CENOZOIC GEOLOGIC EVOLUTION OF THE SOUTHERN TUMACO FOREARC BASIN (SW COLOMBIAN PACIFIC)

Tumaco is a forearc basin that keeps insights into the Cenozoic geologic evolution of SW Colombia. In this region the subduction of the Farallon and Nazca plates beneath South American plate, have controlled subsidence and magmatic activity during Oligocene to Recent times. A synthesis of seismic, stratigraphic, petrographic, geochronologic and biostratigraphic data from outcrops and wells is presented. Tumaco onshore basin has a trough shaped symmetric geometry limited to the east by the Western Cordillera and to the west by the Remolinogrande-Gorgona Structural High where ~8,000 m of sediments were accumulated during the Cenozoic. They are composed of mudrocks, sandstones and conglomerates, which vary in their proportion over time, and were mainly accumulated in open marine and deltaic systems. Calcareous nanofossils, foraminifera and palynomorphs, allowed controlling the depositional time of the sedimentary units; however, the low abundance and preservation and the reworking of microfossils, in some intervals, requires the use of multi-tools to determine the age of the deposits.

Sandstones are mainly litharenires and feldspatic litharenites, texturally immature, and composed of cherts, basic to intermediate volcanic fragments, and crystals such as feldspars (Na and K), pyroxene, amphibole and biotite, which can be associated with basic-intermediate volcanic, plutonic and sedimentary rocks of the current basement of Western Cordillera. Sediment provenance analysis (detrital zircon and heavy minerals) suggests continuous volcanism from Late Oligocene to Pleistocene times, the activity of which increased since the Middle Miocene. The presence of low percentages of pre-Cenozoic zircons and metamorphic rock fragments in the Miocene units, would be related to reworking of ancient sedimentary units or to a partial connection with the Central Cordillera basement. The study of Miocene-Pliocene outcrops and well cores allowed to interpret a shallowing of the basin during the Messinian-Zanclean times. Volcanoclastic fans, as well as fluvial and coastal sediments, associated to the current Patía and Mira rivers are partially covering the Miocene-Pliocene deposits.