The choco block in the northwestern corner of South America: Structural, tectonostratigraphic, and paleogeographic implications

Abstract

The Choco Block, located in the northwestern corner of South America, comprises the isthmus of Panama east of the Canal Zone and northwestern Colombia, including the westernmost flanks of the Cordillera Occidental above latitude 4°N. Three major structural and lithogenetic elements compose this terrain: the Dabeiba and Baudo Arches, the Atrato-Chucunaque Basins, and the Istmina Deformed Zone. The Dabeiba and Baudo Arches outline the external boundaries of the Choco Block and display similar characteristics: (a) mostly positive gravity anomalies and association of igneous bodies of oceanic origin with sedimentary suites, and (b) occurence of Upper Cretaceous to Miocene pelagic and hemipelagic and terrigenous strata in blocks of different ages and environments associated with mafic igneous rocks. The Dabeiba Arch exhibits a melange fabric, particularly at its eastern margin, in which disrupted strata and inclusions of Upper Cretaceous-Paleocene, Eocene-Oligocene, and Miocene exotic blocks are dispersed in a sheared pelitic matrix of middle Miocene age. The Atrato-Chucunaque Basins contain sedimentary fill exceeding 10 km in thickness. Two distinctive stratigraphic sequences comprise: (1) an outcropping and apparently continuous Oligocene to middle Miocene sequence composed mostly of pelagic and hemipelagic strata, overlain by hemipelagic and terrigenous strata of latest Middle Miocene to Pliocene age, and (2) an underlying middle Miocene and older sequence, composed mostly of turbidities, which has been recognized only in subsurface sections. An evaluation and synthesis of the structural and lithogenetic information of the Choco Block indicate the following conclusions. The Choco Block is an exotic terrane with no lithogenetic affinity with South America, accreted onto the northwestern flanks of the Cordillera Occidental during the middle Miocene. The occurence of exotic upper Paleocene planktic foraminiferal assemblages in the Dabeiba Arch suggests an origin for the Choco Block as distant as the northern latitudes of Guatemala and Mexico. The Uramita Fault Zone is the suture between the Choco Block and the Cordillera Occidental in NW South America. The young intermontane Atrato and Chucunaque Basins and the Istmina Deformed Zone were formed as a result of the accretion of the Choco Block to the northwestern flanks of the Cordillera Occidental during middle Miocene time.