Abstract

ECHEVERRI, Sebastián et al. UPPER NEOGENE STRATIGRAPHY IN THE SOUTH OF TUMACO BASIN (COLOMBIAN PACIFIC): CASCAJAL FORMATION, LITHOSTRATIGRAPHIC REDEFINITION PROPOSAL. bol.geol. [online]. 2016, vol.38, n.4, pp.43-60. ISSN 0120-0283.  https://doi.org/10.18273/revbol.v38n4-2016003.

The stratigraphic knowledge of the sedimentary fill of the Tumaco onshore fore-arc basin is relatively poor. Previous works are based on the study of seismic lines and wells for hydrocarbon exploration with minor emphasis on the surface segments. We present a detailed study of the late Miocene - Pliocene rocks that outcrop in the southern Tumaco basin. New stratigraphic data from the ANH-Tumaco 1-ST-S well represent the most complete and best preserved sections for this time interval. This information is integrated with new data from descriptions of ~200 m of stratigraphic sections visited in the coastal cliffs of the Tumaco bay and the Patia river. The detailed descriptions of these locations following parameters from stratigraphic codes allow us to propose the use of the term Cascajal Formation to refer to volcanoclastic rocks of Messinian - Zanclean age. This succession, with an estimated thickness of up to 405 m, is composed mainly of thick lenticular and undulated layers of sandstones and conglomerates, with an important volcanic input. Minor interbedded mudstones and muddy sandstones are present too. Locally, ichnofossils, bivalves, gastropods, foraminifera, echinoderms, crustaceans and well-preserved plant remains were preserved, interpreted as formed in a deltaic system influenced by volcanism.

Keywords : Colombian Pacific; fore arc; Tumaco Basin; stratigraphic code; volcanism.

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