TINACOA Formation

UPPER DEVONIAN ?

State of Zulia, Venezuela

Author of name: Geologists of the Caribbean Petroleum Co. (?).

Original reference: R. A. Liddle, 1943, p. 17.

Original description: ibid.

Probably the Tinacoa formation was originally named by the geologists of the Caribbean Petroleum Company, who were among the first to study the geology of the Perijá mountains. The name was first used in the literature by Liddle (1943, p. 17), although he stated that is was "an established name".

The earlier use of the name was "Río Tinacoa" formation; however, to maintain consistency with similarly named units (Guasare, Cogollo, etc.), the more commonly used term "Tinacoa formation" is preferred.

Liddle (ibid., p. 18) described the formation on Río Tinacoa as predominantly black, nodular, locally graphitic appearing shale which breaks conchoidally or splintery, with thin-bedded, hard, quartzitic sandstones in the lower one-third, and thin, hard, black limestone beds at the base.

W. K. Gaeley (1945, Richmond Exploration Company private report) described the type section: "The Tinacoa formation consists of uniformly thin-bedded, hard, gray-black, nodular, calcareous shale, medium-gray, carbonaceous siltstone, and light-gray, hard, fine-grained, calcareous sandstone. Thickness of individual beds is commonly between two and five centimeters, but may reach a decimeter or more. The formation is stained with limonite and limonitic nodules are locally abundant in the shales and siltstones. The, shales break with a blocky or splintery fracture."

Gealey (ibid.) described the rocks underlying the type Tinacoa formation (calling them "Devonian undifferentiated") in practically the same terms as Liddle (1943, p. 14) used to describe his Caño Grande formation of the Río Cachirí series, and although no fossils have been recovered from the Río Tinacoa locality, these formations are considered correlative. It is probable that Liddle's description of the Río Tinacoa formation included at least some of the older beds. The type section of the Tinacoa formation is therefore restricted to the upper two-thirds of Liddle's description. It begins at a point 1.5 kilometers northwest of "hacienda" Medellín on Río Tinacoa, and continues for 1.6 kilometers northwestward (aerial distances). On the Río Tinacoa it occupies the center of a syncline, both limbs of which are bounded by faults. Faulting and distortion make accurate measurement difficult, but at least 850 meters of thickness are exposed on the west limb of the syncline.

On the Río Cuíba, a short distance to the south, it is overlain apparently conformably by the Macoíta formation, and farther to the south, between Río Cuíba and Cairo de la Cuna (the northern fork of Río Macoita), it is thought to be conformable with the underlying "Devonian undifferentiated" beds mentioned earlier. The Tinacoa formation has been mapped between the last site mentioned and the Arroyo Seco, a tributary of the Río Cogollo, a distance of about 15 kilometers. In this region it is badly crumpled and faulted, but it is thought to extend, intermittently exposed, to the Río Cachirí area some 50 kilometers to the north.

No fossils have been recorded from the Tinacoa formation, but on stratigraphic position and lithologic similarities, it is correlated with the Caño del Oeste formation of the Río Cachirí series, for which Liddle (1943, p. 14-19) gives evidence of Upper Devonian age.