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Sedimentitas marinas del Neógeno en la bahía de Tumaco, Nariño
The Morales Formation (New Unit): Record of Fluvial–Lacustrine Environments and the Beginning of the Miocene Explosive Volcanism in the Patía Sub–basin (SW of Colombia)
By Andrés F. GALLEGO–RÍOS, Andrés PARDO–TRUJILLO and Guillermo A. LÓPEZ–PLAZAS
The sedimentary rocks of the Patía Sub–basin (SW of Colombia) registered 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 (Department of Cauca). 563 m of sedimentary deposits were described in detail and divided from base to top into three lithological assemblages. (i) The top of the Conglomeratic Member of the Esmita Formation is formed by 33 m of thick beds of clast–supported lithic conglomerates interbedded with lenticular beds of litharenites, with trough and planar cross stratification, originated in a braided river environment. (ii) The Morales Formation, which is defined in this research, has ca. 470 m thick and is composed mainly by laminated gray–black mudrocks interbedded with thin beds of sandstones, with parallel and ripple lamination, graded bedding and soft–sediment deformation structures. Plant fragments, pollen and spores are frequent. Toward the top, there is an increase in the sandstone and polymictic conglomerate beds. They are interpreted as lake, marsh and crevasse splay adjacent to fluvial channels. (iii) The base of the Galeón Formation (7 m), composed by sandstones and conglomerates rich in volcanic fragments and crystals, characteristic of braided river deposits and volcanic activity simultaneous to sedimentation.
The petrographic study (conventional and heavy minerals) identified mainly litharenites and feldspathic litharenites, whose fragments suggest multiple igneous (volcanic and plutonic), sedimentary (shales and sandstones) and metamorphic (high pressure metamorphics and graphitic schists) sources, some of them related to the basement of the present–day Central and Western Cordilleras. A sample of detrital zircons obtained from the base of the Galeón Formation provided more than 70 volcanic crystals with U/Pb ages of ca. 14–15 Ma; this linked to the sudden increase in volcanic components, allows interpreting this age as the onset of a strong volcanism that continues nowadays in the Central Cordillera axis. This age and new palynological studies suggest a Burdigalian – early Langhian (ca. 19–15 Ma) age for the Morales Formation, which improves the chronostratigraphy of the region. Lower Miocene mud–dominated successions have been described in different sections of Colombia (Cauca–Patía, Upper Magdalena, Llanos and the Caribbean Basins), some of them related to marine incursions.
Keywords: Patía Sub–basin, Miocene, stratigraphy, volcanism, geochronology.