Correlation and Ar-Ar geochronology of the Cretaceous basement and the upper Eocene - Miocene (lower Aquitanian) sedimentary filling of the Tumaco forearc basin, southwest Colombia

Abstract

The geological record of the Northern Andes includes a series of allochthonous blocks of oceanic origin that were accreted to the continental margin during the late Cretaceous and Cenozoic. A forearc segment from southwestern Colombia contains an extensive Upper Cretaceous to lower Miocene stratigraphic record, which has been characterized using seismic lines, oil exploration wells and scarce surface outcrops. New Ar- Ar geochronological data, integrated with the Remolinogrande-1 well logs indicate the presence of at least 1051 m of a volcano-sedimentary basement characterized by basaltic rocks and mudstones with Ar-Ar ages that range between 82 and 76 Ma. These ages can be correlated with Ar-Ar ages from Gorgona Island, which is localized to the north and lies within the same structural trend, and suggest a long magmatic history independent from the Cretaceous allochthonous blocks that form the Western Cordillera. The increase in volcanic rocks and the presence of gabbroic rocks suggests a volcanic arc setting that experimented and increment in the magmatic flux at ca. 82 Ma. These basement rocks are unconformably overlain by mudstones, siltstones and muddy sandstones of the Lutitas de Remolinogrande formation (proposed unit), of upper Eocene – base of lower Miocene. These sedimentary rocks constrain the age of the accretion of this block, correlated with a Paleogene magmatic arc built over Cretaceous volcanic substratum. This segment could represent the southwestern portion of the Caribbean plate, associated with the Panama Isthmus, or an independent oceanic element accreted to the Northern Andes continental margin during the late Eocene.