ROBLECITO, Formation

TERTIARY (Oligocene)

State of Guárico, Venezuela

Author of name: Geological staff, S. A. Petrolera Las Mercedes.

Original reference: H. D. Hedberg, 1950, p. 1204.

Original description: ibid.

Hedberg (1950, p. 1204) was the first to describe in a publication the "Roblecitos shale" (wrongly spelled) and referred to it as a subsoil unit of the Santa Inés group which underlies the Chaguaramas formation east of the State of Guárico. A few years before this publication, the term Roblecito had been established and used by geologists of the S. A. Petrolera Las Mercedes, operating in the oilfields of Las Mercedes, State or Guárico. Patterson and Wilson (1953, p. 2719-2722) in an important publication, by means of electrographs, correlated the upper limit of the Roblecito shale in the northcentral part of Guárico with the outcrops near Camazas. They defined the upper limit of the Roblecito formation as the base of the overlying Chaguaramas formation and the base of the Roblecito as the upper limit of sand "A", that is the most recent sand of the La Pascua formation. Due to the fact that the Roblecito shale is a well limited unit, it has been elevated to the rank of formation.

The Roblecito formation consists lithologically of dark gray to black shales, massive, with thin bands of claystone and sandstone. However, some sandstones are found at the upper and lower extremities. The shales show abnormal characteristics in the electrograph by means of an amplified resistivity curve. Due to the silty material, some small stimuli show that can be correlated with much precision over great distances. In this way, it is believed that the shales represent deposition conditions of extreme lateral uniformity, and that were not affected by bottom currents before being compacted.

The correlations made by means of electrographs show that the greatest sinking and sedimentation took place in the northern part of the Roblecito basin. The average thickness is about 2,000 feet east of Guárico, and it is known by means of isopatic control that these thicknesses vary from 0 ft. southeast of Guárico to at least 7,000 ft. to the north of it. Towards the south, the Roblecito formation and the underlying La Pascua beds transgress continuously over a peneplain of cretaceous beds.

The extension of the formation is not known. To the northwest of Ruiz, Tucupido and Palacio oilfields, correlations of Roblecito extend all over in an approximate area of 7,000 square miles.

The Roblecito formation can be recognized in the outcrops of the Guarumen sandstone group (see) and it is suggested that Roblecito formation is equivalent approximately to the lower and middle parts of the Guarumen group (Kamen-Kaye and Mencher, private report). Kamen-Kaye and Mencher suggest an Oligocene age for the major part of the formation, but state that the upper beds may be lower Miocene.

The Roblecito formation contains an abundant foraminiferal fauna related to that of the Carapita formation. According to Hedberg (1950, p. 1204), the Roblecito may be equivalent to some part of the Carapita formation and may be the same age as the Bruzual claystone. Patterson and Wilson (1953, p. 2719-2722) place the Roblecito formation entirely within Oligocene time.

S. J. Brown