ANTIMANO, Crystalline Limestone

See ANTIMANO, Formation

ANTIMANO Formation

MIDDLE or UPPER MESOZOIC

Distrito Federal, Venezuela

Author of name: G. Dengo, 1951.

Original reference: G. Dengo, 1951, p. 64.

Original description: ibid., p. 63-64.

Dengo (1951, p. 64) used the name Antímano crystalline limestone or Antímano formation to designate a unit which he described as "crystalline limestone or marble, massive, medium-grained, light gray" (p. 63). It contains pyrite crystals and forms thick lenticular beds alternating with thin micaceous schists. Such schists present lineal structure consisting of microcorrugations of micaceous minerals.

In the Antímano type-locality the limestone is associated with greenstones of igneous origin. Said igneous rocks associated with the Antímano formation are (p. 84-85) amphibolites which extend from a site located 500 meters to the north of Antímano to the Quebrada Mamera; they also outcrop at La Colina (to the north of Los Caobos) and at Plan de Manzano. Smith (1952, p. 369) also found the epidote amphibolite, associated with the Antímano formation, to the south of San Pedro and considered it as the oldest intrusive igneous rock of the region, being the only one to undergo a metamorphism similar to that of the metasedimentary rocks of the Caracas group.

According to Dengo (1951, p. 85) in the vicinity of Antímano the igneous intrusives form possibly a sill., 20 to 25 meters thick, in lower contact with a thin bed of quartzite. Its lower part consists of epidote amphibolite rich in glaucophane. In the upper part appears garnet.

According to Dengo, the maximum thickness is 40 meters, at the type-locality, but it thins out to the east and west.

The type-locality is 0.5 kilometers to the north of Antímano, Distrito Federal. According to Dengo, there are other outcropping areas near Carapa and Carapita, and, generally speaking, along a line that goes from Mamera to Bella Vista along the northern part of the Tacagua Valley; other outcrops are also found in the Mamo River and to the north of Curucutí, in road-cuttings of the old Caracas-La Guaira highway and to the north of La Florida (Caracas).

Dengo points out that the Antímano limestone, which he had previously included (1947, p. 137) in the Zenda phase of the Las Brisas formation, occurs stratigraphically above the Zenda phase and is underlain by quartz micaschists (Las Brisas formation) and overlain by calcareous schists (Las Mercedes formation).

Due to its lenticularity, the Antímano formation disappears locally along the strike. In the places where it is present, is marks the transitional contact of the Las Brisas formation and the Las Mercedes formation. In many places where the Antímano limestone is absent, it has been possible to observe a direct transitional contact between the Las Brisas quartz micaschists and the Las Mercedes calcareous schists.

Dengo (fig. 5, front p. 68) indicates graphically the lenticular character of the formation and its local absence from the stratigraphic section, for instance, in the region of the Sebastopol-Baruta anticline.

Smith (1952, p. 355) indicates that it is commonly difficult to distinguish the Antímano limestone from some of the limestones of the overlying Las Mercedes formation, and verifies tile local lenticular character of the unit and its consequent discontinuity. Nevertheless, he still calls it a formation and places it at the base of the Las Mercedes formation.

Smith (1952, p. 355) mentions limestone outcrops to the south of San Pedro, where he noted a maximum thickness of 300 meters; he also noted the occurrence of graphite, finely distributed within the limestone, as well as a scarce proportion of detrital quartz grains. Smith suggests the possibility that the Antímano limestone might be distinguished from the Zenda microcline and dolomite limestone in the proportion Ca/Mg. Chemical analysis of Antímano limestone show only a small amount of magnesium. Smith suggests a biohermal origin for the limestone. He says that Maxwell and Dengo (1950) found fragments of non-identified shells in the quarries north of Antímano.

Aecording to Smith (1952, p. 357) the limestones mentioned by López (1942, p. 13) to the north of Valencia, State of Carabobo, are approximately equivalent to the Antímano formation, as well as those of the Carúpano section, State of Sucre, described by Maxwell and Dengo (1951, p. 260).

The Antímano formation belongs to the Caracas group and therefore, to the lower part of the Caribbean "series" in the Cordillera de la Costa.

J. M. Sellier de Civrieux