DO U/PB AGE DISTRIBUTIONS ON DETRITAL ZIRCON GRAINS FROM MIOCENE SEDIMENTARY SEQUENCES IN NORTHERN COLOMBIA REALLY IMPLY FULL CLOSURE OF THE CENTRAL AMERICAN SEAWAY (CAS) AND DEVELOPMENT OF A MAJOR FLUVIAL CONNECTION BETWEEN CENTRAL AND SOUTH AMERICA?

Full closure of the Central American Seaway (CAS) cannot be considered irrelevant matter. It involved changes in oceanic circulation between Atlantic and Pacific waters, with potential consequences for northern hemisphere climate variability, onset of glaciations, and evolution of bio-diversity/biological exchange between North and South America. Timing and processes associated to full CAS closure are therefore essential for understanding tectonic and paleo-geographic evolution of the Circum-Caribbean region and the Eastern Equatorial Pacific.

Recently, U/Pb detrital zircon provenance analyses in Miocene fluvial and shallow-marine sequences of northern Colombia have been used to infer full closure of the CAS and to imply major paleogeographic changes in the Middle Miocene, such as the fluvial connection between the Panama Choco Block (PCB) and northern Colombia, capable of delivering a presumably unique Panamanian Eocene DZ signature to this area. Through the analyses of existing geochronology and new U/Pb ages and Hf data in igneous and detrital samples, and from a review of paleogeogprahic data we demonstrate that: 1) an alternative source for Early/Middle Miocene detrital zircons (DZ) from the Norandean Block (NAB), i.e., outside of the PCB litho-structural domain, is completely feasible, and 2) that the proposition of a major fluvial connection between the PCB and NAB is quite improbable.

In addition to U/Pb ages we use zircon morphometric and Hf data to more accurately constrain provenance. We specifically review information on the pelaoegeofgpahic evolution of features such as the WC topographic high and a deep marine bathyal basin situated to the E-NE of the accreted PCB, which create major impediments for the development of the proposed fluvial connection between the PCB and Northern Colombia. In our view, the presence of Early to Middle Miocene zircons in Middle Miocene strata in northern Colombia can be more coherently explained as being deposited by north-to-south flowing Inter-Andean fluvial systems, fulfilling both geochronologic and paleogeographic constraints.