TACAL, Formation

TERTIARY (upper Eocene-?lower Oligocene)

State of Falcón, Venezuela

Author of name: Geologists of' the Standard Oil of Venezuela (present Creole Petroleum Corp.).

Original reference: C. González de Juana, 1938, p. 126.

Original description: ibid.

The name Tacal was published by C. González de Juana (1938, p. 126) for a unit which he stated had been called in private reports, Tacal group, but which he preferred to call "a formation, or at least a facies". Since Senn (1940, p. 1604) and Liddle (1946, p. 378) have cited the unit as a formation, and since no subdivisions have been named, it seems convenient to consider the unit, at least for the present, as a formation.

The name is derived from the Tacal hills, in the eastern part of the District of Buchivacoa, State of Falcón. González de Juana states that due to folding and faulting of the beds, and the fact that these are frequently covered by sediments of the La Puerta formation, it was not possible to measure a detailed section. The thickness in the type region must be more than 1,000 meters. González de Juana believes that these sediments represent the youngest Eocene, stratigraphically higher than the Paují shales. The detailed description reads as follows:

" . . . the upper part is predominantly sandy, and consists of alternating beds of sandstone and sandy clay. The sandstones are gray white, micaceous and locally conglomeratic; the clays are generally of light gray color, with thin beds of coal very characteristic of the upper 350 meters" (from which we would assume that this upper part is more than 350 meters thick).

"Below this sandy facies, the formation has a more clayey character; sandstones are still present, but they are thin and fine-grained, some of them with calcareous cement. Light colors are still present, but dark gray and greenish gray predominate in these 400 meters".

"Below this clayey section, there are observed some 250 meters of sandy beds; the sandstones are coarse-grained, intercalated with dark clays. Below these are found a zone of dark shales whose differentiation from the Paují shales is practically impossible".

González de Juana assumes that the beds mentioned by Halse (1937, p. 183) as "Upper Eocene (Jackson)" which overlie the Tupure shales in southeastern Buchivacoa, correspond to the Tacal formation; these beds were described as chiefly marine clays with sandy, fossiliferous, calcareous bands. Furthermore, González de Juana believes that the Tacal may be represented in the south flank of the Maneta anticline, in the District of Miranda of Zulia; here, above 3,000-5,000 feet or sediments referred to the Paují formation, there are found some 5,000 feet of sandstones and clays, in part marine, in part brackish-water, cross-bedded and with plant remains in the upper part. These beds, whose correlation is uncertain, are thought by González de Juana to represent either the Tacal formation or the lower Oligocene Patiecitos formation.

Senn (1940, p. 1604) states that the base of the Tacal is an unconformity which corresponds to the great upper Eocene orogeny, observable in many regions, which formed the Zulia-Falcón basin. This basin, he says, was itself deformed during the orogeny, so that the upper Eocene-lower Oligocene seas were restricted to two narrow channels, separated by an elevation made-up of folded beds of the Misoa-Trujillo and Paují formations. In the southern channel the Agua Negra unit was laid down, and in the northern one the Tacal, "corresponding to an upper Eocene and perhaps lower Oligocene age". (Seen, 1940, p. 1590). In his correlation table, Senn (1940, p. 1580) correlates the Tacal formation with the Santa Rita conglomerate plus the Jarillal shales, but excluding the Churuguara beds which he correlates with the San Juan de la Vega sand plus the Patiecitos formation.

The description of the Taeal formation given by Liddle (1946, p. 378-381) is unsatisfactory; he describes the lower contact in one line as unconformable in another line as transitional, and more-over, without any explanation changes the ages given by González de Juana for the rocks in the Maneta anticline.

Mencher (1953, p. 709) mentions that in western Falcón the basal bed of the Mitare group (of middle to possibly lower Oligocene age) are bans gressive on the Tacal formation (age given as upper Eocene to possibl lower Oligocene). The Tacal is shown in the correlation table as an approximate equivalent of the Cerro Misión shales of eastern Falcón.

None of the authors cited gives fossil lists.

Frances de Rivero