The Choco block in northwestern South America: structural, tectonostratigraphic, and paleogeographic implications
DOI:
https://doi.org/10.32685/0120-1425/bolgeol31.1.1990.179Keywords:
Colombia, Dabeiba Arch, Atrato Basins, Chucunaque, Istmina, Deformed Zone, Baudo Arch
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Copyright (c) 1990 Servicio Geológico Colombiano
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Abstract
The Chocó Block, in northwestern South America, comprises the Panamanian and Colombian regions east of the Panama Canal Zone to northwestern Colombia, from the northwestern flanks of the Western Cordillera above 4° north latitude. It is characterized by three structural and lithogenic elements: the Dabeiba and Baudó arcs, the Atrato-Chucunaque basins, and the Isthmian Deformed Zone.
The Dabeiba and Baudó arcs, with similar characteristics, delimit the outer margins of the Chocó Block and are distinguished by: (a) positive gravimetric anomalies related to igneous and sedimentary bodies of oceanic origin; and (b) occurrence of pelagic, hemipelagic, and terrigenous strata in blocks without keeping among themselves any stratigraphic or structural coherence, with different ages and environments, and mafic volcanic intercalations. In addition, the Dabeiba Arch shows a "melange" type fabric, particularly in the eastern margin where broken strata and inclusions of exotic blocks of Cretaceous-Paleocene, Eocene-Oligocene, and Miocene age occur dispersed within a chalky pelitic matrix of middle Miocene age.
The Atrato-Chucunaque basins contain a sedimentary fill more than 10 km thick. Two very distinctive stratigraphic sequences characterize them, particularly in the Atrato valley: (1) An outcropping sequence composed mainly of pelagic and hemipelagic strata of Oligocene to middle Miocene age, overlain by a hemipelagic and terrigenous sequence of upper- middle Miocene to Pliocene age, and (2) An underlying sequence of middle Miocene and older age, mainly composed of turbid tic os sediments that could only be recognized in subsurface sections.
An evaluation and synthesis of the structural and lithogenic information of the Chocó Block indicate the following: (1) The Chocó Block is an exotic terrain with no lithogenic affinity to South America, accreted to the continent (Cordillera Occidental) during the middle Miocene. The occurrence of exotic blocks with Paleocene planktonic microfauna in the Dabeiba Arc suggests that the Chocó Block comes from latitudes as far away as Guatemala and Mexico. (2) The Uramita Fault Zone is the suture that brings the Chocó Block into contact with the Western Cordillera of northwestern South America. (3) The juvenile intramountainous Atrato and Chucunaque basins and the Isthmian Deformed Zone were formed due to Middle Miocene accretion and tectonic disturbances.
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