CAÑO GRANDE, Formation

LOWER - MIDDLE DEVONIAN

State of Zulia, Venezuela

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

Original reference: R. A. Liddle, et. al., 1943, part 1, p. 14-17.

Original description: ibid.

The type section of the Caño Grande formation is in Caño Grande, a tributary of the Río Cachirí, and is located in the District of Maracaibo, State of Zulia, approximately 90 kilometers west of the city of Maracaibo.

At the type locality, the Caño Grande formation constitutes the lower portion of the Río Cachirí group and extends from a point 1,130 meters upstream to a point 65 meters downstream from the mouth of Caño del Sur. The base of the formation is in fault contact and discordant with pre-Devonian schists, quartzites and granite of Liddle's (1946, p. 111) Sierra de Perijá "series" whereas above, the Caño Grande formation grades into and is conformable with the overlying Caño del Oeste formation. The thickness of the Caño Grande formation in the type section is calculated at 762 meters (2,500 feet) but in view of the fault at the base and the implication that the lowermost strata are not exposed, the true thickness may be greater than that measured at the surface.

As originally described by Liddle, et al., (1943, p. 14), the Caño Grande formation in the type section consists for the most part of nodular highly fossiliferous gray micaceous sandy shales containing mud pellets and microscopic particles of lignitic matter. The sandy shales are limonitic and calcareous and break with a shattery fracture. In the lowest portion of the formation there are some evenly bedded fine-grained dark gray micaceous quartzitic sandstones and a few shaly sandstones which contain imprints of Spirifer and of corals. In Caño del Oeste, 3.4 kilometers northeast of the type section, the uppermost beds of the Caño Grande formation are composed of highly fossiliferous gray calcereous shales and blackish argillaceous limestones underlain by gray shales in which the body cavities of fossils are filled with powdered limonite.

More than fifty different forms of invertebrate fossils have been described from the Caño Grande formation by Weisbord (1926) and Harris and Wells, in Liddle, et al. (1943) and among these are corals, bryozoa, mollusks, brachiopods and trilobites. Representative species of each of these divisions are Heliopryllum halli Milne, Edwards and Haime, Fenestella venezuelensis Weisbord, Platyostoma ventricosum Conrad, Lepteena rohboidalis (Wilckens), and Phacops argentinus ? Thomas. Many of the fossils in the Caño Grande formation resemble, and some are identical with, species found in the Oriskany, Onandaga and Hamilton formations of eastern United States and it is on this basis that the Caño Grande formation is assigned an upper Lower to lower Middle Devonian age.

The Caño Grande formation has been traced northeast of the type section for a distance of about 7 kilometers. Farther north, fossiliferous float possibly from this formation has been reported from the Río Socuy and Río Guasare, indicating that in its type facies the Caño Grande formation extends northward from Caño Grande for perhaps a distance of 40-50 kilometers along the east flank of the Sierra de Perijá. The formation is doubtless exposed southwest of Caño Grande but its extent in Venezuela in that direction is not known. Liddle (1928, p. 100; 1946, p. 116) reports fossiliferous Devonian rocks in the valley of Río Momboy near Mendoza in the State of Trujillo. The Momboy locality is in the Cordillera de Mérida about 250 kilometers southeast of Caño Grande. Little is known of this particular area either stratigraphically or paleontologically so that equivalency with the Caño Grande formation or with other formations of the Río Cachirí group in the Sierra de Perijá is still uncertain. In Colombia, however, it does seem likely that the Caño Grande formation is equivalent to at least a part of the Middle Devonian deposits reported in (a) the Guajira peninsula, (b) east of Manaure, (c) in the Curumaní-Santa Isabel region and (d) at Floresta, and additional comments on this correlation are given under the Río Cachirí group.

Norman E. Weisbord