FREITES, Formation

TERTIARY (Miocene)

States of Anzoátegui and Monagas, Venezuela

Author of name: H. D. Hedberg, 1936 (private report).

Original reference: H. D. Hedberg, L. C. Sass and H. J. Funkhouser, 1947, p. 2105-2107.

Original description: ibid.

The Freites formation was named by Hedberg (1936, private report) with type locality in wells of the Oficina field, District of Freites, Anzoátegui, and first published by Hedberg et al. (1947, p. 2105-2107) who divided it into three members, based on the presence of sandstones near the top and base of the formation in contrast to the middle and thicker shaly part. The formation thickens northeastward in the Greater Oficina area from 1,100 feet in well Yopales Nº 1 to 2,000 feet in West Guara, Nipa and North Leona. The upper member is 300 feet thick and includes thin, shaly, fine-grained, whitish-gray and slightly glauconitic sandstones, remarkably continuous laterally; the lower sandy member is also 300 feet thick, and besides the typical greenish-gray fissile shales forming the major part of the formation (middle member), includes yellowish-green, medium to coarse-grained, glauconitic, calcareous or sideritic highly fossiliferous sandstones; common calcareous clay-ironstone concretions occur throughout the shales. Sand bodies in the Freites formation are designated (from top to bottom) by the greek letters Sigma, Rho, Mu and Lambda. The formation grades upward into the Las Piedras formation, contact being placed at the top of the first marine horizon in the section (base of the Tau sand). The contact with the conformably underlying Oficina formation is placed where the greenish-gray Freites shales give way to the brownish-gray Oficina sediments. The lower member of the Freites is rich in shallow-water marine fossils; the middle shale member represents a moderately deep-water marine environment, and the upper member is dominantly brackish. Foraminifera and mollusks indicate an upper middle to lower upper Miocene age, and the formation is correlated with the uppermost Santa Inés group of northern Anzoátegui.

According to Funkhouser et al. (1948, p. 1865-1866) the thickness of the Freites formation in the Greater Anaco area ranges from 1,500-2,000 feet southwestward, to 2,600 feet in outcrops along the Aragua de Barcelona road in the northwestern part. In well RG-1 (Santa Rosa field), the formation is divided into (1) the upper member, 1,135 feet thick, of interbedded greenish-gray shale and sandstone, locally glauconitic, with 100 feet of basal mottled claystone, barren of fossils; (2) the middle member, 565 feet thick, of greenish-gray shale, with basal fossiliferous grits and alternating marine and brackish-water faunas; (3) the lower member, 530 feet thick, of fossiliferous grits, black chert conglomerate, sandy limestone, "pepper and salt" sandstones and greenish-gray, shale, with shallow-water mollusks. A slight unconformity with the underlying Oficina formation is suggested on structural highs; the unit is unconformably overlapped by the Sacacual group in this area.

Hedberg (1950, p. 1205) recommends substituting the names of Aragua (pre-occupied by Garner (1926) for sediments now included within the Santa Anita group) and Punche formations for the more widespread term Freites formation. Although he includes Freites formation into the Santa Inés group, all geologists are not in agreement with this designation.

Mencher et al. (1951, p. 68) divide the Freites formation in the Temblador area into four members which are from top to bottom: sandy - shaly - sandy - shaly, with a total thickness of 1,600 feet, increasing northward and decreasing southward. A lower and middle Miocene age is here assigned to the formation.

Cecily Petzall