TABLE MOUNTAIN GEOLOGY AND SURFICIAL GEOCHEMISTRY - CHIMANTA MASSIF, VENEZUELAN GUAYANA SHIELD

The Chimanta Massif consists of eleven table-mountains (up to more than 2600 m in elevation), which are remnants of the Auyan-tepui planation surface (with a possible Cretaceous initial age) cut into siliceous sandstones of the Mataui' Formation, the highest unit of the Precambrian Roraima Group. The sandstones are gently folded and several fracture systems control weathering and erosion. The Mataui' Formation was intruded by Precambrian diabase dikes and sills. This relationship invalidates previous suggestions that the Roraima Group contained an unconfirmity separating Lower Precambrian from upper Mesozoic units. Ombrotrophic peat-bogs are conspicuous features on the summits of the table-mountains. Peat formation probably began in the early Holocene, initiated by algae and lichens, accumulation of humidity and detritus, plant colonization of weathered rock surfaces, a low rate of organic decomposition, and high rainfall. The geochemistry of Ca, K, Fe, Si, Cl, I, and Br is discussed. The nutrient cycle consists of sunlight, which contributes to photosynthetic processes, and rainwater, which partly provides the nutrients, dissolves the bedrock, and transports the nutrients from the bedrock and eventually out of the system as river or stream water. The natural waters (rain-, river and stream, peat, and plant waters) are acid (pH range 3.5-4.7) and act on the sandstone, dissolving the siliceous cement and liberating the sand grains. This is the primary weathering mechanism and produces the karst-like topography of the table-mountain summits. Through geologic time (probably up to about 70 million years), this weathering has produced the spectacular table-mountain and savanna landscape of the Gran Sabana.