Geomorphology of the Gran Sabana, Guayana Shield, southeastern Venezuela

Abstract

The forms of the Gran Sabana have been developed mainly on rocks of the gently folded Precambrian Roraima Group, a sequence of quartzites and siliceous conglomerates, with intercalated jasper, chert, arkose, siltstone, and shale, locally intruded by diabase dikes and sills. Two planation surfaces are preserved on the Gran Sabana: the Auyán-tepui (2000–2900 m elevation) and the Wonken (900–1200 m) Surfaces, with a speculative Mesozoic initial age; they are still extending. Table mountains dominate the landscape. They comprise pediments, foothills, precipitous scarps hundreds of meters high, a rim, and a flattish summit. The table-mountain topography is controlled by joint systems; water percolates along joints and bedding planes, dissolves the siliceous cement, the quartzite disaggregates, and large blocks are isolated and collapse, accumulating at the foot of the scarps and on top of the table-mountain summits. The percolating water forms large and intricate cave systems, which frequently emerge in the scarp zone as high waterfalls. These mark the lowest level of erosion of the table mountains. Karstic structures are prominent in the summit zone. The evolution of the table-mountain topography has been controlled by a combination of tectonic, structural, stratigraphic, climatic, and temporal factors.