LAGUNILLAS Formation

TERTIARY (lower-middle Miocene)

State of Zulia, Venezuela

Authors of name: H. D. Hedberg and L. C. Sass, 1937.

Original reference: H. D. Hedberg and L. C. Sass, 1937, p. 100.

Original description: ibid.

The name Lagunillas formation refers to one of the major Miocene subsurface divisions of the Bolívar Coastal field and is derived from the village and oil field of Lagunillas in the Bolívar District, State of Zulia. The formation here consists of about 300 meters of light gray, light green and whitish sandstone, siltstones and shales, and interlaminated sandstone and shale. There are some mottled claystones in the upper part. Lignites are fairly common and a glauconite zone as much as 30 meters in thickness occurs in the lower part of the formation. According to Sutton (1946, p. 1688) the basal 100 to 150 meters of the formation in the Bolívar Coastal field is known as the Lower Lagunillas Sand, the top of which is usually fixed by the first appearance of shales with sand laminations. In the Lagunillas area the zone on top of the Lower Lagunillas Sand thickens, especially the sands which locally become predominant and are called the Upper Lagunillas Sand. The Lagunillas formation is also well represented in well sections in the district of Urdaneta on the west side of the Lake Maracaibo. The discussions of Hedberg given below are strictly limited to these occurrences. On the west side of the lake the formation thickens rapidly from 350-450 meters near the lake shore to as much as 900 meters in the central part of the district of Urdaneta, State of Zulia, where it grades laterally into the Los Ranchos formation. The division between the two formations is arbitrary. Lithologically, the Lagunillas formation of the west side of the lake consists largely of light green, greenish gray, gray, and light gray sandstones and claystones. There are occasional lignites, thin glauconitic sandstones, and red and gray mottled claystones. The mottled claystones increase rapidly in prominence westward as the formation grades laterally into the Los Ranchos formation. The formation apparently represents a transition from marine to nonmarine conditions. Brackish water to marine foraminifera and ostracods are fairly common in the lower part of the formation, while the upper part is barren. Streblus beccarii (Linn‚) and Miliammina fusca (Brady) are particularly characteristic of the lower beds. Fossils suggest lower to middle Miocene age.

The Lagunillas formation on the west side of the lake in general, rests conformably on the La Rosa formation and grades upward conformably into the La Villa formation. However, both contacts may show disconformity and locally south of the city of Maracaibo, the Lagunillas formation is directly overlain by the Onia Beds. The formation grades laterally westward into the Los Ranchos formation.

From its stratigraphic position immediately overlying the La Rosa formation and from its lithologic character and fossil fauna, the Lagunillas formation is believed to be, at least in part, the time equivalent of the Cerro Pelado formation of the State of Falcón. An oil sand in the Bachaquero field in the upper part of the Lagunillas formation is called Bachaquero sand (Sutton, 1946, p. 1698).

For fossil, lists, see Sutton (1949, p. 1700). The Lagunillas formation crops out in the northeastern Bolívar district and southeastern Miranda in the eastern part of the State of Zulia. It is found in the subsurface throughout the Bolívar coastal area and in the eastern and central parts of the districts of Urdaneta, Maracaibo, and Mara. Hedberg and Sass (1937, p. 100) and Sutton (1946, p. 1697) point out that the Miocene Lagunillas formation should not be confused with the Lagunillas conglomerate of Sievers (1888, p. 8-16).

W. A. Mohler