LA LUNA, Formation

CRETACEOUS (Turonian-Coniacian)

State of Zulia, Venezuela

Author of name: A. H. Garner, 1926.

Original reference: A. H. Garner, 1926, p. 679.

Original description: ibid.

The name La Luna limestone named after the Quebrada La Luna, immediately west of a ranch house called La Luna, in the northwestern Perijá districts, State of Zulia, was first published by Garner (1926, p. 679). Hedberg and Sass (1937, p. 79) give the following description of the La Luna formation: lt consists of thin-bedded and laminated dense dark gray to black carbonaceous-bituminous limestone and calcareous shale. The limestone beds vary from a few inches to about a foot and one-half in thickness. The formation is particularly characterized by hard black ellipsoidal and discoidal limestone coneretions ranging from a few inches to severas feet in diameter. Black chert is common as seams and nodules. The uppermost part of the formation is sometimes lighter in color than is the bulk of the formation. When freshly broken, the limestone frequently has a strong petroleum odor. Details concerning thin sections of La Luna limestone are given by Hedberg (1931, p. 229-246). The La Luna formation conformably overlies the Cogollo formation.

The upper boundary of the La Luna formation has recently been redefined as the base of the Socuy limestone member of the Colón formation by Sellier de Civrieux (1952, p. 237), who stated that this member has sometimes been included in the La Luna formation. Although the Colón formation overlies the La Luna formation conformably there is, according to Hedberg and Sass (1937, p- 81), some suggestion of a time break in deposition at this contact. According to the same authors La Luna type limestone has been found in the middle portion of the Cogollo formation.

The thickness in the type section is, according to Hedberg and Sass (1937, p. 79), nearly 300 meters. With the exception of the type exposure, the thickness along the eastern front of the Sierra de Perijá nowhere exeeeds 183 nieters. In the Carache Valley of Trujillo, the La Luna is 315 meters thick but decreases in a short distance southwestward along the basin-facing flank of the Mérida Andes. In the Rio Buena Vista of south western Trujillo the thickness is 70-100 meters. In the Barco concession, Colombia, Notestein et al. (1944, p. 1182) report thicknesses of 43-56 meters in the southern part of the area and an average of 86 meters in the northern part.

The foraminiferal fauna of the La Luna formation includes species of Globotruncana, Gümbelina and Globigerina (Smith, 1951, p. 59). González de Juana (1951, p. 207) mentions the following macrofossils: Inoceramus, Barroisiceras, Hoplitoides, Neoptychites, Coilopoceras, Didymotis, Peroniceras Texanites.

The age of the La Luna formation is Turonian to Coniacian.

According to Sutton (1946, p. 1649) the La Luna formation is distributed widely throughout western Venezuela and eastern Colombia. The La Luna can be correlated with the Guayuta group in eastern Venezuela. See Rod and Mayne (1954, p. 202-203, fig. 4).

According to Rod and Mayne (1954, p. 207) part of the so-called "La Luna" described by Liddle (1946, p. 230-231) from Quebrada La Gé, belongs to the Middle Apon (Cogollo group) as is evidenced by the specimens of Dufrenoya and Deshayesites. Liddle assigns the bulk of the Cretaceous deposits in Quebrada La Gé to the La Luna formation, but there is no La Luna exposed in the whole area. Hedberg (1931, p. 238) and Hedberg & Sass (1937, p. 81) already reported La Luna type limestone in the middle part of the Cogollo formation. Hedberg (1931, p. 238) discussed the La Luna as a petroleum souree rock.

The sandy facies of the La Luna formation in Táchira is called by Liddle (1946, p. 250) Urumales formation, named after Caño Urumales near the village of Colón in Táchira.

The Rubio horizon of Gerhardt (1897-1898, p. 70), sometimes referred to as Rubio beds, is, according to Liddle (1946, p. 255), an obsolete name for part of the La Luna formation in Táchira. The Táchira formation of Hedberg and Sass (1937, p. 81) usually spoken of as the Táchira cherts in the state of Táchira is, according to Sutton, the equivalent of the La Luna formation. The name "limestone of Táchira" has already been used by Sievers in 1889.

W. A. Mohler