MITO JUAN, Formation

CRETACEOUS (Maestrichtian)

Department of Santander del Norte, Colombia

Author of name: Probably geologists of Caribbean Petroleum Co.

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

Original description: ibid.

The formation name, according to Hedberg and Sass (1937, p. 83), was probably first applied by geologists of the Caribbean Petroleum Company. The Quebrada Mito Juan is a tributary of the Río Sardinata and crosses the north dome of the Petrolea structure on the Barco concession in the Department of Santander del Norte, Colombia. The type section is exposed on Qda. Mito Juan on the east flank of this structure about 2 kilometers due east of the axis, and about one kilometer southwest of the Venezuelan- Colombian international boundary line. Garner's designation (1926, p. 679) of the type locality as "on Cafío Mito Juan near the Colombian border in southern Colón, Zulia", is undoubtedly an error.

Sutton (1946, p. 1653) gives the following lithological description: The formation is brackish water to marine in origin and is composed principally of massive, black, gray or greenish gray shales that are locally sandy. These shales are more prominent in the lower part of the formation and in many places are difficult to recognize lithologically from the underlying Colón shales. The shales of the upper part of the Mito Juan formation locally contain interbedded limestones varying from a few centimeters to 75 centimeters in thickness, and sandstones, generally thinly bedded but in some places attaining a thickness of one meter. The limestones are gray, sandy and generally glauconitic Ellongated concretions of limestone and ironstone are common. The sandstones are light gray to greenish gray in color, generally fine-grained but in places gritty, calcareous, glauconitic and carbonaceous. The Mito Juan formation as described by Sutton (1946, p. 1653-1654) includes both the Mito Juan and the Río de Oro formations of Hedberg and Sass (1937, p. 83-87). In this, the procedure of Notestein et al. (1944, p. 1185) has been followed by Sutton. Due to the fact that the glauconitic limestones and sandstones of the upper part of the formation pinch out toward the south in the Barco region, Notestein et al. prefer to treat them as a local facies rather than as a separate formation. Sutton (1946, p. 1654) considers the Río de Oro beds as locally the upper member of the Mito Juan formation. González de Juana (1951, p. 209) proposes to consider the top of the marine beds as the top of the Mito Juan.

The age of the Mito Juan is upper Cretaceous (Maestrichtian). Hedberg and Sass (1937, p. 86) note the discovery of the ammonites Sphenodiscus and Coabuilites in the base of the Río de Oro in the Barco concession. Smaller foraminifera are described by Cushman and Hedberg (1941, p. 80) from the "Ammobaculites" colombianus zone which comprises the Mito Juan and lowermost 300 feet of overlying "Orocué" formation. The fauna consists largely of arenaceous foraminifera, particularly "Ammobaculites" colombianus and associated Haplophragwaides. Vaginulina navarroana, Gümbelitria cretacea, and a few other calcareous forms are rare. According to Sutton (1946, p. 1654) the Mito Juan formation is ordinarily difficult to differentiate from the subjacent Colón formation, and the two have usually been mapped together either as Colón alone or as Colón-Mito Juan. The Río de Oro member is known to extend northward from the Barco concession to the Quebrada El Mene, a southern tributary of the Río Negro in the Perijá district. According to González de Juana (1951, p. 209) the thickness of the Mito Juan formation varies between 150 and 750 meters.

The Mito Juan conformably rests on Colón shale and is conformably overlain by the Third Coal formation (Catatumbo formation). According to Liddle (1946, p. 275) the Mito Juan shale is part of the Guayuta group of the Guanoco series. The shales of the Colón, Mito Juan and Guasare formations are sometimes called La Paz shales.

W. A. Mohler