CARACAS, "Series"

See CARACAS, Group.

CARACAS, Group

MIDDLE and/or UPPER MESOZOIC

Distrito Federal, Venezuela

Authors of name: S. E. Aguerrevere and G. Zuloaga, 1937.

Original reference: S. E. Aguerrevere and G. Zuloaga, 1937, p. 12.

Original description: ibid.

S. E. Aguerrevere and G. Zuloaga (1937, p. 12) first proposed the name Caracas "series" to designate a sequence of metasediments in north-central Venezuela which constitutes, locally, the lower part of the Caribbean series. According to the same authors, the Caracas "series" starts with a basal conglomerate that lies directly with unconformity on top of the ancient igneous complex (Sebastopol granite). It is constituted by angular fragments of quartz and weathered granite, mainly cemented by siliceous material. "From this conglomerate the series passes gradually through arkose and conglomeratic limestone into a series of calcareous and graphitoid schists and limestone".

Said authors (1938, p. 282) changed the name "series" to Caracas group, including under such name Las Brisas formation which comprises the Zenda phase, and Las Mercedes formation which comprises the Los Colorados phase. Later on, Dengo (1951, chart 2, front p. 70) included two new formations in the definition of the Caracas group: Antímano and Tacagua. On the other hand, this author modified the original con¿ept, in relation with what was supposed to be a basal conglomerate (See LAS BRISAS, Formation).

Aguerrevere and Zuloaga (ibid.) indicate that the Caracas group lies in transitional contact below the rocks that they include in the Villa de Cura "series". This name is obsolete.

R. J. Smith (1952, fig. 5, p. 341 and 363) assumes that the Caracas "series" (group) unconformably underlies the Paracotos formation, of a lower metamorphic grade, though he says (p. 363) that he never saw the unconformity due to the fact that in his whole area, the units were in fault contact.

G. Dengo (1951, p. 110) states that the metamorphic rocks of the Caracas group were originally sediments and igneous rocks (serpentinites, antigorites and anphibolites).

Dengo writes (ibid., p. 110-111): "The original sediments, largely if not entirely of Cretaceous age, consisted of sandstones and conglomerates at the base that graded upward into limestones and limy shales. Basaltic rocks were intruded prior to any major deformation and probably contemporaneously with the deposition of some of the sediments. Some volcanic tuffs were probably deposited in the upper part of the sedimentary sucession". The same author goes on saying: "In the Late Cretaceous, northern Venezuela was deformed, together with a large portion of the Caribbean region. During this deformation, ultramafic rocks were intruded along the axial region of the down buckle. Continued deformation, probably accompanied by remelting of the crust at depth, rise of the isogeotherms, and migration of solution into the sedimentary succesion, produced the regional metamorphism. The sediments were changed into gneiss, schists, and marbles and the basaltic rocks into amphibolites".

Maxwell and Dengo (1951, p. 152-153) state that the metamorphic rocks of the Carúpano area (northeastern Venezuela) are "lithologically similar to the younger metamorphic rocks of the Caracas group". According to the same authors, the structural complexity and the lack of good outcrops make it altogether impossible to measure the exact thickness in the Carúpano region, but it can be said that the metamorphic sections of Carúpano and Caracas, are about the same. According to the same source, the Caracas group has an approximate thickness of 1,500 to 3,000 m. while the metamorphic sequence which outcrops at the Carúpano region would have a maximum thickness of 3,100 m. Maxwell and Dengo believe that "the lithological and structural similarity and the metamorphic grade along the strike are good evidences to believe that the stratigraphic section of the regions of Caracas and Carúpano are approximately the same in age".

The age of the rocks which constitute the Caracas group is still subject to discussion. Many geologists, following Aguerrevere and Zuloaga's suggestion (1937, p. 22) accept the possibility of a Lower Cretaceous age, but the evidences that support such hypothesis are still scarce. Some geologists accept the possibility that at least part of the Caracas group would be Jurassic in age. A more general discussion about the age of the metamorphic rocks in north-central Venezuela is presented in an other article (see CARIBBEAN, "Series").

The upper contacts of the Caracas group are locally uncomformable: a) near La Guaira with Upper Tertiary sediments (Cabo Blanco group); b) near Caracas with Pleistocene (bones of megatheria found near Caracas) and perhaps Holocene alluvium; and c) to the south of Caracas with Cretaceous metasediments of the Paracotos formation.

Further information in relation with the igneous rocks associated with the metasediments of the Caracas group is to be obtained in the article "CORDILLERA DE LA COSTA" (Igneous in).

J. M. Sellier de Civrieux