Geology of southeastern Venezuela
In southeastern Venezuela, in the region of the Gran Sabana, there outcrops the Roraima formation of conglomerates, sandstones, and shales, with interbedded thin tuffs. The greatest thickness, 2400 meters, at Mount Auyan-tepui, represents merely an erosion remnant. Predominant reddish tints, cross-bedding, arkosic materials, lateral and vertical variations, and the complete lack of fossils indicate a terrestrial origin. Thick dikes, sills, and laccoliths of gabbroitic rock intrude the Roraima, as well as a few acid dikes probably related to the same magma. The sediments are highly silicified. The age of the Roraima is undetermined. The Roraima formation was deposited on the peneplaned surface of the Archean basement of the Guayana Shield. In the Gran Sabana area, these old rocks outcrop only along the Brazilian border where they consist of red and gray silicified porphyries. The Roraima beds are essentially horizontal, except for gentle arching due to the gabbroitic intrusions. The present topography of mesas and cuestas is controlled by an original northward dip of the sediments off the old basement combined with the structures resulting from intrusion. The drainage is controlled by stream piracy due to a gentle southward tilting of the older northward-dipping beds. Gold and diamonds in small quantities are associated with the basal conglomerate of the Roraima series and are concentrated in the Recent alluvial deposits eroded from the conglomerate.