DETRITAL ZIRCON TYPOLOGY AND U/PB GEOCHRONOLOGY FOR THE MIOCENE LADRILLEROS-JUANCHACO SEDIMENTARY SEQUENCE, EQUATORIAL PACIFIC (COLOMBIA): NEW CONSTRAINTS ON PROVENANCE AND PALEOGEOGRAPHY IN NORTHWESTERN SOUTH AMERICA

Zircon U/Pb dating (LA-ICP-MS) plus typological and internal texture analyses (optical and SEM-CL/BS microscopy) were performed on detrital materials obtained from 12 samples collected from the Miocene sandstones of the Ladrilleros-Juanchaco Sedimentary Sequence (LJSS) located in Colombia’s Pacific Coast. Based on 114 zircon grains dated we established a maximum depositional age for the LJSS at ca. 10-13 Ma (Tortonian-Serravallian). We also identified typology-age associations as indicators of sediment provenance. Our results show that zircons with S and P dominant typologies have internal structures/zoning indicative of igneous, and potentially also metamorphic, origins. Morphometric results suggest limited transport from source areas. Both U/Pb age spectra and zircon typology data point to the Western Cordillera as the principal source of detrital materials for the LJSS. A paleogeographic reconstruction shows that, during the Late Miocene, significant portions of the Western Cordillera were uplifted and actively eroding, thereby forming a fluvio-topographic barrier that prevented sediments from the Central Cordillera reaching the Pacific basins. Exhumed Miocene plutons located along the axis of the Western Cordillera may also have played a role as geomorphologically active massifs. Our methodological approach to evaluate provenance in a complex litho-structural regime represents a viable development of the standard methodology, considering the quantity of information provided in provenance studies and paleogeographic reconstruction, and should be systematically used in other sedimentary sequences in the Northern Andes.