PASTORA, Series

LOWER OR MIDDLE MESOZOIC ?

State of Bolívar, Venezuela

Authors of name: W. H. Newhouse and G. Zuloaga, 1929.

Original reference: W. H. Newhouse and G. Zuloaga, 1929, p. 798.

Original description: ibid.

W. H. Newhouse and G. Zuloaga (1929, p. 798) introduced the name Pastora with a short description, subsequently enlarged by G. Zuloaga (1930, p. 471-73 ).

The type-section goes from a point located in the Yuruari River, some kilometers west of the village of Pastora, toward the southeast along the river, to another point located on the road to Tumeremo, some kilometers east of El Callao. Rocks forming a long syncline cut by local gabbro and granitic porphry intrusions are found at the type-section. Tuffs pertaining to the same unit outcrop at the town of El Callao.

The rocks from the region of El Callao were first described by Duparc (1922) who called them "grunsteine" (greenstones). Later on, they were included in the Pastora series. Duparc described in detail, the following types: 1) Porphyrites with hornblende, 2) Diabases without olivine, 3) Microgranulites, 4) More or less metamorpnosed tuffs, 5) Schists of detrital origin, 6) Chloritic-quartzose schists, rich in calcite, 7) Sericitic-chloritic schists with calcite, 8) Typical jaspers.

According to Duparc, the greenstones are considered by some authors as pre-Cambrian sediments, while others assigned them to a merely eruptive activity, considered as a primary source of gold mineralization, and iden tiffed them as diabase and porphyrites.

At the region of El Callao, greenstones are cut by aplite, quartz and diabase veins. At the same region, the Pastora series attain a great economical significance due to the fact that auriferous quartz veins are found within the stratified planes of the tuffs located near by gabbro intrusions and concentric around a laccolith.

The Pastora "series" is mainly constituted by green andesitic waterlaid tuffs. The fact that such rocks were referred to as diabase by old authors, in spite of being well-bedded, is pointed out by Zuloaga (1930, p. 472). Almost brecchioid, conglomeratic beds are found interbedded in the several hundred meters thick section. These beds might be seen in the river at Nacupay and in the same river, to the west of El Callao.

Towards the upper part of the series, Zuloaga includes a section of dolomitic marls which overlies the volcanic tuffs at the syncline located between Pastora and El Callao. Those marls outcrop from Caratal to the near vicinity of Nacupay and in the road from Pastora to Cicapra, where they form low hills. These marls, with a blue limestone appearance, are non-fossiliferous. Zuloaga (1934, p. 1183) referred to such rocks as "bluish dolomitic clays". No other author has again mentioned such type of rocks in the Pastora formation.

According to Zuloaga, the tuffs have been metamorphosed near the intrusions and silicified into "green jasper" of compact structure and conchoidal fracture, locally red. Outcrops are found at one of the sides of a laccolith near by the old pumping station of the La Increible Mine.

Zuloaga believes that the Pastora series overlies the Imataca series, although he has not seen the contact. He assigns it (p. 469) to the Cretaceous (?) and points out the fact that it underlies the "Kaieteurian or Roraima series", the age of which is considered by him as Cretaceous. The same opinions in a later article by Zuloaga (1934, p. 1182) have been misinterpreted by Liddle (1946, p. 178) who says that "Zuloaga has separated... beds known further south as the Roraima series into a lower Pastora series... and a Kaieteurian series". Zuloaga did not intend to include the Pastora series into the Roraima series, which is an unfortunate suggestion; he used the name Kaieteurian to designate the Roraima formation only.

On the other hand, according to Rubio et al. (p. 481), andesitic tuff outcrops assigned to the Pastora series, were found by the geologists G. Dengo and L. J. Candiales near La Paragua. Thus, the extension of the Pastora series into the west, was demonstrated. There are also signs of the Pastora extension along a stripe oriented northeast-southwest through the Territorio Amazonas. A few meters thick quartz-sericite-talc-chloritic schists, petrographically identified by C. Martin Bellizzia as Pastora, were found by J. M. S. de Civrieux and C. Carmona (M. S.) immediately under the first sandstones and conglomerates of Roraima to the south of the Duida Plateau, at the hills of La Esmeralda.

Unfortunately, the upper contact (probably with Roraima) and lower contact (probably with a granodiorite of the basement complex) of the remaining narrow section of schists, are concealed by a great ferruginous quartz injection of hydrothermal origin, in which the schists are found scattered as separated masses.

Charles D. Reynolds, in Basset Maguire (1955, p. 49), mentions in the Cerro de la Neblina (Territorio Amazonas), the southernmost region of Venezuela, a serie of metavolcanic rocks that overlie unconformably over the crystalline rocks of the Guiana Shield: "The volcanic rocks are found at the gradient of the slopes of the Cerro de La Neblina from the surface level to the quartzite bluff, at 2100 ft." . . . "The rocks of that volcanic sequence, which have an estimated thickness of 1500 ft., vary from dark grey graywackes (of clastic appearance) to phyllites and actinolithic schists. Some of these rocks have a sedimentary appearance in weathered surfaces, but the microscopic evidences regarding their clastic origin have been simulated by metamorphism probably associated with the tectonic deformation of the rocks"... It is known that the contact between the volcanic rocks, the conglomerates, and the overlying quartzites is present in a zone 50 ft. thick, but it is not exposed in the explored regions.

This metavolcanic section, described by Reynolds, must be assigned without any doubt to the Pastora formation; his field observations in connection with the relative position of the phyllites and metavolcanic schists (placed in between the Crystalline Complex and the conglomerates which begin the quartzite formation) are similar to the undersigned's observations in the southern spurs of the Duida (Esmeralda hills) as well as in the cerro Tukui (Mahekodo Range) in the Upper Orinoco. In this last locality, quartz-sericitic-chloritic or quartz-sericitic-epidotic schists (identified by C. Martin Bellizzia) outcrop under the Roraima quartzites that form the upper levels of the Tukui, showing contact with an intrusive diabase that also cuts the overlying Roraima.

Rubio et al. (1952, p. 482) mentions a possible correlation between the Pastora Series and the Upper Pre-Cambrian "Volcanic Series" of British Guiana. Thus, the age suggested would be much older than that proposed by Zuloaga (Cretaceous) or the Eupaleozoic age later suggested by Mencher et al., (1953, correlation chart). On the other hand, Gansser (1954), considers the "Volcanic Series" of the Guianas (which he finds similar to the volcanic rocks of El Callao) as probable equivalents of similar volcanic sequences of Triassic-Jurassic age in the Andes (Quinta formation) and Ecuador (fossiliferous strata). The correlation of the "Volcanic Series" with Pastora is also admited by A. Bellizzia and C. Martin Bellizzia (M. S.) whom believe it of upper Algonquian age, and propose to include in the Pastora Series extremely silicified rocks that vary from rhyolite porphyry to andesites, including many other intermediated types of rocks. Such outcrops extend south and southeast into the Gran Sabana, in unconformity under the Roraima formation. In more recent works, G. M. Stockley (1955) of the British Guiana Geological Survey Department, includes in a unit unfortunately called "Guyana System", (almost homonymous, but not synonymous with Liddle's "Guayana Serie") a sequence of rocks in part equivalent to the Pastora. The lower part (no name) of that system consists of rhyolites, acid ashes and tuffs, gondites, quartzites, manganese bearing tuffs and aglomerates, a mineralized complex with primary manganese (tuffs and gondites) columbite (in pegmatites) and gold (in quartz veins near the contact zones). This part of the sequence is mostly equivalent to the above mentioned Volcanic Series and therefore, to the Pastora.

In the upper part of said system, it includes the Haimaraka series (red shales, phyllites, jaspilites, hornblend schists, volcanic basic rocks, and epidiorites), which is also lithologically similar to the facies rich in jasper of Pastora series in the Gran Sabana and Territorio Amazonas, Venezuela. In a similar way to what probably happens in the Territorio Amazonas, Stockley shows an intrusive contact of his "Guyana system" (Pastora) with granites and other acid intrusives. In connection with the basic intrusives, it seems that they only affect the lower part (pre-Haimaraka) of the "System". In Surinam, the only probable equivalents of the Pastora serie are the so-called Ballinggesteenten and Bonidoro-serie (lower). These are represented also in the French Guiana (H. Schols in A. Cohen, 1953, p. 147, plate II), by the so-called Roches Vertes and by the Paramaka Serie (Lower Complex) where schists sometimes sandy, green volcanic rocks, gold mineralization in contact zones with intrusive diorites, have been described.

Even though the problem concerning the Pastora's age is merely speculation, and that some geologists are inclined to believe it Pre-Cambrian, we believe that the intensive volcanic activity in north and northeastern South America, suggests rather a Mesozaic age (possibly lower or middle).

For general references, the reader should consult Gansser, A. (1954).

Jean Marc Sellier de Civrieux