URUMACO, Formation

TERTIARY (upper middle Miocene)

State of Falcón, Venezuela

Author of name: S. H. Williston, 1922 (private report).

Original reference: A. H. Garner, 1926, p. 683.

Original description: ibid.

Although the term Urumaco formation was first applied by S. H. Williston (1922) in a private report, A. H. Garner (1926, p. 683) seems to have been the first to describe this formation in publication. Soon thereafter Williston and Nichols (1928, p. 448) mentioned and describes it briefly. The type locality is agreed upon as being north and south of the village of Urumaco, in the District of Democracia.

The Urumaco formation of west-central and western Falcón is essentially the lateral equivalent of the Caujarao of east-central Falcón, but both Urumaco boundaries are slightly higher than the corresponding ones of the Caujarao. In modern usage, these terms have largely supplanted Ralph Arnold's "Damsite" formation, established during private investigations in Venezuela between 1911 and 1916. The "Damsite" formation probably represents only the middle portion of the Urumaco, however. The Berjadín beds, described by Liddle (1928, p. 318-319), near the junction of Quebrada La Danta with Río Borojó, south of Dabajuro, have since been recognized as the local development of the Urumaco formation. The Sabaneta limestone of Garner (1926, p. 678, 682), including fossiliferous, brown, soft limestone beds and brown and white clays and shales in the vicinity of the town of Sabaneta, forms a portion of the Urumaco as herein described. The topmost portion of the Boca de Don Diego beds of eastern Falcón (Liddle, 1946, p. 431) may correlate with the Urumaco.

The formation has been divided into lower, middle, and upper members at the type locality. The lower Urumaco consists of both marine and nonmarine interbedded gypsiferous, blue, brown and mottled shales, with a lesser quantity of thin fossiliferous limestone and a few sandstone beds. The shales contain a few thin coal seams and silicified wood. The middle Urumaco is very similar to the lower Urumaco, but the more numerous marine mollusks testify to a more marine environment of deposition. The upper Urumaco is a somewhat more terrestrial deposit, with more sandstone beds and coal seams than in the two older members. Near the top of the Urumaco a bone bed extends widely north of the village of the same name. The bones are of turtle, crocodile, and some unidentified mammals. If the upper contact is placed at a lithologic change about 20 meters stratigraphically above the bone bed, the total Urumaco thickness is about 1,675 meters. Since the latter figure corresponds closely with that of Williston and Nichols (1928, p. 448) -5,200 feet or 1,585 meters- their formation boundaries must have been drawn at about the same places in the local section.

The Urumaco formation contains an abundant molluskan fauna at various stratigraphic levels. Bones of reptiles and some mammals are especially numerous in the so-called "bone bed" near the upper boundary of the formation, and they occur somewhat sporadically elsewhere in the upper member. The abundant fauna indicates an upper middle Miocene age.

W. M. Chapell