Sedimentological evidence for early uplift (Oligocene) of the Venezuelan Andes

Conference · Mon Feb 01 00:00:00 EST 1993 · AAPG Bulletin (American Association of Petroleum Geologists); (United States)

OSTI ID:6064411

Higgs, R ^[1]

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1. Maraven S.A., Caracas (Venezuela)

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; 300m) are dark gray and bioturbated. Pyrite is absent and the fauna is restricted to benthonic forms (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 form 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 Lake 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). They are thus regarded as the initial Andean molasse deposits and their deposition has continued to the present day.

• OSTI ID: 6064411
• Report Number(s): CONF-930306-; CODEN: AABUD2
• Journal Information: AAPG Bulletin (American Association of Petroleum Geologists); (United States), Vol. 77:2; Conference: International congress of the American Association of Petroleum Geologists (AAPG), Caracas (Venezuela), 14-17 Mar 1993; ISSN 0149-1423
• Country of Publication: United States
• Language: English

Related Subjects

02 PETROLEUM
58 GEOSCIENCES
ANDES
OROGENESIS
MOUNTAINS
AGE ESTIMATION
DEPOSITION
DEPOSITS
FOSSILS
PYRITE
SEA LEVEL
SEDIMENTARY BASINS
SHALES
TERTIARY PERIOD
VENEZUELA
CENOZOIC ERA
DEVELOPING COUNTRIES
GEOLOGIC AGES
GEOLOGIC STRUCTURES
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LEVELS
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SOUTH AMERICA
SULFIDE MINERALS
020200* - Petroleum- Reserves
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580000 - Geosciences