AMBROSIO, Formation

TERTIARY (upper Eocene)

State of Zulia, Venezuela

Author of name: H. D. Hedberg, 1927 (private report).

Original reference: H. H. Hedberg and L. C. Sass, 1937, p. 95.

Original description: F. A. Sutton, 1946, p. 1683-1685.

The name Ambrosio, introduced into print by Hedberg and Sass (1937, p. 95) is taken from the village of Ambrosio on the eastern shore of Lake Maracaibo, northern part of the Bolívar District. The type section is in wells of the Ambrosio field, northern Bolívar Coastal field, but scattered outcrops occur in the vicinity of the seeps of El Mene de Ambrosio.

Sutton (1946, p. 1683-1685) first described the formation, stating that it is composed largely of interbedded shales, clay shales and siltstones. The shales and clay-shales are mottled, the siltstones are generally light green, friable, carbonaceous and micaceous, and the sandstones are greenish gray to buff. Fossiliferous, calcareous sandstones and shales have been found in the subsurface. The thickness in subsurface varies from zero to over 350 feet, the maximum thickness being along the axis of the Ambrosio syncline.

The upper part of the formation is moderately fossiliferous; the middle part contains a rich foraminiferal fauna, the most common of which is Anomalina, providing the Anomalina zone. Larger foraminifera (Sutton, 1946, p. 1684-1685) indicate an upper Eocene age.

The outcrop area of the Ambrosio formation is limited to the vicinity of the El Mene de Ambrosio seeps and the subsurface distribution is limited to the Ambrosio syncline. Towards the north and the south it is truncated against the post-Eocene unconformity, whereas to the east is passes laterally into the Churuguarita formation and towards the west it changes facies into the Orumo formation.

Hedberg and Sass (1937, p. 95) correlated the Ambrosio with the Orumo formation and believed that the Ambrosio overlies the El Mene formation conformably. There is divergence of opinion among authors on the correlation with the Orumo, but all agree on its correlation with the Churuguarita formation. In present day usage, the Ambrosio is correlated with the upper part of the Orumo formation. Sutton (1946, p. 1685) and González de Juana (1951, p. 279) both postulate an unconformity between the Ambrosio and the underlying formation (El Mene o Las Flores) but subsurface well data indicate a complete fossil, mineral and lithologic vertical and lateral intergradation with the uppermost Ell Mene formation.

Gordon A. Young