Geology of North-Central Guayana Shield, Venezuela

Abstract

The geological work for this paper was based on the reconnaissance mapping of more than 8000 square miles of savanna during a period of 7 months by one geologist, who was equipped with a helicopter for a quarter of this time. In the north-central part of the Guayana Shield in Venezuela, the lowest sequence of rocks, referred to as the Imataca Complex, comprises quartz-feldspar gneisses, amphibolite and amphibole-pyroxene gneisses, iron-formation, and migmatite. The grade of metamorphism ranges from almandine-amphibolite to granulite, with granites restricted to areas of lower-grade metamorphism. Overlying the gneisses on the south are strata of the Pastora-Carichapo Assemblage. The Carichapo Formation consists of amphibolite derived from mafic lavas and is interlayered with sedimentary, including manganiferous, strata. Perhaps unconformable above the Carichapo Formation is the Pastora Series, a sequence comprising volcanic wackes, shale, and mafic flows, all in the greenschist facies. On the north side of the Imataca Complex are quartzites, conglomerates, and amphibolites of the Real Corona-El Torno Assemblage. From considerations of metamorphic grade, these rocks undoubtedly overlie the Imataca sequence and from lithologic considerations, may be equivalent to the Pastora-Carichapo Assemblage. The occurrence of aluminous hematitic pebbles in one of the conglomerates suggests that the conglomerates have not been derived from the Imataca terrain. Both elongate, gneissic folds and more equidimensional domes have formed in the Imataca Complex. Some of the folds are strongly overturned but show no consistent regional pattern. The Complex is traversed by the El Pao and the Ciudad Piar-Guri faults, the former associated with as much as 3 km of flaser gneiss and mylonite and the latter with several hundred meters of mylonite. Despite these differences in the widths of cataclastic belts, the El Pao fault has been the locus of less movement, for like a splay from the Ciudad Piar-Guri fault, it disappears in fold structures. From a consideration of structural relationships and isotopic age determinations, it appears that Imataca rocks must have been folded prior to 2.0 b.y. before the present (B.P.), that Imataca and Carichapo rocks were folded together about 2.0 b.y. B.P., and that Pastora strata may have been recrystallized again about 1.3 b.y. B.P. in some places. The Roraima Formation of quartzites and shales delimits a very old, stable, tectonic element to the south. The major lithologic differences and major faults are indicated also by regional gravity and magnetic data.