DETRITAL ZIRCON EVIDENCE FOR EARLY MIOCENE ARRIVAL OF PROTO-ORINOCO RIVER DELTA IN TRINIDAD

The Cenozoic geologic history of northern South America can be characterized as a diachronous, west-to-east change from north-facing passive margin, to zone of collision with the Great Arc of the Caribbean, and to oblique, eastward transport of the Great Arc of the Caribbean and plate. This complex tectonic history and related large-scale strike-slip displacement controlled the on- and offshore sedimentation along the northern South America-Caribbean plate boundary zone. As the current eastern end of the Caribbean-South America plate boundary, the Trinidad area records the most recent tectonic regime transition and provides a more recent tectonic analog to understanding similar collisional and strike-slip events that earlier affected areas to the west of Trinidad in Venezuela, Colombia and offshore areas. Five sedimentary outcrop samples ranging in age from Middle Eocene to Pliocene were collected from the central range of the Trinidad to understand rapid, tectonically-controlled sediment dispersal changes that affected the Trinidad area. 684 detrital zircon grains were separated from these samples and analyzed using the U-Pb LA-ICP-MS method. Results show that pre-early Miocene sedimentary rocks are dominated by Precambrian age grains typical of the Guyana shield south and southwest of Trinidad with age peaks at ~1400, ~1800, and ~2000 Ma. In contrast, all the post-early Miocene samples show a much more mixed population of ages that include Precambrian, Paleozoic, and Mesozoic peaks that typify rocks presently cropping out in the Andes Mountains of northwestern South America. We interpret this abrupt transition in early Miocene zircon age populations as the initial collision of the Great Arc of the Caribbean in Trinidad, formation of the Venezuelan Coastal Range that produced the valley of the proto-Orinoco river, and the establishment of the proto-Orinoco river that connected eroded areas of the Andes Mountains in Venezuela and Colombia with its delta in the Trinidad area.