magdalena Valley (seismically transparent section at the top of the Real Group.

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