Paleozoic Tectonics and Basin Evolution Along the Northwestern South American Margin. Insights From Detrital-Zircon U-Pb Geochronology
Ibanez-Mejia, Mauricio1; Ruiz, Joaquin; De Freitas, Mario G.; Mora, Alejandro; and Mora, Andres
Paleozoic sedimentary sequences of northern South America have the potential of being important oil- and gas-producing horizons, and are recently becoming the focus of active resource exploration by the energy industry. However, given the prolific character of the Meso- Cenozoic plays in the region, the Paleozoic systems have commonly been overlooked and our knowledge about the regional basin configuration and its evolution with time is still very limited. In this contribution we present new detrital-zircon U-Pb geochronological results from Paleozoic and inferred Neoproterozoic (meta)-sedimentary units that outcrop in the Quetame and Floresta Cordilleran massifs, as well as from deeply-buried but coeval clastic sequences that have been drilled below the Colombian Llanos foreland basin. Our new geochronological constraints clearly show that the pre-Devonian stratigraphy of the Cordillera is not allochthonous to South America as previous hypotheses propose, but instead represents a remobilized Neoproterozoic to Lower Paleozoic passive-margin sequence that was developed in very close proximity to the Amazonian margin of NW Gondwana. Passive-margin sedimentation lasted at least until the Lower Ordovician, time at which subduction was likely initiated along the northern proto-Andes giving birth to a long-lived accretionary margin that was active until the final assembly of Pangea in the upper Permian. Mid-Ordovician through Devonian units from the Llanos basin have zircon-age signatures that agree with this interpretation as they 1) display a progressive increase in the proportion of 1.2-0.9 Ga (Putumayo-age) grains that imply the exhumation of basement uplifts like the Garzón massif and/or reworking of correlative units to the Quetame group, and 2) show an increasing abundance of zircon grains with ages close to those of sedimentation, indicating the proximity of an active volcanic arc. Additionally, the presence of a prominent upper-Silurian to lower-Devonian zircon population in the Devo-Carboniferous units of the Llanos basin argues for a period of major magmatic activity in the neighboring arc. An intense period of orogenesis along this accretionary margin, if coupled with enhanced shortening and surface uplift, might be a potential explanation for the conspicuous absence of thick Silurian sedimentary units in the Eastern Cordillera and the regionally observed mid-Devonian unconformity.
AAPG Search and Discovery Article #90166©2013 AAPG International Conference & Exhibition, Cartagena, Colombia, 8-11 September 2013