MISOA, Formation

TERTIARY (middle and upper Eocene)

State of Zulia, Venezuela

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

Original reference: A. H. Garner, 1926, p. 678, 680.

Original description: ibid.

Garner (1926, p. 678, 680) stated that the "Misoa Hill Formation" forms Misoa Hill at its type locality in the extreme south-central part of District of Bolívar, State of Zulia. He described the formation as consisting of "alternating beds, hard, coarse, white to dark brown sandstone and brown to black, slaty shale". In his text he considered the formation to be Eocene in age. On his chart (p. 678) the "Misoa Hill Formation" is shown to be middle and upper Eocene in age and to have a thickness of 2000 feet.

Liddle (1946, p. 313-335) cited the "Misoa sandstone member", replacing the "Mirador sandstone member" of Liddle (1928, p. 181-208) as the upper member of his "Misoa-Trujillo formation". He claimed that the Misoa sandstone is named from the Sierra Misoa in the District of Bolívar, State of Zulia, where it is well exposed. Undoubtedly, both Garner's and Liddle's localities are the same. Liddle considered the "Misoa sandstone member" to be lower middle Eocene in age as shown on his correlation chart and as the heading of his discussion of the "Misoa-Trujillo formation". Discussing the age of this latter formation, he stated, however, that it included beds of Midway and Wilcox age (Paleocene and lower Eocene).

Sutton (1946, p. 1666-1669) stated that he used the name Misoa "to designate the thick, essentially sandstone deposits which constitute the Upper Eocene part of the original Misoa-Trujillo formation". He pointed out that the type area of the Misoa formation is the Cerro Misoa east of Lake Maracaibo in the District of Sucre, Zulia. The best exposures are along the Río Misoa, where this stream cuts thróugh the western flank of the Serranía de Trujillo along the Sucre-Bolívar district boundary in eastern Zulia. He described the formation as consisting of "hard, quartzitic, light gray to brownish gray, medium to coarse-grained, locally conglomeratic, thick-bedded to massive, sandstone". In the lower part of the formation are subordinate amounts of fine to medium, gray and dark gray, carbonaceous, micaceous, laminated and cross-laminated sandstone. Interbedded with the sandstones are gray to dark gray shales that probably comprise 15 per cent of the formation. Sutton pointed out that "locally, the base of the formation consists of orbitoidal limestone, sandy limestone and calcareous sandstone. These beds are known as the Cuicas limestone member of the Misoa. They crop out along the upper part of the side road to the town of Cuicas in northeastern Trujillo, as well as along the main Carache valley road". According to Sutton, the Misoa formation is 1,725 meters (5,658 feet) thick at the type section. This is the maximum that has been recorded in the basin. Sutton stated that the Misoa formation is considered to be unconformable with the underlying Trujillo and equivalent formation. It is conformable and gradational above with the Paují and equivalent formations. Sutton pointed out that the formation is distribued widely throughout western Venezuela and eastern Colombia. He cited Halymenites as the commonest fossil in the Misoa sandstones. A detailed list of further fossils listed is given by Sutton (1946, p. 1668). Based on these fossils, Sutton considered the Misoa formation to be upper Eocene in age. He pointed out that the Misoa formation of his paper includes the "second orbitoid level", which he considered equivalent to the Cuicas limestone member, at the top of the "Lower Misoa-Trujillo" of Tash (1937, p. 167-172) and the massive quartzitic sandstones forming the lower part of Tash's "Upper MisoaTrujillo". Sutton assigned the "transition zone", included by Tash in his "Misoa-Trujillo", bo the Paují formation. Sutton pointed out that the "Bocorrón sandstone" of Garner (1926, p. 680) in southwestern Trujillo is another Misoa equivalent, as is also the "Corritos" formation of Mackenzie (1937, p. 262-263) on the southwest flank of the Mérida Andes in the State of Barinas. Sutton claimed that the Mirador sandstone was traced around the south edge of the Maracaibo basin from southwestern Zulia into the type Misoa, thus confirming the correlation made by Liddle (1928, p. 181). Sutton listed as an other correlative of the Misoa formation, part of the upper half of the Paso Diablo formation of Hedberg and Sass (1937, p. 90-91) in northwestern Zulia. The "La Rosa sandstone" of Garner (1926, p. 680) is another equivalent of the Misoa formation in northwestern Zulia but this name, like his "La Rosa coal measures", must be rejected to avoid confusion with the La Rosa formation. Then the name La Rosa is an obvious error for Santa Rosa, the type section occurring on the north bank of the Río Guasare, just below the mouth of its tributary, the Caño Santa Rosa, in the southwestern part of the District of Páez, Zulia.

Schaub (1948, p. 215-227) expressed the opinion that the upper part of the Misoa formation of Sutton (1946, p. 1666-1669) could be tentatively correlated with the Potreritos formation of Sutton (1946, p. 1675-1677). He pointed out that the Pueblo Viejo Main Sandstone, a quartzitic sandstone body very similar to, but not identical with, the sandstones in the upper part of the Misoa formation is in fact situated below, or in the lower part of, the Potreritos formation. Howewer, there is also reason to assume that the Pueblo Viejo Main Sandstone is situated more than 1,000 meters below the top of the Misoa formation as defined by Sutton, although still within it.

De Cizancourt and Frizzell (1949, p. 496-497) recorded Ferayina coralliformis Frizzell together with a number of other larger foraminifera from what they called the middle Eocene Cuicas formation of Venezuela.

Van Raadshoven (1951, p. 6-7) pointed out that limestones from the Quebrada Grande and Río San Juan (since named Quebrada Grande limestone and San Juan limestone) in the Districts of Sucre and Bolívar of the State of Zulia, are known in the literature as "second orbitoid level" and for a long time were believed to belong to the upper Eocene. Van Raadshoven stated that "Mme. de Cizancourt, however, published a typical Middle Eocene larger foraminifera fauna from the Cuicas limestone member of the Carache area (northern Trujillo) which is identical with the fauna found by the writer in the 'second orbitoid level' of the Bolívar and Sucre Districts. This Cuicas limestone member was also correlated with the 'second orbitoid level' by Sutton". Van Raadshoven claimed that the Quebrada Grande limestone and the San Juan limestone, which were first thought to represent a single stratigraphic horizon, were found to have a slight difference in age, the former being the older. He expresses the opinion that both limestones belong to the middle Eocene and as they are both developed as Lithotbamnium limestones with some quartz, no difference in facies seems to exist.

Mencher et al. (1951, p. 18) stated that "it is suggested that the lower part of the Misoa formation represents the last phases of the regression that began in Cretaceous times while the upper part is the beginning of a new transgression which reached its culmination in the Late Eocene with the deposition of the marine Paují shales". They pointed out that the age of the Misoa formation is somewhat in doubt, some geologists favoring the middle Eocene and others the late Eocene. They claimed that "it is possible that the regressive phase began in the middle Eocene while the transgressive phase did not end until late Eocene".

González de Juana (1951, p. 274-275) discussed Sutton's usage of the Misoa formation and added that he assigns the limestones, which Sutton placed at the base of his Misoa formation, to the Trujillo formation. González de Juana pointed out that the unconformity postulated by Sutton at the base of Sutton's Misoa formation is scarcely apparent, if it exists at all.

Information taken from private reports indicates that the middle Eocene Quebrada Grande and San Juan limestones of Van Raadshoven occur within the lower part of the Misoa sandstone and could, therefore, be considered as members of the Misoa formation. Other limestone occurring within the upper part of the Misoa, carry upper Eocene larger foraminifera. Thus the age of the Misoa remains established as middle and upper Eocene. Most geologists prefer to follow González de Juana's usage of the Misoa formation, i.e., excluding the Cuicas limestone member, which they consider to be a member of the Valle Hondo formation.

Leo Weingeist