The sedimentary rocks of the Patía Sub-basin (SW Colombia) record the Cenozoic geologic evolution of western Colombia from the Paleogene to the Holocene. In this research, we report a new Miocene stratigraphic unit near Mercaderes town (Cauca Department). A 563 m-thick sequence is described in detail and divided from bottom to top into three lithological assemblages. (I) The upper segment of the conglomeratic member of the Esmita Formation is formed by 33 m-thick beds of clast-supported lithic conglomerates interbedded with lenticular litharenites, with trough and planar cross-bedding. They are interpreted as braided river deposits. (II) The Morales Formation, which is defined in this work, presents 470 m thick composed mainly of parallel laminated gray-black mudstones interbedded with thin sandstone beds, with parallel and ripple laminations, graded bedding, and soft-sediment deformation structures. Plant fragments, pollen, and spores are common. Towards the top, sandstone and polymictic conglomerate beds increase. They are interpreted as lake, marsh, and crevasse splay deposits adjacent to fluvial channels. (III) The bottom of the Galeón Formation is composed by 7 m thick of sandstones and conglomerates rich in volcanic fragments accumulated in a fluvial environment with simultaneous volcanic activity.The petrographic study (conventional and heavy minerals) identifies mainly litharenites and feldspathic litharenites, whose fragments suggest igneous (volcanic and plutonic), sedimentary (mudstones and sandstones), and metamorphic (high-pressure metamorphic rocks and graphitic schists) sources, which are correlated with the basements exposed at the present in the Central and Western Cordilleras.We review a volcaniclastic sandstone obtained from the bottom of the Galeón Formation that provides the youngest population of U/Pb ages in detrital zircons of ca. 15.4 Ma, interpreted as the depositional age. The sudden increase in volcanic components allows us to interpret this time as the onset of strong volcanism in the basin, which continues currently in the Central Cordillera. This age and new palynological studies suggest a Burdigalian -early Langhian (ca. 19-15.4 Ma) age for the Morales Formation, which improves the chronostratigraphy of the region. Lower Miocene mud-dominated successions have been described in different basins of Colombia (the Cauca-Patía, Neogene Supplementary Information: S: https://www2.sgc.gov.co/ LibroGeologiaColombia/tgc/ sgcpubesp37201913s.pdf 396 GALLEGO-RÍOS et al.Middle and Upper Magdalena, Llanos, and Caribbean Basins) and can be related to a regional period of tectonically induced accommodation.