GEOLOGY AND GEOMORPHOLOGY OF THE RORAIMA GROUP, SOUTHEASTERN VENEZUELA
Abstract
A layered sequence of Proterozoic sedimentary rocks cover 50,000 km('2) in south-eastern Venezuela, locally is more than 3,000 m thick, and rests on older pre-Cambrian igneous-metamorphic rocks of the Guiana Shield. This Roraima "Formation" (Schwark and Anglade, 1956) extends also into Brazil and Guyana. The region is wild and inaccessible except by air; remoteness, climate, and jungle kept the region virgin until the 1960's. Geologic and geomorphologic mapping has been sparse owing to lack of accurate base maps for an inventory of natural resources. Radar imagery and reasonably good high-altitude aerial photography was acquired during the 1970's. The major purpose of this study was to prepare a moderately detailed, accurate geologic-geomorphologic map of the Roraima region at a 1:250,000 scale. This will provide a suitable base for future, more detailed studies. It is recommended that the Roraima "Formation" be raised to Group status and include three apparently conformable but lithologically separable Formations: from the base upwards, these are the Canaima, Guaiquinima, and Auyantepui formation. Two composite stratigraphic sections are established and formational thicknesses determined near Santa Elena de U. and Canaima. Five members of the Canaima Formation are distinguised at Santa Elena, and eight members in the Canaima district. Radar imagery and aerial photo interpretation allows improved mapping of regional and local structures. Broad, gentle, post-depositional folding of the Roraima into anteclises and syneclises apparently was followed by intrusion of basic dikes and skills; the principal host is the Canaima Formation. Drainage evolution and migration of cuesta-scarps, the major relief element of the region, has been controlled by regional, or local structures. Black- and down-wasting of scarps through time has resulted in successive basinward shifts and lowered base levels of major strike valleys, such as along Caroni River; shifting of divides and stream captures resulted from tributary piracy. Outliers are erosional relics of former cuestas. Local fracture-joint control determines the courses of major reaches of tributary streams. Diamond-bearing placers in the present alluvial valleys are the result of continuous deposition, reworking, and redeposition of older, more geographically disjunct floodplain and deltaic sediments. Future development of the region should be limited, by planning, to areas underlain by the Canaima Formation; geology and geomorphology elsewhere strongly argues for preservation of the existing natural state.
Degree
Ph.D.
Subject Area
Geology
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