ONIA, Formation

TERTIARY (Pliocene)

Author of name: H. D. Hedberg and L. C. Sass, 1937.

Original reference: H. D. Hedberg and L. C. Sass, 1937, p 106-107.

Original description: ibid.

The term "Onia beds" was introduced by Hedberg and Sass (1937, p. 106-107) for young Tertiary sediments encountered in the subsurface of the Lake Maracaibo basin. The name was derived from the Río Onia, a tributary of the Río Escalante in the northern part of the State of Mérida. However, most of the information concerning the formation was derived from well sections on both the eastern and western borders of the Lake Maracaibo and it is not certain if the outcrops on Río Onia are representative. No type section has been measured on Río Onia, but Manger (1938, p. 78) described a good section in the Rita water well, 2 kilometers east of the village of La Rita, District of Bolívar, that could be considered as a type section.

The Onia formation was described (Hedberg and Sass, 1937, p, 106) as consisting of light gray and greenish-gray, friable, micaceous sandstones and light gray to yellowish-brown mottled claystones and siftstones. The formation attains a thickness of up to 3900 feet (1220 is thin: 250 feet at La Rita, 85 feet at Cabimas and missing at Lagunillas. In the Baralt District, it is over 3000 feet thick (well Nejeme-1).

The Onia formation is in transitional stratigraphic contact with the underlying La Villa formation in some areas, but near Maracaibo it rests unconformably upon the La Villa and Lagunillas formations. The uppermost Onia grades vertically and laterally into the overlying El Milagro formation, with an interfingering of the sediments of these two formations (Manger, 1938, p. 77).

No fossils have been found but Hedberg and Sass (1937, p. 107) correlate the Onia formation with the Codore formation (of Mencher et al., 1951, Correlation Chart), State of Falcón, which contains fossils of a lower Pliocene age. It is probable that the Onia is in part lower Pliocene. Sutton (1946, p. 1710) states that the type Onia beds of Hedberg and Sass (1937, p. 106107), or Onia formation of Douglas (1938, p. 88-89) form the lower part of his "Betijoque formation" on Rio Onia. However, from stratigraphic relationships, it appears that the Onia of the Bolívar Coastal fields is equivalent to the upper Betijoque group, the Sanalejos and Vichú formations.

Gordon A. Young