IMATACA, Series

PRECAMBRIAN? (Lower Algonquian?)

State of Bolívar and Territorio Federal Delta Amacuro, 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.

The name Imataca series was introduced in geological literature by Newhouse and Zuloaga (1929, p. 798). According to the original description, the unit is made-up of medium-grained, well-stratified, iron-gray quartzites. The quartzites are composed of essential: quartz, magnetite, hematite and accesory: pyroxene, apatite, zircon and garnet. In this series is where the large iron deposits of the Venezuelan Guiana are found, the reserves of which exceed a 1.000.000.000 metric tons of high-law ore. The name Imataca series was subsequently changed by Zuloaga and Tello (1930, p. 412-416) to Imataca formation.

The rocks of the Imataca series are confined to the northern border of the Venezuelan Guiana highlands, where they form an approximately 400 Kms. long orogenic belt that goes from the Aro River, S.W. of Cindad Bolívar, to the vicinity of the British Guiana frontier. The Imataca series strikes N-70-E and dips south. It probably extends much farther into the west than what is known today.

The observations carried out by the geologists G. Dengo, N. J. Perfetti and L. Candiales, in the region of Santa Barbara, where the quartzite and the gneiss outcrop a short distance apart, show that the strikes and dips are perfectly parallel. This suggests a conformable contact or an interbedding.

A. Bellizzia and C. Martín Bellizzia (Rubio et al., 1952, p. 480) observed at El Pao, in fresh outcrops, the conformable contact between the quartzites and a highly-weathered gneiss. The quartzites interbedded with the gneisses were observed, on the road between El Pao and Palúa. Concordant intrusions of basic rocks (gabbros and diabases) were also found. One of the best outcrops, where the interbedding of the gneisses and the quartzites is observed, is the "Las Vueltas de Las Marias", some 10 kms. from Upata on the road to Guasipati.

From what was said above, Newhouse and Zuloaga's, and Zuloaga and Tello's original idea of restricting the Imataca series to the iron quartzites and the iron deposits should be modified, since the new investigations indicate that the Imataca series is constituted by a complex of metamorphosed sedimentary rocks, varying from ferruginous quartzites and hematitic gneiss to feldspar gneiss and schists which represent metamorphosed clay sediments. Such gneisses and schists, when highly metamorphosed, can easily be confused with intrusive rocks. They are frequently cut by numerous quartz, aplite, and pegmatite veins. In some regions they are injected by granitic materials which develop large migmatic zones. Recent studies (Perfetti and Candiales, unpublished information) have demonstrated the existence of rather important manganese deposits in rocks of the Imataca series located at the regions of Upata and the Aro River.

The gneisses of the Imataca series contain interbeddings of amphibolitic schists, amphibolites and garnetiferous amphibolites, similar to those described in the Archean. They seem to constitute the same post-Pastora intrusive series of basic rocks and to represent different intrusive cycles subsequent to the Pre-Cambrian. These rocks were changed into amphibolitic schist and amphibolite, in the first case by post-Pastora orogenesis and in the latter by diastrophic processes which affected the region to the end of the Pre-Cambrian period. It is possible that the amphibolitic rocks woud represent metamorphosed clay sediments. Sometimes the rocks of the metamorphic complex are intruded by dikes and sills made-up of basic igneous rocks (diabases, gabbros, and nyorites) older than those already mentioned, since they present no metamorphism.)

The petrographical identification carried out by Cecilia Martin Bellizzia (1953, p. 75) on a dolomitic marble pertaining to a rock collection pickedup by R. P. Morrison in the Guacurìpla region is considered of a remarkable importance. R. P. Morrison (1953, p. 50-51) proposed the name Imataca group in order to include the following formational units: a) ferruginous quartzites, b) dolomitic marble (Guacuripia formation; see), c) hornblende schists and paragneiss.

The age of the Imataca series is dubious since no fossils have ever been found and it is very possible that they will never be found on account of the intense metamorphism of the rocks. The Imataca quartzites have been correlated by Newhouse and Zuloaga (1929, p. 798), Zuloaga (1930, p. 471 and 1934, p. 1183), Zuloaga and Tello (1939, p. 416) with the Itabira formation of the Minas Geraes Department of Brazil, on the basis of lithological similarities and stratigraphic relations. Later on (1953), A. Bellizzia and C. Martín Bellizzia identified in exploratory core-drills of the El Pao ferriferous zone, the anortosite rock, the presence of which is considered by modern petrologists as index of Pre-Cambrian age. Basing themselves on such criterion and in the stratigraphic relations with the Pastora series, the above mentioned authors assigned to the Imataca series a lower Algonquian age. The series appears as a beach deposit, being found at the northern border of the Guayana shield in a position similar to that of the Itabira formation in the southern border of the Brazilian Shield.

The Imataca Series was referred by Liddle (1928) to his "Guayana Series". Later on, he modified the description of said series in order to include in it younger rocks of the Pastora series.

Cecilia Martín Bellizzia y Alirio Bellizzia