CAÑO DEL OESTE, Formation

MIDDLE DEVONIAN

State of Zulia, Venezuela

Author of name: R. A. Liddle, et al., 1943.

Original reference: R. A. Liddle, et al., 1943, p. 17-19.

Original description: ibid.

The Caño del Oeste formation is named from the Caño del Oeste branch of the Río Cachirí located in the District of Maracaibo about 87 kilometers west of the city of Maracaibo. In Caño del Oeste, the type section extends from a point 3 kilometers upstream to a point 1.55 kilometers upstream from the junction of Caño del Norte. The Caño del Oeste formation, which forms the middle portion of the Rio Cachirí group, is conformable with both the underlying Caño Grande and overlying Campo Chico formations, and its thickness is given as approximately 3,500 feet (1,067 meters). In the type section, the base of the Caño del Oeste formation is represented by a black unctuous shale which conformably overlies gray calcareous shale at the top of the Caño Grande formation. The greater part of the formation, however, is composed of bluish black fine-grained micaceous ferruginous quartzite, dark gray micaceous nodular limonitic unfossiliferous shale which has a shattery fracture and contains microscopic particles of lignite, black micaceous slaty and splintery shale and an occasional dark gray fossiliferous shale and argillaceous limestone containing many corals and some crinoid stems. In Caño Grande, some 3 kilometers southwest of the type section, there are locally indurated shales caused by a nine-meter sill of basalt in the lower part of the formation. On either side of the sill the shales are slaty.

Although Liddle, et al. (1943, p. 18) state that "no fossils have been found in the Caño del Oeste formation, "they report on p. 45 and p. 46 that sample 37, collected from "calcareous shale and shaly limestone" in the lower third of the formation, contains many corals and some crinoid stems. The corals identified are Heliophyllum halli Milne, Edwards and Haime and Heterophrentis venezuelensis (Weisbord), and both of these species occur in the underlying Caño Grande formation which is considered to be upper Lower to lower Middle Devonian in age. In view of the presence of at least two identical fossil species in both formations and the transitional nature of the one into the other, the age of the Caño del Oeste formation is thought by the present writer to be Middle Devonian.

Liddle, et al. (1943, p. 17-19), believe that the Caño del Ooeste formation can be correlated with the Río Tinacoa formation as exposed in Río Tinacoa about 52 kilometers southwest of Caño del Oeste. The Río Tinacoa formation has been mapped from the Río Tinacoa for at least 15 kilometers to the southwest by Donald MacArthur (unpublished report) and, on the assumption that the Caño del Oeste formation is exposed for some distance northward from Caño del Oeste, the Caño del Oeste-Río Tinacoa formations have a known, if perhaps interrupted, extent of at least 70 kilometers along the east flank of the Sierra de Perijá in Venezuela. In the adjoining Republic of Colombia, the Caño del Oeste formation seems to be equivalent in part to Middle Devonian deposits reported or described from the Guajira peninsula, Manaure, Curumaní-Santa Isabel and Floresta, and for further comments on this correlation the reader is referred to the discussion of the Río Cachirí group.

Norman E. Weisbord