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HIGGS, ROGER, Maraven S.A., Caracas, Venezuela

ABSTRACT: Sedimentological Evidence for Early Uplift (Oligocene) of the Venezuelan Andes

The ongoing Andean orogeny is generally believed to have started in Miocene time. However, sedimentological studies of a Cenozoic clastic section in the northwestern foothills of the Venezuelan Andes (Rio Chama) yield two lines of evidence that uplift was already underway in the Oligocene. (1) Thick Oligocene shales (Leon Formation; 300 m) are dark gray and bioturbated. Pyrite is absent and the fauna is restricted to benthonic forams (R. Pittelli), suggesting deposition in a brackish lake rather than the sea. The shales occur throughout the plains northwest of the Andes. Such a large, long-lived lake implies isolation from the sea, suggesting that the Andes were already high in the Oligocene, forming a topographically confined basin similar to the modern Lake Maracaibo. Like modern La e Maracaibo, there was a tenuous connection with the sea, allowing marine incursions whenever eustatic sea level was high enough, as shown by horizons with marine fossils at other localities. (2) The overlying Oligocene-Miocene succession which caps the Rio Chama section (Caracol Member; Chama Formation; basal Betijoque Formation) includes fluvial channel sands with pebbles which can be matched to Cretaceous cherts of the adjacent Andes (Ftanita de Tachira). The first pebbles appear in the Caracol Member (Oligocene), which is thus regarded as the initial Andean molasse deposits, whose deposition has continued to the present day.

AAPG Search and Discovery Article #90988©1993 AAPG/SVG International Congress and Exhibition, Caracas, Venezuela, March 14-17, 1993.