RORAIMA, Formation

UPPER MESOZOIC?

State of Bolívar, Venezuela

Author of name: L. V. Dalton, 1912.

Original reference: L. V. Dalton, 1912, p. 39.

Original description: ibid.

The name Roraima beds has been originally used and published by Dalton (1912, p. 39), to designate a sequence of continental sediments with the type locality in the Cerro Roraima, on the border between Venezuela, British Guiana and Brasil.

Dalton describes the unit as uplifted beds of horizontally stratified material that have not yet been completely denuded from the ancient foundation of gneiss. Then, he mentions specifically, sandstones which have been protected from erosion by a cap of harder igneous rocks.

C. B. Brown (1871), C. B. Brown and J. G. Sawkins (1875, p. 14) have previously described the same sequence, in Kaieteur, Sierra Pakaraima, British Guiana, under the name, "Sandstone formation" Said authors described it as a series of conglomerates and sandstones with scarce intercalations of shales, with a thickness of 7,000 feet. The outcrops between Kaieteur Falls (Potaro River) and the Ireng River have also been studied by C. W. Anderson and W. A. Dunn (1895) who have used for the first time the name "Kaieteurian", to designate the sediments today generally known as Roraima. Even though this last name has been generally used, the priority of the name "Kaieteurian" should be recognized. Later studies on the outcrops in the British Guiana should be accredited to H. J. C. Conolly (1925) and to Bracewell (1927).

Tate (1930, p. 59), describes in detail the section exposed in the Cerro Roraima; he defines it as a sequence of prominent quartzites and sandstones which varies gradually, from base to top, from a very compact and hardened pink quartzite to a fairly hard but more porous sandstone. Colors vary from banded white to pink and red, dull pink being more frequent to wards the base. The sandstone grains tend to increase in size from base to top. Cross bedding and ripple marks are frequent. According to S. E. Aguerrevere et al. (1939, p. 651), the sedimentary section exposed in the Cerro Roraima is constituted by some 500 meters of sandstones which lie on top of a diabase laccolith In that place, the lower section is not visible in Venezuela.

Tate and Hitchcock (1939, p. 43-45) describe the same unit in the Cerro Duida, largely a cross-bedded pink quartzite with intercalations of red slate, 12 to 16 feet thick; both types of rocks showing abundant ripple marks. Hitchcock (1931, p. 284) described it as "intensely folded series of reddish and white conglomeratic sandstones and purple-colored shales".

Aguerrevere S. E. et al. (1939, p. 647) use for the first time the word formation to designate the Roraima sediments and this term is at present time of general use. Based on observations in the Gran Sabana and the Kamarata Valley, they enlarged the description with a local definition of a basal conglomerate, whose variation they describe on the basis of four localities in the vicinity of Santa Elena. Said conglomerate, whose thickness varies between 3 and 10 meters, contains porphyre fragments from the Basal Complex and, locally, rounded or angular, very ferruginous sandstone fragments with diameters that vary from 1 to 12 centimeters, embedded in a siliceous-hematitic sandstone matrix. In the Sierra Mahekodo (Sierra Gushariba on the old maps), Upper Orinoco, the writer of the present article has observed at the base of the Roraima formation, a cobble conglomerate about 10 meters thick. The cobbles are constituted by quartzite, similar to the matrix, and quartz, but never by igneous rocks or by metavolcanic rocks of the Pastoral In Yacurai (Las Carmelitas), Upper Ventuari, there is a conglomerate identical to the forementioned. In the Territorio Amazonas, the writer, and also B. Maguire, have observed conglomerates near the base of the Roraima which do not contain any clastic elements derived from the igneous basement.

Aguerrevere et al. (1939, loc. cit.) mention the occurrence of shales and describe sandstones made up essentially of sub-rounded to sub-angular quartz grains and sub-angular plagioclase grains, almost always altered. Locally, specially towards the lower part of the section, there is abundance of mica. Lenticular conglomerate beds are intercalated in the section; their thicknesses reach some 10 meters and their horizontal extension, 6 kilometers. Jasper beds are frequent and they are extremely fine-grained, conchoidal frac- tured, very hard, pale green to red; they represent volcanic tuffs and indicate igneous activity, perhaps a continuation of the volcanic cycle so notable in the underlying Pastora formation. López et al. (1942) on the basis of petrographic studies, describe with great detail the different rocks in the formation. In the Gran Sabana, the sediments have been affected, in the lower part of the section, by gabbro sills that represent a great intrusion with local variations. Other gabbro intrustons have taken place in the Auyantepuy section.

Liddle (1946, p. 177) affirms that the maximum thickness observed in the Roraima formation is 2,000 feet. Nevertheless, in the Auyantepuy, Tate (1939, p. 651) estimates it around 2,000 meters, while Aguerrevere S. E. et al. believe it 2 600 meters. Such section seems to be more complete and representative than the one exposed in Cerro Roraima.

Kugler et al. (1942-1944, p. 22-24) are of the opinion that the maximum thickness of the Roraima formation (7,000 feet according to Brown and Sawkins, 1875) (loc. cit.) is probably along the western slope of the old shield and to the west of the Sierra Parakaima. In the Tafelberg Plateau, Surinam, Ijzerman (1931) estimated a thickness of more than 650 meters.

The stratigraphic relations of the formation depend on the different localities of its extensive distribution area.

The formation overlies the peneplaned Guayana series or locally the Pastora series (see). In turn, the Roraima sediments have been exposed to erosion during the whole Tertiary and in many localities during the Quaternary, being a source of sediments for the geosynclines located to the north and northeast. Locally, the formation is in unconformity with Plio-Pleistocene sediments known in British Guiana as White Sand Series.

In the type locality, Dalton (loc. cit.) refers to the "old gneiss basement" underlying the strata, but Tate (1930, p. 60) mentions the occurrence of a diabase sill at the foot of the Cerro Roraima, which in turn rests on a green jasper that, in a microscopic section, Prof. R. J. Colony of Columbia, has identified as volcanic ash. Zuloaga (1930 and 1934) is of the opinion that the green jasper at the foot of the Roraima might belong to the Pastora Series; he mentions that the Pastora series (tuffs) in its type locality, lies directly beneath the Roraima "series"; this same relationship, according to Zuloaga, exists at another locality situated a few kilometers to the north of San Pedro in La Paragua. Today, other references exist in relation to this contact in the Gran Sabana and the Territorio Amazonas. (See PASTORA Series.) Aguerrevere S. E. et al. (1939, p. 649) mention that in a locality one kilometer northeast of Santa Elena, the basal conglomerate of the formation lies in direct contact over the porphyries of the fundamental complex. They also mention an indirect evidence, to the west of the locality, for the existence of a pre-Roraima sedimentary formation not found in situ. This evidence consists of the occurrence, within the basal conglomerate, of pebbles of a well stratified, well consolidated ferruginous sandstone with distorted layers.

It is interesting to mention here the existence of post-Pastora and pre-Roraima sedimentary formations, in the upper part of the "Basal Complex" located in the stratigraphic sections of Suriname and French Guiana. These formations (Orapo, Makambie, Maabo-Schisten, etc.) are conglomeratic, quartzitic and schistose.

Gansser (1954) compares the Imataca itabirites with the hematitic quartzites removed from the basal conglomerate of Santa Elena and suggests a partial origin of the Roraima sediments from the northeast.

The most exhaustive studies about the Roraima formation have been carried out by British Guiana geologists. Martin Kaye (1952) is the author of the most complete work on the subject. In Suriname, where only a residual outcrop of the formation occurs in the massive of Tafelberg, the references are Ijzerman (1931, p. 80) and Maguire (1945, p. 517). The only available references about the distribution of the Roraima in Venezuela were until recently, very scarce. After the study of Aguerrevere et al., (1939), concerning the Gran Sabana, very few contributions have been given to the knowledge of the same in the southwest of the country. Gansser (1954) only mentions the notes of Tate, G. H. H. (1930) concerning the Duida Plateau, and of Hitchcock, C. B. in the Ventuari River region (Yavi and Guanay Hills), as indicators of the westward extension of the formation. Today, however, geologists of the Ministry of Mines and Hydrocarbons have private reports that have a greater diversity of references concerning the outcrops in the Amazonas Territory. A. Bellizzia and J. M. S. de Civrieux (1950) found and studied the formation in Cerro Yapakana, Marahuaka Range and Cerro Kushamakari; De Civrieux and Carmona (1951) in the Mahekodo and Tukui (64° 40'-65° west latitude in the Upper Orinoco River), the Manaviche Range and the Mavaca Range. On the other hand, Schomburgk, C. A. (1841) mentions the formation of sandstones in the upper Caura and upper Metakuni. Cruxent and Kamen-Kaye (1949, p. 314-315) in the Autana, B. Maguire (1955, p. 27-51) in the Imery Range, and the headwaters of the Siapa and Baria; this last in the southern Venezuelan limits (Neblina, Avispa and Aracamuni hills). All these references outline the distribution of the Roraima formation in Venezuela.

In the adjoining region between Venezuela and Brazil, in the Parima and Pakaraima Ranges, the sources of information are Hamilton Rice (1928), Oliveira (1929), Holdridge (1933), de Paiva (1939-40), Oliveira and Leonardos (1937).

In Colombia, the so-called San Jos‚ sandstone, which has extensive outcrops in the Guaviare, Inirida, Vaupé‚ and Caquetá Valleys, is usually referred to by Colombian geologists as Cretaceous; anyway, it must be identified without any doubt with the Roraima formation. The western limit of the formation is probably at the Macarena Range. See Martin, F. O. (1929) and Philipson et al. (1951).

Thus, the continental extension of the Roraima (including the so-called San José‚ sandstone in the Colombian plains) from west to east, covers approximately from 74° W (La Macarena, Colombia), in the Andean foothills, to 54° W (Tafelberg, Surinam). To the south, it reaches the austral hemisphere in the banks of the Caquet (Maine Hanari, Colombia), which to the north seems to extend itself to the eastern proximities of El Callao, Venezuela (fide, Zuloaga, 1930).

Age.-No paleontological evidence can help to resolve the difficult problem of Roraima's age. Harrison, J. B. (1908) and Oliveira and Leonardos (1943, p. 24) proposed a Pre-Cambrian age based on a correlation with the Lavras series of Minas Geraes and Bahía, Brazil, whose upper part could really cover a Cambrian section. The Lithology and diamond content of Lavras are similar to Roraima's. The Brazilian geologists also compare the Roraima with Scotland's Torridonian sandstone. Outside of Brazil, such ideas were not generally accepted.

C. B. Brown (1871), L. V. Dalton (1912), R. A. Liddle (1928 and 1946, p. 177) were in favor of giving a Cretaceous age to the Roraima; Liddle suggested correlation with the Barranquin formation (Lower Cretaceous). Aguerrevere et al. (1939) have also suggested on the basis of vulcanism, a correlation with the Cretaceous; they mention the lower Cretaceous as a period of high volcanic activity in northern South America, including Colombia and the Caribbean Dutch Islands; in north-central Venezuela, eugeosyncline sequences in part volcanic (Guárico, Arrayanes, Tiara) or metavolcanic (Tacagua, Paracotos) suggest at present time, a Cretaceous climax of the local effusive activity, which extends itself into the Paleocene. Aguerrevere et al. (ibid.) mention the vulcanism in the Girón and La Quinta formations, both considered by them as Jurassic-Cretaceous, and with the vulcanism in the Pastora, which underlies the Roraima to the north of the Gran Sabana. They refute the idea of the Brazilian geologists with regards to the Pre-Cambric age of the Roraima. López et al. (1942), give similar ideas concerning this respect, mentioning again this vulcanism in La Quinta.

A correlation with the european continental Trias (Bunter sandstones) appears in Brown and Sawkins (1875, p. 126).

The idea of correlating the Roraima with the La Quinta, but considering this last as Triassic-Jurassic, reappears in Kugler (1935, p. 30), who also proposes correlation with the Carrizal-Hato Viejo and the gypsiferous metamorphic sequence of Macuro, Península of Paria.

Triassic-Jurassic age appears in relation with the San José‚ sandstone (Colombia) and the Roraima formation, in the Geological maps of South America, published by the Geological Society of America, 1950.

Gansser (1954) prefers to compare the vulcanism of La Quinta formation, considered Triassic-Jurassic, and the Chapiza-Misahually formation from Ecuador, proven of the same age (on paleontological basis) with the one of the "Volcanic Series" (Pastora Series in Venezuela), underlying the Roraima; on the other hand, he is in favor of a Cretaceous age for the Roraima.

Gansser, following the Colombian geologists, is also in favor of a Cretaceous age for the San José‚ sandstone which, in the sub-Andean region of the Macarena, appoximate western limit of the formation, occurs above the Silurian and Devonian? and under the Eocene-Oligocene; on the other hand, in the Colombian Andean foothills, the Albian-Cenomanian with sandy facies increase in thickness towards the shield located to the east, and Gansser is of the opinion that the whole Cretaceous section goes from marine to clastic fluviatil facies until it integrates itself over the shield with the San José formation (Roraima).

Jean Marc Sellier de Civrieux

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